Hyponatremia
Hyponatremia is a serum sodium concentration below 135 mEq/L, resulting from a relative excess of water to sodium in the extracellular fluid.
Hyponatremia
1. Definition
Hyponatremia is defined as a serum sodium concentration < 135 mmol/L [1][2]. It is the most common electrolyte disorder encountered in clinical practice, particularly in hospitalised patients.
The name itself tells you the condition: "hypo" = low, "natr-" (from Latin natrium) = sodium, "-emia" = in the blood. So literally: low sodium in the blood.
Critical Concept: Sodium Reflects Water, Not Sodium Content
[Na⁺] does not reflect the absolute content of Na⁺ in the body → but reflects the amount of solvent (water) [1][3]. This is the single most important concept in understanding hyponatremia. In most cases:
- Hyponatremia = relative water excess (dilution/fluid overload)
- Hypernatremia = relative water deficit (dehydration)
The situation becomes more complex when there is concomitant Na⁺ loss or retention [3].
Na is primarily an extracellular cation [3]. Serum [Na⁺] is determined by the ratio of total body sodium (plus its accompanying anions) to total body water (TBW):
This "Edelman equation" (simplified) explains why hyponatremia can result from:
- Excess water relative to sodium (most common) — dilutional
- Loss of sodium relative to water — depletional
- A combination of both
| Severity | Serum [Na⁺] (mmol/L) | Clinical Relevance |
|---|---|---|
| Mild | 130–134 | Often asymptomatic, may cause subtle cognitive impairment |
| Moderate | 125–129 | Nausea, headache, confusion |
| Severe | < 125 | Risk of seizures, coma, cerebral herniation, death |
| Category | Time Frame | Significance |
|---|---|---|
| Acute | Develops within < 48 hours | Brain has NOT adapted → higher risk of cerebral oedema → more aggressive correction warranted |
| Chronic | Develops over ≥ 48 hours | Brain HAS adapted (lost organic osmolytes) → rapid correction risks osmotic demyelination syndrome (ODS) |
High Yield: If the duration is unknown, assume chronic to avoid the catastrophic complication of osmotic demyelination syndrome (ODS/central pontine myelinolysis) from overly rapid correction [1][2].
- Most common electrolyte abnormality in hospitalised patients [1][2]
- Prevalence in hospitalised patients: ~15–30% (mild hyponatremia), ~1–4% (moderate-severe)
- More common in:
- Elderly (reduced ability to excrete free water, polypharmacy, multiple comorbidities) [5]
- Hospitalised patients (iatrogenic causes: hypotonic IV fluids, post-operative state)
- Psychiatric patients (psychogenic polydipsia, SSRIs)
- ICU patients (multi-organ dysfunction, fluid management)
- Patients with chronic diseases: heart failure, cirrhosis, CKD, malignancy
- Hong Kong relevance: High prevalence of elderly population, frequent use of thiazide diuretics, and common comorbidities (HF, CKD, liver disease) make hyponatremia a bread-and-butter clinical problem. Prescribing in older people is a key exam topic — thiazide-related hyponatremia is particularly important in the elderly Hong Kong population [5].
Understanding risk factors requires knowledge of the mechanisms that can lower serum [Na⁺]:
| Category | Risk Factors | Mechanism |
|---|---|---|
| Age | Elderly (> 65) | Reduced renal diluting capacity, ↓ GFR, ↓ total body water, polypharmacy [5] |
| Sex | Female (especially post-menopausal) | Oestrogen impairs brain Na⁺-K⁺-ATPase → reduced adaptation to hypo-osmolality; lower body water volume |
| Drugs | Thiazides, SSRIs, carbamazepine, PPIs, opioids, NSAIDs, desmopressin, antipsychotics, loop diuretics (less common than thiazides) [1][5] | See below for mechanism per drug |
| Comorbidities | Heart failure, cirrhosis, nephrotic syndrome, CKD, hypothyroidism, adrenal insufficiency | ↓ Effective circulating volume → ↑ ADH; or direct hormonal effect |
| Hospitalisation | Post-operative state, hypotonic fluid administration | Non-osmotic ADH release (pain, nausea, anaesthesia) + hypotonic fluids |
| Psychiatric illness | Psychogenic polydipsia | Overwhelming the kidney's diluting capacity |
| Malignancy | SCLC, head/neck tumours, lymphoma | Ectopic ADH (SIADH) |
| Pulmonary disease | Pneumonia, TB, COPD | SIADH |
| CNS disease | Stroke, SAH, meningitis, TBI | SIADH or cerebral salt wasting |
| Exercise | Marathon runners | Excessive hypotonic fluid intake + non-osmotic ADH release |
| Diet | "Tea and toast" diet, beer potomania | Very low solute intake → reduced obligatory free water excretion |
Why Thiazides > Loop Diuretics for Hyponatremia
Thiazides act on the distal convoluted tubule (DCT), which is in the cortex, and do NOT impair the medullary concentration gradient. This means the kidney can still concentrate urine via ADH (aquaporin-2 insertion in collecting ducts). So you lose NaCl at the DCT but can still reabsorb free water at the collecting duct → net effect: sodium loss with water retention → hyponatremia.
Loop diuretics act on the thick ascending limb of the Loop of Henle, which DOES impair the medullary concentration gradient (by blocking the Na⁺-K⁺-2Cl⁻ cotransporter, NKCC2). This prevents the kidney from creating the hyperosmolar medullary interstitium needed to reabsorb water. So loop diuretics cause a relatively "isotonic" fluid loss and are actually used to treat hyponatremia in some settings.
This is a favourite exam question.
4. Anatomy and Physiology of Water and Sodium Homeostasis
-
Total body water (TBW) ≈ 60% of body weight in men, ~50% in women and elderly
- Intracellular fluid (ICF): 2/3 of TBW (~28L in 70kg male)
- Extracellular fluid (ECF): 1/3 of TBW (~14L)
- Interstitial fluid: 3/4 of ECF (~10.5L)
- Intravascular (plasma): 1/4 of ECF (~3.5L)
-
Na⁺ is the primary extracellular cation [3] — it determines ECF osmolality and volume
-
K⁺ is the primary intracellular cation
-
Water moves freely across cell membranes following osmotic gradients
Most of sodium (70%) is reabsorbed at the proximal tubules, 25% at the Loop of Henle, very little at the remaining places [3].
| Nephron Segment | % Na⁺ Reabsorbed | Transporter | Notes |
|---|---|---|---|
| Proximal tubule | ~65–70% | Na⁺/H⁺ exchanger (NHE3), Na⁺-glucose cotransporter (SGLT2) | Isotonic, obligatory; follows GFR |
| Thick ascending limb (TAL) | ~25% | Na⁺-K⁺-2Cl⁻ (NKCC2) | Water-impermeable → creates dilute tubular fluid & hyperosmolar medulla; site of loop diuretic action |
| Distal convoluted tubule (DCT) | ~5% | Na⁺-Cl⁻ cotransporter (NCC) | Site of thiazide action |
| Collecting duct | ~1–3% | Epithelial Na⁺ channel (ENaC) | Regulated by aldosterone; site of amiloride/spironolactone action |
Antidiuretic hormone (ADH), also called arginine vasopressin (AVP), is the master regulator of water balance:
- Synthesis: Supraoptic and paraventricular nuclei of the hypothalamus
- Storage and release: Posterior pituitary (neurohypophysis)
- Action: Binds V2 receptors on the basolateral membrane of collecting duct principal cells → activates cAMP → inserts aquaporin-2 (AQP2) channels on the apical membrane → allows free water reabsorption from the tubular lumen back into the hyperosmolar medullary interstitium
Stimuli for ADH release:
| Osmotic (physiological) | Non-osmotic (pathological or stress-related) |
|---|---|
| ↑ Serum osmolality (detected by hypothalamic osmoreceptors) — threshold ~280 mOsm/kg | ↓ Effective circulating volume (detected by carotid/aortic baroreceptors, atrial stretch receptors) |
| Nausea, pain, stress, post-operative state | |
| Drugs: SSRIs, carbamazepine, opioids, DDAVP | |
| Ectopic production: SCLC, other malignancies |
Key principle: In hypovolemia, the body prioritises volume over osmolality. Even if [Na⁺] is low (low osmolality), the volume-depletion signal is stronger and ADH is still released → water retention → worsening hyponatremia. This is why hypovolemic hyponatremia exists.
- Osmoreceptors in the hypothalamus also drive thirst when osmolality rises
- Thirst threshold (~290 mOsm/kg) is slightly higher than ADH release threshold (~280 mOsm/kg)
- In the elderly, thirst perception is blunted → greater susceptibility to both hypernatremia (if not drinking) and hyponatremia (if given hypotonic fluids)
5. Etiology and Pathophysiology
The approach to etiology follows a systematic framework based on serum osmolality and volume status. This is the standard clinical approach taught in the GC lecture and data interpretation sessions [1][2][4].
5.1 Step 1: Assess Serum Osmolality — Is This True Hyponatremia?
- Mechanism: Laboratory artefact. Normal serum is ~93% water and ~7% solids (lipids, proteins). Sodium is dissolved only in the water phase. When there is massive elevation of lipids or proteins (e.g., severe hypertriglyceridaemia, multiple myeloma with paraprotein), the solid fraction increases → the water fraction per unit volume decreases → the measured [Na⁺] in a given volume of serum appears low, but the [Na⁺] in the water phase is actually normal.
- This artefact occurs with indirect ion-selective electrode (ISE) methods which require dilution of the sample. Direct ISE (used in blood gas analysers / point-of-care) measures [Na⁺] in the water phase directly and will show a normal result.
- Causes:
- Severe hyperlipidaemia (hypertriglyceridaemia)
- Severe hyperproteinaemia (multiple myeloma, Waldenström's macroglobulinaemia)
- Management: No treatment needed for the sodium itself — treat the underlying condition. The sodium is actually normal!
- Mechanism: Presence of an osmotically active solute in the ECF (that does NOT freely cross cell membranes) draws water from the ICF to the ECF → dilutes [Na⁺].
- Causes:
- Hyperglycaemia (most common) — correction factor: for every 5.6 mmol/L (100 mg/dL) rise in glucose above normal, serum [Na⁺] falls by ~2.4 mmol/L (some sources say 1.6 mmol/L; the more recent Hillier 1999 correction uses 2.4) [1]
- Mannitol infusion
- Radiocontrast agents (rarely)
- Glycine irrigation fluid (post-TURP syndrome)
- Management: Treat the underlying cause (e.g., insulin for hyperglycaemia); [Na⁺] will correct as the osmotically active solute clears
Exam Trap: Hyperglycaemia and Hyponatremia
Always correct the measured [Na⁺] for glucose in a diabetic patient before diagnosing true hyponatremia. A patient in DKA with glucose 30 mmol/L and Na⁺ 128 may actually have a corrected Na⁺ that is normal or even elevated. This is high yield in data interpretation stations [1][4].
Corrected Na⁺ ≈ measured Na⁺ + 2.4 × [(glucose − 5.6)/5.6]
This is the clinically important category. Further sub-classified by volume status (Step 2).
This is the bread-and-butter classification. The pathophysiology of each category differs fundamentally.
Wait — let me correct that. In hypovolemic hyponatremia: both total body Na⁺ and TBW are decreased, but Na⁺ is lost MORE than water (or water is retained secondarily via ADH). The net effect is that [Na⁺] falls.
Pathophysiology: Volume depletion → baroreceptor-mediated non-osmotic ADH release (body prioritises volume over osmolality) → water retention despite low osmolality → [Na⁺] falls further.
Use urine Na⁺ to differentiate renal from extrarenal losses [1][2][4]:
| Urine [Na⁺] | Source of Loss | Causes |
|---|---|---|
| < 20 mmol/L (kidney retaining Na⁺) | Extrarenal losses | GI losses (diarrhoea, vomiting, NG suction), third-spacing (pancreatitis, burns, bowel obstruction), sweating |
| > 20 mmol/L (kidney wasting Na⁺) | Renal losses | Thiazide diuretics, mineralocorticoid deficiency (Addison's), salt-wasting nephropathy, cerebral salt wasting, osmotic diuresis, post-obstructive diuresis |
Why does vomiting cause hyponatremia if you're losing HCl (acid) rather than NaCl?
- Vomiting causes loss of H⁺ and Cl⁻ → metabolic alkalosis
- Volume depletion triggers proximal tubular Na⁺-HCO₃⁻ reabsorption AND ADH release
- The net effect is Na⁺ loss + water retention → hyponatremia
- Additionally, the alkalosis stimulates NaHCO₃ excretion in urine (paradoxical aciduria comes late)
Cerebral Salt Wasting Syndrome (CSWS) [6]
Cause: head pathologies, e.g. cerebral OT, head injury, SAH, cerebral tumour [6].
Pathophysiology: idiopathic natriuresis + diuresis secondary to cerebral disorder [6].
Important d/dx of SIADH [6]:
- Both can follow head pathologies
- CSWS results in renal Na loss → hypovolemic hyponatremia
- cf. SIADH results in renal water retention → euvolemic hyponatremia (different Tx) [6]
| Feature | CSWS | SIADH |
|---|---|---|
| Volume status | Hypovolemic | Euvolemic |
| Urine Na⁺ | High (> 20) | High (> 20) |
| Treatment | Saline replacement (give volume) | Fluid restriction (opposite!) |
| CVP | Low | Normal/slightly elevated |
| Body weight | Decreased | Normal/increased |
High Yield: Getting CSWS vs SIADH wrong means giving the exact opposite treatment — restricting fluids in a volume-depleted patient or giving saline to a euvolemic patient with SIADH (which worsens hyponatremia). This is a classic exam pitfall [6].
5.4 Euvolemic Hyponatremia (Normal Total Body Na⁺, ↑ Total Body Water)
This is the most common category in clinical practice. The patient is clinically euvolemic — no oedema, no signs of dehydration. TBW is mildly expanded (up to ~3L excess before oedema appears) but Na⁺ content is normal.
The most common cause of euvolemic hyponatremia and a favourite exam topic.
Pathophysiology:
- All hypoNa is accompanied with an excessive amount of ADH secretion [6]
- ↑ADH in hypovolemic and hypervolemic hypoNa results from ↓effective circulatory volume [6]
- ↑ADH in euvolemic hypoNa results from a true non-physiological ↑ADH [6]
- Those [euvolemic cases] that cannot be explained by secondary causes are grouped as SIADH [6]
In SIADH, ADH is released despite:
- Normal/low serum osmolality (should suppress ADH)
- Normal blood volume (no baroreceptor stimulus)
The inappropriate ADH → AQP2 insertion → free water reabsorption from collecting ducts → dilution of [Na⁺]. The mild volume expansion triggers natriuresis (via ANP/BNP and suppression of aldosterone), which explains the high urine [Na⁺].
| Category | Examples |
|---|---|
| Malignancy | Small cell lung cancer (SCLC) (most classic), head/neck tumours, lymphoma, other carcinomas |
| Pulmonary disease | Pneumonia, TB, lung abscess, COPD, positive-pressure ventilation |
| CNS disorders | Stroke, SAH, meningitis, encephalitis, head trauma, brain tumour |
| Drugs | SSRIs, carbamazepine, cyclophosphamide, vincristine, oxytocin, NSAIDs, opioids, ecstasy (MDMA), PPIs |
| Post-operative | Pain, nausea, anaesthesia → non-osmotic ADH release |
| Others | HIV, acute intermittent porphyria, idiopathic |
Diagnosis of SIADH is by exclusion [6]:
- ↓ Serum osmolality (i.e. true hypoNa)
- ↑ Urine osmolality > 200 mOsm/kg (inappropriate concentration of urine despite dilute serum) [6]
- Urine [Na⁺] > 20 mmol/L — both indicating inappropriate natriuresis [6]
- Normal RFT, cardiac and liver function (i.e. in normal euvolemic state) [6]
- Normal TFT and adrenal function [6]
- No related drugs [6]
- Response to fluid restriction [6]
- ↑ Plasma ADH [6] (not routinely measured clinically)
Understanding Urine Osmolality in SIADH
Note that urine osmolality > 200 mOsm/kg is not a so-called 'normal range' of urine osmolality. Urine osmolality is largely controlled by ADH to correct changes in serum osmolality. A hypo-osmolar state should be accompanied with a hypo-osmolar urine. Therefore urine osmolality should be interpreted together with serum osmolality [6].
In other words: if serum osmolality is LOW, the kidney should produce maximally DILUTE urine (< 100 mOsm/kg) by suppressing ADH. If urine is > 200 mOsm/kg despite low serum osmolality, ADH is inappropriately present.
- Severe hypothyroidism (myxoedema) → reduced cardiac output → ↓ GFR → impaired free water excretion
- Also possible direct stimulation of ADH release
- Mild/moderate hypothyroidism is a rare cause; only severe cases cause significant hyponatremia
- Always check TFT when working up hyponatremia [6]
- Cortisol normally suppresses ADH release. In cortisol deficiency → loss of this tonic inhibition → ADH is released → water retention → hyponatremia
- Cortisol deficiency also → ↓ cardiac output → ↓ GFR → impaired water excretion
- If primary adrenal insufficiency (Addison's) → also lose aldosterone → additional renal Na⁺ wasting → hypovolemic component
- If secondary adrenal insufficiency (pituitary ACTH deficiency) → aldosterone production is largely maintained (RAAS still works) → presents more as euvolemic hyponatremia
- Always check adrenal function (9am cortisol, ACTH stimulation test) [6]
- Intake of > 10–15 L/day overwhelms the kidney's maximum diluting capacity (~18 L/day)
- ADH is appropriately suppressed → urine is maximally dilute (< 100 mOsm/kg)
- Differentiating feature from SIADH: urine osmolality is LOW (< 100 mOsm/kg) in polydipsia vs. inappropriately HIGH (> 200) in SIADH
- Common in psychiatric patients (schizophrenia)
- Low solute intake → fewer osmoles to excrete → limited obligatory water excretion
- Normal kidneys can dilute urine to ~50 mOsm/kg, but if total daily solute is only 200 mOsm (from low protein, low salt diet or beer), maximum daily urine volume = 200/50 = 4L. Any water intake above this → retained → hyponatremia
- Urine osmolality is LOW (< 100 mOsm/kg), similar to polydipsia
- ADH is regulated normally but around a lower set point (e.g., 270 instead of 280 mOsm/kg)
- Chronic mild hyponatremia that is stable
- Seen in: pregnancy, chronic illness, elderly, quadriplegia
- Key feature: urine CAN be diluted normally if water load is given → helps distinguish from SIADH
Both Na⁺ and water are increased, but water proportionally more so → [Na⁺] falls. These patients have oedema (peripheral oedema, ascites, pulmonary oedema).
The unifying mechanism: ↓ effective arterial blood volume (EABV) despite total body volume expansion → baroreceptor-mediated ADH release + RAAS activation → water and Na⁺ retention (but water retention dominates).
| Condition | Why ↓ EABV? | Urine [Na⁺] |
|---|---|---|
| Congestive heart failure | Reduced cardiac output → poor forward flow | Usually < 20 (kidney retaining Na⁺) unless on diuretics |
| Cirrhosis | Splanchnic vasodilation → effective hypovolemia; portal hypertension → ascites | Usually < 20 unless on diuretics |
| Nephrotic syndrome | Hypoalbuminaemia → reduced oncotic pressure → fluid shifts to interstitium | Usually < 20 |
| Advanced CKD / Renal failure | Impaired free water excretion from reduced GFR | > 20 (kidneys cannot retain Na⁺) |
Why does cirrhosis cause hyponatremia?
- Portal hypertension → splanchnic NO-mediated vasodilation → ↓ EABV
- Baroreceptors detect "underfilling" → activate RAAS + sympathetic nervous system + ADH
- ADH-mediated water retention is disproportionate to Na⁺ retention → dilutional hyponatremia
- This is why overdiuresis, excess paracentesis of ascites fluid without adequate albumin replacement [7] can worsen hyponatremia in cirrhotic patients
Drug-induced hyponatremia is extremely important, especially in the elderly population [5].
| Drug Class | Mechanism | Clinical Pearl |
|---|---|---|
| Thiazide diuretics | NaCl loss at DCT without impairing medullary gradient → ADH can still reabsorb water | #1 drug cause of hyponatremia; onset typically within 2 weeks of starting; more common in elderly women |
| SSRIs (e.g., sertraline, fluoxetine) | Stimulate ADH release (SIADH-like) | Very common in elderly; onset 1–4 weeks |
| Carbamazepine, oxcarbazepine | Enhance renal sensitivity to ADH | Anti-epileptic — check Na⁺ regularly |
| NSAIDs | Potentiate ADH effect; reduce prostaglandin-mediated renal water excretion | Common co-prescription in elderly |
| PPIs | SIADH (mechanism unclear) | Rare but recognised |
| Opioids | Non-osmotic ADH release | Perioperative context |
| DDAVP (desmopressin) | Exogenous ADH analogue | Used for DI/nocturnal enuresis — can cause hyponatremia if fluid intake not restricted |
| Ecstasy (MDMA) | Stimulates ADH release + excessive water intake | Young adults; can be fatal |
| Cyclophosphamide | SIADH + concomitant IV hydration | Oncology setting |
| ACEi/ARBs | Mild impairment of water excretion (unclear mechanism); potentiate natriuresis | Usually mild |
Beers Criteria & Prescribing in the Elderly
The AGS Beers Criteria (2023) flags drugs to avoid or use with caution in the elderly [5]. Thiazides, SSRIs, and carbamazepine — all common causes of hyponatremia — are frequently prescribed in older patients. Always check [Na⁺] within 1–2 weeks of starting these medications in the elderly.
| Serum Osmolality | Volume Status | Key Causes |
|---|---|---|
| High (> 295) | — | Hyperglycaemia, mannitol |
| Normal (275–295) | — | Pseudohyponatremia (↑↑ lipids/proteins) |
| Low (< 275) — TRUE | Hypovolemic (UNa < 20) | GI losses, third-spacing, burns |
| Low (< 275) — TRUE | Hypovolemic (UNa > 20) | Thiazides, Addison's, CSWS, salt-wasting nephropathy |
| Low (< 275) — TRUE | Euvolemic | SIADH (most common), hypothyroidism, adrenal insufficiency, polydipsia, beer potomania, reset osmostat |
| Low (< 275) — TRUE | Hypervolemic | Heart failure, cirrhosis, nephrotic syndrome, advanced CKD |
6. Classification
- Hypertonic (translocational)
- Isotonic (pseudohyponatremia)
- Hypotonic (true)
- Hypovolemic
- Euvolemic
- Hypervolemic
- Acute (< 48h)
- Chronic (≥ 48h or unknown duration)
- Mild (130–134): Subtle symptoms, often incidental finding
- Moderate (125–129): Nausea, confusion, headache
- Severe (< 125): Seizures, coma, cerebral oedema, herniation
- Symptomatic (requires more urgent management) vs. Asymptomatic (incidental finding, manage cause)
7. Clinical Features
The symptoms of hyponatremia are primarily neurological, because the brain is the organ most vulnerable to osmotic shifts.
Pathophysiological basis: When serum [Na⁺] falls → serum osmolality drops → water moves from the hypotonic ECF into brain cells (which have relatively higher intracellular osmolality) → cerebral oedema → raised intracranial pressure → neurological symptoms.
The severity of symptoms depends on:
- Degree of hyponatremia (how low)
- Rate of development (acute vs. chronic) — this is the more important factor
In chronic hyponatremia, brain cells undergo volume regulatory decrease: they expel intracellular solutes (Na⁺, K⁺, Cl⁻ first, then organic osmolytes like myo-inositol, glutamate, taurine) over 24–48 hours. This partially restores brain cell volume → fewer symptoms despite low [Na⁺]. This adaptation is why chronic hyponatremia can be asymptomatic even at Na⁺ ~115–120.
However, this adaptation also makes the brain vulnerable to osmotic demyelination if [Na⁺] is corrected too rapidly — because the brain cells have already lost their osmolytes, rapid increase in ECF osmolality pulls water OUT of now-"shrunken" brain cells → dehydration of neurons → demyelination (particularly in the central pons → central pontine myelinolysis, now called osmotic demyelination syndrome).
| Symptom | Pathophysiological Basis | Acuity/Severity Association |
|---|---|---|
| Nausea, malaise | Mild cerebral oedema → stimulation of emetic centre | Early/mild |
| Headache | ↑ ICP from cerebral oedema | Moderate |
| Confusion, disorientation | Neuronal dysfunction from oedema and electrolyte imbalance | Moderate |
| Drowsiness, lethargy | Progressive cerebral oedema → depressed consciousness | Moderate-severe |
| Muscle cramps, weakness | Altered neuronal excitability, hypo-osmolar extracellular environment affecting NMJ transmission | Moderate |
| Seizures | Severe cerebral oedema → cortical irritation; also direct effect of hypo-osmolality on neuronal membrane potential (lowered threshold) → hyponatremia is a recognised cause of seizures [8] | Severe / acute |
| Coma | Severe cerebral oedema → brainstem compression | Severe |
| Respiratory arrest | Brainstem herniation (transtentorial) → compression of respiratory centres | Terminal event |
| Falls and gait instability | Subtle cognitive and neuromuscular dysfunction, even in "mild" hyponatremia | Chronic, underappreciated; important in elderly [5] |
| Cognitive impairment | Chronic low-grade cerebral oedema; impaired neurotransmission | Chronic, even mild |
| Osteoporosis (chronic) | Hyponatremia → ↑ osteoclast activity and bone resorption (recently recognised) | Chronic |
Even 'Mild' Chronic Hyponatremia Matters
Recent evidence shows that even mild chronic hyponatremia (130–134) is associated with:
- Increased risk of falls in the elderly
- Osteoporosis and fracture risk
- Cognitive impairment
- Increased all-cause mortality
This is why chronic "asymptomatic" hyponatremia should still be investigated and managed, not ignored [5].
7.2 Signs
Volume status assessment is the cornerstone of physical examination in hyponatremia — it guides the etiological workup and determines treatment:
| Sign | Pathophysiological Basis |
|---|---|
| Postural hypotension (drop in SBP ≥ 20 mmHg or DBP ≥ 10 mmHg on standing) | Reduced intravascular volume → inadequate baroreceptor-mediated vasoconstriction on standing |
| Tachycardia | Compensatory sympathetic activation for reduced cardiac output |
| Dry mucous membranes | Reduced total body water |
| Reduced skin turgor | ↓ Interstitial fluid → skin tenting, especially on sternum/forehead in elderly (skin turgor unreliable on limbs in elderly due to loss of elasticity) |
| Sunken eyes | ↓ Periorbital fat pad hydration |
| Reduced JVP | ↓ Venous return from reduced intravascular volume |
| Reduced urine output | Kidney retaining water and sodium; ↓ GFR |
| Confusion/altered mental state | Both from hyponatremia itself and from hypoperfusion |
| Sign | Pathophysiological Basis |
|---|---|
| Peripheral (pitting) oedema | Excess ECF → transudation into interstitial space; gravitationally dependent (ankles in ambulatory, sacrum in bed-bound) |
| Raised JVP | ↑ Central venous pressure from volume overload (especially in heart failure) |
| Pulmonary crackles (crepitations) | Pulmonary oedema from ↑ pulmonary capillary pressure (LVF) or low oncotic pressure |
| Ascites | Portal hypertension + hypoalbuminaemia (cirrhosis); or ↑ hydrostatic pressure in hepatic veins (RHF). Why does RHF cause ascites? Because of hydrostatic back-pressure in the hepatic veins |
| Hepatomegaly | Hepatic congestion from RHF |
| S3 gallop | Volume overload in a failing ventricle |
| Pleural effusion | Transudation from ↑ hydrostatic pressure or ↓ oncotic pressure |
- Absence of both hypovolemic and hypervolemic signs
- Clinical euvolemia is essentially a diagnosis of exclusion on examination
- Remember: up to 3L of fluid can be retained before oedema becomes clinically apparent, so patients with SIADH may have mild subclinical volume expansion without detectable oedema
| Finding | Suggests |
|---|---|
| Cushingoid features (moon face, buffalo hump, striae) | Cortisol excess — but also consider adrenal insufficiency if recently stopped steroids |
| Hyperpigmentation (skin creases, buccal mucosa) | Primary adrenal insufficiency (Addison's) — ↑ ACTH → ↑ MSH (from same POMC precursor) |
| Dry skin, bradycardia, delayed ankle jerk relaxation, periorbital oedema | Hypothyroidism |
| Lymphadenopathy, clubbing, chest signs | Malignancy (SCLC → SIADH) |
| Stigmata of chronic liver disease (spider naevi, palmar erythema, jaundice, gynaecomastia, caput medusae) | Cirrhosis |
| Signs of heart failure (displaced apex, S3, murmurs) | Congestive heart failure |
| Neurosurgical scar, papilloedema | CNS cause (SIADH or CSWS) |
8. Pathophysiology of Complications (Brief Preview)
(Full complications section to follow in subsequent response)
- Previously called "central pontine myelinolysis" — but demyelination can also be extrapontine
- Mechanism: In chronic hyponatremia, brain cells lose organic osmolytes to maintain volume. If serum [Na⁺] is corrected too rapidly, the now osmolyte-depleted brain cells cannot retain water → cell shrinkage → endothelial damage → disruption of blood-brain barrier → oligodendrocyte death → demyelination
- Risk factors for ODS: Chronic hyponatremia, [Na⁺] < 120, alcoholism, malnutrition, liver disease, hypokalaemia
- Presentation: 2–6 days after overcorrection — dysarthria, dysphagia, paraparesis/quadriparesis, "locked-in syndrome"
- Safe rate of correction: No more than 10 mmol/L in first 24 hours, and no more than 8 mmol/L in any subsequent 24-hour period (some guidelines say ≤ 6–8 mmol/L/day for high-risk patients)
Golden Rule of Hyponatremia Correction
Correct slowly in chronic hyponatremia: maximum 10 mmol/L in 24 hours (6–8 in high-risk patients). Overcorrection is more dangerous than the hyponatremia itself in chronic cases. If you accidentally overcorrect, give D5W (5% dextrose) or DDAVP to bring [Na⁺] back down. This is the single most tested concept in hyponatremia management [1][2].
High Yield Summary
- Hyponatremia = serum [Na⁺] < 135 mmol/L — the most common electrolyte disorder in hospitalised patients.
- [Na⁺] reflects water balance, not sodium content. In most cases, hyponatremia = relative water excess.
- Step 1: Check serum osmolality → exclude pseudohyponatremia (isotonic, ↑↑ lipids/proteins) and translocational hyponatremia (hypertonic, hyperglycaemia).
- Step 2: Assess volume status → hypovolemic (Na⁺ and water lost, but Na⁺ more), euvolemic (SIADH, hypothyroidism, adrenal insufficiency), hypervolemic (HF, cirrhosis, nephrotic syndrome).
- Step 3: Check urine [Na⁺] and urine osmolality → distinguish renal vs. extrarenal losses in hypovolemic; distinguish SIADH (high urine Osm, UNa > 20) from polydipsia (low urine Osm).
- SIADH is the most common cause of euvolemic hyponatremia — diagnosis by exclusion; causes: malignancy (SCLC), pulmonary disease, CNS disease, drugs (SSRIs, carbamazepine).
- CSWS vs SIADH: both follow CNS pathology, both have high UNa; CSWS = hypovolemic (give saline), SIADH = euvolemic (restrict fluids).
- Thiazides are the #1 drug cause; they impair NaCl reabsorption at DCT but preserve medullary concentration gradient → kidney can still retain water via ADH.
- Symptoms are neurological (cerebral oedema): nausea → confusion → seizures → coma → death. Severity depends on rate of onset more than absolute level.
- Correction speed: acute symptomatic → can correct faster (3% NaCl); chronic → max 10 mmol/L/24h to avoid osmotic demyelination syndrome.
- Always correct for glucose in diabetic patients before diagnosing true hyponatremia.
- Chronic mild hyponatremia is NOT benign — associated with falls, osteoporosis, cognitive impairment, and mortality.
Active Recall - Hyponatremia (Definition, Epidemiology, Etiology, Pathophysiology & Clinical Features)
[1] Lecture slides: GC 044. Electrolyte and Acid-Base Disorders.pdf [2] Lecture slides: Chemical Pathology Seminar 1_Sodium and water.pdf [3] Senior notes: Block A - Electrolyte and Acid-Base Disorders.pdf (Sodium section) [4] Senior notes: Block A – Nephrology Data Interpretation.pdf [5] Lecture slides: GC 079. Prescribing in older people.pdf; GC 079 (supp-4) AGS Beers Criteria for potentially inappropriate med use_Pocket Guide_2023.pdf [6] Senior notes: Ryan Ho Chemical Path.pdf (SIADH and CSWS sections) [7] Senior notes: Block A - A jaundiced and incoherent patient_ liver failure.pdf (hepatic encephalopathy precipitants) [8] Senior notes: MBBS Final MB (Pediatrics) (Felix PY Lai).pdf (seizure causes – hyponatremia)
Differential Diagnosis of Hyponatremia
The differential diagnosis of hyponatremia is not a simple "list of diseases." It is a systematic diagnostic pathway — a stepwise algorithm that narrows the differential at each branch point. Hyponatremia is a biochemical diagnosis — start from the laboratory report [2]. The lab result itself is the starting point, and then you work through the physiology to arrive at the cause.
The key principle: the diagnostic pathway for differential diagnosis is built on three sequential questions [2]:
- Is this TRUE hyponatremia? → Check serum osmolality
- What is the volume status? → Clinical assessment (hypovolemic / euvolemic / hypervolemic)
- Where is the sodium (or water) going? → Urine osmolality and urine [Na⁺]
Step 1: Exclude Non-Hypotonic Causes (Is It Real?)
Before diving into the "true" differentials, you must exclude the two scenarios where serum [Na⁺] is low but the patient does NOT have hypotonic hyponatremia. This is the first branch of the algorithm.
| Differential | Mechanism | Key Clue |
|---|---|---|
| Hyperglycaemia (DKA, HHS) | Glucose is osmotically active but cannot freely cross cell membranes → draws water from ICF to ECF → dilutes [Na⁺] in ECF | Always correct Na⁺ for glucose before diagnosing true hyponatremia. Corrected Na⁺ = measured Na⁺ + 2.4 × [(glucose − 5.6)/5.6] [1][2] |
| Mannitol infusion | Same osmotic mechanism as glucose; mannitol is confined to ECF | Iatrogenic — check drug chart; used for raised ICP |
| Glycine irrigation fluid (post-TURP syndrome) | Hypotonic glycine (1.5%) used for bladder irrigation is absorbed systemically → initially dilutes Na⁺; glycine itself is osmotically active | Post-urological surgery context; may also cause visual disturbance (glycine neurotoxicity) [11] |
| Differential | Mechanism | Key Clue |
|---|---|---|
| Severe hypertriglyceridaemia | ↑ Lipid fraction in serum → ↓ water fraction per unit volume → indirect ISE underestimates [Na⁺] | Lipaemic (milky) serum; check with direct ISE / blood gas analyser — will show normal Na⁺ |
| Severe hyperproteinaemia (multiple myeloma, Waldenström's macroglobulinaemia) | Same principle as above — protein occupies volume, displacing water; indirect ISE gives falsely low reading | Check serum protein electrophoresis, serum free light chains; myeloma kidney + Tam-Horsfall casts are a related exam topic [4] |
Pseudohyponatremia: A Lab Artefact, Not a Clinical Problem
A common exam trap is to treat pseudohyponatremia. The sodium is actually NORMAL — the problem is the measurement technique. Direct ISE (point-of-care blood gas analyser) gives the true Na⁺ concentration because it measures [Na⁺] in the water phase directly. If the blood gas Na⁺ is normal but the lab Na⁺ is low, think pseudohyponatremia.
Step 2: True (Hypotonic) Hyponatremia — Differential by Volume Status
Once you have confirmed hypotonic hyponatremia (serum osmolality < 275 mOsm/kg), the next step is clinical assessment of volume status [1][2][6]. This is done at the bedside and is the most important differentiating step.
A. Hypovolemic Hyponatremia (↓ ECF Volume)
Unifying pathophysiology: Loss of both sodium and water, but sodium loss predominates OR secondary ADH release from volume depletion retains water disproportionately → [Na⁺] falls.
Clinical clue: Signs of dehydration/volume depletion — postural hypotension, tachycardia, ↓ skin turgor, dry mucous membranes, ↓ JVP, ↓ urine output.
The kidney detects hypovolemia → avidly retains Na⁺ (and water via ADH) → urine Na⁺ is low. The sodium was lost somewhere OUTSIDE the kidney.
| Differential | Mechanism / Why It Causes Hyponatremia | Distinguishing Features |
|---|---|---|
| GI losses — Diarrhoea | Direct loss of Na⁺-rich GI fluid; volume depletion → ADH release → water retention | History of diarrhoea; metabolic acidosis (loss of HCO₃⁻ from lower GI) |
| GI losses — Vomiting / NG suction | Loss of H⁺ and Cl⁻ → metabolic alkalosis; volume depletion → RAAS activation + ADH release; proximally reabsorbed NaHCO₃ → paradoxically may see urine Na⁺ elevated (bicarbonaturia) early on | History of vomiting; metabolic alkalosis; may have paradoxical alkaline urine with elevated urine Na⁺ if bicarbonaturia is present — a potential exam pitfall |
| Third-space losses — Pancreatitis, bowel obstruction | Fluid sequestered into third spaces (peritoneum, bowel lumen) → effective hypovolemia → ADH release | Acute abdomen; elevated amylase/lipase (pancreatitis); distended bowel on imaging (obstruction) |
| Burns | Massive loss of protein-rich fluid through denuded skin → hypovolemia | Clinical context obvious |
| Excessive sweating | Sweat is hypotonic but large volume loss → hypovolemia → ADH-mediated water retention | Endurance athletes, heat stroke |
Despite hypovolemia, the kidney cannot retain sodium. Something is forcing renal Na⁺ wasting.
| Differential | Mechanism / Why It Causes Hyponatremia | Distinguishing Features |
|---|---|---|
| Thiazide diuretics | Block NCC at DCT → Na⁺ wasting; intact medullary gradient → ADH can still reabsorb water → net Na⁺ loss with water retention. Most common drug cause of hyponatremia | Drug history; onset typically < 2 weeks; elderly women especially susceptible; often with concurrent hypokalaemia [1][5] |
| Mineralocorticoid deficiency (Addison's disease) | Loss of aldosterone → cannot retain Na⁺ at collecting duct (ENaC) → renal Na⁺ wasting + K⁺ retention → hypovolemia → ADH release | Hyperkalaemia (characteristic!); hyperpigmentation; postural hypotension; low morning cortisol, high ACTH; may also have metabolic acidosis (type 4 RTA from hyperkalaemia) [9] |
| Cerebral salt wasting syndrome (CSWS) | Idiopathic natriuresis + diuresis secondary to cerebral disorder → renal Na⁺ loss → hypovolemia | Follows head pathologies: cerebral OT, head injury, SAH, cerebral tumour [6]; hypovolemic (low CVP, weight loss) — contrast with SIADH which is euvolemic |
| Salt-wasting nephropathy | Intrinsic renal tubular damage → impaired Na⁺ reabsorption | Chronic kidney disease, tubulointerstitial disease, medullary cystic disease; progressive CKD features |
| Osmotic diuresis | Osmotically active solutes in tubular fluid (glucose, urea, mannitol) drag water AND sodium with them → volume depletion | Diabetes with glycosuria; post-obstructive diuresis; high urea states; high urine osmolality |
| Renal tubular acidosis (RTA) | Especially Type 1 (distal) and Type 2 (proximal): impaired tubular function → Na⁺ and HCO₃⁻ wasting; Type 4: hyperkalaemic, hypoaldosteronism | Normal anion gap metabolic acidosis; Type 1 associated with nephrocalcinosis; Type 4 associated with diabetic nephropathy (hyporeninemic hypoaldosteronism) [10] |
| Post-obstructive diuresis | After relief of bilateral obstruction → impaired tubular concentrating ability → obligatory Na⁺ and water loss | Context of relieved urinary obstruction; massive polyuria |
High Yield: Renovascular hypertension (renal artery stenosis) → reduced renal perfusion → RAAS activation → secondary hyperaldosteronism → hypokalemia and hyponatremia [9]. This is a secondary hypertension cause where hyponatremia is an associated finding, not the primary presentation.
Unifying pathophysiology: Total body sodium is essentially normal. The problem is excess water retention (dilution) — either from inappropriate ADH or excessive water intake.
Clinical clue: No oedema, no signs of dehydration. The patient "looks normal" on volume assessment — this is a diagnosis of exclusion on physical exam.
Further differentiate using urine osmolality and urine [Na⁺] [1][2][6]:
| Differential | Mechanism | Urine Osm | Urine Na⁺ | Key Distinguishing Features |
|---|---|---|---|---|
| SIADH | Non-physiological ↑ADH → water retention → dilution of Na⁺; mild volume expansion → natriuresis (ANP/BNP rise, aldosterone suppression) [6] | > 200 mOsm/kg (inappropriately concentrated) | > 20 mmol/L | Diagnosis by exclusion: must have normal RFT, cardiac and liver function, normal TFT and adrenal function, no related drugs, response to fluid restriction [6]. Must then find underlying cause: CNS (meningitis, SAH, CVA, head trauma), respiratory (CA lung, chest infection, atypical pneumonia, COVID), drugs (SSRI, ecstasy, carbamazepine), hypothyroidism [3] |
| Hypothyroidism | Severe hypothyroidism → ↓ cardiac output → ↓ GFR → impaired free water excretion; also possible direct ADH stimulation | > 200 | > 20 | Must rule out hypothyroidism (TSH) before diagnosing SIADH [3][6]; look for clinical features: bradycardia, dry skin, delayed reflex relaxation |
| Adrenal insufficiency (secondary — ACTH deficiency) | Cortisol normally suppresses ADH; cortisol deficiency → loss of tonic ADH suppression → water retention; aldosterone is preserved (RAAS-dependent) → presents as euvolemic | > 200 | > 20 | Rule out adrenal insufficiency (morning cortisol, ACTH stimulation test) before diagnosing SIADH [3][6]; fatigue, weight loss, hypotension; K⁺ is usually NORMAL (aldosterone preserved — unlike Addison's where K⁺ is HIGH) |
| Primary (psychogenic) polydipsia | Intake > 10–15 L/day overwhelms renal diluting capacity; ADH is appropriately suppressed | < 100 mOsm/kg (maximally dilute) | < 20 | Psychiatric history (schizophrenia common); massive fluid intake; urine is maximally dilute — this distinguishes it from SIADH |
| Beer potomania | Very low solute diet (beer has minimal protein/salt) → ↓ obligatory osmole excretion → ↓ maximum free water excretion capacity | < 100 | < 20 | History of chronic heavy beer consumption with poor nutrition; responds rapidly to normal diet |
| Tea-and-toast diet | Same low-solute mechanism as beer potomania | < 100 | < 20 | Elderly living alone with poor diet; very common in Hong Kong elderly |
| Reset osmostat | ADH regulation normal but around a lower set point (e.g., 270 mOsm/kg) | Variable | Variable | Chronic stable mild hyponatremia; can dilute urine normally with water load; pregnancy, chronic illness, quadriplegia |
| Exercise-associated hyponatremia | Non-osmotic ADH release (exertion, pain) + excessive hypotonic fluid intake | > 200 (inappropriate) | Variable | Marathon runners, endurance athletes; acute onset; can be fatal |
SIADH: The Diagnosis of Exclusion — Must-Exclude Checklist
Before diagnosing SIADH, you MUST exclude [2][3][6]:
- Hypothyroidism → check TSH
- Adrenal insufficiency → check morning cortisol (± ACTH stimulation test)
- Drug causes → review full medication list
- Renal impairment → check RFT
- Heart failure → clinical assessment, NT-proBNP
- Liver disease → check LFT
- Primary polydipsia → distinguish by urine osmolality (< 100 in polydipsia vs. > 200 in SIADH)
SIADH is confirmed by: ↓ serum osmolality, ↑ urine osmolality > 200, urine Na > 20, euvolemic, normal TFT, normal adrenal function, no drugs, response to fluid restriction [6].
Once SIADH is confirmed, you must then find the UNDERLYING CAUSE: CNS, respiratory, drugs, malignancy [3].
SIADH vs. CSWS — The Classic Exam Comparison
This comparison is heavily tested because both can occur after CNS pathology, both have high urine Na⁺, but treatment is diametrically opposite [6]:
| Feature | SIADH | CSWS |
|---|---|---|
| Volume status | Euvolemic | Hypovolemic |
| Mechanism | Renal water retention | Renal Na⁺ loss (natriuresis) |
| CVP | Normal / slightly elevated | Low |
| Body weight | Stable or slightly increased | Decreased |
| Urine output | Normal or decreased | Increased |
| Urine Na⁺ | > 20 (natriuresis from volume expansion) | > 20 (primary natriuresis) |
| Uric acid | Low (dilutional) | Low (renal wasting) |
| Treatment | Fluid restriction | IV normal saline (volume replacement) |
| Danger of wrong Tx | Giving saline to SIADH worsens hyponatremia (Na⁺ excreted, water retained) | Restricting fluids in CSWS worsens hypovolemia → shock |
Unifying pathophysiology: Both total body Na⁺ and total body water are increased, but water is retained proportionally more → [Na⁺] falls. These conditions share ↓ effective arterial blood volume (EABV) despite total volume overload → baroreceptor-mediated ADH release + RAAS activation → Na⁺ and water retention (water > Na⁺).
Clinical clue: Oedema — peripheral pitting oedema, ascites, pulmonary oedema, elevated JVP.
Urine Na⁺ differentiates avid renal Na⁺ retention from renal impairment [1][2]:
| Differential | Mechanism | Urine Na⁺ | Key Distinguishing Features |
|---|---|---|---|
| Congestive heart failure | ↓ Cardiac output → ↓ EABV → RAAS + ADH activation → Na⁺ and water retention (water > Na⁺) | < 20 (kidney avidly retaining Na⁺) — unless on diuretics | Orthopnoea, PND, elevated JVP, S3, pulmonary crackles, displaced apex; elevated NT-proBNP; CXR shows cardiomegaly, pulmonary congestion [12] |
| Hepatic cirrhosis | Portal hypertension → splanchnic NO-mediated vasodilation → ↓ EABV → RAAS + ADH; hypoalbuminaemia → ↓ oncotic pressure → oedema/ascites | < 20 (unless on diuretics or hepatorenal syndrome) | Stigmata of chronic liver disease (spider naevi, palmar erythema, jaundice, caput medusae); ascites; overdiuresis or excess paracentesis without albumin replacement worsens hyponatremia [7] |
| Nephrotic syndrome | Massive proteinuria → hypoalbuminaemia → ↓ oncotic pressure → fluid shift to interstitium → ↓ EABV → RAAS + ADH (underfill theory); also overfill theory with primary Na⁺ retention | < 20 | Periorbital oedema (morning) + ankle oedema (evening); proteinuria > 3.5g/day; hypoalbuminaemia < 30 g/L; hyperlipidaemia; lipiduria; Muehrcke's bands [13] |
| Advanced CKD / Renal failure | Severely reduced GFR → impaired free water excretion; reduced nephron mass → cannot dilute urine adequately | > 20 (kidneys cannot retain Na⁺ due to reduced function) | Elevated creatinine and urea; metabolic acidosis; hyperkalaemia; small kidneys on ultrasound suggest chronic; normal-sized kidneys suggest acute [14]; anaemia of CKD |
Heart Failure vs Nephrotic Syndrome: The Bedside Trick
Presence or absence of periorbital oedema helps differentiate cardiac from renal causes of bilateral leg oedema [13]:
- Heart failure → orthopnoea → cannot lie flat → fluid does NOT redistribute to the face overnight → NO periorbital oedema
- Nephrotic syndrome → no orthopnoea → lies flat at night → fluid redistributes to face → periorbital oedema present (morning, resolves during the day with gravity)
This is a classic bedside teaching point.
Once you've narrowed the differential by volume status, use the following biochemical parameters to refine:
| Parameter | What It Tells You | How to Interpret |
|---|---|---|
| Serum osmolality | True vs pseudo/translocational | < 275 = true hypotonic |
| Urine osmolality | Is ADH present? | < 100 = ADH suppressed (polydipsia, beer potomania); > 200 = ADH active (SIADH, hypovolemia, hypervolemia) |
| Urine [Na⁺] | Renal vs extrarenal Na⁺ handling | < 20 = kidney retaining Na⁺ (extrarenal loss or ↓EABV states); > 20 = kidney wasting Na⁺ (renal loss, SIADH, CKD) |
| Serum K⁺ | Helps narrow the differential | ↑K⁺: Addison's (aldosterone deficiency), Type 4 RTA, advanced CKD; ↓K⁺: thiazides, vomiting, Bartter/Gitelman, secondary hyperaldosteronism |
| Serum uric acid | SIADH causes low uric acid (dilutional + uricosuric effect) | Low in SIADH and CSWS; helps confirm but not specific |
| TSH | Exclude hypothyroidism | Must check before diagnosing SIADH [3][6] |
| Morning cortisol / ACTH stim test | Exclude adrenal insufficiency | Must check before diagnosing SIADH [3][6] |
| Serum glucose | Correct for translocational component | Correct Na⁺ if glucose is elevated |
| Serum lipids and protein | Exclude pseudohyponatremia | If markedly elevated, check direct ISE |
| Differential | Category | Mechanism | Clinical Pearl |
|---|---|---|---|
| Bartter syndrome | Hypovolemic, renal loss | Genetic defect in NKCC2 (thick ascending limb) → mimics loop diuretic use → NaCl wasting, hypokalaemia, metabolic alkalosis, polyuria | Presentation: premature neonate with polyhydramnios, polyuria, hypokalaemia, hyponatremia, metabolic alkalosis [10]; consanguineous parents |
| Gitelman syndrome | Hypovolemic, renal loss | Genetic defect in NCC (DCT) → mimics thiazide diuretic use → NaCl wasting, hypokalaemia, metabolic alkalosis, hypomagnesaemia, hypocalciuria | Milder than Bartter; often presents in adolescents/adults with muscle cramps; incidental finding of hypokalaemia during body check-up [10] |
| Renal Fanconi syndrome | Hypovolemic, renal loss | Generalised proximal tubular dysfunction → wasting of Na⁺, K⁺, glucose, amino acids, PO₄, HCO₃⁻, uric acid | Short stature, rickets, hypophosphataemia, glycosuria with normal blood glucose, normal anion gap metabolic acidosis [10] |
| Tumour lysis syndrome | Acute, multifactorial | Massive cell lysis → release of intracellular contents (K⁺, PO₄, uric acid, nucleic acids); hyponatremia from associated fluid shifts and renal impairment; seizure from hyponatremia [15] | Context of haematological malignancy after chemotherapy; hyperkalemia → arrhythmia; hyponatremia → seizure; hyperuricaemia → AKI [15] |
| Acute intermittent porphyria | Euvolemic (SIADH) | SIADH from autonomic neuropathy or direct hypothalamic effect | Young woman with abdominal pain, psychiatric symptoms, peripheral neuropathy; urine darkens on standing; check urinary porphobilinogen |
In practice, the differential narrows dramatically when you know the clinical context:
| Clinical Scenario | Most Likely Differentials |
|---|---|
| Elderly patient on new medication | Thiazide diuretics (#1), SSRIs, carbamazepine, PPIs [5] |
| Post-operative patient | Non-osmotic ADH release (pain, nausea, anaesthesia) + hypotonic IV fluids → SIADH-like picture |
| Patient with known heart failure | Hypervolemic hyponatremia from ↓ EABV; worsened by overdiuresis |
| Patient with liver cirrhosis | Hypervolemic hyponatremia; precipitated by overdiuresis, excessive paracentesis without albumin [7] |
| Patient after SAH or neurosurgery | CSWS vs SIADH — must distinguish by volume status (CVP, body weight) [6] |
| Patient with new lung mass | SIADH from ectopic ADH — especially small cell lung cancer (SCLC) [3] |
| Patient with pneumonia / TB | SIADH from pulmonary disease; TB meningitis can also cause SIADH or CSWS [16] |
| Psychiatric patient with polydipsia | Primary polydipsia (low urine osmolality < 100); also consider SSRI-induced SIADH |
| Patient with DKA | Translocational hyponatremia — always correct for glucose |
| Marathon runner / endurance athlete | Exercise-associated hyponatremia (acute, can be fatal) |
| Neonate with polyhydramnios and consanguineous parents | Bartter syndrome [10] |
| Elderly with poor diet, living alone | Tea-and-toast diet / beer potomania (low solute intake) |
| Serum Osm | Volume | Urine Na⁺ | Urine Osm | Differentials |
|---|---|---|---|---|
| High | — | — | — | Hyperglycaemia, mannitol, glycine |
| Normal | — | — | — | Pseudohyponatremia (lipids, proteins) |
| Low | Hypovolemic | < 20 | > 200 | GI losses, burns, third-spacing, sweat |
| Low | Hypovolemic | > 20 | > 200 | Thiazides, Addison's, CSWS, RTA, Bartter, Gitelman, osmotic diuresis |
| Low | Euvolemic | > 20 | > 200 | SIADH (after excluding hypothyroidism, adrenal insufficiency) |
| Low | Euvolemic | < 20 | < 100 | Primary polydipsia, beer potomania, tea-and-toast |
| Low | Hypervolemic | < 20 | > 200 | Heart failure, cirrhosis, nephrotic syndrome |
| Low | Hypervolemic | > 20 | Variable | Advanced CKD |
High Yield Summary — Differential Diagnosis of Hyponatremia
- Always start from the lab — hyponatremia is a biochemical diagnosis. Check serum osmolality FIRST.
- Exclude pseudohyponatremia (isotonic: ↑↑ lipids/proteins → check direct ISE) and translocational hyponatremia (hypertonic: correct Na⁺ for glucose).
- For true hypotonic hyponatremia, the three pillars of differential are: (a) volume status, (b) urine osmolality, (c) urine [Na⁺].
- Hypovolemic + UNa < 20 = extrarenal loss (GI, burns). Hypovolemic + UNa > 20 = renal loss (thiazides, Addison's, CSWS).
- Euvolemic + Urine Osm > 200 + UNa > 20 = SIADH (after excluding hypothyroidism and adrenal insufficiency). Urine Osm < 100 = polydipsia or low-solute diet.
- SIADH is a diagnosis of EXCLUSION — must rule out hypothyroidism (TSH), adrenal insufficiency (cortisol), drugs, renal/cardiac/liver disease.
- CSWS vs SIADH: both follow CNS pathology, both have high UNa. CSWS = hypovolemic (give saline); SIADH = euvolemic (fluid restriction). Getting this wrong = giving opposite treatment.
- Hypervolemic: HF, cirrhosis, nephrotic syndrome (UNa < 20) vs. advanced CKD (UNa > 20).
- Drugs: thiazides (#1), SSRIs, carbamazepine — especially in elderly. Always review medication list.
- Serum K⁺ is a helpful differentiator: ↑K⁺ → Addison's, CKD, type 4 RTA; ↓K⁺ → thiazides, vomiting, Bartter/Gitelman.
Active Recall - Differential Diagnosis of Hyponatremia
References
[1] Lecture slides: GC 044. Electrolyte and Acid-Base Disorders.pdf [2] Lecture slides: Chemical Pathology Seminar 1_Sodium and water.pdf [3] Senior notes: Block A - Electrolyte and Acid-Base Disorders.pdf (SIADH and hyponatremia sections) [4] Senior notes: Block A – Nephrology Data Interpretation.pdf [5] Lecture slides: GC 079. Prescribing in older people.pdf [6] Senior notes: Ryan Ho Chemical Path.pdf (SIADH and CSWS sections) [7] Senior notes: Block A - A jaundiced and incoherent patient_ liver failure.pdf (hepatic encephalopathy precipitants) [9] Senior notes: Block A - High blood pressure_ hypertension.pdf (renovascular hypertension, mineralocorticoid hypertension) [10] Senior notes: Block A - Nephrotology Teaching Clinic RTD.pdf (Bartter, Gitelman, Fanconi, RTA cases) [11] Senior notes: Ryan Ho Urogenital.pdf [12] Senior notes: Block A - Shortness of breath on exertion_ heart failure.pdf [13] Senior notes: Block A - Glomerular and Tubulo-interstitial Diseases and Acute Kidney Injury.pdf (nephrotic syndrome) [14] Senior notes: Block A - Chronic Kidney Disease and its Complications.pdf (kidney sizes on US) [15] Senior notes: Block A - High white cell count_ acute and chronic leukaemia; bone marrow transplantation; immunogenetics.pdf (tumour lysis syndrome) [16] Senior notes: Ryan Ho Respiratory.pdf (TB meningitis complications)
Diagnostic Criteria, Diagnostic Algorithm and Investigations for Hyponatremia
Hyponatremia does not have "diagnostic criteria" in the way autoimmune or infectious diseases do (e.g., SLICC for SLE). It is a biochemical diagnosis — start from the laboratory report [2]. The diagnosis is made the moment the lab result shows:
However, the real clinical challenge is not diagnosing hyponatremia itself — it is diagnosing the cause. This requires a structured workup. The most important "diagnostic criteria" to know are those for SIADH, because it is a diagnosis of exclusion and a favourite exam topic.
Diagnostic Criteria for SIADH (Bartter & Schwartz Criteria, Modified)
SIADH is a diagnosis by exclusion [2][6]. All of the following must be present:
| Criterion | Explanation (First Principles) |
|---|---|
| 1. ↓ Serum osmolality (< 275 mOsm/kg) | Confirms true hypotonic hyponatremia — excludes pseudo and translocational causes [6] |
| 2. ↑ Urine osmolality > 200 mOsm/kg | Inappropriately concentrated urine despite dilute serum — the kidney should be producing maximally dilute urine (< 100) when serum osmolality is low. If urine osm is > 200, ADH must be present and active, which is "inappropriate" [6] |
| 3. Urine [Na⁺] > 20 mmol/L | Inappropriate natriuresis — the mild volume expansion from water retention triggers ANP release and suppresses aldosterone → kidney excretes Na⁺ instead of retaining it [6] |
| 4. Clinical euvolemia | No signs of dehydration (excludes hypovolemic) and no oedema (excludes hypervolemic). Normal RFT, cardiac and liver function [6] |
| 5. Normal thyroid function (TFT) | Must rule out hypothyroidism — severe hypothyroidism causes euvolemic hyponatremia via ↓ GFR and direct ADH stimulation [3][6] |
| 6. Normal adrenal function | Must rule out adrenal insufficiency — cortisol deficiency removes tonic suppression of ADH → water retention [3][6] |
| 7. No related drugs | Must exclude drug-induced SIADH (SSRIs, carbamazepine, etc.) before labelling as "idiopathic" SIADH [6] |
| 8. Response to fluid restriction | SIADH should improve with fluid restriction (confirms water retention as the mechanism) [6] |
| 9. ↑ Plasma ADH (supportive, not required) | Not routinely measured clinically; expensive and difficult to assay (short half-life). Copeptin (surrogate) may be used in research settings [6] |
Exam Approach: Diagnosing SIADH Step by Step
- Confirm true hypotonic hyponatremia (serum osm < 275)
- Confirm urine is inappropriately concentrated (urine osm > 200)
- Confirm natriuresis (urine Na⁺ > 20)
- Confirm euvolemia clinically
- Exclude hypothyroidism → check TSH
- Exclude adrenal insufficiency → check morning cortisol (± short Synacthen test)
- Exclude drugs → full medication review
- Exclude renal, cardiac, liver disease → RFT, NT-proBNP, LFT
Only after ALL of the above are satisfied can you diagnose SIADH. Then you must find the underlying cause of SIADH (CNS, pulmonary, malignancy, drugs).
The diagnostic algorithm mirrors the diagnostic pathway for differential diagnosis [2] discussed in the previous section, but now we operationalise it with specific investigations at each step.
3. Investigation Modalities — Detailed Interpretation
The investigations for hyponatremia are structured in tiers: essential first-line, second-line directed, and specialized tests. Let me walk through each with the "why" and expected findings.
3.1 Essential First-Line Investigations
These should be ordered for EVERY patient with hyponatremia. They answer the three core questions of the algorithm.
| Parameter | Details |
|---|---|
| What it is | Total concentration of all osmotically active solutes in serum, measured by freezing-point depression |
| Why order it | Answers Step 1: Is this true hyponatremia? |
| Normal range | 275–295 mOsm/kg |
| Interpretation |
| Result | Interpretation | Next Step |
|---|---|---|
| < 275 mOsm/kg | True hypotonic hyponatremia — proceed to volume assessment | Assess volume status clinically |
| 275–295 mOsm/kg | Isotonic = pseudohyponatremia — artefact from ↑↑ lipids or proteins | Check lipid panel, total protein, serum protein electrophoresis; confirm with direct ISE |
| > 295 mOsm/kg | Hypertonic = translocational — effective osmole drawing water into ECF | Check glucose (most common), mannitol, glycine history |
Calculated vs Measured Osmolality
Calculated osmolality = 2 × [Na⁺] + [Urea] + [Glucose] (all in mmol/L)
The osmolar gap = Measured − Calculated. Normal is < 10 mOsm/kg.
A high osmolar gap suggests unmeasured osmoles: ethanol, methanol, ethylene glycol, isopropanol, mannitol. This is particularly important in the toxicology setting [6].
In pseudohyponatremia, the calculated osmolality (which uses the lab [Na⁺]) will appear low, but the measured osmolality will be normal — because the osmometer measures all solutes in the water phase directly.
| Parameter | Details |
|---|---|
| Why order it | Exclude translocational hyponatremia from hyperglycaemia (the most common cause); allows calculation of corrected Na⁺ |
| Correction formula | Corrected Na⁺ = measured Na⁺ + 2.4 × [(glucose − 5.6)/5.6] [1] |
| Interpretation | If corrected Na⁺ is normal → translocational hyponatremia from glucose → treat the hyperglycaemia, not the "hyponatremia" |
| Parameter | Details |
|---|---|
| Why order it | Assess renal function; exclude CKD/AKI as a cause; required for SIADH criteria (normal RFT required) [6] |
| Interpretation | ↑ Creatinine → renal impairment contributing to impaired free water excretion; when creatinine rises, GFR has already been reduced by at least 50% [17] |
| Urea:Creatinine ratio | Elevated ratio ( > 100:1 in SI units) suggests pre-renal (hypovolemia) or upper GI bleeding; low ratio may suggest low protein intake or liver disease |
| Parameter | Details |
|---|---|
| What it is | Concentration of all osmotically active solutes in urine |
| Why order it | Determines whether ADH is active or suppressed — critical for differentiating SIADH from polydipsia |
| Normal context | In the setting of hyponatremia (low serum osm), the kidney should produce maximally dilute urine (< 100 mOsm/kg) by suppressing ADH |
| Result | Interpretation | Points to |
|---|---|---|
| < 100 mOsm/kg | ADH is appropriately suppressed — kidney is doing its job, producing dilute urine | Primary polydipsia, beer potomania, tea-and-toast diet — the problem is excessive water intake or low solute intake, not ADH |
| > 200 mOsm/kg | ADH is present and active — kidney is concentrating urine despite low serum osm, which is "inappropriate" | SIADH, hypovolemia, hypervolemia, hypothyroidism, adrenal insufficiency — ADH is present for some reason (ectopic production, non-osmotic stimulus, or physiological response to hypovolemia) |
| 100–200 mOsm/kg | "Grey zone" — partial ADH activity or submaximal dilution | May be seen in partial polydipsia, early SIADH, beer potomania, or recovering states |
Note that urine osmolality > 200 mOsm/kg is not a so-called 'normal range' of urine osmolality. Urine osmolality is largely controlled by ADH to correct changes in serum osmolality. A hypo-osmolar state should be accompanied with a hypo-osmolar urine. Therefore urine osmolality should be interpreted TOGETHER with serum osmolality [6].
| Parameter | Details |
|---|---|
| Why order it | Differentiates renal from extrarenal causes; critical in both hypovolemic and euvolemic categories |
| Result | In Hypovolemic | In Euvolemic | In Hypervolemic |
|---|---|---|---|
| < 20 mmol/L | Extrarenal Na⁺ loss (GI, burns) — kidney avidly retaining Na⁺ | Unusual for euvolemic causes (except low-solute diet) | HF, cirrhosis, nephrotic syndrome — kidney retaining Na⁺ due to ↓ EABV |
| > 20 mmol/L | Renal Na⁺ wasting (thiazides, Addison's, CSWS, RTA) | SIADH (natriuresis from mild volume expansion) [6] | Advanced CKD — kidney cannot retain Na⁺ |
Urine Na+ Pitfall: Diuretics
A patient on diuretics will have urine Na⁺ > 20 regardless of their volume status — the diuretic forces renal Na⁺ excretion. This makes urine Na⁺ unreliable in patients currently on diuretics. If possible, check urine Na⁺ after holding diuretics for at least 24–48 hours, or use fractional excretion of urea (FEUrea) instead (< 35% suggests pre-renal/hypovolemic, > 50% suggests intrinsic renal/SIADH), which is not affected by diuretics.
| Parameter | Details |
|---|---|
| Why order it | Helps narrow the differential significantly; also important for safe correction (hypokalaemia increases ODS risk) |
| Potassium Pattern | Differential Associations |
|---|---|
| ↑ K⁺ (Hyperkalaemia) | Addison's disease (loss of aldosterone → K⁺ retention + Na⁺ wasting); Type 4 RTA (hyporeninemic hypoaldosteronism — most common in diabetic nephropathy [17]); advanced CKD; ACEi/ARB use |
| ↓ K⁺ (Hypokalaemia) | Thiazide diuretics; vomiting (metabolic alkalosis + renal K⁺ wasting); Bartter syndrome (mimics loop diuretic); Gitelman syndrome (mimics thiazide) [10]; secondary hyperaldosteronism (HF, cirrhosis); SIADH (mild, via dilutional) |
| Normal K⁺ | SIADH (usually); primary polydipsia; hypothyroidism; secondary adrenal insufficiency (aldosterone preserved → K⁺ normal) |
| Parameter | Details |
|---|---|
| Why order it | Assess acid-base status, which helps narrow the differential |
| Acid-Base Pattern | Differential Associations |
|---|---|
| Metabolic alkalosis | Vomiting (loss of HCl); thiazides (contraction alkalosis); Bartter/Gitelman (renal H⁺ wasting); primary hyperaldosteronism |
| Metabolic acidosis, normal AG | Diarrhoea (loss of HCO₃⁻); RTA (Type 1: distal; Type 2: proximal); Addison's (Type 4 RTA) |
| Metabolic acidosis, raised AG | DKA (with translocational hyponatremia); lactic acidosis; uraemia (advanced CKD) |
3.2 Second-Line Directed Investigations
These are ordered based on the clinical context and the results of first-line tests, to confirm or exclude specific aetiologies.
| Parameter | Details |
|---|---|
| Why order it | Must rule out adrenal insufficiency before diagnosing SIADH [3][6] |
| What to check | 9am serum cortisol; if equivocal (100–400 nmol/L), proceed to short Synacthen test (ACTH stimulation test) |
| Interpretation | Morning cortisol < 100 nmol/L → likely adrenal insufficiency; > 400 nmol/L → adrenal insufficiency excluded; 100–400 = indeterminate → do Synacthen test. Also check ACTH: ↑ ACTH + ↓ cortisol = primary (Addison's); ↓ ACTH + ↓ cortisol = secondary (pituitary) |
| Parameter | Details |
|---|---|
| Why order it | Exclude hepatic cirrhosis as a cause of hypervolemic hyponatremia; normal liver function required for SIADH diagnosis [6] |
| Key findings | Hypoalbuminaemia (↓ synthetic function), ↑ bilirubin, ↑ transaminases, reversed albumin:globulin ratio, prolonged INR |
| Parameter | Details |
|---|---|
| Why order it | Exclude heart failure as a cause of hypervolemic hyponatremia |
| Interpretation | NT-proBNP can be used to rule OUT heart failure but not rule IN (very sensitive, not specific) [12]. Age-specific cut-offs for HF: < 50y: > 450 pg/mL; 50–75y: > 900 pg/mL; > 75y: > 1800 pg/mL [18]. Below these thresholds, HF is effectively excluded. |
| Parameter | Details |
|---|---|
| Why order it | Screen for renal pathology contributing to Na⁺ wasting or CKD |
| Key findings | Proteinuria (nephrotic syndrome, CKD); haematuria/RBC casts (glomerulonephritis); WBC casts (tubulointerstitial nephritis); WBC casts seen in acute TIN — e.g. TB drug-induced ATIN [10] |
| Parameter | Details |
|---|---|
| Why order it | Screen for infection (WCC), haematological malignancy (tumour lysis), anaemia (CKD) |
| Key findings | Eosinophilia → allergic interstitial nephritis, Addison's; leucocytosis → infection/sepsis; anaemia → CKD, malignancy |
| Parameter | Details |
|---|---|
| Why order it | If pseudohyponatremia suspected (normal serum osmolality) |
| What to check | Triglycerides, total protein; if elevated, check serum protein electrophoresis and free light chains (for myeloma, Waldenström's) |
| Parameter | Details |
|---|---|
| Why order it | Helps confirm SIADH (low) and distinguish from CSWS |
| Interpretation | SIADH → low serum uric acid (dilutional + increased renal uric acid clearance from volume expansion). CSWS → also low (renal wasting). Both are low — so this does not differentiate between them well, but a LOW uric acid supports the diagnosis of SIADH over hypovolemic causes (where uric acid is usually normal or high due to haemoconcentration) |
These are ordered when the cause is still unclear after first and second-line investigations, or when a specific diagnosis is suspected.
| Investigation | When to Order | Key Findings / Interpretation |
|---|---|---|
| Chest X-ray | All patients — screen for lung pathology causing SIADH (pneumonia, TB, SCLC), pulmonary oedema (HF), pleural effusion | Mass/consolidation → SCLC, pneumonia; pulmonary oedema → HF; pleural effusion → HF, nephrotic syndrome, cirrhosis |
| CT thorax | Suspected lung malignancy (SCLC) causing ectopic ADH | Hilar mass, mediastinal lymphadenopathy |
| CT / MRI brain | Suspected CNS cause (SAH, tumour, meningitis, stroke) — differentiating SIADH from CSWS after neurosurgery/head trauma [6] | SAH, cerebral tumour, pituitary lesion, infarct. In diabetes insipidus workup: loss of pituitary bright spot on MRI suggests AVP-D [19] |
| Echocardiography | Suspected heart failure | LV systolic/diastolic dysfunction, valvular disease, pericardial effusion |
| Ultrasound kidneys | Assess kidney size and exclude obstruction | Small kidneys → CKD (chronic); normal-sized kidneys with high creatinine → worry about AKI (acute) [14]; enlarged kidneys → polycystic, infiltration (amyloid), acute obstruction; bilateral dilated pelvicalyceal system → obstruction [19] |
| Paired plasma/urine osmolality + plasma electrolytes | In suspected DI to differentiate from primary polydipsia [20] | DI: high-normal plasma Na/osmo, U/P ratio < 1; primary polydipsia: ↓ plasma Na/osmo, ↓ urine osmo [20] |
| Water deprivation test | Suspected DI (relevant if presenting with hypernatremia, but occasionally DI patients present with normal or borderline-low Na⁺ if they drink enough water) | Normal/polydipsia: adequate concentration of urine (U/P ratio ≥ 2); DI: with plasma osm > 300, U/P still < 2; Cranial DI: ↑ ≥ 50% urine osm after DDAVP; Nephrogenic DI: no change after DDAVP [20] |
| Fractional excretion of sodium (FENa) | Differentiating pre-renal from intrinsic renal causes of AKI with concurrent hyponatremia | FENa < 1% → pre-renal (avid Na⁺ retention); FENa > 2% → intrinsic renal (ATN, etc.). Note: unreliable with diuretics — use FEUrea instead (< 35% pre-renal, > 50% intrinsic) |
| Fractional excretion of urea (FEUrea) | Same as above but reliable even when patient is on diuretics | < 35% → pre-renal; > 50% → intrinsic renal |
| 24-hour urine protein | Suspected nephrotic syndrome | > 3.5 g/day → nephrotic range proteinuria |
| Adrenal function tests — Short Synacthen test | Suspected adrenal insufficiency (both primary and secondary) | Inadequate cortisol rise (< 550 nmol/L at 30 min after 250μg IV Synacthen) → adrenal insufficiency |
| ACTH level | Differentiate primary (high ACTH) from secondary (low ACTH) adrenal insufficiency | ↑ ACTH + ↓ cortisol = Addison's; ↓ ACTH + ↓ cortisol = pituitary/hypothalamic |
| Aldosterone-renin ratio | Suspected primary hyperaldosteronism (typically presents with hypertension + hypokalaemia, but mild hyponatremia possible) | 1° hyperaldosteronism: ↓ PRA, ↑ aldosterone, ARR > 30; 2° hyperaldosteronism: ↑ PRA, ↑ aldosterone [9] |
| Copeptin | Surrogate for ADH levels (more stable, longer half-life); better for children as you don't deprive them of that much water [19] | High copeptin in polyuric patient = nephrogenic DI/AVP-resistance (ADH is being produced but kidney doesn't respond); Low copeptin = cranial DI/AVP-deficiency [19] |
| Serum/urine drug screen | Suspected drug-induced hyponatremia or toxin-related | Identify offending agent (SSRIs, carbamazepine, ecstasy/MDMA) |
This table is a high-yield summary for rapid data interpretation in exam settings [1][2][4]:
| Category | Serum Osm | Urine Osm | Urine Na⁺ | Serum K⁺ | Acid-Base | Key Extra Finding |
|---|---|---|---|---|---|---|
| Pseudohyponatremia | Normal | N/A | N/A | Normal | Normal | Lipaemic serum or ↑↑ protein; direct ISE normal |
| Translocational (glucose) | High | Variable | Variable | Variable | May have ↑ AG acidosis (DKA) | Corrected Na⁺ is normal |
| GI losses (diarrhoea) | Low | High (> 200) | < 20 | ↓ (or ↑ in severe diarrhoea) | NAGMA (HCO₃⁻ loss) | History of diarrhoea |
| Vomiting | Low | Variable | Variable (may be > 20 early — bicarbonaturia) | ↓ | Metabolic alkalosis | History of vomiting |
| Thiazide | Low | High (> 200) | > 20 | ↓ | Metabolic alkalosis | Drug history; onset < 2 weeks; elderly woman |
| Addison's | Low | High | > 20 | ↑ | NAGMA (type 4 RTA) | Hyperpigmentation; ↓ cortisol, ↑ ACTH |
| CSWS | Low | High | > 20 | Normal or ↓ | Normal or mildly acidotic | Post-CNS insult; hypovolemic on exam |
| SIADH | Low | > 200 | > 20 | Normal | Normal | Euvolemic; low serum uric acid; normal RFT, TFT, cortisol [6] |
| Hypothyroidism | Low | High | > 20 | Normal | Normal | ↑ TSH, ↓ fT4 |
| Primary polydipsia | Low | < 100 | < 20 | Normal | Normal | Psychiatric history; massive fluid intake |
| Beer potomania | Low | < 100 | < 20 | Normal or ↓ | Normal | Alcoholic with poor diet |
| Heart failure | Low | High | < 20 | Variable | Normal | Elevated NT-proBNP; CXR cardiomegaly |
| Cirrhosis | Low | High | < 20 | Variable | Resp alkalosis (early), met acidosis (late) | Stigmata of CLD; ↓ albumin; ascites |
| Nephrotic syndrome | Low | High | < 20 | Normal | Normal | Heavy proteinuria; ↓ albumin; hyperlipidaemia |
| Advanced CKD | Low | Variable (often iso-osmolar, impaired concentrating AND diluting ability) | > 20 | ↑ | NAGMA + ↑ AG | ↑↑ Creatinine; small kidneys on USS [14] |
| Bartter syndrome | Low (mild) | High | > 20 | ↓↓ | Met alkalosis | Premature neonate with polyhydramnios [10] |
| Gitelman syndrome | Low (mild) | High | > 20 | ↓↓ | Met alkalosis + ↓ Mg | Incidental hypokalaemia on body check-up; hypocalciuria [10] |
4. Special Diagnostic Scenarios
- Context: Very common in surgical patients. Multiple non-osmotic stimuli for ADH: pain, nausea, anaesthesia, opioids. Combined with hypotonic IV fluid administration → dilutional hyponatremia.
- Investigation: Check serum osm (confirm true hyponatremia), urine osm (will be > 200 — ADH active from pain/nausea), review IV fluid chart (hypotonic fluids?), check cortisol (surgical stress should elevate cortisol — if cortisol is low, consider adrenal crisis). Review the drug chart for medications such as opioids, SSRIs, NSAIDs [5].
- Key point: The most common iatrogenic cause of hyponatremia in hospitalised patients is hypotonic IV fluid administration in the setting of non-osmotic ADH release. Prevention is using isotonic fluids (NS or Hartmann's) unless there is a specific indication for hypotonic fluid.
- Context: Prescribing in older people — this is a specific GC lecture topic [5]. Thiazides, SSRIs, and carbamazepine are the most common culprits.
- Investigation: Check [Na⁺] within 1–2 weeks of starting these medications. If hyponatremia develops, check timing of drug initiation (temporal correlation), urine osm and urine Na⁺. AGS Beers Criteria (2023) flags many drugs that can cause hyponatremia [5].
- Key point: Drug-induced hyponatremia is a diagnosis of exclusion (like SIADH) — you must still exclude other causes. But temporal correlation with drug initiation and resolution after drug withdrawal is strong evidence.
In data interpretation stations [4], you will be given a clinical vignette with lab values. The approach is:
- Identify hyponatremia: Serum Na⁺ < 135
- Check serum osm: Rule out pseudo/translocational
- Read clinical scenario for volume clues: Oedema? Dehydration signs? Or neither?
- Check urine osm: ADH active (> 200) or suppressed (< 100)?
- Check urine Na⁺: Renal (> 20) or extrarenal (< 20)?
- Check K⁺ and acid-base: Narrow the differential
- Check glucose: Calculate corrected Na⁺ if elevated
- Synthesise: Volume + urine osm + urine Na⁺ = diagnosis
High Yield Summary — Diagnosis and Investigations
- Hyponatremia is a biochemical diagnosis — start from the lab report [2].
- The diagnostic algorithm is a 3-step process: (1) Serum osm (true vs pseudo/translocational), (2) Volume status (hypo/eu/hyper), (3) Urine osm + urine Na⁺ (renal vs extrarenal, ADH active vs suppressed).
- SIADH diagnostic criteria: ↓ serum osm, ↑ urine osm > 200, urine Na > 20, euvolemic, normal RFT, normal TFT, normal adrenal function, no drugs, response to fluid restriction — diagnosis by exclusion [6].
- Always check TSH and morning cortisol before diagnosing SIADH.
- Urine osm < 100 in a hyponatremic patient = ADH is OFF → primary polydipsia or low-solute diet (not SIADH).
- Urine osm > 200 + urine Na > 20 in euvolemic patient = SIADH (after exclusion).
- Urine Na⁺ is unreliable if patient is on diuretics — use FEUrea instead.
- Serum K⁺ is a powerful differentiator: ↑K⁺ → Addison's, CKD; ↓K⁺ → thiazides, vomiting, Bartter/Gitelman.
- Kidney size on US: small = CKD; normal with high creatinine = AKI; large = polycystic/infiltrative/obstruction [14].
- Correct Na⁺ for glucose in all diabetic patients before diagnosing true hyponatremia.
- Copeptin is a surrogate for ADH — high copeptin in DI = nephrogenic/AVP-resistance; low = cranial/AVP-deficiency [19].
Active Recall - Diagnostic Criteria, Algorithm & Investigations for Hyponatremia
References
[1] Lecture slides: GC 044. Electrolyte and Acid-Base Disorders.pdf [2] Lecture slides: Chemical Pathology Seminar 1_Sodium and water.pdf [3] Senior notes: Block A - Electrolyte and Acid-Base Disorders.pdf (SIADH and hyponatremia sections) [4] Senior notes: Block A – Nephrology Data Interpretation.pdf [5] Lecture slides: GC 079. Prescribing in older people.pdf; GC 079 (supp-4) AGS Beers Criteria for potentially inappropriate med use_Pocket Guide_2023.pdf [6] Senior notes: Ryan Ho Chemical Path.pdf (SIADH and CSWS sections) [9] Senior notes: Block A - I have fluctuating BP_ cushing syndrome; adrenal diseases and tumours; other endocrine tumours.pdf (hyperaldosteronism investigations) [10] Senior notes: Block A - Nephrotology Teaching Clinic RTD.pdf (Bartter, Gitelman, Fanconi, RTA cases) [12] Senior notes: Block A - Introduction to CVS investigations (including ECG).pdf (NT-proBNP) [14] Senior notes: Block A - Chronic Kidney Disease and its Complications.pdf (kidney sizes on US) [17] Senior notes: Block A - Nephrology Interactive Tutorial.pdf (creatinine interpretation, AKI criteria) [18] Senior notes: MBBS Final MB (Pediatrics) (Felix PY Lai).pdf (BNP/NT-proBNP cut-offs) [19] Senior notes: Block A - I keep on bumping into people on my side_ pituitary tumours; hypopituitarism.pdf (copeptin, pituitary bright spot); Block A - Two cases of polyuria and polydipsia.pdf [20] Senior notes: Ryan Ho Endocrine.pdf (water deprivation test protocol and interpretation); Ryan Ho Fundamentals.pdf (polyuria/polydipsia workup)
Management of Hyponatremia
The management of hyponatremia is guided by three simultaneous decisions:
- How urgent is the situation? → Determined by symptoms (not the absolute number)
- How fast can I safely correct? → Determined by chronicity (acute vs. chronic)
- What is the underlying cause? → Determines definitive treatment (cause-specific)
The overarching philosophy: Comes quick, correct quick; comes slow, correct slow [21].
- Rapid correction is indicated to rapidly correct hypoNa in acute, symptomatic hypoNa [21]
- Slow correction is indicated to slowly correct hypoNa in chronic hypoNa [21]
Why? Because:
- Cerebral neurones already well-compensated by ↓ intracellular osmolarity [21] in chronic states. They have expelled organic osmolytes over 24–48 hours to reduce brain cell swelling.
- Rapid correction → rapid water efflux → neurones shrink and detach from myelin sheath → central pontine myelinolysis (CPM) [21]
The goals of treatment [21]:
- Prevent brain herniation due to cerebral oedema
- Relieve symptoms due to acute hypoNa
- Avoid overcorrection and osmotic demyelination syndrome (ODS)
- Treat the underlying cause
Treatment Modalities — By Clinical Scenario
This is the life-threatening situation — seizures, coma, respiratory distress, signs of brainstem herniation. The Na⁺ level is usually < 120, but symptoms matter more than the number.
Hypertonic Saline (3% NaCl)
| Parameter | Details |
|---|---|
| What it is | 3% NaCl — contains 513 mmol/L of Na⁺ (compared to 154 mmol/L in 0.9% NS). "Hypertonic" because its tonicity vastly exceeds plasma [21][22] |
| Mechanism | Infusing a solution with very high [Na⁺] rapidly raises serum [Na⁺]. The hypertonic ECF draws water OUT of brain cells → reduces cerebral oedema → relieves symptoms |
| Indication | Severe symptomatic hyponatremia with seizures, coma, or respiratory distress — regardless of acuity or cause [1][3] |
| Dosing | 100–150 mL IV bolus over 10–20 minutes — can repeat up to 2 more times (total 3 boluses) if symptoms persist or Na⁺ has not risen by ≥ 5 mmol/L [current European/American guidelines] |
| Target | Raise Na⁺ by 4–6 mmol/L in the first 1–2 hours — this is usually enough to reverse cerebral oedema and stop seizures. Then STOP boluses and transition to cause-specific management |
| Monitoring | Recheck serum Na⁺ every 1–2 hours during and after boluses. Do NOT exceed 10 mmol/L in first 24 hours (or 8 in high-risk patients) |
| Contraindications | Relative: severe uncontrolled heart failure (risk of fluid overload); must weigh risk of pulmonary oedema against risk of brain herniation — in practice, brain herniation is more immediately lethal |
| Where to give | Should only be used in very experienced hands [3] — ICU or high-dependency setting with close monitoring |
The '100 mL Bolus' Rule — Simplified Emergency Protocol
For severe symptomatic hyponatremia: give 100 mL of 3% NaCl IV over 10 minutes. Recheck Na⁺. If still seizing or no clinical improvement, repeat the bolus (up to 3 times total). Each 100 mL bolus raises Na⁺ by approximately 1.5–2 mmol/L in a 70 kg patient.
This approach is simpler and safer than continuous infusion for the acute emergency phase. Once symptoms are controlled, STOP boluses and switch to cause-specific management with slower correction.
Estimation formula (Adrogue-Madias):
Where TBW = 0.6 × body weight (males) or 0.5 × body weight (females/elderly).
This estimates the change in serum Na⁺ per 1 L of infusate. For 3% NaCl (513 mmol/L) in a 70 kg male with Na⁺ of 110:
So 100 mL of 3% NaCl would raise Na⁺ by ~0.94 mmol/L. In practice, several boluses are needed.
This is the single most tested concept in hyponatremia management and has significant medicolegal implications [3].
| Scenario | Maximum Correction Rate | Rationale |
|---|---|---|
| Acute hyponatremia (< 48h, known) | Can correct faster — up to 1–2 mmol/L/hour initially, max 10–12 mmol/L/24h | Brain has NOT adapted → no osmolyte loss → no risk of ODS from rapid correction |
| Chronic hyponatremia (≥ 48h or unknown duration) | < 0.5 mmol/L/hour, or < 10–12 mmol/L/day [3]; 6–8 mmol/L/day in high-risk patients | Brain HAS adapted (lost osmolytes) → rapid correction dehydrates brain cells → central pontine myelinolysis → patients become tetraplegic [3] |
| Unknown duration | Assume chronic — use the slower rate | You cannot be sure the brain hasn't adapted |
| High-risk for ODS | < 6–8 mmol/L/24h | Risk factors for ODS: Na⁺ < 105, alcoholism, malnutrition, liver disease, hypokalaemia, burns |
High Yield: The rate of correction should be slow for chronic hyponatremia → over 2 days. Less than 0.5 mmol/L/hour, or < 12 mmol/L/day. If correction occurs too rapidly, may result in a complication called central pontine myelinolysis [3]. Hypertonic saline should only be used in very experienced hands [3].
3. Cause-Specific Management — By Volume Status
| Treatment | Details |
|---|---|
| IV Normal Saline (0.9% NaCl) | The mainstay of treatment. NS has 154 mmol/L Na⁺ — still hypertonic relative to the patient's serum in severe hyponatremia, so it will raise [Na⁺]. More importantly, it restores intravascular volume → turns OFF the non-osmotic ADH stimulus → kidney can now excrete free water → Na⁺ corrects |
| Rate | Replace the volume depletion with normal saline cautiously. Initial ⅓ can be given in first 8 hours, reduce speed afterwards. Monitor the [Na⁺] regularly [3] |
| Treat the cause | Stop thiazides; replace cortisol/fludrocortisone if Addison's; treat diarrhoea/vomiting; IV saline for CSWS |
Danger: 'Autocorrection' After Volume Repletion
When you give NS to a hypovolemic patient, you restore volume → the non-osmotic ADH stimulus is switched off → ADH drops suddenly → the kidney dumps a huge amount of free water (dilute urine). This causes Na⁺ to rise very rapidly — sometimes exceeding the safe limit. This is called "autocorrection" or "unintentional overcorrection".
Solution: Monitor Na⁺ every 2–4 hours during the first 24 hours of NS infusion. If Na⁺ is rising too fast, slow the infusion and give 5% Dextrose (D5W) ± DDAVP to re-lower Na⁺ into the safe zone. Think of this as putting your foot on the brake.
| Cause | Specific Treatment |
|---|---|
| GI losses | IV NS; treat underlying cause (anti-emetics, anti-diarrhoeals); correct concurrent K⁺ and HCO₃⁻ disturbances |
| Thiazide-induced | Stop the offending thiazide; NS for volume repletion; switch to an alternative antihypertensive (NOT another thiazide); monitor Na⁺ |
| Addison's disease | IV NS for volume repletion + IV hydrocortisone 100 mg stat then Q6–8H (cortisol replaces the tonic ADH suppression); once stable, oral hydrocortisone + fludrocortisone (mineralocorticoid replacement for aldosterone deficiency) |
| CSWS | IV NS — volume replacement is the KEY [6]. This is the opposite of SIADH management. May need hypertonic saline if severe. Fludrocortisone (0.1–0.2 mg/day) can also help by promoting Na⁺ retention. The cerebral pathology usually self-resolves in 2–4 weeks |
B. Euvolemic Hyponatremia — Mainly SIADH
The most common cause of euvolemic hyponatremia. Management follows a stepwise escalation.
| Parameter | Details |
|---|---|
| Mechanism | By limiting water intake, you reduce the dilution of Na⁺. The kidney still excretes some electrolyte-free water (though less than normal because of inappropriate ADH), so restricting intake below output allows Na⁺ to gradually rise |
| How much | 800–1000 mL/day total fluid intake (some guidelines say 500–1000 mL/day; should be < daily urine output + insensible losses) [21][22] |
| Indication | SIADH — first-line treatment; response to fluid restriction is part of the diagnostic criteria [6] |
| Monitoring | Daily weights (should decrease slightly), serum Na⁺ daily, fluid balance chart |
| Limitation | Many patients find strict fluid restriction very difficult to comply with, especially long-term. Takes 3–5 days to see effect. May not be sufficient alone if urine osmolality is very high (concentrated urine "locks in" water) |
When Will Fluid Restriction NOT Work in SIADH?
Fluid restriction is less likely to work if:
- Urine osmolality > 500 mOsm/kg — the urine is very concentrated, meaning the kidney is reabsorbing a lot of water. Even with restricted intake, the patient may not be able to generate enough free water clearance to raise Na⁺.
- Urine Na⁺ + Urine K⁺ > Serum Na⁺ — this means the kidney is excreting more electrolytes per litre of urine than the serum contains, so every litre of urine output effectively returns free water to the body (the "desalination" effect). This is a sign of refractory SIADH.
In these cases, escalate to pharmacological therapy.
| Parameter | Details |
|---|---|
| Oral NaCl tablets | Increase solute intake → increase obligatory water excretion (the kidney must excrete more water to excrete the additional solute). 3–9 g/day of NaCl tablets added to fluid restriction |
| Loop diuretic (furosemide) | May seem paradoxical — why give a diuretic to a euvolemic patient? Because furosemide blocks NKCC2 in the TAL → disrupts the medullary concentration gradient → the kidney produces more dilute urine → promotes free water excretion even in the presence of ADH. Combined with oral salt (to prevent Na⁺ loss from the diuretic), this effectively "forces" free water clearance |
| Indication | Fluid restriction alone insufficient; add-on to fluid restriction |
Apart from saline, two other medications that can be used in SIADH: demeclocycline and V2 antagonist (tolvaptan) [3].
| Drug | Mechanism | Indication | Dosing | Key Points |
|---|---|---|---|---|
| Tolvaptan | Vasopressin V2-receptor antagonist — "vaptan" = vasopressin antagonist. Blocks V2 receptors on the collecting duct → prevents AQP2 insertion → blocks water reabsorption → produces dilute urine (aquaresis without natriuresis) | Chronic SIADH refractory to fluid restriction [3] | 15 mg PO daily, titrate up to max 60 mg/day | Must be initiated in hospital with close Na⁺ monitoring (risk of overcorrection). Contraindicated in hypovolemic hyponatremia (would worsen volume depletion without addressing the cause). Hepatotoxicity risk — monitor LFTs. Should NOT be used for > 30 days (FDA black box warning for liver injury). Fluid restriction should be RELAXED when starting tolvaptan (otherwise Na⁺ rises too fast) |
| Demeclocycline | Tetracycline antibiotic that causes nephrogenic diabetes insipidus as a side effect — it makes the collecting duct resistant to ADH → impairs water reabsorption | SIADH refractory to fluid restriction (largely historical — tolvaptan preferred now) | 600–1200 mg/day PO divided doses | Onset is SLOW (3–5 days). Dose-dependent effect. Risk of nephrotoxicity (especially in cirrhotic patients — avoid in liver disease). Photosensitivity. Largely superseded by tolvaptan in current practice but still may be tested |
| Urea (oral) | Osmotic diuresis — urea is freely filtered by the glomerulus → creates obligatory water excretion → promotes free water clearance | Chronic SIADH, sometimes first-line in European guidelines | 15–60 g/day PO (dissolved in orange juice to mask taste) | Very effective; cheap; long track record in Europe. Poorly tolerated (unpleasant taste, GI upset). Not widely available in HK. Relatively safe — fewer overcorrection concerns than tolvaptan |
These patients are oedematous. The problem is ↓ EABV despite total body volume overload → ADH + RAAS activation → water retention > Na⁺ retention → dilutional hyponatremia.
The principle: treat the underlying disease + fluid and salt restriction + diuretics carefully.
| Condition | Specific Management |
|---|---|
| Heart failure | Fluid restriction (1–1.5 L/day); salt restriction; optimise HF therapy (ACEi/ARB, beta-blocker, MRA, SGLT2i); diuretics (loop diuretics — furosemide) for decongestion but monitor Na⁺ closely. Tolvaptan can be considered for refractory hyponatremia in HF (approved for this indication). All diuretics should be discontinued if there is severe hyponatremia [23] |
| Hepatic cirrhosis | Fluid restriction (800–1000 mL/day when Na⁺ < 120–125) [23]; salt restriction 0.5–2 g/day [23]; spironolactone is preferred since it is an aldosterone antagonist and liver cirrhosis is associated with hyperaldosteronism [23]; spironolactone + frusemide is the standard therapy to maintain normokalaemia [23]. Target weight loss = 1 kg/day [23]. All diuretics should be discontinued if there is severe hyponatremia, progressive renal failure, worsening hepatic encephalopathy or incapacitating muscle cramps [23]. Tolvaptan can be considered short-term. Albumin infusion after large-volume paracentesis to prevent worsening hyponatremia [7] |
| Nephrotic syndrome | Treat underlying glomerular disease; salt restriction; loop diuretics + albumin infusion (to maintain intravascular volume); ACEi/ARB for proteinuria reduction |
| Advanced CKD | Fluid and salt restriction; optimise dialysis (if on renal replacement therapy — dialysis directly corrects Na⁺); haemodialysis is the last-resort treatment for refractory electrolyte imbalances [24] |
Cirrhosis: When to STOP Diuretics
All diuretics should be discontinued if there is [23]:
- Severe hyponatremia (Na⁺ < 120)
- Progressive renal failure (hepatorenal syndrome)
- Worsening hepatic encephalopathy (overdiuresis → dehydration → ↓ hepatic perfusion → ↑ ammonia)
- Incapacitating muscle cramps
- Severe hyperkalaemia (stop spironolactone)
- Severe hypokalaemia (stop frusemide)
In cirrhotic patients, overdiuresis is a common precipitant of both worsening hyponatremia AND hepatic encephalopathy [7].
| Step | Action |
|---|---|
| 1. Identify the offending drug | Most common: thiazides, SSRIs, carbamazepine. Review full drug list including OTC medications [5] |
| 2. Stop the offending drug | If clinically possible. Na⁺ usually corrects within days to 1–2 weeks. Monitor for overcorrection! |
| 3. Switch to alternative | For HT: switch thiazide to CCB or ACEi/ARB. For depression: switch SSRI to an alternative (mirtazapine has lower risk). For epilepsy: switch carbamazepine to levetiracetam or lamotrigine |
| 4. Support | NS if hypovolemic; fluid restriction if euvolemic/SIADH-like picture. Monitor Na⁺ daily until stable |
Overcorrection is defined as Na⁺ rising > 10 mmol/L in 24 hours (or > 8 mmol/L in high-risk patients). This is a medical emergency because of the risk of ODS.
| Rescue Strategy | Details |
|---|---|
| 5% Dextrose (D5W) infusion | Dextrose is metabolised by the liver → leaves free water → dilutes serum Na⁺ back down. Infuse at 3–6 mL/kg/hour and recheck Na⁺ every 1–2 hours until Na⁺ is back within the safe correction range [22] |
| DDAVP (Desmopressin) 2 mcg IV/SC | Exogenous ADH → inserts AQP2 → promotes water reabsorption → re-dilutes Na⁺. Given Q8H as needed. Also prevents further free water excretion (stops the "autocorrection" phenomenon after NS in hypovolemic patients) |
| Combination | D5W + DDAVP for maximum effect in severe overcorrection |
Key exam point: The ability to recognise and manage overcorrection is as important as knowing how to correct hyponatremia. If you accidentally overshoot, give D5W ± DDAVP to bring Na⁺ back down. Document carefully — this has medicolegal implications [3].
| Cause | Treatment |
|---|---|
| Hypothyroidism | Thyroid hormone replacement (levothyroxine). Na⁺ usually corrects as thyroid function normalises. Fluid restriction in the interim if symptomatic |
| Primary adrenal insufficiency (Addison's) | Cortisol replacement (IV hydrocortisone 100 mg in acute crisis, then oral hydrocortisone 15–25 mg/day in divided doses) + fludrocortisone (0.05–0.2 mg/day for mineralocorticoid replacement) + IV NS for volume repletion |
| Secondary adrenal insufficiency | Cortisol replacement only (no fludrocortisone needed — aldosterone is RAAS-dependent and preserved) |
7. Special Situations
- Prevention is key: use isotonic fluids (NS or Hartmann's) perioperatively, NOT hypotonic solutions (D5W, half-normal saline) [22]
- Avoid excessive free water administration in the setting of non-osmotic ADH release (pain, nausea, opioids)
- If hyponatremia develops: stop hypotonic fluids, check Na⁺, treat as acute symptomatic if < 48h with symptoms
- Prescribing in older people is a key GC exam topic [5]
- AGS Beers Criteria (2023) recommends caution with thiazides, SSRIs, carbamazepine in the elderly [5]
- Monitor Na⁺ within 1–2 weeks of starting thiazides or SSRIs in elderly patients
- Lower threshold for stopping offending medications
- Greater risk of ODS (often malnourished, concurrent liver disease, hypokalaemia)
- Must correct K⁺ alongside Na⁺ — hypokalaemia increases the risk of ODS
- Every 1 mmol/L of K⁺ given effectively raises serum Na⁺ by approximately the same amount (because K⁺ is exchanged for Na⁺ across cell membranes — giving K⁺ causes K⁺ to enter cells and Na⁺ to leave cells)
- If you correct K⁺ from 2.5 to 3.5 mmol/L, that also raises Na⁺ by ~1 mmol/L — factor this into your correction rate calculation
| Category | First-Line Treatment | Adjuncts | What NOT to Do |
|---|---|---|---|
| Hypovolemic | IV 0.9% NS | Treat underlying cause (stop thiazide, replace cortisol for Addison's, NS for CSWS) | Do NOT fluid restrict (will worsen hypovolemia) |
| Euvolemic / SIADH | Fluid restriction 800–1000 mL/day | Oral NaCl tablets + loop diuretic; tolvaptan; demeclocycline; oral urea | Do NOT give NS (will worsen hyponatremia — Na⁺ excreted, water retained) |
| Hypervolemic | Fluid + salt restriction; treat underlying disease | Optimise HF/cirrhosis Rx; diuretics (careful); tolvaptan for refractory cases | Do NOT give NS (will worsen oedema) |
| Severe symptomatic (any cause) | 3% Hypertonic saline 100 mL bolus × up to 3 | ICU monitoring; hourly Na⁺ checks | Do NOT correct > 10 mmol/L/24h (ODS risk) |
| Drug-induced | Stop offending drug | NS if hypovolemic; fluid restriction if euvolemic | Do NOT ignore — can be fatal |
| Overcorrection | D5W ± DDAVP | Re-check Na⁺ every 1–2h | Do NOT continue NS or hypertonic saline |
| Drug | Class | MoA | Indication | Key Side Effects / Contraindications |
|---|---|---|---|---|
| 0.9% NaCl (NS) | Isotonic crystalloid | Volume replacement; 154 mmol/L Na⁺ restores ECF | Hypovolemic hyponatremia | Overcorrection risk (autocorrection); hyperchloraemic acidosis with large volumes [22] |
| 3% NaCl | Hypertonic crystalloid | 513 mmol/L Na⁺; rapidly raises serum [Na⁺]; draws water out of brain cells | Life-threatening euvolemic hypoNa [22]; severe symptomatic (any cause) | Overcorrection → ODS; fluid overload (HF patients); use only in experienced hands [3] |
| Tolvaptan | V2-receptor antagonist | Blocks vasopressin V2R → prevents AQP2 insertion → aquaresis (electrolyte-free water excretion) | Chronic refractory SIADH [3]; also approved for HF-associated hyponatremia | C/I: hypovolemic hyponatremia; hepatotoxicity (monitor LFTs; limit to ≤ 30 days); must initiate in hospital; relax fluid restriction when starting |
| Demeclocycline | Tetracycline antibiotic | Induces nephrogenic DI → collecting duct ADH resistance | SIADH refractory to fluid restriction [3] | Slow onset (3–5 days); nephrotoxicity; avoid in liver disease; photosensitivity; largely superseded by tolvaptan |
| Oral urea | Osmotic agent | ↑ Solute excretion → obligatory free water excretion | Chronic SIADH (European guidelines) | Unpleasant taste; GI upset; relatively safe |
| Furosemide | Loop diuretic | Blocks NKCC2 → disrupts medullary gradient → dilute urine → free water clearance | Add-on to fluid restriction + oral salt in SIADH | Hypokalaemia; hypomagnesaemia; volume depletion if not combined with salt |
| Oral NaCl | Salt supplementation | ↑ Solute intake → ↑ obligatory water excretion | Add-on in SIADH | Hypertension (in susceptible); oedema (in HF/cirrhosis) |
| DDAVP | ADH analogue | Prevents further free water excretion | Rescue for overcorrection (combined with D5W) | Can worsen hyponatremia if used incorrectly |
| D5W | Hypotonic crystalloid | Provides free water → re-lowers Na⁺ | Rescue for overcorrection | Can cause hyperglycaemia |
| Hydrocortisone | Glucocorticoid | Restores cortisol → reinstates tonic ADH suppression + improves GFR | Adrenal insufficiency | Usual steroid side effects with chronic use |
| Fludrocortisone | Mineralocorticoid | Replaces aldosterone → promotes Na⁺ retention at ENaC | Addison's disease; adjunct in CSWS | Hypertension; hypokalaemia; oedema |
| Spironolactone | MRA | Blocks aldosterone at ENaC | Cirrhotic ascites (hyperaldosteronism) [23] | Hyperkalaemia; gynaecomastia |
High Yield Summary — Management of Hyponatremia
- Comes quick, correct quick; comes slow, correct slow [21].
- Severe symptomatic (seizures/coma): 3% NaCl 100 mL bolus IV over 10–20 min, repeat up to ×3. Target: raise Na⁺ by 4–6 in first 1–2 hours.
- Chronic hyponatremia correction rate: < 10 mmol/L/24h (6–8 in high-risk). Overcorrection → osmotic demyelination syndrome → tetraplegia [3].
- Hypovolemic: IV 0.9% NS for volume repletion + treat cause. Watch for autocorrection.
- Euvolemic / SIADH: Fluid restriction 800–1000 mL/day (1st line) → add oral NaCl + loop diuretic → tolvaptan or demeclocycline for refractory cases.
- Hypervolemic: Fluid + salt restriction + treat underlying disease (HF, cirrhosis, CKD). Stop diuretics if Na⁺ < 120.
- Drug-induced: STOP the offending drug. Most common: thiazides, SSRIs, carbamazepine.
- Overcorrection rescue: D5W infusion ± DDAVP 2 mcg IV/SC to re-lower Na⁺.
- Tolvaptan: V2-receptor antagonist for refractory SIADH. C/I in hypovolemia. Must initiate in hospital. Hepatotoxicity risk.
- Correct concurrent hypokalaemia — K⁺ correction also raises Na⁺ and reduces ODS risk.
- In cirrhosis: stop ALL diuretics if severe hyponatremia, progressive renal failure, worsening HE, or incapacitating cramps [23].
- Prevention: use isotonic fluids perioperatively; monitor Na⁺ when starting thiazides/SSRIs in elderly [5].
Active Recall - Management of Hyponatremia
References
[1] Lecture slides: GC 044. Electrolyte and Acid-Base Disorders.pdf [3] Senior notes: Block A - Electrolyte and Acid-Base Disorders.pdf (hyponatremia management, correction rates, CPM) [5] Lecture slides: GC 079. Prescribing in older people.pdf; GC 079 (supp-4) AGS Beers Criteria for potentially inappropriate med use_Pocket Guide_2023.pdf [6] Senior notes: Ryan Ho Chemical Path.pdf (SIADH diagnosis and CSWS) [7] Senior notes: Block A - A jaundiced and incoherent patient_ liver failure.pdf (hepatic encephalopathy precipitants, paracentesis) [21] Senior notes: Ryan Ho Urogenital.pdf (hyponatremia management approach, CPM mechanism) [22] Senior notes: Ryan Ho Fluids and Nutrition.pdf (crystalloids, hypertonic saline indications) [23] Senior notes: MBBS Final MB (Surgery) (Felix PY Lai).pdf (cirrhotic ascites management, diuretic precautions) [24] Senior notes: Ryan Ho Critical Care.pdf (AKI management, haemodialysis indications)
Complications of Hyponatremia
Complications of hyponatremia fall into two broad categories:
- Complications of the hyponatremia itself — direct consequences of low serum osmolality on the brain and body
- Complications of treatment (iatrogenic) — consequences of overcorrection, which can be more devastating than the hyponatremia itself
This distinction is critical: most serious complications result not from the disorder itself but from inappropriate treatment — a principle that applies to both hyponatremia and hypernatremia [2][6].
A. Complications of Hyponatremia Itself
This is the fundamental pathological consequence of hyponatremia and the mechanism underlying all neurological symptoms.
Pathophysiology (from first principles):
- Serum [Na⁺] falls → serum osmolality drops → an osmotic gradient is created between the now-hypotonic ECF and the relatively hypertonic intracellular fluid (ICF) of brain cells
- Water moves DOWN this osmotic gradient: from ECF INTO brain cells
- Brain cells swell → cerebral oedema
- The skull is a rigid box — the brain cannot expand beyond the fixed cranial vault → raised intracranial pressure (ICP)
- The severity depends on the rate of decline more than the absolute number: a Na⁺ of 115 that developed over 5 days is often better tolerated than a Na⁺ of 125 that dropped in 6 hours, because the brain has had time to adapt in the former case
Clinical consequences of cerebral oedema:
| Consequence | Mechanism | Clinical Feature |
|---|---|---|
| Mild ↑ ICP | Early oedema → compression of neural tissue | Nausea, malaise, headache |
| Moderate ↑ ICP | Progressive oedema → impaired cortical function | Confusion, disorientation, drowsiness, lethargy |
| Severe ↑ ICP | Significant cortical oedema → neuronal hyperexcitability (lowered seizure threshold) | Seizures — hyponatremia is a recognised cause of acute symptomatic seizures [8] |
| Critical ↑ ICP / Herniation | Massive oedema → transtentorial herniation (brain pushes through the tentorium cerebelli) → compression of the brainstem | Coma, respiratory arrest, death |
Brain herniation is the MOST fatal complication of hyponatremia [25]. It occurs almost exclusively in: (1) patients with intracranial pathology, (2) patients who have massive water ingestion associated with psychosis, competitive exercises such as marathon runners, or use of recreational drug ecstasy, (3) parenteral fluid administration in post-operative patients who have ADH hypersecretion associated with surgery [25].
Who Is Most at Risk for Fatal Cerebral Oedema?
- Pre-menopausal women: Oestrogen impairs the brain's Na⁺-K⁺-ATPase, reducing the ability to extrude solute during adaptation → worse cerebral oedema for a given degree of hyponatremia. This is why young women on SSRIs or post-operative hypotonic fluids are a particularly dangerous combination.
- Children: Higher brain-to-skull ratio → less room for brain expansion → more vulnerable to herniation.
- Patients with existing intracranial pathology (tumour, recent neurosurgery, SAH): Already reduced intracranial compliance → even small increases in brain volume → herniation [25].
Seizures from hyponatremia are an acute symptomatic seizure — they arise from the pathological lowering of the seizure threshold [8].
Pathophysiology:
- Cerebral oedema → stretching and distortion of neuronal membranes → altered ion channel function
- Hypo-osmolality itself directly lowers the resting membrane potential of neurons (makes them more excitable) because the dilute ECF alters transmembrane ion gradients
- Combined effect: lowered seizure threshold → cortical hyperexcitability → generalised tonic-clonic seizures
Key points:
- Seizure from hyponatremia is listed as a characteristic sign of tumour lysis syndrome (which causes hyponatremia among other electrolyte derangements: seizure → hyponatremia) [15]
- Usually occurs when Na⁺ < 115–120, especially if acute
- Represents a medical emergency requiring 3% hypertonic saline [1][3]
- These are provoked seizures → do NOT start long-term anti-epileptic drugs; correct the sodium
This is an increasingly recognised complication of even mild chronic hyponatremia (Na⁺ 130–134).
Pathophysiology:
- Subtle cerebral oedema → impaired attention, psychomotor slowing, impaired postural reflexes
- Hyponatremia also causes direct osteoporosis: low Na⁺ stimulates osteoclast activity (osteoclasts have Na⁺-sensitive signalling pathways) and promotes bone resorption. Animal studies show chronic hyponatremia causes bone density loss comparable to vitamin D deficiency.
- Combined effect: impaired balance + weaker bones → increased risk of falls AND fractures
Clinical significance:
- Highly relevant in elderly patients [5] — this population already has baseline fall risk from polypharmacy, sarcopenia, and visual impairment
- Thiazide-induced chronic mild hyponatremia → falls → hip fractures → significant morbidity/mortality
- This is why even "asymptomatic" mild hyponatremia should not be ignored
Pathophysiology:
- Chronic low-grade cerebral oedema impairs neurotransmission
- Altered glutamate and GABA signalling (organic osmolytes like glutamate and taurine are expelled from brain cells during adaptation → reduced excitatory neurotransmission)
- Results in attention deficits, impaired concentration, slowed processing, and poor memory
Clinical significance:
- Often mistaken for dementia or depression in the elderly
- Reversible with correction of Na⁺
- Under-recognised cause of "failure to thrive" in nursing home residents
Pathophysiology:
- Severe hyponatremia → hypotonic extracellular environment → water influx into skeletal muscle cells → cell swelling → myocyte lysis → release of myoglobin, CK, K⁺, PO₄
- Myoglobin precipitates in renal tubules → pigment nephropathy → AKI
- Hyperkalaemia from cell lysis → cardiac arrhythmias
This is uncommon but is tested in exams because it creates a vicious cycle: hyponatremia → rhabdomyolysis → AKI → worsening electrolyte derangement.
Hyponatremia is an independent predictor of mortality in hospitalised patients, even when mild (130–134):
| Setting | Impact |
|---|---|
| Heart failure | Hyponatremia at admission predicts worse outcomes, higher 30-day mortality |
| Cirrhosis | Na⁺ < 130 is incorporated into the MELD-Na score (used for liver transplant prioritisation) — lower Na⁺ = worse prognosis |
| Pneumonia | Hyponatremia (often from SIADH) as a systemic complication is associated with increased ICU admission and mortality [26][27] |
| Post-operative | Perioperative hyponatremia independently predicts longer hospital stay and higher mortality |
| General inpatient | Association with increased all-cause in-hospital mortality across all diagnoses |
Why does hyponatremia predict mortality even when mild?
- It is a marker of underlying disease severity (worse HF, worse cirrhosis, more extensive malignancy)
- It also has direct deleterious effects (cognitive impairment → falls; cardiac effects → arrhythmia susceptibility)
B. Complications of Treatment — Iatrogenic
These are arguably more important for exams than the complications of the condition itself, because they are preventable and have medicolegal implications [3].
This is the single most feared complication in the management of hyponatremia.
Old name: Central pontine myelinolysis (CPM) — because the pons was the classic site of demyelination. Current name: Osmotic demyelination syndrome (ODS) — because demyelination can also be extrapontine (basal ganglia, thalamus, cerebellum, cortex).
Pathophysiology (from first principles):
-
In chronic hyponatremia, brain cells adapt over 24–48 hours by expelling intracellular osmolytes (first electrolytes: Na⁺, K⁺, Cl⁻; then organic osmolytes: myo-inositol, glutamate, taurine, glycerophosphorylcholine). This restores brain cell volume toward normal despite the hypotonic ECF.
-
If serum [Na⁺] is then corrected too rapidly, the ECF becomes relatively hypertonic compared to the osmolyte-depleted brain cells.
-
Water rushes OUT of brain cells → acute cellular dehydration → endothelial cell damage in the brain vasculature → disruption of the blood-brain barrier → complement activation and inflammatory cascade → oligodendrocyte apoptosis → demyelination
-
The pons is particularly vulnerable because it has a unique anatomical relationship between grey and white matter — the white matter tracts in the central pons are closely interdigitated with grey matter nuclei, creating a watershed zone susceptible to osmotic stress.
If correction occurs too rapidly, may result in a complication called central pontine myelinolysis [3]. Central pontine myelinolysis is very severe → patients become tetraplegic [3].
Risk factors for ODS:
| Risk Factor | Why It Increases ODS Risk |
|---|---|
| Chronic hyponatremia (> 48h) | Brain has fully adapted (lost osmolytes) → most vulnerable to rapid osmotic shifts [1][3] |
| Na⁺ < 105 mmol/L | Greater degree of osmolyte depletion |
| Alcoholism | Chronic malnutrition → depleted organic osmolytes; concurrent liver disease |
| Malnutrition | Reduced intracellular osmolyte stores |
| Liver disease / Cirrhosis | Impaired synthetic function → low osmolytes; concurrent malnutrition |
| Hypokalaemia | K⁺ correction raises effective serum tonicity → contributes to "hidden" overcorrection of Na⁺. Also, K⁺ depletion may directly impair oligodendrocyte function |
| Burns | Massive fluid shifts; malnutrition |
| Correction rate > 10 mmol/L/24h | Direct cause (exceeding safe limits) |
Clinical features of ODS:
- Typically present 2–6 days after overcorrection (the "lucid interval" — patient initially improves with Na⁺ correction, then deteriorates days later)
- Pontine ODS (classic):
- Dysarthria ("dys" = difficulty, "arthria" = articulation)
- Dysphagia ("dys" = difficulty, "phagia" = swallowing)
- Paraparesis → quadriparesis (corticospinal tract demyelination)
- "Locked-in syndrome": patient is fully conscious but cannot move or speak — only able to communicate through vertical eye movements and blinking (because the pontine lesion spares the reticular activating system above and the oculomotor pathways)
- Tetraplegic [3]
- Extrapontine ODS:
- Behavioural changes, movement disorders (parkinsonism, dystonia), mutism, catatonia
- Seizures
- Cognitive dysfunction
Diagnosis:
- MRI brain (T2/FLAIR): hyperintense signal in the central pons ("trident" or "bat-wing" pattern) or extrapontine structures. MRI changes may lag behind clinical presentation by 1–2 weeks.
Treatment:
- Largely supportive — there is no proven specific treatment once ODS has occurred
- Some case reports suggest re-lowering Na⁺ back toward hyponatremic levels (with D5W ± DDAVP) within the first 24 hours of overcorrection may help, before irreversible damage occurs
- Rehabilitation for neurological deficits
- Prognosis is highly variable: some patients recover substantially over weeks to months; others have permanent devastating neurological deficits
ODS Prevention: The Golden Rules
The rate of correction should be slow for chronic hyponatremia → over 2 days. Less than 0.5 mmol/L/hour, or < 12 mmol/L/day [3]. Current guidelines recommend:
- Standard patients: ≤ 10 mmol/L in first 24h, ≤ 8 mmol/L in any subsequent 24h
- High-risk patients (alcoholism, malnutrition, liver disease, K⁺ < 3.0, Na⁺ < 105): ≤ 6–8 mmol/L/24h
- If unknown duration: assume chronic → use the slower rate
- Monitor Na⁺ every 2–4 hours during active correction
- If overcorrection occurs: immediately give D5W ± DDAVP to re-lower Na⁺
- Correct concurrent hypokalaemia — but factor K⁺ correction into the total Na⁺ correction rate
Hypertonic saline should only be used in very experienced hands [3]. This carries medicolegal implications [3].
Mechanism: Overly aggressive IV saline administration (especially NS or hypertonic saline) in patients with limited cardiac reserve → fluid overload → pulmonary oedema.
At-risk populations:
- Heart failure patients (already volume-overloaded)
- Elderly patients with reduced cardiac reserve
- CKD patients (reduced ability to excrete the fluid load)
Prevention:
- Judicious fluid administration with close monitoring (fluid balance charts, daily weights, lung auscultation)
- In hypervolemic hyponatremia, saline is generally NOT indicated — use fluid restriction + treat the underlying disease
- In euvolemic hyponatremia (SIADH), NS is generally NOT indicated either (it can worsen hyponatremia AND cause fluid overload)
Mechanism: When treating hyponatremia with NS or hypertonic saline, the osmotic shift of water out of cells can cause K⁺ to move INTO cells (along with water). Additionally, if hyponatremia is corrected with aggressive diuresis (loop diuretics), K⁺ is lost renally.
Clinical significance:
- Hypokalaemia is itself a risk factor for ODS
- Cardiac arrhythmias (hypokalaemia + hyponatremia is a particularly dangerous combination)
- Must monitor K⁺ alongside Na⁺ and replace aggressively
Mechanism: After stopping hypertonic saline or NS, the underlying cause of hyponatremia may persist (e.g., SIADH, ongoing ADH stimulus). The patient can rapidly re-develop hyponatremia if cause-specific treatment is not initiated concurrently.
Prevention: Always initiate cause-specific management (fluid restriction, drug cessation, hormone replacement) alongside acute correction.
| Complication | Category | Mechanism | Key Point |
|---|---|---|---|
| Cerebral oedema | Disease | ↓ Serum osm → water influx into brain cells → swelling | Severity depends on rate more than absolute level |
| Brain herniation | Disease | Severe oedema → transtentorial herniation → brainstem compression | Most fatal complication [25]; requires emergency 3% NaCl |
| Seizures | Disease | Cerebral oedema + altered neuronal membrane potential → cortical hyperexcitability | Acute symptomatic seizure; do NOT start AEDs long-term |
| Falls / Fractures | Disease | Subtle cognitive impairment + osteoporosis | Even mild chronic hyponatremia; elderly at highest risk |
| Cognitive impairment | Disease | Chronic low-grade oedema + altered neurotransmission | Reversible; often mistaken for dementia |
| Rhabdomyolysis | Disease | Hypotonic cell swelling → myocyte lysis | Rare; can cause AKI from myoglobin nephropathy |
| Increased mortality | Disease | Marker of disease severity + direct effects | Independent predictor across all diagnoses |
| ODS / CPM | Treatment | Rapid correction of chronic hyponatremia → brain cell dehydration → demyelination | Most feared iatrogenic complication; patients become tetraplegic [3]; onset 2–6 days post-correction |
| Volume overload | Treatment | Excessive IV saline → fluid overload | Avoid NS in hypervolemic and euvolemic causes |
| Hypokalaemia | Treatment | Osmotic shifts + diuretic use during correction | Risk factor for ODS; monitor and replace K⁺ |
| Rebound hyponatremia | Treatment | Underlying cause persists after stopping acute Tx | Must address the underlying cause concurrently |
High Yield Summary — Complications of Hyponatremia
- Brain herniation is the MOST fatal complication of hyponatremia [25] — occurs in acute severe cases, especially with intracranial pathology, psychogenic polydipsia, marathon runners, ecstasy use, and post-operative hypotonic fluid administration.
- Cerebral oedema is the pathophysiological basis of all neurological complications — water influx into brain cells due to osmotic gradient.
- Seizures are acute symptomatic seizures from hyponatremia — treat the Na⁺, not with long-term AEDs.
- Osmotic demyelination syndrome (ODS/CPM) is the most feared treatment complication — caused by too rapid correction of chronic hyponatremia. Patients become tetraplegic [3]. Presents 2–6 days after overcorrection with dysarthria, dysphagia, quadriparesis, and locked-in syndrome.
- Safe correction limits: ≤ 10 mmol/L/24h standard, ≤ 6–8 mmol/L/24h for high-risk patients (alcoholism, malnutrition, liver disease, hypokalaemia, Na⁺ < 105).
- Even mild chronic hyponatremia (130–134) is NOT benign — causes falls, osteoporosis, fractures, cognitive impairment, and increased mortality.
- Hyponatremia is an independent predictor of mortality in HF, cirrhosis, pneumonia, and general inpatients.
- Treatment complications (ODS, volume overload, hypokalaemia, rebound hyponatremia) are preventable — careful monitoring and adherence to correction limits.
- If overcorrection occurs → rescue with D5W ± DDAVP immediately.
Active Recall - Complications of Hyponatremia
References
[1] Lecture slides: GC 044. Electrolyte and Acid-Base Disorders.pdf [2] Lecture slides: Chemical Pathology Seminar 1_Sodium and water.pdf [3] Senior notes: Block A - Electrolyte and Acid-Base Disorders.pdf (hyponatremia management, correction rates, CPM) [5] Lecture slides: GC 079. Prescribing in older people.pdf [6] Senior notes: Ryan Ho Chemical Path.pdf (hyponatremia and hypernatremia sections) [8] Senior notes: MBBS Final MB (Pediatrics) (Felix PY Lai).pdf (seizure causes — hyponatremia as metabolic encephalopathy) [15] Senior notes: Block A - High white cell count_ acute and chronic leukaemia; bone marrow transplantation; immunogenetics.pdf (tumour lysis syndrome — seizure from hyponatremia) [25] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (brain herniation as most fatal complication, settings) [26] Senior notes: Ryan Ho Respiratory.pdf (pneumonia complications — hypoNa due to SIADH) [27] Senior notes: Adrian Lui Pediatrics Notes.pdf (pneumonia complications — electrolyte abnormalities, hypoNa due to SIADH)
High Yield Summary
- Hyponatremia = serum [Na⁺] < 135 mmol/L — the most common electrolyte disorder in hospitalised patients.
- [Na⁺] reflects water balance, not sodium content. In most cases, hyponatremia = relative water excess.
- Step 1: Check serum osmolality → exclude pseudohyponatremia (isotonic, ↑↑ lipids/proteins) and translocational hyponatremia (hypertonic, hyperglycaemia).
- Step 2: Assess volume status → hypovolemic (Na⁺ and water lost, but Na⁺ more), euvolemic (SIADH, hypothyroidism, adrenal insufficiency), hypervolemic (HF, cirrhosis, nephrotic syndrome).
- Step 3: Check urine [Na⁺] and urine osmolality → distinguish renal vs. extrarenal losses in hypovolemic; distinguish SIADH (high urine Osm, UNa > 20) from polydipsia (low urine Osm).
- SIADH is the most common cause of euvolemic hyponatremia — diagnosis by exclusion; causes: malignancy (SCLC), pulmonary disease, CNS disease, drugs (SSRIs, carbamazepine).
- CSWS vs SIADH: both follow CNS pathology, both have high UNa; CSWS = hypovolemic (give saline), SIADH = euvolemic (restrict fluids).
- Thiazides are the #1 drug cause; they impair NaCl reabsorption at DCT but preserve medullary concentration gradient → kidney can still retain water via ADH.
- Symptoms are neurological (cerebral oedema): nausea → confusion → seizures → coma → death. Severity depends on rate of onset more than absolute level.
- Correction speed: acute symptomatic → can correct faster (3% NaCl); chronic → max 10 mmol/L/24h to avoid osmotic demyelination syndrome.
- Always correct for glucose in diabetic patients before diagnosing true hyponatremia.
- Chronic mild hyponatremia is NOT benign — associated with falls, osteoporosis, cognitive impairment, and mortality.
High Yield Summary — Differential Diagnosis of Hyponatremia
- Always start from the lab — hyponatremia is a biochemical diagnosis. Check serum osmolality FIRST.
- Exclude pseudohyponatremia (isotonic: ↑↑ lipids/proteins → check direct ISE) and translocational hyponatremia (hypertonic: correct Na⁺ for glucose).
- For true hypotonic hyponatremia, the three pillars of differential are: (a) volume status, (b) urine osmolality, (c) urine [Na⁺].
- Hypovolemic + UNa < 20 = extrarenal loss (GI, burns). Hypovolemic + UNa > 20 = renal loss (thiazides, Addison's, CSWS).
- Euvolemic + Urine Osm > 200 + UNa > 20 = SIADH (after excluding hypothyroidism and adrenal insufficiency). Urine Osm < 100 = polydipsia or low-solute diet.
- SIADH is a diagnosis of EXCLUSION — must rule out hypothyroidism (TSH), adrenal insufficiency (cortisol), drugs, renal/cardiac/liver disease.
- CSWS vs SIADH: both follow CNS pathology, both have high UNa. CSWS = hypovolemic (give saline); SIADH = euvolemic (fluid restriction). Getting this wrong = giving opposite treatment.
- Hypervolemic: HF, cirrhosis, nephrotic syndrome (UNa < 20) vs. advanced CKD (UNa > 20).
- Drugs: thiazides (#1), SSRIs, carbamazepine — especially in elderly. Always review medication list.
- Serum K⁺ is a helpful differentiator: ↑K⁺ → Addison's, CKD, type 4 RTA; ↓K⁺ → thiazides, vomiting, Bartter/Gitelman.
High Yield Summary — Diagnosis and Investigations
- Hyponatremia is a biochemical diagnosis — start from the lab report [2].
- The diagnostic algorithm is a 3-step process: (1) Serum osm (true vs pseudo/translocational), (2) Volume status (hypo/eu/hyper), (3) Urine osm + urine Na⁺ (renal vs extrarenal, ADH active vs suppressed).
- SIADH diagnostic criteria: ↓ serum osm, ↑ urine osm > 200, urine Na > 20, euvolemic, normal RFT, normal TFT, normal adrenal function, no drugs, response to fluid restriction — diagnosis by exclusion [6].
- Always check TSH and morning cortisol before diagnosing SIADH.
- Urine osm < 100 in a hyponatremic patient = ADH is OFF → primary polydipsia or low-solute diet (not SIADH).
- Urine osm > 200 + urine Na > 20 in euvolemic patient = SIADH (after exclusion).
- Urine Na⁺ is unreliable if patient is on diuretics — use FEUrea instead.
- Serum K⁺ is a powerful differentiator: ↑K⁺ → Addison's, CKD; ↓K⁺ → thiazides, vomiting, Bartter/Gitelman.
- Kidney size on US: small = CKD; normal with high creatinine = AKI; large = polycystic/infiltrative/obstruction [14].
- Correct Na⁺ for glucose in all diabetic patients before diagnosing true hyponatremia.
- Copeptin is a surrogate for ADH — high copeptin in DI = nephrogenic/AVP-resistance; low = cranial/AVP-deficiency [19].
High Yield Summary — Management of Hyponatremia
- Comes quick, correct quick; comes slow, correct slow [21].
- Severe symptomatic (seizures/coma): 3% NaCl 100 mL bolus IV over 10–20 min, repeat up to ×3. Target: raise Na⁺ by 4–6 in first 1–2 hours.
- Chronic hyponatremia correction rate: < 10 mmol/L/24h (6–8 in high-risk). Overcorrection → osmotic demyelination syndrome → tetraplegia [3].
- Hypovolemic: IV 0.9% NS for volume repletion + treat cause. Watch for autocorrection.
- Euvolemic / SIADH: Fluid restriction 800–1000 mL/day (1st line) → add oral NaCl + loop diuretic → tolvaptan or demeclocycline for refractory cases.
- Hypervolemic: Fluid + salt restriction + treat underlying disease (HF, cirrhosis, CKD). Stop diuretics if Na⁺ < 120.
- Drug-induced: STOP the offending drug. Most common: thiazides, SSRIs, carbamazepine.
- Overcorrection rescue: D5W infusion ± DDAVP 2 mcg IV/SC to re-lower Na⁺.
- Tolvaptan: V2-receptor antagonist for refractory SIADH. C/I in hypovolemia. Must initiate in hospital. Hepatotoxicity risk.
- Correct concurrent hypokalaemia — K⁺ correction also raises Na⁺ and reduces ODS risk.
- In cirrhosis: stop ALL diuretics if severe hyponatremia, progressive renal failure, worsening HE, or incapacitating cramps [23].
- Prevention: use isotonic fluids perioperatively; monitor Na⁺ when starting thiazides/SSRIs in elderly [5].
High Yield Summary — Complications of Hyponatremia
- Brain herniation is the MOST fatal complication of hyponatremia [25] — occurs in acute severe cases, especially with intracranial pathology, psychogenic polydipsia, marathon runners, ecstasy use, and post-operative hypotonic fluid administration.
- Cerebral oedema is the pathophysiological basis of all neurological complications — water influx into brain cells due to osmotic gradient.
- Seizures are acute symptomatic seizures from hyponatremia — treat the Na⁺, not with long-term AEDs.
- Osmotic demyelination syndrome (ODS/CPM) is the most feared treatment complication — caused by too rapid correction of chronic hyponatremia. Patients become tetraplegic [3]. Presents 2–6 days after overcorrection with dysarthria, dysphagia, quadriparesis, and locked-in syndrome.
- Safe correction limits: ≤ 10 mmol/L/24h standard, ≤ 6–8 mmol/L/24h for high-risk patients (alcoholism, malnutrition, liver disease, hypokalaemia, Na⁺ < 105).
- Even mild chronic hyponatremia (130–134) is NOT benign — causes falls, osteoporosis, fractures, cognitive impairment, and increased mortality.
- Hyponatremia is an independent predictor of mortality in HF, cirrhosis, pneumonia, and general inpatients.
- Treatment complications (ODS, volume overload, hypokalaemia, rebound hyponatremia) are preventable — careful monitoring and adherence to correction limits.
- If overcorrection occurs → rescue with D5W ± DDAVP immediately.
Diabetes Mellutus
Diabetes mellitus is a chronic metabolic disorder characterized by persistent hyperglycemia resulting from defects in insulin secretion, insulin action, or both.
Hypernatremia
Hypernatremia is a serum sodium concentration greater than 145 mEq/L, typically resulting from a deficit of total body water relative to sodium, leading to hyperosmolality and cellular dehydration.