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.

Exam domain	One-glance essentials
Definition / diagnosis	*Serum Na⁺ > 145 mmol/L reflects hypertonicity and too little water relative to sodium* ^[1]^[2] Classify as acute < 48 h or chronic/unknown because brain adaptation determines correction speed ^[3]
Epidemiology / risks	Infants, frail older adults, cognitively impaired and intubated patients cannot reliably access or request water ^[1] HHS/osmotic diuresis, fever/GI loss and diabetes insipidus are common mechanisms; lithium is a key nephrogenic DI drug ^[3]^[4]
Core mechanism	Hypertonic ECF → water leaves brain cells → cell shrinkage and traction on cerebral vessels ^[3] Chronic brain cells accumulate idiogenic osmoles; rapid rehydration then drives water inward → cerebral oedema ^[3]
Clinical picture	Thirst, irritability, lethargy and confusion may progress to seizures or coma ^[3] Polyuria/polydipsia suggests DI; dehydration signs suggest hypovolaemic water loss ^[1]^[3]
Investigations	*Obtain paired serum and urine osmolality plus glucose, K⁺ and Ca²⁺* ^[2]^[3] Urine Osm < 300 mOsm/kg despite hypernatraemia strongly suggests DI; DDAVP response supports AVP deficiency, absent response AVP resistance ^[3] Calculate free-water deficit and add ongoing urinary, GI and insensible losses ^[3]
Management	Shock/hypotension → *restore circulation with 0.9% saline first, then replace free water orally/enterally or with IV D5 ^[1]^[3] Chronic/unknown duration: lower Na⁺ by < 8–10 mmol/L per 24 h* with frequent monitoring ^[3] AVP deficiency → desmopressin; AVP resistance → remove cause, low-solute diet ± thiazide/amiloride ^[3]^[4]
Complications	Severe disease → encephalopathy, seizures, ICH/SAH, shock and pre-renal AKI ^[3] Over-rapid chronic correction → cerebral oedema, herniation and death ^[2]^[3]
Prevention / follow-up	Prescribe and document adequate free water for dependent, intubated and cognitively impaired patients ^[1] Monitor Na⁺, urine output and renal function with lithium, major GI losses, HHS or post-pituitary surgery ^[4]
Exam traps	*In hypovolaemic shock, isotonic resuscitation precedes hypotonic correction* ^[1] Hypernatraemia + dilute urine is already diagnostic of DI; do not water-deprive an already hypernatraemic patient ^[3] Desmopressin overtreatment plus free-water intake can cause dangerous hyponatraemia ^[3]

References

^[1] Lecture slides / Senior notes: Block A - Electrolyte and Acid-Base Disorders.pdf (Sodium section, Hypernatraemia algorithm) ^[2] Lecture slides: Chemical Pathology Seminar 1_Sodium and water.pdf ^[3] Senior notes: Maksim Medicine Notes.pdf (Nephrology section - Hypernatraemia, p208); Ryan Ho Urogenital.pdf (Hypernatraemia diagnostic approach and management, p20–21) ^[4] Senior notes: Block A - Drugs and the Kidney.pdf (Lithium section); Block A - Two cases of polyuria and polydipsia.pdf

Hypernatraemia is defined as a serum sodium concentration > 145 mmol/L ^[1]^[2]. It always reflects a state of hyperosmolality (i.e., elevated effective serum osmolality), because sodium is the dominant extracellular cation and the primary determinant of plasma tonicity.

The word itself: "hyper" (Greek: above/excessive) + "natr-" (Latin: natrium = sodium) + "-aemia" (Greek: haima = blood). So hypernatraemia literally means "too much sodium in the blood."

Key Conceptual Point – What Sodium Really Reflects

[Na⁺] does not reflect the absolute content of Na⁺ in the body — but reflects the amount of solvent (water) ^[1]. In the absence of sodium retention or loss:

HypoNa → water retention (dilution, fluid overload)
HyperNa → water depletion (dehydration)

The situation becomes more complicated when there is concomitant Na⁺ loss or retention. So when you see hypernatraemia, think: "This patient has lost water, or gained sodium, or both." ^[1]

Because hypernatraemia ≡ hyperosmolality, it always creates an osmotic gradient pulling water out of cells, causing cellular dehydration — this is why neurological symptoms dominate the picture.

Epidemiology and Risk Factors

Hypernatraemia is common in hospitalized patients, particularly in the ICU (prevalence ~2–6% of ICU admissions) ^[3].
Community-acquired hypernatraemia is less frequent but carries a high mortality (40–60% in severe cases), partly because it reflects severe underlying disease and partly because the brain injury itself is devastating ^[3].
Hospital-acquired hypernatraemia is often iatrogenic (inadequate free water prescription, hypertonic fluid administration) and is an independent predictor of in-hospital mortality.
Bimodal age distribution: very young (neonates/infants) and very old (elderly > 65 years) ^[2]^[4].

The fundamental requirement for hypernatraemia to develop is either impaired access to water or impaired thirst. If a conscious person with a working hypothalamic thirst centre has access to water, they will simply drink and correct any mild rise in sodium. So the key risk factors are:

Risk Factor	Why
Extremes of age (neonates, elderly)	Neonates depend on caregivers for water; elderly have diminished thirst sensation and reduced renal concentrating ability
Altered mental status / impaired consciousness	Cannot perceive or communicate thirst (delirium, dementia, stroke, sedation)
Intubated / ICU patients	Rely entirely on prescribed IV fluids for free water
Diabetes insipidus (DI) — central or nephrogenic	Massive free water losses via dilute urine; if intake cannot keep up → hypernatraemia
Infective gastroenteritis (especially in children)	Hypotonic fluid losses (diarrhoea > vomiting)
Nursing home residents	Limited access to water, cognitive impairment
Drugs: lithium, demeclocycline	Cause nephrogenic DI → free water loss ^[5]
Osmotic diuresis (hyperglycaemia, mannitol, urea)	Obligate water loss exceeding sodium loss
Febrile illness / burns / excessive sweating	Insensible losses of hypotonic fluid
Hyperaldosteronism (Conn syndrome)	Na⁺ retention → hypervolaemic hypernatraemia ^[1]

Exam Pearl

Elderly patients with a reduced appreciation of thirst, difficulty drinking, or those on diuretics are especially prone to severe hyperosmolality ^[6]. This is a classic exam scenario — an elderly, confused nursing home patient brought in dehydrated.

Anatomy and Physiology of Water and Sodium Homeostasis

To understand hypernatraemia, you need to understand the normal system that keeps [Na⁺] in the tight range of 135–145 mmol/L. This is fundamentally a water balance problem, not a sodium balance problem.

Total body water (TBW) ≈ 60% of body weight in men, ~50% in women and elderly (less lean mass = less water)
TBW is divided:
- Intracellular fluid (ICF): 2/3 of TBW (~40% BW)
- Extracellular fluid (ECF): 1/3 of TBW (~20% BW)
  - Interstitial fluid: 3/4 of ECF
  - Plasma: 1/4 of ECF

Key Hormones

Produced in the supraoptic and paraventricular nuclei of the hypothalamus
Transported along axons and stored in the posterior pituitary
Released in response to:
1. ↑ Serum osmolality (detected by hypothalamic osmoreceptors — exquisitely sensitive, threshold ~280 mOsm/kg)
2. ↓ Effective circulating volume (detected by baroreceptors in carotid sinus, aortic arch, and cardiac atria — less sensitive, requires > 10% volume loss)
3. Nausea, pain, stress
Mechanism of action: AVP binds V2 receptors on the basolateral membrane of collecting duct principal cells → activates cAMP → aquaporin-2 (AQP2) channels are inserted into the apical membrane → water is reabsorbed from the tubular lumen into the hypertonic medullary interstitium → concentrated urine
Co-secreted with copeptin (the C-terminal fragment of pre-pro-vasopressin). Copeptin is a surrogate marker for AVP production because AVP itself has a very short half-life (~6 minutes) and is difficult to measure reliably ^[7]^[8].

Osmoreceptors in the organum vasculosum of the lamina terminalis (OVLT) detect ↑ osmolality → trigger thirst
Thirst is the primary defence against hypernatraemia
Threshold for thirst (~295 mOsm/kg) is slightly higher than for ADH release

Regulates sodium reabsorption in the distal nephron (principal cells of cortical collecting duct) via ENaC
Primarily regulates volume, not osmolality
Excess aldosterone (Conn syndrome) → Na⁺ retention → mild hypernatraemia + hypertension + hypokalaemia

Most of sodium (~70%) is reabsorbed at the proximal tubules, ~25% at the loop of Henle, very little at remaining places ^[1]
The countercurrent multiplier (loop of Henle) creates the medullary concentration gradient
AVP-mediated AQP2 insertion in collecting ducts allows maximal urine concentration up to ~1200 mOsm/kg
Without AVP → urine is dilute (~50–100 mOsm/kg) → massive free water loss

Etiology (Focus on Hong Kong Context)

Conceptual Framework

Three main categories of hypernatraemia: hypervolaemic (e.g. Conn syndrome) vs. isovolaemic/hypovolaemic (fluid loss, or DI) — separated based on serum vs. urine osmolality ^[1]. The key question: Is the kidney responding appropriately (concentrating urine) or inappropriately (dilute urine despite high serum osmolality)?

Detailed Aetiological Breakdown

The patient has lost both sodium and water, but proportionally more water than sodium (hypotonic fluid loss).

Source	Examples	Hong Kong Relevance
GI losses	Diarrhoea (especially in children), vomiting, NG suction, osmotic diarrhoea (lactulose)	Infectious gastroenteritis (rotavirus, norovirus) very common in HK paediatric population
Renal losses	Osmotic diuresis (hyperglycaemia/DKA/HHS, mannitol), loop diuretics, post-obstructive diuresis	HK has high prevalence of T2DM → HHS common in elderly
Skin/respiratory	Burns, excessive sweating (HK subtropical climate), fever, hyperventilation	Hot and humid HK summers increase insensible losses

Pathophysiology: Hypotonic fluid loss → relative excess of Na⁺ remaining → [Na⁺] rises. ECF volume is contracted → signs of dehydration.

2. Euvolaemic (Isovolaemic) Hypernatraemia (Pure water loss)

The patient has lost water without proportional sodium loss, or has impaired water intake.

This is the classic exam-tested cause of euvolaemic hypernatraemia with dilute urine.

Feature	AVP-Deficiency (Central DI)	AVP-Resistance (Nephrogenic DI)
Old name	Central / Cranial DI	Nephrogenic DI
New name (2022)	AVP-D (Arginine Vasopressin Deficiency)	AVP-R (Arginine Vasopressin Resistance)
Pathophysiology	Deficiency of ADH — inadequate production/secretion from hypothalamus/posterior pituitary ^[7]	Resistance to ADH — kidneys cannot respond to ADH ^[7]
Causes	Idiopathic (most common), infective (meningoencephalitis), neoplastic (craniopharyngioma, metastases), head trauma, vascular (Sheehan syndrome), iatrogenic (TSS) ^[4]	Lithium toxicity (most common drug cause), metabolic (hypokalaemia, hypercalcaemia), hereditary (XR mutation in V2 receptor, AD/AR mutation in AQP-2), chronic obstruction ^[4]^[5]
Copeptin level	Low (no AVP production → no copeptin)	High (lots of AVP being produced, but kidneys don't respond) ^[7]^[8]
Response to DDAVP	↑ urine osmolality after DDAVP (kidneys respond when you give exogenous ADH) ^[7]	No increase in urine osmolality after DDAVP (kidneys are resistant) ^[7]

2022 Name Change

The 2022 consensus proposal replaced "diabetes insipidus" with AVP-D (central) and AVP-R (nephrogenic) to eliminate confusion with diabetes mellitus and endorse a name that more accurately reflects the condition ^[7]. Know both old and new names for exams.

Why does hypercalcaemia cause nephrogenic DI? → Hypercalcaemia can cause autophagic degradation of aquaporin-2 channels → less water reabsorption → more urine output and water loss ^[9]. This directly creates nephrogenic DI → hypernatraemia. This is why hypercalcaemia patients are often dehydrated.

Why does lithium cause nephrogenic DI? → Lithium enters principal cells via ENaC, accumulates intracellularly, and decreases expression of AQP2 ^[5]^[10]. Lithium is the most common drug inducing nephrogenic DI ^[5].

Why does hypokalaemia cause nephrogenic DI? → Chronic hypokalaemia downregulates AQP2 expression in the collecting duct and impairs the medullary concentration gradient (by stimulating prostaglandin E2 production, which antagonises ADH action).

Insensible losses (fever, hyperventilation, mechanical ventilation with dry gases) without adequate free water replacement
Hypodipsia / Adipsia: rare, due to hypothalamic lesions (tumour, infiltration, congenital)
Depression of sensorium with lack of thirst response (when serum osmolality > 340 mmol/L) ^[6] — a vicious cycle: hypernatraemia causes confusion → confused patient can't drink → hypernatraemia worsens

The patient has gained sodium in excess of water. This is the least common category.

Cause	Mechanism
Iatrogenic hypertonic saline / NaHCO₃	Direct Na⁺ loading (e.g. during cardiac arrest resuscitation, TPN with high Na content)
Primary hyperaldosteronism (Conn syndrome) ^[1]	Aldosterone excess → Na⁺ retention via ENaC in collecting duct → mild hypernatraemia, hypertension, hypokalaemia
Cushing syndrome	Cortisol excess → mineralocorticoid effect
Salt poisoning (child abuse, accidental)	Direct Na⁺ ingestion
Sea water drowning	Hypertonic sea water ingestion
Hypertonic dialysis	Dialysate with high sodium concentration

High prevalence of T2DM → osmotic diuresis (glucosuria) and HHS are major causes
Elderly population with comorbidities (dementia, stroke) → impaired thirst and access to water
Lithium use in psychiatric patients (bipolar disorder treatment) → nephrogenic DI
Subtropical climate → significant insensible losses in summer
Paediatric gastroenteritis → dehydrating illness in young children

Relevant Pathophysiology

This is the most important pathophysiological concept:

Hypernatraemia → ↑ plasma osmolality → osmotic gradient between ECF and ICF
Water moves out of brain cells (down the osmotic gradient) → brain cell shrinkage
Brain cell shrinkage → mechanical traction on meningeal vessels and bridging veins → risk of cerebral venous sinus thrombosis, intracerebral haemorrhage (ICH), and subarachnoid haemorrhage (SAH) ^[4]
Clinical manifestations: irritability → confusion → seizures → coma → death

If hypernatraemia develops slowly (over > 48 hours, i.e. chronic), brain cells mount a protective response:
- Brain cells generate idiogenic osmoles (organic osmolytes: taurine, glutamine, myoinositol, betaine, sorbitol)
- These intracellular osmolytes draw water back into the cells, partially restoring cell volume
- The brain "adapts" to the high osmolality environment
The danger of rapid correction: If you now rapidly lower serum Na⁺ / osmolality, these idiogenic osmoles are still inside the brain cells. Water rushes into the cells (down the new osmotic gradient) → cerebral oedema. This can cause herniation and death.

Rate of correction: < 8–10 mmol/24h to avoid cerebral oedema ^[4]. This is the mirror-image of the hyponatraemia correction limit (where rapid correction causes osmotic demyelination syndrome).

Critical Safety Rule

Rate of correction for hypernatraemia: < 8–10 mmol/L per 24 hours ^[4]. If the patient has acute hypernatraemia (< 48 hours duration, e.g. iatrogenic salt loading), faster correction is safer because brain adaptation has not yet occurred. But if duration is unknown or chronic, always assume chronic and correct slowly.

Classification

Category	Duration	Brain Adaptation	Correction Speed
Acute	< 48 hours	None/minimal → high risk of acute brain shrinkage	Can correct more rapidly (1–2 mmol/L/hr in first few hours, but still monitor closely)
Chronic	≥ 48 hours or unknown	Idiogenic osmoles present → adapted	Must correct slowly: < 8–10 mmol/24h ^[4]

Severity	[Na⁺] (mmol/L)
Mild	146–150
Moderate	151–159
Severe	≥ 160

Volume Status	Mechanism	Urine Osm	Urine Na⁺	Examples
Hypovolaemic	Hypotonic fluid loss	> 600–800 (renal response appropriate) OR < 300 (if renal loss is the cause)	< 20 (extrarenal) OR > 20 (renal)	Diarrhoea, burns, osmotic diuresis
Euvolaemic	Pure water loss / ↓ intake	< 300 (DI) or variable	Variable	DI, insensible loss, hypodipsia
Hypervolaemic	Na⁺ gain	> 600–800	> 20	Iatrogenic NaHCO₃, Conn syndrome

Urine Osmolality – The Key Discriminator

If urine osmolality < serum osmolality in the setting of hypernatraemia, this is inappropriate and highly suggestive of diabetes insipidus ^[1]^[4]. A normal kidney should be producing maximally concentrated urine (> 800 mOsm/kg) in response to hypernatraemia. If the urine is dilute, the kidney is either not receiving ADH (AVP-D) or not responding to it (AVP-R).

Clinical Features

The clinical features of hypernatraemia are predominantly neurological, reflecting the osmotic effects on brain cells. The severity of symptoms correlates with both the degree of hypernatraemia and the rapidity of its development.

Symptom	Pathophysiological Basis
Thirst (early)	↑ Osmolality detected by hypothalamic osmoreceptors → conscious desire to drink. This is the earliest and most sensitive symptom. If the patient cannot perceive thirst (elderly, dementia) or cannot access water (intubated, infant), severe hypernatraemia ensues
Headache ^[4]	Brain cell shrinkage → traction on pain-sensitive meningeal structures
Irritability, restlessness ^[4]	Early neuronal dysfunction from cellular dehydration
Lethargy, drowsiness	Progressive neuronal dehydration → impaired neurotransmission
Confusion, disorientation	Cerebral cortex dysfunction from osmotic stress; depression of sensorium with lack of thirst response (when serum osmolality > 340 mmol/L) ^[6] — creates a vicious cycle
Nausea, vomiting	CNS dysfunction (brainstem emetic centres); also contributes to further water loss
Muscle weakness, cramps	Cellular dehydration affecting muscle cells; also altered membrane potential (hypernatraemia shifts resting membrane potential)
Seizures ^[4]	Severe brain cell shrinkage → cortical irritability; or can occur during overly rapid correction (cerebral oedema)
Coma	Severe, progressive brain cell dehydration → global cerebral dysfunction
Polyuria, polydipsia (if DI is the cause)	Copious quantity of dilute urine → inadequate rise in urine osmolality despite high serum osmolality ^[7]; compensatory polydipsia if thirst is intact
Nocturia (if DI)	Inability to concentrate urine at night → continued high urine output during sleep

Special Populations

Neonates/infants: irritability, high-pitched cry, hyperthermia (dehydration), lethargy, "doughy" skin texture (distinct from the "tenting" of hypovolaemia — hypernatraemic dehydration causes cells to lose water but interstitial fluid is relatively preserved by the high osmolality, giving the skin a "doughy" rather than tenting quality)
Elderly: may present as delirium — an acute confusional state — which is the most common presentation in the elderly hospitalized patient ^[11]

Sign	Pathophysiological Basis
Signs of dehydration (hypovolaemic hypernatraemia):
— Loss of skin turgor = ~5% BW loss ^[1]	ECF volume depletion → reduced interstitial fluid
— Dry mucous membranes	Reduced mucosal hydration
— Sunken eyes (especially children)	Loss of retro-orbital fat pad hydration
— Postural hypotension = ~10% BW loss ^[1]	ECF contraction → reduced preload → ↓ cardiac output → orthostatic drop
— Shock (tachycardia, hypotension) = ~15% BW loss ^[1]	Severe ECF depletion → haemodynamic compromise
— Reduced urine output (if not DI)	Appropriate renal response to hypovolaemia
— Depressed fontanelle (infants)	Loss of intracranial fluid volume
Neurological signs
— Hyperreflexia, increased muscle tone	Neuronal shrinkage → increased excitability
— Myoclonus	Cortical irritability
— Asterixis	Metabolic encephalopathy (non-specific)
— Focal neurological deficits (rare)	If ICH or cerebral venous sinus thrombosis occurs from traction on bridging veins
— ICH / SAH (rare) ^[4]	Shearing of bridging veins as brain shrinks away from the skull
— Decreased level of consciousness → coma	Progressive brain dehydration
Signs of hypervolaemia (hypervolaemic hypernatraemia):
— Peripheral oedema	Na⁺ retention → ECF expansion
— Hypertension	Volume expansion → ↑ cardiac output
— Pulmonary oedema (if severe)	Fluid overload
Signs of underlying cause:
— Bitemporal hemianopia, ± other visual field defects	Pituitary tumour compressing optic chiasm → if central DI
— Features of hypopituitarism	If central DI from pituitary/hypothalamic disease
— Cushing syndrome features	If mineralocorticoid excess

Clinical Pearl – Why Skin Turgor is 'Doughy' in Hypernatraemic Dehydration

In hyponatraemic dehydration, the low osmolality drives water into cells, leaving the interstitial space very depleted → classic skin tenting. In hypernatraemic dehydration, the high ECF osmolality actually holds some water in the interstitial space (water is drawn out of cells instead) → the skin feels "doughy" or "thick" rather than producing classic tenting. This is a subtle but important clinical distinction, especially in paediatrics.

Acute post-operative / traumatic DI shows a triphasic pattern: transient DI (hours to days) → antidiuresis (2–14 days, due to release of stored ADH from dying neurons) → return of DI (may be permanent) ^[4]. This is important to recognise post-neurosurgery (e.g. after transsphenoidal surgery for pituitary adenoma). The second phase can paradoxically cause hyponatraemia if free water intake is not restricted.

When you encounter hypernatraemia, the clinical approach before moving to formal diagnosis and management involves:

Confirm true hypernatraemia: Check serum osmolality — should be elevated (> 295 mOsm/kg). If serum osm is normal or low, consider pseudohypernatraemia (extremely rare laboratory artefact).
Assess volume status: Is the patient hypovolaemic, euvolaemic, or hypervolaemic? This immediately narrows the differential.
Check urine osmolality and volume: The kidney's response tells you where the problem is.
Review medication history: Especially lithium ^[4]^[5].
Assess neurological status: GCS, focal deficits, seizure activity.
Determine chronicity: Acute (< 48h) vs. chronic (> 48h) — this determines safe correction rate.

Investigations: serum & urine osmolality, serum glucose, Na, K, Ca ^[4]. Urine osmolality < 300 mOsm/kg in hypernatraemia is highly suggestive of DI ^[4].

High Yield Summary

Hypernatraemia = serum [Na⁺] > 145 mmol/L, always reflecting hyperosmolality and relative water deficit
[Na⁺] reflects water balance, not absolute sodium content — hypernatraemia fundamentally means too little water relative to sodium
Three categories by volume status: hypovolaemic (hypotonic fluid loss), euvolaemic (DI, insensible loss), hypervolaemic (Na⁺ gain / Conn syndrome)
Diabetes insipidus: now renamed AVP-D (central) and AVP-R (nephrogenic) per 2022 consensus; differentiated by DDAVP response and copeptin levels
Lithium is the most common drug cause of nephrogenic DI; hypercalcaemia causes nephrogenic DI via autophagic degradation of AQP2
Brain cell shrinkage causes neurological symptoms: irritability, seizures, ICH/SAH (rare), coma
Brain adaptation (idiogenic osmoles) occurs in chronic hypernatraemia → must correct slowly: < 8–10 mmol/L per 24 hours to avoid cerebral oedema
Urine osmolality < serum osmolality in hypernatraemia = inappropriate = DI
Key investigations: paired serum and urine osmolality, serum glucose, Na, K, Ca, medication review
Volume depletion assessment: 5% BW loss = ↓ skin turgor; 10% = postural hypotension; 15% = shock

Active Recall - Hypernatraemia (Definition to Clinical Features)

^[1] Lecture slides / Senior notes: Block A - Electrolyte and Acid-Base Disorders.pdf (Sodium section, Hypernatraemia algorithm) ^[2] Lecture slides: GC 044. Electrolyte and Acid-Base Disorders.pdf ^[3] Lecture slides: MBBS IV Electrolytes_2024.pdf ^[4] Senior notes: Maksim Medicine Notes.pdf (Nephrology section - Hypernatraemia, p208) ^[5] Senior notes: Block A - Drugs and the Kidney.pdf (Lithium section) ^[6] Senior notes: Block A - Polyuria and polydipsia_ glucose metabolism; diabetes mellitus; diabetic ketoacidosis.pdf (Hyperosmolar coma section) ^[7] Senior notes: Block A - I keep on bumping into people on my side_ pituitary tumours; hypopituitarism.pdf (DI section) ^[8] Senior notes: Chemical Pathology Data interpretation.pdf (Case 1 - DI) ^[9] Senior notes: Block A - Confused and dehydrated_ hypercalcaemia; hypocalcaemia.pdf (Hypercalcaemia and nephrogenic DI) ^[10] Senior notes: Block A - Two cases of polyuria and polydipsia.pdf (AVP resistance - lithium) ^[11] Senior notes: Ryan Ho Fundamentals.pdf (Delirium section, p325)

Differential Diagnosis of Hypernatraemia

Before listing differentials, let's think about this from first principles. Hypernatraemia means the ratio of sodium to water in the ECF is too high. There are only three ways this can happen:

You lost water (most common) — either through the kidneys, the GI tract, the skin, or the lungs
You gained sodium (least common) — iatrogenic or mineralocorticoid excess
You didn't drink enough water — impaired thirst or impaired access

The differential diagnosis is therefore systematically approached by asking two key questions in sequence:

Question 1: What is the patient's volume status? → Hypovolaemic, euvolaemic, or hypervolaemic?

Question 2: What is the urine osmolality relative to the serum osmolality? → Is the kidney concentrating urine appropriately or not?

The Golden Rule of Urine Osmolality in Hypernatraemia

In hypernatraemia, the kidney should be producing maximally concentrated urine (> 800 mOsm/kg). If you see a urine osmolality that is inappropriately low (< 300 mOsm/kg, or any urine osmolality < serum osmolality), the kidney's concentrating mechanism is broken — this is highly suggestive of diabetes insipidus ^[4]^[8]. The U:P osmolality ratio is the single most discriminating test ^[12].

Differential Diagnosis Organised by Volume Status

The patient has lost both sodium and water, but the water loss is proportionally greater. The ECF is contracted → clinical signs of dehydration are present. The key sub-question is: Is the loss renal or extrarenal? Answer: check the urine sodium ^[12]^[13].

Differential	Pathophysiology	Urine Na⁺	Urine Osm	Key Clues
GI losses — Diarrhoea	Stool is hypotonic relative to plasma → net loss of water > Na⁺. Especially prominent in children with gastroenteritis (rotavirus, norovirus)	< 20	> 600 (kidney appropriately concentrating)	History of diarrhoea, vomiting; very common in HK paediatric population
GI losses — Vomiting / NG suction	Loss of gastric fluid (which is hypotonic) + inability to drink	< 20	> 600	Metabolic alkalosis (loss of HCl)
Burns	Extensive skin loss → massive evaporative hypotonic fluid loss	< 20	> 600	Obvious burn history; insensible losses dramatically elevated
Excessive sweating	Sweat is hypotonic (~20–50 mmol/L Na) → net water loss	< 20	> 600	HK subtropical climate; exercise, fever
Osmotic diuresis — Hyperglycaemia / HHS ^[6]	Glucose in tubular lumen obligates water excretion; water loss exceeds Na loss because glucose acts as an additional osmole retaining water in the lumen	> 20	~300–600 (iso-osmotic, driven by glucose)	Severe hyperglycaemia and dehydration; glycaemia can be higher in HHS compared to DKA; dehydration can be worse in HHS compared to DKA ^[6]; elderly T2DM
Osmotic diuresis — Mannitol	Mannitol is freely filtered but not reabsorbed → osmotic diuresis	> 20	~300	Iatrogenic (neurosurgical setting for ↓ ICP)
Osmotic diuresis — Urea	High urea load (high-protein feeds, post-obstructive diuresis, catabolic states) → urea diuresis	> 20	~300 (U:P ≈ 1)	U:P osmo ratio = 1 → intact ADH activity but urine is not concentrated → osmotic diuresis ^[12]
Loop diuretics	Inhibit NKCC2 in thick ascending limb → impair medullary concentration gradient → cannot maximally concentrate urine; also cause natriuresis, but water loss proportionally greater	> 20	< 600 (impaired concentrating ability)	Drug history; metabolic alkalosis + hypokalaemia
Post-obstructive diuresis	After relief of bilateral obstruction → accumulated urea and Na cause osmotic diuresis + tubular damage impairs concentrating ability	> 20	~300	Bilateral dilated pelvicalyceal system on imaging ^[14]; massive polyuria after catheterisation
Intrinsic renal disease (recovering ATN, CKD)	Tubular damage impairs ability to maximally concentrate urine	> 20	< 600	History of AKI, drug exposure, raised creatinine

HHS — A Classic Exam Scenario for Hypernatraemia

The tendency to hyperosmolality may be worsened in elderly people, who may have reduced appreciation of thirst, or difficulty in drinking enough water to compensate for their osmotic diuresis, and may be on diuretics ^[6]. This is a favourite exam vignette: an elderly confused T2DM patient on diuretics, presenting with severe dehydration and hypernatraemia. The glucose-driven osmotic diuresis causes massive water loss, and the confused patient cannot drink.

2. Euvolaemic (Isovolaemic) Hypernatraemia (Pure water loss or inadequate water intake)

The patient has lost water without proportional sodium loss. There are no obvious signs of ECF volume depletion (no tachycardia, no postural hypotension) because the water is being lost from the total body water (mostly ICF), not specifically the ECF. The critical discriminator here is the urine osmolality and U:P osmolality ratio ^[12].

Differential	Pathophysiology	Urine Osm	U:P Ratio	Key Clues
AVP-Deficiency (Central DI) ^[7]	Deficiency of ADH — lesions of the hypothalamus, pituitary stalk or posterior pituitary → no ADH release → collecting duct remains impermeable to water → dilute urine	< 300	< 1	Polyuria, polydipsia, +/- hypernatraemia; copious quantity of dilute urine ^[7]; increase in urine osmolality after DDAVP ^[7]; copeptin LOW ^[8]; post-neurosurgical triphasic pattern
AVP-Resistance (Nephrogenic DI) ^[7]	Resistance to ADH → kidneys cannot respond → AQP2 not inserted → dilute urine despite high ADH levels	< 300	< 1	No increase in urine osmolality after DDAVP ^[7]; copeptin HIGH ^[7]^[8]; drug history (lithium — most common drug cause ^[5]); metabolic (hypercalcaemia, hypokalaemia); hereditary (XR mutation in V2 receptor, AD/AR mutation in AQP-2 ^[4]); chronic obstruction → tubular dysfunction ^[14]
Insensible losses (fever, hyperventilation, mechanical ventilation)	Evaporative losses of pure water through skin and respiratory tract, without electrolyte loss	> 800	> 1 (kidney responding appropriately)	Febrile patient, ventilated ICU patient not receiving adequate free water; U:P ratio appropriate
Hypodipsia / Adipsia	Damage to hypothalamic thirst centres (tumour, infiltration, granulomatous disease, congenital) → absent thirst drive despite rising osmolality	> 800	> 1	Rare; patient does not feel thirsty despite hypernatraemia; exclude structural hypothalamic lesion with MRI
Primary polydipsia (as a differential to EXCLUDE)	Differentiation between AVP-D and primary polydipsia requires hypertonic saline infusion done in ICU, supervised condition ^[14]	Variable (dilute if currently drinking)	Variable	Psychiatric history; serum Na tends to be low or normal, NOT high — so this is actually a differential to rule OUT rather than rule IN for hypernatraemia

The gold standard method to diagnose diabetes insipidus is the water deprivation test + DDAVP ^[7]. However, copeptin test is better for children as you don't deprive them of that much water ^[7]^[8].

Feature	AVP-D (Central DI)	AVP-R (Nephrogenic DI)
Copeptin	Low	High — diagnostic for AVP resistance ^[8]
Water deprivation test	Urine osm remains < 300 despite dehydration	Urine osm remains < 300 despite dehydration
After DDAVP 2 µg IM	↑↑ U/P ratio (urine osmolality rises significantly) ^[12]	No change in U/P ratio ^[12]
Classic causes	Idiopathic (MC), craniopharyngioma, head trauma, Sheehan syndrome, meningoencephalitis	Lithium, hypercalcaemia, hypokalaemia, hereditary (AVPR2 deletion on X chromosome ^[8])

Copeptin — The Modern Discriminator

Copeptin is a surrogate marker for AVP production — AVP half-life is too short, so we use copeptin instead ^[8]. Copeptin raised ⇒ basically diagnostic for AVP resistance (nephrogenic DI) — because the body IS producing ADH (hence high copeptin), but the kidneys aren't responding ^[14]. Copeptin LOW → the body is NOT producing ADH → central DI (AVP-D).

Category	Causes	Mechanism
Drugs	Lithium (most common drug cause) ^[5]; demeclocycline; amphotericin B; foscarnet; cidofovir	Lithium decreases expression of AQP2 ^[5]^[10]; demeclocycline blocks V2 receptor signalling
Metabolic	Hypercalcaemia ^[9]; Hypokalaemia	Hypercalcaemia → autophagic degradation of AQP2 channels ^[9]; Hypokalaemia → ↓ AQP2 expression + prostaglandin-mediated ADH antagonism
Hereditary	XR mutation in AVPR2 gene (V2 receptor) ^[8]; AD/AR mutation in AQP2 gene ^[4]	Non-functional V2 receptor or non-functional aquaporin-2 water channels; X-linked recessive pattern means boys are predominantly affected
Chronic renal disease	CKD (reduced functioning nephrons), tubulointerstitial disease, post-obstructive uropathy	Damaged medullary architecture impairs concentrating gradient; tubular damage impairs AQP2 response
Pregnancy (rare)	Placental vasopressinase degrades ADH	Excessive enzymatic breakdown of AVP → functional deficiency
Chronic obstruction ^[14]	Dilatation of the system causes tubular dysfunction → impaired AQP2 signalling	Mechanical pressure + ischaemia damage collecting duct epithelium

Category	Causes
Idiopathic (most common) ^[4]	Autoimmune destruction of ADH-producing neurons; ~30% of cases
Neoplastic	Craniopharyngioma, pituitary metastases (breast, lung), lymphoma, germinoma
Traumatic	Head trauma, post-neurosurgery (transsphenoidal surgery (TSS) ^[4])
Vascular	Sheehan syndrome (post-partum pituitary necrosis) ^[4]
Infective	Meningoencephalitis ^[4], tuberculosis, syphilis
Infiltrative	Sarcoidosis, Langerhans cell histiocytosis, haemochromatosis
Congenital	Familial AVP deficiency (extremely rare) ^[14]; Wolfram syndrome (DIDMOAD: DI, DM, optic atrophy, deafness)
Post-operative triphasic pattern	Transient DI (hours to days) → antidiuresis (2–14 days, from release of stored ADH) → return of DI (may be permanent) ^[4]

The patient has gained sodium in excess of water. This is the least common category. The patient shows signs of volume overload (oedema, hypertension).

Differential	Pathophysiology	Urine Osm	Urine Na⁺	Key Clues
Iatrogenic — hypertonic saline / NaHCO₃	Direct administration of sodium-rich solutions (NaHCO₃ during cardiac arrest, hypertonic saline for raised ICP)	Variable	> 20	Review IV fluid charts; post-cardiac arrest; TPN with high sodium content
Primary hyperaldosteronism (Conn syndrome) ^[1]	Aldosterone excess → ENaC-mediated Na⁺ retention in cortical collecting duct → ECF expansion → mild hypernatraemia + hypokalaemia + hypertension	> 600 (concentrated, kidney function intact)	Low (paradoxically, due to reset osmostat) or variable	Polyuria, polydipsia, and muscle weakness due to hypokalaemia ^[15]; resistant hypertension; suppressed renin
Cushing syndrome	Cortisol excess → mineralocorticoid receptor activation (cortisol has affinity for MR when 11β-HSD2 is overwhelmed) → Na⁺ retention	> 600	Variable	Cushingoid features; moon face, buffalo hump, striae; hypokalaemia
Salt poisoning	Direct ingestion of large sodium load	> 600	> 20	Child abuse (Munchausen by proxy), accidental ingestion of salt
Sea water ingestion	Sea water ≈ 3.5% NaCl (~500 mmol/L Na)	> 600	> 20	Drowning/near-drowning history
Hypertonic dialysis	High sodium dialysate	Variable	Variable	Dialysis history

Since hypernatraemia commonly presents as delirium in the elderly ^[11], the differential of the confused patient must also include other causes of delirium that may coexist or mimic hypernatraemia:

Differential	How to Distinguish
Hypernatraemia (the actual diagnosis)	Serum Na > 145, ↑ serum osmolality
Hypercalcaemia	Serum Ca²⁺ elevated; can coexist with hypernatraemia (hypercalcaemia causes nephrogenic DI → hypernatraemia)
Uraemic encephalopathy	↑ urea and creatinine; can coexist with hypernatraemia in dehydrated patients
Hepatic encephalopathy	Liver disease features, raised ammonia, flapping tremor
Hypoglycaemia / Hyperglycaemia ^[13]	Check blood glucose; HHS can cause both hypernatraemia AND confusion
Drug toxicity (lithium)	Lithium can cause nephrogenic DI AND direct neurotoxicity ^[5]
Sepsis / CNS infection	Fever, raised inflammatory markers; LP if meningism
Alcohol withdrawal / Wernicke encephalopathy	Alcohol history; ophthalmoplegia, ataxia in Wernicke's
Subdural haematoma	Head trauma history; focal neurology; CT brain

Clinical Pearl — Hypercalcaemia and Hypernatraemia Often Coexist

Why are patients with hypercalcaemia often dehydrated? → Nephrogenic diabetes insipidus → hypernatraemia ^[9]. Hypercalcaemia can cause autophagic degradation of aquaporin-2 channels → less water reabsorption, more urine output and water loss ^[9]. So when you see hypernatraemia + hypercalcaemia together, the hypercalcaemia is likely the upstream cause. Always check calcium in a hypernatraemic patient.

This ratio tells you about the kidney's ability to concentrate urine ^[12]:

U:P Osmo Ratio	Interpretation	Differentials
< 1	Insufficient ADH activity → Diabetes insipidus ^[12]	AVP-D or AVP-R; confirm with DDAVP test / copeptin
= 1 (iso-osmotic)	Intact ADH activity but urine not concentrated → Osmotic diuresis ^[12]	Uncontrolled hyperglycaemia, uraemia, mannitol, post-obstructive diuresis ^[12]
> 1	Appropriate urine concentration → Extrarenal fluid loss ^[12]	GI losses, insensible losses, inadequate intake, skin losses

Exam High-Yield — U:P Osmo Ratio Summary

U:P < 1 = DI (kidney not concentrating → the problem is the kidney or ADH) ^[12]
U:P = 1 = osmotic diuresis (kidney overwhelmed by osmoles) ^[12]
U:P > 1 = extrarenal losses or inadequate intake (kidney doing its job properly) ^[12]

This is the single most useful test in working up hypernatraemia. It immediately classifies the mechanism.

Volume Status	Mechanism	Common Differentials	Urine Osm	Urine Na
Hypovolaemic	Hypotonic fluid loss	Diarrhoea, vomiting, burns, sweating	> 600 if extrarenal; < 600 if renal	< 20 if extrarenal; > 20 if renal
	Osmotic diuresis	HHS, mannitol, post-obstructive	≈300 (iso-osmotic)	> 20
	Renal losses	Loop diuretics, recovering ATN, CKD	< 600	> 20
Euvolaemic	DI — AVP-D	Idiopathic, tumour, trauma, Sheehan	< 300	Variable
	DI — AVP-R	Lithium, hypercalcaemia, genetic	< 300	Variable
	Insensible losses	Fever, ventilation	> 800	Variable
	Inadequate intake	Hypodipsia, impaired access	> 800	Variable
Hypervolaemic	Na gain	Iatrogenic (NaHCO₃, hypertonic saline)	Variable	> 20
	Mineralocorticoid excess	Conn, Cushing	> 600	Variable

High Yield Summary — Differential Diagnosis of Hypernatraemia

Classify by volume status first: hypovolaemic (most common), euvolaemic, or hypervolaemic
U:P osmolality ratio is the key discriminator: < 1 = DI; = 1 = osmotic diuresis; > 1 = extrarenal loss/inadequate intake
Urine osmolality < 300 in hypernatraemia is highly suggestive of DI ^[4]
Differentiate AVP-D vs AVP-R with: DDAVP response (↑ urine osm = AVP-D; no change = AVP-R) and copeptin (low = AVP-D; high = AVP-R)
Most common causes in clinical practice: dehydration in elderly with impaired thirst (extrarenal losses + inadequate intake), osmotic diuresis from HHS/hyperglycaemia, and drug-induced nephrogenic DI (lithium)
Always check calcium — hypercalcaemia and hypernatraemia frequently coexist via nephrogenic DI mechanism
HHS is a favourite exam scenario: elderly T2DM + dehydration + confusion + hypernatraemia from osmotic diuresis
Post-neurosurgical DI shows the triphasic pattern — transient DI → antidiuresis → permanent DI

Active Recall - Hypernatraemia Differential Diagnosis

References

^[1] Lecture slides / Senior notes: Block A - Electrolyte and Acid-Base Disorders.pdf (Sodium section, Hypernatraemia algorithm) ^[4] Senior notes: Maksim Medicine Notes.pdf (Nephrology section - Hypernatraemia, p208) ^[5] Senior notes: Block A - Drugs and the Kidney.pdf (Lithium section) ^[6] Senior notes: Block A - Polyuria and polydipsia_ glucose metabolism; diabetes mellitus; diabetic ketoacidosis.pdf (HHS section) ^[7] Senior notes: Block A - I keep on bumping into people on my side_ pituitary tumours; hypopituitarism.pdf (DI section) ^[8] Senior notes: Chemical Pathology Data interpretation.pdf (Case 1 - DI, copeptin) ^[9] Senior notes: Block A - Confused and dehydrated_ hypercalcaemia; hypocalcaemia.pdf (Hypercalcaemia and nephrogenic DI) ^[10] Senior notes: Block A - Two cases of polyuria and polydipsia.pdf (AVP resistance - lithium and AQP2) ^[11] Senior notes: Ryan Ho Fundamentals.pdf (Delirium section, p325) ^[12] Senior notes: Ryan Ho Urogenital.pdf (Hypernatraemia diagnostic approach, p20) ^[13] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (Hypernatremia section, p38-40) ^[14] Senior notes: Block A - Two cases of polyuria and polydipsia.pdf (Chronic obstruction and tubular dysfunction) ^[15] Senior notes: Block A - High blood pressure_ hypertension.pdf (Primary aldosteronism symptoms)

Diagnostic Criteria, Diagnostic Algorithm, and Investigations for Hypernatraemia

Hypernatraemia itself is a laboratory diagnosis, not a clinical syndrome with formal "diagnostic criteria" like SLE or rheumatic fever. The diagnosis is straightforward:

Hypernatraemia = Plasma [Na⁺] > 145 mmol/L ^[4]^[13]^[16]

However, you must confirm it is true hypernatraemia by checking that:

Serum osmolality is concurrently elevated (> 295 mOsm/kg) ^[8]
The formula for serum osmolality: Serum Osmolality = 2[Na⁺] + [Urea] + [Glucose] (all in mmol/L; normal: 285–295 mOsm/kg) ^[13]
Osmolar gap = Measured osmolality – Calculated osmolality (normal: < 10 mmol/kg). A widened osmolar gap suggests presence of other osmotically active substances (e.g. alcohol, mannitol, ethylene glycol) ^[13]

Why Confirm True Hypernatraemia?

Unlike hyponatraemia (where pseudohyponatraemia from hyperlipidaemia or hyperproteinaemia is common), pseudohypernatraemia is exceedingly rare. However, confirming elevated serum osmolality is important because it: (1) confirms the hypernatraemia is clinically significant (i.e., truly hyperosmolar), and (2) helps calculate the osmolar gap which may reveal additional unmeasured osmoles contributing to the clinical picture.

Severity grading:

Severity	Serum [Na⁺]
Mild	146–150 mmol/L
Moderate	151–159 mmol/L
Severe	≥ 160 mmol/L

For the specific diagnosis of Diabetes Insipidus (the key differential within hypernatraemia), there are defined diagnostic criteria:

Hypernatraemia + dilute urine are diagnostic of DI ^[4]. Specifically:

Urine Osm < 300 mOsm/kg in the setting of hypernatraemia (Na > 145 mmol/L) is highly suggestive of DI ^[4] — this alone can be considered diagnostic without needing a water deprivation test ^[16]^[17]

U/P osmolality ratio < 1 → insufficient ADH activity → diabetes insipidus ^[12]

Diagnostic Algorithm — Step-by-Step Clinical Approach

The diagnostic workup for hypernatraemia follows a logical, stepwise approach. Think of it as answering a series of questions, each one narrowing the differential:

Check serum Na⁺ and serum osmolality simultaneously
True hypernatraemia requires the concurrent elevation of serum osmolality ^[8]
Calculate the osmolar gap

This is a bedside assessment — the single most important clinical step:

Correlate physical exam findings of volume depletion or excess to % lost or gained of body weight: ^[1]

Mildly dehydrated = loss of skin turgor = 5% BW loss
Moderate = postural hypotension = 10% BW loss
Severe = shock = 15% BW loss
Mild oedema = 5% BW excess

Volume status categorises the patient into one of three groups:

Hypovolaemic → hypotonic fluid loss (most common)
Euvolaemic → pure water loss (DI, insensible losses, inadequate intake)
Hypervolaemic → sodium gain (iatrogenic, mineralocorticoid excess)

Investigations: serum & urine OsM, serum glucose, Na, K, Ca ^[4]

The paired plasma and urine osmolality is the cornerstone investigation. You MUST request both simultaneously.

The U:P osmolality ratio indicates the kidney's ability to concentrate urine ^[12]:

U:P Osmo Ratio	Interpretation	Next Step
> 1	Appropriate urine concentration → extrarenal fluid loss ^[12]	Look for GI losses, insensible losses, inadequate intake
≈ 1	Intact ADH activity but urine not concentrated → osmotic diuresis ^[12]	Check glucose (HHS), urea, mannitol history
< 1	Insufficient ADH activity → diabetes insipidus ^[12]	Proceed to DDAVP test or copeptin to differentiate AVP-D vs AVP-R

GC Lecture Slide High-Yield Point

The ratio is only used as a cut-off for investigation of DI. In patients with severe polyuria in whom DI is suspected, a U/S ratio > 1 but < 1.9 is still compatible with the diagnosis of DI ^[18]. So don't dismiss DI just because the U:P ratio is slightly above 1 — partial DI exists!

Two approaches:

A. Water Deprivation Test + DDAVP (Gold standard) ^[7]^[16]^[17]

Indication: in all suspected patients EXCEPT if U/P ratio < 1 or Na > 145 + U osmo < 300 (these are already diagnostic) ^[16]^[17]
Why not test those patients? → Because if the patient already has hypernatraemia with dilute urine, depriving them of water is dangerous and unnecessary — the diagnosis is already clear

B. Copeptin Test (Modern alternative) ^[7]^[8]

Better for children as you don't deprive them of that much water ^[7]
Copeptin is a surrogate marker for AVP → but AVP half-life is too short, so we use this instead ^[8]

If AVP-D → MRI pituitary/hypothalamus + anterior pituitary hormone panel
If AVP-R → Review medication history (lithium) ^[4], check Ca²⁺, K⁺, renal ultrasound
If osmotic diuresis → check glucose, urea
If extrarenal losses → clinical history (diarrhoea, burns, sweating)

Investigation Modalities — Detailed Interpretation

Investigation	What You're Looking For	Interpretation
Serum Na⁺	Confirms hypernatraemia	> 145 mmol/L diagnostic ^[4]^[13]
Serum Osmolality	Confirms true hyperosmolality	> 295 mOsm/kg expected; true hypernatraemia requires concurrent elevation of serum osmolality ^[8]
Serum Glucose	Osmotic diuresis from hyperglycaemia?	↑↑ Glucose → HHS/DKA; note that hyperglycaemia also causes dilutional hyponatraemia (1.6 mmol/L ↓ Na per 5.6 mmol/L ↑ glucose), so corrected Na may be even higher
Serum K⁺	Hypokalaemia causes nephrogenic DI; also important for ECG monitoring	↓ K⁺ → consider renal tubular dysfunction, diuretic use, mineralocorticoid excess
Serum Ca²⁺	Hypercalcaemia causes nephrogenic DI	↑ Ca²⁺ → investigate for hyperparathyroidism, malignancy
Serum Urea and Creatinine	Assess renal function; pre-renal AKI from dehydration?	Creatinine high → dehydration ^[14] or intrinsic renal disease; disproportionately raised urea:creatinine ratio (> 100:1 in SI units) suggests pre-renal state
Venous Blood Gas	Acid-base status	Metabolic acidosis may suggest DKA, lactic acidosis from shock, or RTA

Sequence of Investigations — The Endocrine Principle

When working up any suspicion of hormonal excess or deficiency: ^[19]

History and PE
Baseline blood test → CBC / LRFT
Screening blood biochemistry tests
Confirmatory tests (sometimes dynamic suppression/stimulation tests)
Imaging with MRI / CT
Invasive tests (venous sampling)

General principle: less invasive before more invasive. Endocrine is unique in that you need biochemistry before imaging — resolution is too high with modern imaging, and you might find incidental benign lesions unrelated to the condition ^[19].

Investigation	What You're Looking For	Interpretation
Urine Osmolality	Is the kidney concentrating urine appropriately?	Urine Osm < 300 in hyperNa is highly suggestive of DI ^[4]; Normal response to hypernatraemia should be > 800 mOsm/kg
U:P Osmolality Ratio	The key discriminator	< 1 = DI; ≈ 1 = osmotic diuresis; > 1 = extrarenal losses ^[12]^[18]
Urine Na⁺	Renal vs extrarenal loss in hypovolaemic patients	< 20 mmol/L = extrarenal loss (kidney conserving Na); > 20 mmol/L = renal loss
Urine Volume (24h)	Document polyuria	> 2L/day (adults) or > 2L/m²/day (children) = polyuria ^[8]; > 3L/day in adults is significant
Urine Glucose	Glucosuria → osmotic diuresis	Present in DM with blood glucose > renal threshold (~10 mmol/L)

Paired plasma & urine osmolarity & electrolytes: Hypernatraemia + dilute urine are diagnostic of DI; ↓Na & ↓Osm suggests primary polydipsia ^[4]

This is the gold standard method to diagnose diabetes insipidus ^[7]. It works by removing the external water supply and observing whether the kidney can concentrate urine (i.e., whether ADH is present and functional).

Principle: To induce ↑ADH by creating a hyperosmolar state and to detect the response to ↑ADH ^[16]^[17]

Indication: In all suspected patients EXCEPT if U/P ratio < 1 or Na > 145 + U osmo < 300 (these are already diagnostic — no need to dehydrate a patient who is already hypernatraemic with dilute urine) ^[16]^[17]

Procedure: ^[16]^[17]

No fluid intake for 8 hours (0730–1630); no tea, coffee, smoking; only dry food allowed
Close supervision with hourly body weight, urine volume, and U/P osmolality
Stop the test when:
- End-point reached: U/P ≤ 2 with plasma Osm > 300, OR
- > 3% decrease in body weight (safety cut-off — further dehydration is dangerous)
At end-point → give DDAVP 2 µg IM
Monitor urine osmolality for 2–4 hours after DDAVP

Interpretation of Results:

Finding	Diagnosis	Why
Adequate urine concentration during deprivation (U/P ratio ≥ 2)	Normal or Primary Polydipsia ^[16]^[17]	ADH is being released and kidneys are responding → urine concentrates normally
Plasma osm > 300 but U/P ratio still ≤ 1.9 (U < 600)	Diabetes Insipidus confirmed ^[16]^[17]	Despite maximal ADH stimulus (high plasma osm), urine cannot concentrate
After DDAVP: ↑ ≥ 50% urine osm	Cranial DI (AVP-D) ^[16]^[17]	The kidneys CAN respond to exogenous ADH → the problem was lack of ADH production
After DDAVP: No change in urine osm	Nephrogenic DI (AVP-R) ^[16]^[17]	The kidneys CANNOT respond even to exogenous ADH → the problem is renal resistance

Pitfall — Primary Polydipsia Can Mimic DI on Water Deprivation Test

In cases of primary polydipsia, chronic polydipsia may lead to washout of the corticomedullary gradient and therefore failure of urine concentration despite water deprivation/DDAVP administration. However, the initial serum osmolality/Na should be low or normal ^[16]^[17]. This is the key distinguishing point — in primary polydipsia, the patient starts with LOW sodium (dilutional), whereas in DI, the patient starts with HIGH-normal or HIGH sodium.

Why Water Deprivation Tests Are Falling Out of Favour

These water deprivation tests are definitely not good — they subject paediatric patients to long periods of water deprivation ^[8]. Additionally, they are:

Uncomfortable and require close inpatient monitoring for 8+ hours
Potentially dangerous in patients with severe DI (rapid dehydration)
Have limited ability to distinguish partial DI from primary polydipsia

So we have a better test → copeptin ^[8]. The hypertonic saline stimulation test with copeptin measurement is increasingly preferred, especially in children and in distinguishing partial AVP-D from primary polydipsia.

Principle: Copeptin is the C-terminal portion of pre-pro-vasopressin, co-secreted equimolarly with AVP. AVP half-life is too short (~6 minutes), so we use copeptin as a surrogate marker instead ^[8].

Copeptin Level	Interpretation
Low	No AVP being produced → AVP-Deficiency (Central DI)
High	AVP IS being produced but kidneys are resistant → AVP-Resistance (Nephrogenic DI) — copeptin raised ⇒ basically diagnostic for AVP resistance ^[8]^[14]
Intermediate (with hypertonic saline stimulation)	Helps differentiate partial AVP-D from primary polydipsia

Differentiation between AVP-D and primary polydipsia requires hypertonic saline infusion done in ICU, under supervised conditions — or else sodium falls too quickly after the test, risking demyelination ^[14].

Hypertonic Saline Stimulation Protocol:

Infuse 3% hypertonic saline IV to raise plasma osmolality to > 300 mOsm/kg
Measure copeptin at defined osmolality thresholds
If copeptin fails to rise appropriately → AVP-D
If copeptin rises → Normal or primary polydipsia
Must be done in ICU with close Na monitoring

Investigation	Indication	Key Findings
MRI Pituitary/Hypothalamus	If AVP-D (central DI) confirmed → look for structural cause	Normal posterior pituitary shows a "bright spot" on T1-weighted imaging (reflecting stored ADH granules); absence of the bright spot suggests central DI ^[14]; look for tumour (craniopharyngioma, germinoma, metastasis), thickened pituitary stalk (infiltrative disease), empty sella
Renal Ultrasound	If AVP-R (nephrogenic DI) confirmed → evaluate renal structure	Normal kidney size: 10–12 cm, symmetrical ^[20]; look for polycystic kidneys, bilateral dilated pelvicalyceal system (obstruction) ^[14], small/scarred kidneys (CKD), hydronephrosis
CT Brain (non-contrast)	If confusion/altered consciousness → rule out structural cause	Cerebral oedema (if overcorrected), ICH, SAH (from brain shrinkage in severe hypernatraemia)
CT/CXR	If malignancy suspected (ectopic or metastatic causing central DI)	Lung mass, mediastinal lymphadenopathy

Post-AVP-D Diagnosis: Don't Forget the Anterior Pituitary!

For all endocrine investigations, after we realise the posterior pituitary is not good, then cannot stop there — have to check the anterior pituitary hormones as well, holistic plan to not miss anything: ^[14]

fT4 and TSH (thyroid axis)
LH, FSH, oestradiol/testosterone (gonadal axis)
Morning cortisol ± ACTH (adrenal axis)
Prolactin (often elevated if stalk effect from mass lesion)
IGF-1 (growth hormone axis)

A lesion causing central DI often affects other pituitary axes too!

Suspected Cause	Investigation	Expected Finding
HHS / DKA	Blood glucose, HbA1c, serum ketones, venous blood gas	Glucose > 30 mmol/L (HHS); ketones elevated (DKA)
Primary hyperaldosteronism	Aldosterone-renin ratio (ARR) screening → confirmatory saline infusion test → CT/MRI adrenals → adrenal venous sampling ^[21]	↑ Aldosterone, ↓ Renin, ↑ ARR
Cushing syndrome	Overnight dexamethasone suppression test / 24h urinary free cortisol / midnight salivary cortisol	Failure to suppress cortisol
Lithium toxicity	Serum lithium level	Therapeutic: 0.6–1.2 mmol/L; toxic > 1.5 mmol/L
Hereditary nephrogenic DI	Chromosomal microarray / genetic testing for AVPR2 gene (X-linked) or AQP2 gene	Homozygous deletion of AVPR2 gene (X-chromosome) ^[8]
In presence of renal impairment	Try avoiding contrast CT to prevent toxicity ^[14]	Use non-contrast imaging or MRI instead

This is essential for guiding replacement therapy:

Free Water Deficit = Body Weight (kg) × 0.6 × (Measured [Na⁺] – 140) / 140 ^[4]

Why this formula works:

TBW ≈ 0.6 × body weight (adjust to 0.5 for women/elderly)
The fraction (Measured Na – 140) / 140 represents the proportional excess of sodium relative to normal → this tells you how much water is "missing" from the total body water pool
Example: 70 kg man with Na = 160 mmol/L → Free water deficit = 70 × 0.6 × (160 – 140)/140 = 6 litres

Important: The Formula Underestimates True Deficit

The free water deficit formula only calculates the water needed to bring Na back to 140 mmol/L. It does NOT account for ongoing losses (insensible, urinary, GI). You must add estimated ongoing losses to the calculated deficit. Also, you must not replace the entire deficit rapidly — rate of correction < 8–10 mmol/24h ^[4].

Investigation	Key Finding	What It Tells You
Serum Na > 145 + Serum Osm > 295	True hypernatraemia	Confirms diagnosis
U:P Osm < 1	Dilute urine despite hyperosmolality	DI (AVP-D or AVP-R) ^[12]
U:P Osm ≈ 1	Iso-osmotic urine	Osmotic diuresis ^[12]
U:P Osm > 1	Concentrated urine	Extrarenal loss or inadequate intake ^[12]
Urine Osm < 300 + Na > 145	Already diagnostic	DI — no water deprivation test needed ^[16]^[17]
DDAVP → ↑ urine osm ≥ 50%	Kidneys respond to exogenous ADH	AVP-D (Central DI) ^[16]^[17]
DDAVP → no change in urine osm	Kidneys resistant to ADH	AVP-R (Nephrogenic DI) ^[16]^[17]
Copeptin LOW	No AVP production	AVP-D ^[8]
Copeptin HIGH	AVP being produced but kidneys resist	AVP-R ^[8]
MRI: absent pituitary bright spot	Loss of stored ADH granules	Central DI ^[14]
Renal US: bilateral hydronephrosis	Obstruction causing nephrogenic DI	Post-obstructive tubular dysfunction ^[14]

High Yield Summary — Diagnosis of Hypernatraemia

Diagnostic criterion: Serum Na > 145 mmol/L with elevated serum osmolality
Serum osmolality formula: 2[Na⁺] + [Urea] + [Glucose]; normal 285–295 mOsm/kg
Essential investigations: serum and urine osmolality, serum glucose, Na, K, Ca — get paired samples
U:P osmolality ratio is the single most important discriminator: < 1 = DI; ≈ 1 = osmotic diuresis; > 1 = extrarenal loss
Urine Osm < 300 with Na > 145 = diagnostic for DI without needing a water deprivation test
Water deprivation test: no fluids for 8h → stop if > 3% BW loss or plasma osm > 300 → give DDAVP → ≥ 50% rise in urine osm = AVP-D; no change = AVP-R
Copeptin: modern alternative — LOW = AVP-D; HIGH = AVP-R
After diagnosing AVP-D: MRI pituitary + check ALL anterior pituitary hormones
After diagnosing AVP-R: drug history (lithium!), check Ca/K, renal ultrasound, consider genetic testing
Free water deficit = BW × 0.6 × (Measured Na – 140) / 140; add ongoing losses; correct at < 8–10 mmol/24h

Active Recall - Hypernatraemia Diagnosis

References

^[1] Lecture slides / Senior notes: Block A - Electrolyte and Acid-Base Disorders.pdf (Sodium section, Hypernatraemia algorithm, volume depletion assessment) ^[4] Senior notes: Maksim Medicine Notes.pdf (Nephrology section - Hypernatraemia, p208) ^[7] Senior notes: Block A - I keep on bumping into people on my side_ pituitary tumours; hypopituitarism.pdf (DI diagnosis section) ^[8] Senior notes: Chemical Pathology Data interpretation.pdf (Case 1 - DI, copeptin, AVPR2 gene) ^[12] Senior notes: Ryan Ho Urogenital.pdf (Hypernatraemia diagnostic approach, U:P ratio, p20) ^[13] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (Hypernatremia section, osmolality formula, p38-40) ^[14] Senior notes: Block A - Two cases of polyuria and polydipsia.pdf (DDAVP response, MRI bright spot, renal US, copeptin, hypertonic saline stimulation) ^[16] Senior notes: Ryan Ho Chemical Path.pdf (Hypernatremia workup, p11) ^[17] Senior notes: Ryan Ho Endocrine.pdf (Water deprivation test procedure and interpretation, p115) ^[18] Lecture slides: Chemical Pathology Seminar 1_Sodium and water.pdf (Diagnostic Pathway of Hypernatremia, U:P ratio interpretation, p31) ^[19] Senior notes: Block A - Introduction to Endocrine investigations.pdf (Sequence of investigations principle) ^[20] Senior notes: Block A - Chronic Kidney Disease and its Complications.pdf (Kidney size on US) ^[21] Senior notes: Block A - I have fluctuating BP_ cushing syndrome; adrenal diseases and tumours; other endocrine tumours.pdf (Conn syndrome investigation sequence)

Management of Hypernatraemia

Before diving into specific treatments, understand the three fundamental management principles of hypernatraemia:

Identify and treat the underlying cause — hypernatraemia is always secondary to something else
Replace the water deficit — but at a safe, controlled rate
Prevent complications of treatment — the most serious complications of hypernatraemia result from inappropriate treatment rather than the disorder per se! ^[12]

The #1 Management Danger

Most serious complications of hypernatraemia result from inappropriate treatment rather than the disorder per se! ^[12] Overly rapid correction causes water to rush into brain cells that have adapted by accumulating idiogenic osmoles → cerebral oedema → herniation → death. This is the mirror image of osmotic demyelination syndrome in hyponatraemia. Always correct slowly in chronic hypernatraemia.

The management strategy depends entirely on two axes:

Volume status (hypovolaemic vs euvolaemic vs hypervolaemic)
Acuity (acute < 48 hours vs chronic ≥ 48 hours / unknown)

Scenario	Safe Correction Rate	Rationale
Chronic hypernatraemia (≥ 48h or unknown duration)	< 8–10 mmol/L per 24 hours ^[4]^[12]	Brain has adapted with idiogenic osmoles; rapid correction → cerebral oedema
Acute hypernatraemia (< 48h)	Can correct faster: 1–2 mmol/L per hour (up to 10–12 mmol/L in first 24h)	Brain has NOT yet adapted; acute brain shrinkage is the immediate danger; faster correction is safe and necessary
Regardless of acuity	> 12 mEq/L change in serum [Na] in chronic hyperNa may result in cerebral oedema due to water rushing into neurones ^[12]	This is the hard ceiling — never exceed this

Practical tip: Check serum Na⁺ every 2–4 hours during active correction, and adjust the infusion rate accordingly. The calculated free water deficit is only an estimate — you need real-time monitoring.

Management by Volume Status — Detailed

This is the most common clinical scenario. The patient has lost both water and sodium, but proportionally more water. They are haemodynamically compromised.

Step 1: Restore circulating volume FIRST

Correct volume status by isotonic fluid (NS), especially if haemodynamically unstable ^[4]

Use normal saline (0.9% NaCl) for initial resuscitation
Why NS and not a hypotonic fluid first? → Because the immediate danger is cardiovascular collapse from hypovolaemia, not the hypernatraemia itself. NS is isotonic to plasma (~308 mOsm/L) but still hypotonic relative to the hypernatraemic patient's serum, so it will gradually lower Na⁺ while also restoring intravascular volume
Replace the volume depletion with normal saline cautiously; initial ⅓ can be given in the first 8 hours, reduce speed afterwards ^[1]

Step 2: Once euvolaemic, switch to free water replacement

Replace free water with hypotonic fluid (D5 or ½:½) Q6–8H, closely monitor [Na] & glucose ^[4]

D5 (Dextrose 5%): The dextrose is metabolised by the liver → leaves behind pure free water that distributes across all body compartments (ICF + ECF). This is the traditional choice for replacing free water deficit ^[22]
Half-half solution (0.45% NaCl + 2.5% dextrose): Provides some sodium (less ECF expansion than NS) plus free water. Minimises fluctuations of blood glucose level (better for DM patients) ^[22]

Step 3: Treat the underlying cause

Stop offending diuretics
Treat diarrhoea/vomiting
Control hyperglycaemia (insulin for HHS/DKA)

For HHS specifically:

If patient has hypernatraemia, use ½ normal saline → if just giving water, the rapid decrease of sodium would result in disequilibrium (especially in the brain cells) ^[6]

Fluid replenishment is the key → have to be careful of how much you're replacing, so guide it using CVP or pulmonary wedge pressure monitoring ^[6]

2. Euvolaemic (Isovolaemic) Hypernatraemia

The patient has lost pure water (DI, insensible losses) or has inadequate intake. There are no signs of significant ECF volume depletion.

Step 1: Replace free water deficit

The preferred routes, in order:

Oral water / tap water via NG tube — safest, most physiological, least risk of rapid overcorrection
IV D5 (Dextrose 5%) — if unable to drink or unconscious
IV half-half solution (0.45% NaCl + 2.5% dextrose) Q6–8H ^[12] — provides electrolytes + free water

Free Water Deficit = BW (kg) × 0.6 × (measured [Na⁺] – 140) / 140 ^[4]

Use 0.5 for women/elderly (lower TBW proportion)
Remember: This formula only calculates the static deficit — you must also add ongoing losses (insensible, urinary) to the total replacement plan

Step 2: Specific DI treatment (if DI is the cause)

Treatment	Details	Rationale
DDAVP (Desmopressin) ^[4]^[7]	Acute: 4–8 µg IV/SC Q3–4H PRN ^[12]; Chronic: 10–40 µg daily intranasally in 1–2 divided doses ^[12]; also available PO and sublingual	DDAVP is a synthetic analogue of ADH that acts on V2 receptors in the collecting duct → inserts AQP2 channels → allows water reabsorption. It has no V1 (vasoconstrictor) activity, so no pressor effect
Low Na diet ^[4]	Reduces obligate solute excretion → reduces urine volume	Less solute to excrete means less water obligated to follow
Chlorpropamide (rarely used) ^[4]	Potentiates ADH action at the collecting duct + stimulates residual ADH release	Only works if there is partial ADH production (not complete AVP-D); largely historical

Dosing principle: adjusted based on clinical symptoms → minimal effective dose to control polyuria (to ↓ risk of hypoNa). Should prevent nocturia but allow a degree of polyuria from time to time before next dose ^[17]

DDAVP Side Effects

S/E of DDAVP: excessive treatment → water intoxication and hyponatraemia; inadequate treatment → hypernatraemia ^[17]. This is a narrow therapeutic window — you must monitor sodium regularly, especially when initiating therapy.

This is trickier because the kidneys are resistant to ADH, so giving more ADH (DDAVP) won't work (or works only partially).

Treatment	Details	Rationale
Treat the underlying cause	Stop offending medication (e.g. lithium) ^[17]; correct hypercalcaemia; correct hypokalaemia; relieve obstruction	Removes the insult that is causing AQP2 dysfunction
Low Na/protein diet ^[17]	Reduces solute load → reduces obligate water excretion	Fewer osmoles to excrete = less urine volume needed
Thiazide diuretics ^[4]^[12]^[17]	Hydrochlorothiazide 25 mg daily ^[12] or indapamide 2.5 mg daily ^[12]	Paradoxical effect: thiazides cause mild Na depletion → ↓ ECF → enhances proximal tubular Na and water reabsorption → less fluid reaches the collecting duct → ↓ urine volume. Also, thiazides reduce the diluting capacity of the distal nephron
Amiloride ^[4]^[12]^[17]	Amiloride 5 mg daily ^[12]	(1) K⁺-sparing diuretic to offset thiazide-induced hypokalaemia; (2) specifically useful in lithium-induced DI because lithium enters principal cells via ENaC — amiloride blocks ENaC, so it blocks lithium entry into cells, protecting the collecting duct from further lithium toxicity
NSAIDs ^[4]^[17]	e.g. indomethacin ^[4]	Prostaglandins normally antagonise ADH action in the collecting duct. NSAIDs inhibit prostaglandin synthesis → potentiate whatever residual ADH action exists → ↑ water reabsorption. Also reduce GFR → ↓ urine volume
Consider DDAVP if refractory ^[17]	Partial effect in incomplete AVP-R	Most patients with non-hereditary nephrogenic DI have partial ADH resistance only ^[17] → may get some benefit from supraphysiological DDAVP doses

Why Thiazides Work Paradoxically in DI

This is a classic exam question that confuses students. Thiazides are diuretics — how can a diuretic treat polyuria? Here's the mechanism:

Thiazides inhibit NCC in the distal convoluted tubule → natriuresis → mild volume depletion
Mild volume depletion activates RAAS and the sympathetic nervous system
This enhances proximal tubular Na⁺ and water reabsorption (to compensate)
Less fluid is delivered to the collecting duct (where the ADH defect is)
Net effect: ↓ urine volume by 30–50%

Additionally, thiazides impair the diluting segment's ability to generate dilute urine, which further concentrates the urine.

Triphasic pattern: transient DI (hours to days) → antidiuresis (2–14 days, release of stored ADH from dying neurons) → return of DI (may be permanent) ^[4]

Management:

Allow oral hydration if can drink + thirsty ^[4]
IV fluids + DDAVP if unconscious ^[4]
Phase 2 (antidiuresis): Restrict fluids! Because ADH is being released uncontrollably → risk of hyponatraemia if free water intake is not restricted
Phase 3: Reassess — some patients recover, others need lifelong DDAVP

The patient has gained sodium in excess of water. They are volume-overloaded.

Step	Treatment	Details	Rationale
1	Treat underlying cause ^[12]	Stop hypertonic saline/NaHCO₃; treat mineralocorticoid excess	Removes the sodium source
2	D5 infusion ^[4]	Replace free water	Dilutes the excess sodium
3	Furosemide 40–80 mg IV/PO Q12–24H ^[4]^[12]	Promote natriuresis	Loop diuretics cause excretion of isotonic urine (roughly equal Na and water loss) → when combined with D5 (pure water replacement), the net effect is sodium removal
4	Dialysis (if refractory)	Especially if concurrent renal failure	Definitive sodium removal when kidneys cannot excrete the excess

For Conn syndrome / Cushing syndrome:

Treat the underlying hormonal excess (surgery for aldosterone-producing adenoma, medical treatment for Cushing)
Spironolactone / eplerenone for primary hyperaldosteronism (blocks aldosterone at the MR)

Fluid	Composition	Na Content	Distribution	Indication in Hypernatraemia
Normal Saline (0.9% NaCl) ^[22]	154 mmol/L NaCl	154 mmol/L	Mainly ECF (~1/3 stays intravascular)	Initial volume resuscitation in hypovolaemic patients — still hypotonic relative to hypernatraemic plasma ^[1]
D5 (5% Dextrose) ^[22]	50 g/L glucose	0 mmol/L (once metabolised)	All compartments (free water) → ~1/13 stays intravascular	Free water replacement — the main fluid for correcting hypernatraemia ^[4]^[12]
Half-half (0.45% NaCl + 2.5% dextrose) ^[22]	77 mmol/L NaCl + 25 g/L glucose	77 mmol/L	Intermediate distribution	Free water replacement with some Na; better for DM patients (less glucose) ^[4]^[12]^[22]
Hypotonic saline (0.45%, 0.33%, 0.2%) ^[22]	Variable	77 / 56 / 34 mmol/L	Intermediate	May be useful in life-threatening hypernatraemia ^[22]; risk of RBC lysis → add dextrose
Tap water (oral or NG) ^[12]	0 mmol/L	0 mmol/L	All compartments	Safest and most physiological route; preferred whenever the patient can drink or has NG tube access

Why Not Just Give Pure Water IV?

You cannot infuse pure water intravenously — it would lyse red blood cells due to extreme hypotonicity. D5 is the IV vehicle for free water delivery: the dextrose provides just enough osmolality to prevent haemolysis during infusion, and then it is rapidly metabolised by the liver, leaving behind pure free water. But monitor blood glucose closely — D5 can cause hyperglycaemia, especially in diabetic patients ^[4]^[12].

Parameter	Frequency	Why
Serum [Na⁺]	Every 2–4 hours during active correction	Monitor the [Na⁺] regularly ^[1]; ensure correction rate < 8–10 mmol/24h
Serum glucose	Every 4–6 hours (more frequent if on D5 or DM patient)	Closely monitor [Na] and [Glc] during treatment → correct hyperglycaemia if necessary ^[12]
Urine output	Hourly (catheterise if unconscious)	Guides ongoing fluid therapy; documents DI response to DDAVP
Urine osmolality	Q4–6H during DI workup/treatment	Confirms DDAVP response (AVP-D); documents ongoing losses
Body weight	Daily	Quick assessment of total body water changes
Neurological status / GCS	Continuous	Detect complications: cerebral oedema from overcorrection, or worsening from undertreated hypernatraemia
Fluid balance (I/O chart)	Continuous	Essential to calculate ongoing losses and net fluid balance
CVP / haemodynamic monitoring	If severe dehydration or HHS	Guide fluid replacement using CVP or pulmonary wedge pressure monitoring ^[6]

Treatment	Contraindication / Caution	Why
Rapid correction (> 10–12 mmol/L per 24h)	Chronic hypernatraemia	Cerebral oedema from osmotic shift
D5 infusion	Uncontrolled diabetes mellitus (relative)	Worsens hyperglycaemia → may paradoxically worsen osmotic diuresis; use half-half solution instead
DDAVP	Primary polydipsia (must exclude first!)	DDAVP + continued excessive water intake → life-threatening hyponatraemia
DDAVP	Nephrogenic DI (complete resistance)	No benefit — kidneys cannot respond to ADH; no increase in urine osmolality after DDAVP, since there is resistance ^[7]
NSAIDs (indomethacin)	Renal impairment, GI bleeding risk, elderly	NSAIDs reduce GFR and increase GI/renal toxicity
Thiazides	Hypovolaemic patients	Further volume depletion; only appropriate in euvolaemic DI patients
Normal saline alone	As sole therapy in hypernatraemia with intact kidney function	NS (154 mmol/L Na) is hypertonic relative to normal plasma — in a patient with mild hypernatraemia it may be adequate, but in severe cases you need hypotonic fluid
Contrast CT imaging	In the presence of renal impairment, try avoiding contrast CT to prevent toxicity ^[14]	Dehydrated patients have reduced GFR; contrast may cause contrast-induced nephropathy
Lithium (for those already on it)	Must balance psychiatric need vs renal toxicity	Lithium causes nephrogenic DI, CKD, and chronic tubulointerstitial nephropathy ^[23]; reducing dose or switching to alternative mood stabiliser may be necessary, but must involve psychiatry

Cause	Specific Treatment
HHS	Fluid replenishment is key; if hypernatraemia present, use ½ NS; less insulin required compared to DKA ^[6]; guide volume replacement with CVP; VTE prophylaxis
Diarrhoeal illness	Oral rehydration salts (ORS) in mild cases; IV NS then switch to hypotonic fluids; treat infective cause
Drug-induced nephrogenic DI (lithium)	Stop offending medication ^[17]; amiloride is specifically useful — blocks lithium entry via ENaC ^[23]; consult psychiatry for alternative mood stabiliser
Central DI from pituitary tumour	DDAVP for immediate symptomatic control; check and replace all anterior pituitary hormone deficiencies ^[14]; neurosurgical resection if indicated
Hypercalcaemia-induced nephrogenic DI	Aggressive IV NS hydration ^[9]; treat hypercalcaemia (bisphosphonates, calcitonin, treat underlying malignancy/hyperparathyroidism); once Ca normalises, AQP2 function recovers
Conn syndrome	Spironolactone / eplerenone (MR antagonist); surgical adrenalectomy for aldosterone-producing adenoma
Insensible losses (fever, ventilation)	Treat fever; humidify ventilator gases; ensure adequate free water prescription in ventilated patients
Inadequate intake (elderly / impaired access)	Ensure adequate water access; educate caregivers; consider regular scheduled oral fluids

Case: 70 kg man (chronic hypernatraemia), serum Na⁺ = 160 mmol/L

Step 1: Calculate free water deficit

Free water deficit = BW × 0.6 × (measured Na – 140) / 140 ^[4] = 70 × 0.6 × (160 – 140) / 140 = 42 × 20/140 = 6 litres

Step 2: Determine safe correction rate

Target: lower Na by no more than 10 mmol/L in 24 hours (from 160 → 150)
This means we need to replace approximately 6 × (10/20) = 3 litres of free water in the first 24 hours (half the total deficit)

Step 3: Add ongoing losses

Estimate insensible losses (~500–1000 mL/day) + urinary free water loss (calculate from urine output and urine Na)
Total first 24h replacement ≈ 3L free water deficit portion + ~1L ongoing losses = ~4L

Step 4: Choose fluid

If haemodynamically stable → D5 or half-half solution
If haemodynamically unstable → start with NS for resuscitation, then switch

Step 5: Monitor

Check serum Na every 2–4 hours
Adjust infusion rate to keep correction < 10 mmol/24h
Monitor glucose (D5 provides 200 kcal/L)

High Yield Summary — Management of Hypernatraemia

Principle: Identify and treat cause + replace free water deficit + correct slowly
Correction rate: < 8–10 mmol/L per 24 hours for chronic hypernatraemia — rapid correction causes cerebral oedema
Most serious complications result from inappropriate treatment, not the hypernatraemia itself
Hypovolaemic: NS first for volume resuscitation → switch to D5 or half-half for free water replacement
Euvolaemic: Free water replacement (oral > IV D5 > half-half); treat DI specifically
Hypervolaemic: D5 + furosemide 40–80 mg Q12–24H to remove excess sodium
AVP-D: DDAVP (intranasal/SC/PO/IV); low Na diet; minimal effective dose to control polyuria
AVP-R: Treat cause (stop lithium!); low Na diet; thiazide + amiloride; NSAIDs (indomethacin); consider DDAVP if partial resistance
Free water deficit formula: BW × 0.6 × (measured Na – 140) / 140 — add ongoing losses
Post-op DI: Triphasic pattern — fluid restriction during antidiuretic phase to prevent hyponatraemia
HHS with hypernatraemia: Use ½ NS, not pure water — rapid Na drop causes disequilibrium
Monitor: Serum Na Q2–4H, glucose, urine output, neurological status

Active Recall - Hypernatraemia Management

References

^[1] Lecture slides / Senior notes: Block A - Electrolyte and Acid-Base Disorders.pdf (Sodium section, volume repletion rate, Hypernatraemia algorithm) ^[4] Senior notes: Maksim Medicine Notes.pdf (Nephrology section - Hypernatraemia management, p208) ^[6] Senior notes: Block A - Polyuria and polydipsia_ glucose metabolism; diabetes mellitus; diabetic ketoacidosis.pdf (HHS treatment, ½ NS, CVP monitoring) ^[7] Senior notes: Block A - I keep on bumping into people on my side_ pituitary tumours; hypopituitarism.pdf (DI definition, DDAVP response) ^[8] Senior notes: Chemical Pathology Data interpretation.pdf (Copeptin, genetic DI) ^[9] Senior notes: Block A - Confused and dehydrated_ hypercalcaemia; hypocalcaemia.pdf (Hypercalcaemia rehydration, nephrogenic DI mechanism) ^[12] Senior notes: Ryan Ho Urogenital.pdf (Hypernatraemia management approach, fluid choices, correction rate, p21) ^[14] Senior notes: Block A - Two cases of polyuria and polydipsia.pdf (Post-DI anterior pituitary workup, contrast CT avoidance, renal US) ^[17] Senior notes: Ryan Ho Endocrine.pdf (DI management — DDAVP dosing, nephrogenic DI treatment, p115) ^[22] Senior notes: Ryan Ho Fluids and Nutrition.pdf (Crystalloid types, D5, half-half, hypotonic saline, p4) ^[23] Senior notes: Ryan Ho Psychiatry.pdf (Lithium side effects — nephrogenic DI, amiloride, p53)

Complications of Hypernatraemia

The complications of hypernatraemia fall into two major categories: (1) complications of the hypernatraemia itself (the disease), and (2) complications of its treatment (iatrogenic). Understanding this distinction is critical because, as we've emphasised throughout:

Most serious complications result not from the disorder itself but from inappropriate treatment of hypernatraemia ^[16]^[24]

This is a recurring theme in the lecture slides and senior notes — it is almost certainly examinable.

A. Complications of Hypernatraemia Itself (The Disease)

These result from the hyperosmolar state driving water out of cells, predominantly affecting the brain.

Complication	Pathophysiology	Clinical Features
Altered mental status / Encephalopathy	↑ Serum osmolality → osmotic gradient pulls water out of brain cells → neuronal shrinkage → impaired neurotransmission	Irritability → lethargy → confusion → obtundation → coma; severity correlates with degree and rapidity of hypernatraemia
Seizures ^[4]	Severe brain cell dehydration → cortical neuronal membrane instability → abnormal electrical discharges. Also can occur paradoxically during treatment (see below)	Generalised tonic-clonic seizures; may be refractory to anticonvulsants if the electrolyte disturbance is not corrected
Intracerebral haemorrhage (ICH) ^[4]	As the brain shrinks away from the inner table of the skull, bridging veins (which span from the cortical surface to the dural venous sinuses) are stretched and may rupture. This is the same mechanism as ICH in shaken baby syndrome or in elderly patients with brain atrophy	Sudden focal neurological deficit, headache, decreased consciousness, may require neurosurgical intervention
Subarachnoid haemorrhage (SAH) ^[4]	Same mechanism — traction on meningeal vessels as the brain retracts from the dura → rupture of small pial or cortical vessels into the subarachnoid space	Thunderclap headache, meningism, decreased consciousness
Cerebral venous sinus thrombosis (CVST)	Severe dehydration → haemoconcentration → ↑ blood viscosity → stasis in cerebral venous sinuses → thrombosis. Neonates and infants are particularly susceptible	Headache, seizures, focal deficits, papilloedema; may present with haemorrhagic venous infarction on CT
Depression of sensorium with loss of thirst response	When serum osmolality > 340 mmol/L ^[6] → severe neuronal dysfunction in hypothalamic thirst centres	Creates a vicious cycle: hypernatraemia → confusion → inability to perceive/communicate thirst → further dehydration → worsening hypernatraemia

The Vicious Cycle of Severe Hypernatraemia

Hypernatraemia causes confusion → confused patient cannot drink → hypernatraemia worsens → further depression of sensorium → complete loss of thirst response at serum osmolality > 340 mmol/L ^[6]. This explains why severe hypernatraemia carries such high mortality (40–60%), particularly in the elderly, who already have a diminished baseline thirst response.

Why is the brain so vulnerable?

The brain is enclosed within the rigid skull. Unlike other organs, it cannot expand or shrink freely. When the brain loses water and shrinks:

The parenchyma pulls away from the meninges and skull
Bridging veins and meningeal vessels are placed under tension
Small tears → ICH or SAH (rare but devastating)
Additionally, brain cells are exquisitely sensitive to osmolar shifts because of their high metabolic rate and dependence on precise ionic gradients for neurotransmission

Complication	Pathophysiology
Hypovolaemic shock	When hypernatraemia is caused by significant hypotonic fluid loss (diarrhoea, burns, HHS), the ECF volume is depleted → ↓ preload → ↓ cardiac output → tissue hypoperfusion → multi-organ failure if untreated
Pre-renal AKI	Severe dehydration → ↓ renal perfusion → ↓ GFR → rising creatinine and urea; disproportionately ↑ urea:creatinine ratio; usually reversible with rehydration
Rhabdomyolysis	Severe cellular dehydration of muscle cells → sarcolemmal disruption → release of myoglobin, CK, potassium, phosphate → risk of pigment nephropathy (myoglobin-induced AKI)
Venous thromboembolism (DVT/PE)	Haemoconcentration → ↑ blood viscosity → ↑ thrombotic risk; In HHS, due to the high risk of thrombosis, you may need to give heparin prophylaxis ^[6]

Complication	Pathophysiology
Hyperglycaemia	Dehydration concentrates blood glucose; also, in HHS, the hyperglycaemia is the primary driver of the osmotic diuresis causing the hypernatraemia in the first place — a bidirectional relationship
Hypokalaemia	Often coexists, particularly in diarrhoeal illness (GI K⁺ loss), osmotic diuresis (urinary K⁺ loss), and mineralocorticoid excess (Conn syndrome). Hypokalaemia itself can worsen nephrogenic DI, perpetuating the hypernatraemia
Metabolic acidosis	In severe dehydration → lactic acidosis from tissue hypoperfusion; in DKA, the acidosis is the primary pathology

B. Complications of Treatment (Iatrogenic) — The Bigger Danger

Exam High-Yield

Most serious complications result from inappropriate treatment rather than the disorder per se! ^[12]^[16]^[24] This point is explicitly stated on the GC Chemical Pathology lecture slides. The complications of overcorrection are potentially more devastating than the hypernatraemia itself.

This is the most feared complication of hypernatraemia management.

Pathophysiology — explained from first principles:

In chronic hypernatraemia (≥ 48 hours), brain cells have adapted by generating idiogenic osmoles (organic osmolytes: taurine, glutamine, myoinositol, betaine, sorbitol)
These idiogenic osmoles draw water back into the brain cells, partially restoring cell volume — the brain has "reset" to the new hyperosmolar environment
If you now rapidly lower the serum sodium/osmolality (e.g. by aggressive D5 infusion), the ECF becomes relatively hypotonic compared to the brain cells (which still contain all those idiogenic osmoles)
Water rushes into the brain cells down the osmotic gradient
Brain cells swell → cerebral oedema
Within the rigid skull → ↑ intracranial pressure → risk of cerebral herniation → death

Too rapid correction of sodium in hypernatraemia will result in cerebral oedema → due to the rapidly reduced plasma osmolality, causes an influx of water into cells ^[1]

> 12 mEq/L change in serum [Na] in chronic hyperNa may result in cerebral oedema due to water rushing into neurones ^[12]

Clinical features of cerebral oedema from overcorrection:

Headache, nausea, vomiting (rising ICP)
Deteriorating consciousness (worsening GCS)
Seizures (new-onset during correction — should raise immediate alarm)
Papilloedema
Cushing reflex (bradycardia + hypertension + irregular breathing) → imminent herniation
Coma and death if not recognised and reversed

Management if overcorrection occurs:

Stop all hypotonic fluids immediately
Administer hypertonic saline (3% NaCl) to raise serum Na back to a safe level
May need mannitol or hypertonic saline bolus for acute ICP management
Neurosurgical consultation if signs of herniation
CT brain to assess for oedema and exclude other pathology

The Mirror Image of Osmotic Demyelination

The relationship between hyponatraemia and hypernatraemia correction complications is a mirror image:

Hyponatraemia corrected too rapidly → water rushes OUT of brain cells → osmotic demyelination syndrome (central pontine myelinolysis) → patients become tetraplegic ^[1]
Hypernatraemia corrected too rapidly → water rushes INTO brain cells → cerebral oedema → herniation → death

Both involve an osmotic mismatch between adapted brain cells and the rapidly changing ECF. The lesson: always respect the brain's adaptation time.

Aspect	Detail
Mechanism	D5 contains 50 g/L of dextrose. In diabetic patients or those with insulin resistance (stress, sepsis, steroid use), this glucose load may not be metabolised efficiently → hyperglycaemia
Consequence	Hyperglycaemia → glucosuria → osmotic diuresis → further free water loss → paradoxically worsens the hypernatraemia you are trying to correct
Prevention	Closely monitor [Na] and [Glc] during treatment → correct hyperGly if necessary ^[12]; consider using half-half solution (0.45% NaCl + 2.5% dextrose) instead of D5 in diabetic patients; add insulin sliding scale if needed

Aspect	Detail
Mechanism	Especially in elderly patients with compromised cardiac function or CKD — excessive fluid administration exceeds the heart's and kidneys' capacity to handle the volume
Consequence	Pulmonary oedema, peripheral oedema, ↑ JVP, respiratory failure
Prevention	Fluid replacement rate dependent on age and comorbid conditions → heart failure, CKD ^[9]; guide replacement using CVP or pulmonary wedge pressure monitoring ^[6]; auscultate lungs regularly; monitor fluid balance charts

Aspect	Detail
Mechanism	As hypernatraemia is corrected and volume is replaced: (1) dilution of serum K⁺; (2) insulin (if given for HHS) drives K⁺ intracellularly; (3) correction of acidosis shifts K⁺ into cells; (4) D5 can stimulate endogenous insulin release
Consequence	Cardiac arrhythmias (QT prolongation, U waves, risk of Torsades de Pointes), muscle weakness, ileus
Prevention	Monitor K⁺ frequently; replace K⁺ proactively; add KCl to IV fluids when K < 4.0 mmol/L

Aspect	Detail
Mechanism	In patients with central DI treated with DDAVP: if DDAVP dose is too high or water intake is excessive, urine becomes maximally concentrated while free water intake continues → dilutional hyponatraemia
Consequence	Water intoxication, hyponatraemic encephalopathy, seizures
Prevention	Dosing adjusted based on clinical symptoms → minimal effective dose to control polyuria; should prevent nocturia but allow a degree of polyuria from time to time before next dose ^[17] — this "breakthrough polyuria" confirms the DDAVP has worn off and prevents dangerous water accumulation

These are not directly caused by the hypernatraemia but are consequences of the disease process that produced it.

Underlying Cause	Associated Complications
HHS	High mortality ^[6]; VTE; aspiration pneumonia (if obtunded); severe hyperosmolality with serum osmolality > 340 causes depression of sensorium ^[6]
Lithium-induced nephrogenic DI	Chronic kidney disease → chronic tubulointerstitial nephropathy ^[5]; cysts and tubular dilatation ^[5]; hypothyroidism, hyperthyroidism, hyperparathyroidism/hypercalcaemia ^[5]; acute lithium intoxication in overdose ^[5]
Hypercalcaemia	Renal stones, nephrocalcinosis, pancreatitis, cardiac arrhythmias (short QT), peptic ulcer disease, constipation, psychiatric symptoms ("bones, stones, abdominal moans, and psychic groans")
Central DI from pituitary tumour	Mass effect (bitemporal hemianopia, headache), anterior pituitary hormone deficiencies (hypopituitarism), pituitary apoplexy
Severe dehydration in neonates	Neonatal seizures, long-term neurodevelopmental impairment, CVST

Category	Complications	Key Point
Disease itself — Neurological	Encephalopathy, seizures, ICH, SAH ^[4], CVST	Due to brain cell shrinkage from hyperosmolality
Disease itself — Haemodynamic	Shock, pre-renal AKI, rhabdomyolysis, VTE	Due to ECF depletion and haemoconcentration
Disease itself — Metabolic	Hyperglycaemia, hypokalaemia, metabolic acidosis	Coexisting or contributing derangements
Treatment — Most Important	Cerebral oedema from rapid correction ^[1]^[12]^[24]	Water rushes into adapted brain cells; correct < 8–10 mmol/24h
Treatment — Metabolic	Hyperglycaemia (D5), hypokalaemia (insulin/dilution), fluid overload	Monitor Na, glucose, K, fluid balance closely
Treatment — DDAVP	Hyponatraemia from over-treatment	Use minimal effective dose; allow breakthrough polyuria

High Yield Summary — Complications of Hypernatraemia

Most serious complications result from inappropriate treatment (overly rapid correction) rather than the disorder per se — this is the single most important teaching point
Neurological complications of the disease: encephalopathy, seizures, ICH/SAH (from bridging vein traction) ^[4], cerebral venous sinus thrombosis
Vicious cycle: hypernatraemia → confusion → cannot drink → worsening hypernatraemia; loss of thirst at serum osm > 340 ^[6]
Cerebral oedema is the most feared treatment complication: caused by rapid Na correction in chronic hypernatraemia → water rushes into brain cells containing idiogenic osmoles → brain swelling → herniation
Safe correction rate: < 8–10 mmol/L per 24h; > 12 mmol/L change may cause cerebral oedema ^[12]
Mirror image of hyponatraemia: rapid correction of hypoNa → osmotic demyelination; rapid correction of hyperNa → cerebral oedema. Both are catastrophic and preventable
Monitor during treatment: Na Q2–4H, glucose (D5-induced hyperglycaemia), potassium (dilution/insulin shifts), fluid balance (risk of overload in elderly/HF/CKD)
DDAVP over-treatment: causes dilutional hyponatraemia; use minimal effective dose with scheduled "breakthrough polyuria"
VTE risk: haemoconcentration in dehydrated patients; heparin prophylaxis may be needed in HHS ^[6]
Lithium complications extend beyond DI: CKD from chronic tubulointerstitial nephropathy, multiple endocrine side effects ^[5]

Active Recall - Complications of Hypernatraemia

References

^[1] Lecture slides / Senior notes: Block A - Electrolyte and Acid-Base Disorders.pdf (Cerebral oedema from rapid correction; hyponatraemia correction complications for comparison, p22–24) ^[4] Senior notes: Maksim Medicine Notes.pdf (Nephrology section - Hypernatraemia clinical features: headache, irritability, seizures, ICH/SAH, p208) ^[5] Senior notes: Block A - Drugs and the Kidney.pdf (Lithium — nephrogenic DI, chronic tubulointerstitial nephropathy, endocrine side effects, p11) ^[6] Senior notes: Block A - Polyuria and polydipsia_ glucose metabolism; diabetes mellitus; diabetic ketoacidosis.pdf (HHS — severe hyperosmolality, loss of thirst at osm > 340, heparin prophylaxis, CVP monitoring, ½ NS for hypernatraemia, p14) ^[9] Senior notes: Block A - Confused and dehydrated_ hypercalcaemia; hypocalcaemia.pdf (Fluid replacement rate dependent on comorbidities, p10) ^[12] Senior notes: Ryan Ho Urogenital.pdf (Most serious Cx from inappropriate Tx; correction rate; monitor Na and Glc, p21) ^[16] Senior notes: Ryan Ho Chemical Path.pdf (Most serious complications from inappropriate treatment, p11) ^[17] Senior notes: Ryan Ho Endocrine.pdf (DDAVP dosing — minimal effective dose, breakthrough polyuria, S/E, p115) ^[24] Lecture slides: Chemical Pathology Seminar 1_Sodium and water.pdf (Most serious complications result from inappropriate treatment, p29)

High Yield Summary

Hypernatraemia = serum [Na⁺] > 145 mmol/L, always reflecting hyperosmolality and relative water deficit
[Na⁺] reflects water balance, not absolute sodium content — hypernatraemia fundamentally means too little water relative to sodium
Three categories by volume status: hypovolaemic (hypotonic fluid loss), euvolaemic (DI, insensible loss), hypervolaemic (Na⁺ gain / Conn syndrome)
Diabetes insipidus: now renamed AVP-D (central) and AVP-R (nephrogenic) per 2022 consensus; differentiated by DDAVP response and copeptin levels
Lithium is the most common drug cause of nephrogenic DI; hypercalcaemia causes nephrogenic DI via autophagic degradation of AQP2
Brain cell shrinkage causes neurological symptoms: irritability, seizures, ICH/SAH (rare), coma
Brain adaptation (idiogenic osmoles) occurs in chronic hypernatraemia → must correct slowly: < 8–10 mmol/L per 24 hours to avoid cerebral oedema
Urine osmolality < serum osmolality in hypernatraemia = inappropriate = DI
Key investigations: paired serum and urine osmolality, serum glucose, Na, K, Ca, medication review
Volume depletion assessment: 5% BW loss = ↓ skin turgor; 10% = postural hypotension; 15% = shock

High Yield Summary — Differential Diagnosis of Hypernatraemia

Classify by volume status first: hypovolaemic (most common), euvolaemic, or hypervolaemic
U:P osmolality ratio is the key discriminator: < 1 = DI; = 1 = osmotic diuresis; > 1 = extrarenal loss/inadequate intake
Urine osmolality < 300 in hypernatraemia is highly suggestive of DI ^[4]
Differentiate AVP-D vs AVP-R with: DDAVP response (↑ urine osm = AVP-D; no change = AVP-R) and copeptin (low = AVP-D; high = AVP-R)
Most common causes in clinical practice: dehydration in elderly with impaired thirst (extrarenal losses + inadequate intake), osmotic diuresis from HHS/hyperglycaemia, and drug-induced nephrogenic DI (lithium)
Always check calcium — hypercalcaemia and hypernatraemia frequently coexist via nephrogenic DI mechanism
HHS is a favourite exam scenario: elderly T2DM + dehydration + confusion + hypernatraemia from osmotic diuresis
Post-neurosurgical DI shows the triphasic pattern — transient DI → antidiuresis → permanent DI

High Yield Summary — Diagnosis of Hypernatraemia

Diagnostic criterion: Serum Na > 145 mmol/L with elevated serum osmolality
Serum osmolality formula: 2[Na⁺] + [Urea] + [Glucose]; normal 285–295 mOsm/kg
Essential investigations: serum and urine osmolality, serum glucose, Na, K, Ca — get paired samples
U:P osmolality ratio is the single most important discriminator: < 1 = DI; ≈ 1 = osmotic diuresis; > 1 = extrarenal loss
Urine Osm < 300 with Na > 145 = diagnostic for DI without needing a water deprivation test
Water deprivation test: no fluids for 8h → stop if > 3% BW loss or plasma osm > 300 → give DDAVP → ≥ 50% rise in urine osm = AVP-D; no change = AVP-R
Copeptin: modern alternative — LOW = AVP-D; HIGH = AVP-R
After diagnosing AVP-D: MRI pituitary + check ALL anterior pituitary hormones
After diagnosing AVP-R: drug history (lithium!), check Ca/K, renal ultrasound, consider genetic testing
Free water deficit = BW × 0.6 × (Measured Na – 140) / 140; add ongoing losses; correct at < 8–10 mmol/24h

High Yield Summary — Management of Hypernatraemia

Principle: Identify and treat cause + replace free water deficit + correct slowly
Correction rate: < 8–10 mmol/L per 24 hours for chronic hypernatraemia — rapid correction causes cerebral oedema
Most serious complications result from inappropriate treatment, not the hypernatraemia itself
Hypovolaemic: NS first for volume resuscitation → switch to D5 or half-half for free water replacement
Euvolaemic: Free water replacement (oral > IV D5 > half-half); treat DI specifically
Hypervolaemic: D5 + furosemide 40–80 mg Q12–24H to remove excess sodium
AVP-D: DDAVP (intranasal/SC/PO/IV); low Na diet; minimal effective dose to control polyuria
AVP-R: Treat cause (stop lithium!); low Na diet; thiazide + amiloride; NSAIDs (indomethacin); consider DDAVP if partial resistance
Free water deficit formula: BW × 0.6 × (measured Na – 140) / 140 — add ongoing losses
Post-op DI: Triphasic pattern — fluid restriction during antidiuretic phase to prevent hyponatraemia
HHS with hypernatraemia: Use ½ NS, not pure water — rapid Na drop causes disequilibrium
Monitor: Serum Na Q2–4H, glucose, urine output, neurological status

High Yield Summary — Complications of Hypernatraemia

Most serious complications result from inappropriate treatment (overly rapid correction) rather than the disorder per se — this is the single most important teaching point
Neurological complications of the disease: encephalopathy, seizures, ICH/SAH (from bridging vein traction) ^[4], cerebral venous sinus thrombosis
Vicious cycle: hypernatraemia → confusion → cannot drink → worsening hypernatraemia; loss of thirst at serum osm > 340 ^[6]
Cerebral oedema is the most feared treatment complication: caused by rapid Na correction in chronic hypernatraemia → water rushes into brain cells containing idiogenic osmoles → brain swelling → herniation
Safe correction rate: < 8–10 mmol/L per 24h; > 12 mmol/L change may cause cerebral oedema ^[12]
Mirror image of hyponatraemia: rapid correction of hypoNa → osmotic demyelination; rapid correction of hyperNa → cerebral oedema. Both are catastrophic and preventable
Monitor during treatment: Na Q2–4H, glucose (D5-induced hyperglycaemia), potassium (dilution/insulin shifts), fluid balance (risk of overload in elderly/HF/CKD)
DDAVP over-treatment: causes dilutional hyponatraemia; use minimal effective dose with scheduled "breakthrough polyuria"
VTE risk: haemoconcentration in dehydrated patients; heparin prophylaxis may be needed in HHS ^[6]
Lithium complications extend beyond DI: CKD from chronic tubulointerstitial nephropathy, multiple endocrine side effects ^[5]

Hypernatremia

Definition

Epidemiology and Risk Factors

Epidemiology

Risk Factors (who is vulnerable?)

Anatomy and Physiology of Water and Sodium Homeostasis

Body Water Distribution

Key Hormones

Arginine Vasopressin (AVP / ADH)

Thirst Mechanism

Aldosterone (RAAS)

Renal Concentrating Mechanism

Etiology (Focus on Hong Kong Context)

Classification by Volume Status

Detailed Aetiological Breakdown

1. Hypovolaemic Hypernatraemia (Net water loss > sodium loss)

2. Euvolaemic (Isovolaemic) Hypernatraemia (Pure water loss)

Diabetes Insipidus (DI)

Other Causes of Euvolaemic Hypernatraemia

3. Hypervolaemic Hypernatraemia (Sodium gain)

Hong Kong Specific Considerations

Relevant Pathophysiology

Why does hypernatraemia cause neurological symptoms?

Brain Adaptation — The Reason You Must Correct Slowly

Pathophysiology by Mechanism Summary

Classification

By Rate of Onset

By Severity

By Volume Status (Most Clinically Useful)

Clinical Features

Symptoms (with pathophysiological basis)

Signs (with pathophysiological basis)

Triphasic Pattern of Post-operative/Post-traumatic DI

Summary of Pre-Diagnostic Approach

Active Recall - Hypernatraemia (Definition to Clinical Features)

References

Conceptual Framework for the Differential

Master Diagnostic Algorithm

Differential Diagnosis Organised by Volume Status

1. Hypovolaemic Hypernatraemia (Loss of hypotonic fluid — more water lost than sodium)

2. Euvolaemic (Isovolaemic) Hypernatraemia (Pure water loss or inadequate water intake)

Differentiating AVP-D from AVP-R — A Key Exam Topic

Causes of AVP-R (Nephrogenic DI) — Expanded

Causes of AVP-D (Central DI) — Expanded

3. Hypervolaemic Hypernatraemia (Sodium gain exceeds water gain)

Differentials for the Confused Hypernatraemic Patient

U:P Osmolality Ratio — The Key Discriminating Tool

Summary Table of All Major Differentials

Active Recall - Hypernatraemia Differential Diagnosis

Diagnostic Criteria

Diagnostic Algorithm — Step-by-Step Clinical Approach

Step 1: Confirm True Hypernatraemia

Step 2: Assess Volume Status Clinically

Step 3: Obtain Paired Serum and Urine Studies

Step 4: Calculate U:P Osmolality Ratio

Step 5: If DI Suspected — Differentiate AVP-D vs AVP-R

Step 6: Investigate the Underlying Cause

Master Diagnostic Algorithm — Mermaid Diagram

Investigation Modalities — Detailed Interpretation

A. Baseline Blood Tests (Mandatory for ALL patients with hypernatraemia)

B. Urine Studies (Critical — must be paired with serum)

C. Water Deprivation Test (Dynamic Test)

D. Copeptin Test (Modern Alternative)

E. Imaging Studies

F. Additional Investigations Based on Suspected Cause

G. Calculating Free Water Deficit

Summary of Key Investigations and Their Interpretation

Active Recall - Hypernatraemia Diagnosis

Guiding Principles — Think Before You Act

Rate of Correction — The Safety Rail

Management Algorithm — Mermaid Diagram

Management by Volume Status — Detailed

1. Hypovolaemic Hypernatraemia

2. Euvolaemic (Isovolaemic) Hypernatraemia

AVP-D (Central DI) Management

AVP-R (Nephrogenic DI) Management

Acute Post-operative / Post-traumatic DI — Special Situation

3. Hypervolaemic Hypernatraemia

Fluid Choice Summary

Monitoring During Treatment