• AVP-D is rare overall — estimated prevalence ~1 in 25,000 ^[5].
• Familial AVP deficiency is extremely rare. ^[3]
• More commonly acquired (post-surgical, traumatic, neoplastic, or idiopathic).
• No significant sex predilection in acquired forms; familial forms (autosomal dominant) affect both sexes equally.
• All ages can be affected — from neonates (congenital/genetic forms) to elderly (post-surgical, metastatic disease).

• One of the most common acquired forms encountered clinically.
• Transsphenoidal surgery (TSS) for pituitary adenomas carries a ~10–20% risk of transient AVP-D and ~1–5% risk of permanent AVP-D ^[5].
• Triphasic pattern after acute post-operative/traumatic DI: transient DI (hours to days) → antidiuresis (2–14 days, from uncontrolled AVP release from dying neurons) → return of DI (may be permanent). ^[4]

• In Hong Kong, the most relevant acquired causes are:
  • Post-neurosurgical (given the volume of pituitary surgery at QMH/PWH).
  • Idiopathic (most common overall cause of central DI — up to 30–50% of cases historically, though some turn out to be autoimmune on further investigation).
  • Craniopharyngiomas (relatively common sellar/suprasellar tumour in paediatric and young adult populations in HK).
  • Metastatic disease (lung, breast — common cancers in HK).
  • Langerhans cell histiocytosis (LCH) — important paediatric cause.

Key anatomical points:

1. Supraoptic (SON) and Paraventricular (PVN) nuclei of the hypothalamus are where AVP is synthesized as part of a larger precursor molecule — pre-pro-vasopressin.
2. Pre-pro-vasopressin is cleaved into:
   • AVP (the active hormone — a nonapeptide: 9 amino acids)
   • Neurophysin II (carrier protein)
   • Copeptin (C-terminal glycopeptide — used as a surrogate marker; see below) ^[3]
3. The AVP + neurophysin II + copeptin complex is transported down the axon within neurosecretory granules along the hypothalamo-hypophyseal tract (pituitary stalk/infundibulum).
4. They are stored in Herring bodies in the posterior pituitary (neurohypophysis).
5. Upon stimulation, exocytosis releases AVP into the perivascular capillaries of the posterior pituitary → systemic circulation.

AVP release is regulated by two main stimuli:

Other modulators:

• Nausea — potent stimulus for AVP release (hence why nausea can cause hyponatraemia).
• Pain, stress, hypoglycaemia, angiotensin II — all stimulate AVP release.
• Alcohol — inhibits AVP release (hence the copious dilute urine after drinking alcohol).
• Atrial natriuretic peptide (ANP) — inhibits AVP release.

This is the key pathway that is disrupted in both AVP-D (no hormone) and AVP-R (hormone present, kidney can't respond):

• V2 receptor — "V2" = vasopressin receptor type 2 (Gs-coupled, in kidney). In AVP-R, this receptor or its downstream pathway (including AQP-2) is defective.
• AQP-2 (Aquaporin 2) — the regulated water channel. Without AVP signalling, AQP-2 remains sequestered in intracellular vesicles and is NOT inserted into the luminal membrane → water cannot be reabsorbed → dilute urine.
• AQP-3 and AQP-4 — constitutively expressed on the basolateral membrane; they allow water that entered through AQP-2 to exit into the interstitium.

In AVP-D, there is no AVP to bind V2 receptors → no AQP-2 insertion → copious dilute urine. Exogenous DDAVP can restore this pathway, which is why urine osmolality increases after DDAVP in AVP-D. ^[1][2]

On a gene standpoint, copeptin can be cleaved from the same molecule (pre-pro-vasopressin) → so levels of vasopressin and copeptin correlate greatly. ^[3]

• AVP itself is difficult to measure clinically because:
  • Very short half-life (~10–20 minutes)
  • Unstable in vitro (binds to platelets)
  • Requires special handling (ice, rapid processing)
• Copeptin is:
  • Released in equimolar amounts with AVP
  • Much more stable (longer half-life)
  • Easier to assay
  • Used as a surrogate marker for AVP ^[2][3]
• Copeptin raised → diagnostic for AVP-R (the body IS making AVP — copeptin proves it — but the kidney doesn't respond) ^[2]
• Copeptin low → consistent with AVP-D (the body is NOT making AVP)
• Better for children as you don't deprive them of that much water ^[2]

The final common pathway is always the same: insufficient AVP reaches the systemic circulation → V2 receptors on the collecting duct are not activated → AQP-2 is not inserted → water is not reabsorbed → large volumes of dilute urine.

But the mechanism of reaching that endpoint varies:

1. Destruction of > 80% of magnocellular neurons (SON/PVN):

   • The posterior pituitary has remarkable reserve — you need to lose > 80% of AVP-producing neurons before clinical DI becomes apparent.
   • This is why pituitary adenomas (which arise from the anterior pituitary) rarely cause DI unless they are very large or surgery damages the stalk.
   • Pituitary adenoma causing DI is unusual — if a patient with a pituitary mass develops DI, think of craniopharyngioma, germinoma, metastasis, or infiltrative disease rather than a simple adenoma. ^[5]

2. Stalk transection/compression:

   • If the stalk is cut/compressed high (near the hypothalamus), DI is more likely to be permanent because the remaining axon stumps above the transection are too short to form a functional "mini-neurohypophysis".
   • If the stalk is cut low (near the pituitary), DI is more likely to be transient because the remaining long axon stumps can form new neurosecretory terminals above the transection point.

3. Autoimmune destruction:

   • Lymphocytic infiltration of the stalk/posterior pituitary → selective destruction of AVP-producing neurons.
   • Anti-AVP antibodies may be present.

4. Genetic — misfolded pro-vasopressin:

   • The AD form is a "toxic gain-of-function" mechanism: the mutant protein misfolds, aggregates in the endoplasmic reticulum, triggers ER stress → apoptosis of magnocellular neurons → progressive DI.
   • This is why familial AVP-D is progressive — neurons die gradually.

No longer nephrogenic or central — the new names immediately tell you about the pathophysiology. ^[2]

Partial AVP-D can be difficult to differentiate from primary polydipsia — this is where the hypertonic saline-stimulated copeptin test becomes valuable.

As outlined in Section 5: Acquired vs. Genetic/Familial.

Triphasic pattern after acute post-operative/traumatic DI ^[4]:

Phase	Timing	Mechanism
Phase 1: Transient DI	Hours to days post-injury	Acute damage → impaired AVP release
Phase 2: SIADH/Antidiuresis	2–14 days	Uncontrolled release of stored AVP from dying neurons → water retention → risk of hyponatraemia
Phase 3: Permanent DI	After Phase 2 resolves	If > 80% of neurons are destroyed, DI becomes permanent

Presentation: polyuria, polydipsia ± hypernatraemia. ^[1]

• Lesions of the hypothalamus, pituitary stalk, or posterior pituitary ^[1]
• Urine osmolality increases with DDAVP treatment (> 50% rise confirms AVP-D) ^[1][8]
• Serum Na: high-normal to elevated
• Copeptin: low (body is not making AVP) ^[2][3]
• Causes: idiopathic, post-surgical, traumatic, neoplastic, granulomatous, infectious, vascular, autoimmune, genetic (see Part 1 notes)

AVP-R = previously Nephrogenic DI = no response to vasopressin; genetic/acquired. ^[1]

Why does this mimic AVP-D? Because the end result is the same — dilute urine despite high serum osmolality. The difference is where the problem lies:

• In AVP-D: the hormone is absent → giving exogenous DDAVP restores the pathway.
• In AVP-R: the hormone is present but the kidney cannot respond → no increase in urine osmolality after DDAVP (< 9% rise) ^[8].

Causes of AVP-R (important differentials within this category):

This is a critical differential because it can mimic DI clinically but the mechanism is completely different:

• The patient drinks excessively (often > 10 L/day) → dilutes the plasma → suppresses AVP appropriately → produces dilute urine.
• The AVP axis is intact — the problem is upstream (behavioural/psychiatric).
• Serum Na is low or low-normal (↓ Na and ↓ osmolality) — this is the key distinguishing feature from DI, where Na is high/high-normal ^[4].

↓ Na and ↓ serum osmolality suggests primary polydipsia rather than DI. ^[4]

Why is the distinction important? Treating primary polydipsia with DDAVP would be dangerous — you'd cause water retention in someone already overloaded with water → severe dilutional hyponatraemia → seizures, cerebral oedema, death.

Differentiation between AVP-D and primary polydipsia may require hypertonic saline-stimulated copeptin test — done in ICU under supervised conditions, or else sodium falls too quickly after the test → risk of demyelination. ^[3]

• Document urine output: > 3 L/day in adults or > 50 mL/kg/day ^[5][11]
• Chart intake and output (I/O) to objectively measure volume — don't rely on subjective complaint alone ^[5][11]
• Rule out urinary frequency masquerading as polyuria (total volume is normal in BPH/OAB/UTI)

Paired plasma/urine osmolality + plasma electrolytes is the critical first-line investigation ^[4][5][11]:

Water deprivation test + DDAVP = traditional gold standard for diagnosing and differentiating DI ^[2][5][11]

Indication:

In all suspected patients EXCEPT if U/P ratio < 1 or Na > 145 + urine osmolality < 300 (already diagnostic). ^[5][11]

In other words, you only do this test when the baseline paired osmolality is equivocal — the patient has polyuria but Na is normal or borderline and you're not sure if it's DI or primary polydipsia.

After confirming DI (either basally or via water deprivation test), the DDAVP challenge separates the two:

Once AVP-D is confirmed, you must investigate the underlying aetiology (MRI brain, anterior pituitary hormone profile, etc. — see Section 4 below).

To induce increased ADH secretion by creating a hyperosmolar state, and to detect the response to increased ADH. ^[5][11]

In a normal person: dehydration → plasma osmolality rises → osmoreceptors stimulate AVP release → AVP acts on collecting duct → urine concentrates. If you deprive someone of water and their urine doesn't concentrate, something is wrong with the AVP axis.

No fluid intake for 8 hours (e.g., 0730/0830–1630), no tea, coffee, smoking; only dry food allowed. ^[5][11]

During the test:

• Close supervision with hourly body weight, urine volume, and U/P osmolality ^[5][11][14]
• The patient must be supervised (risk of surreptitious drinking in primary polydipsia patients, risk of severe dehydration in true DI)

When to STOP:

Stop when endpoint reached: U/P ratio ≤ 2 with plasma osmolality > 300, OR > 3% decrease in body weight. ^[5][11][14]

Why > 3% body weight loss? Because this represents clinically significant dehydration — further deprivation would be unsafe, particularly in children or patients with complete DI who can lose water extremely rapidly.

Then: Give DDAVP 2 μg IM ^[5][11] (or 1–4 μg IV/SC, or 10–20 μg intranasal depending on protocol). Continue monitoring urine osmolality for 2–4 hours after DDAVP.

MRI of the Sella and Brain (with Gadolinium)

This is the most important aetiological investigation.

For this patient, no longer have pituitary bright spot → consistent with AVP-D ^[14]

For all endocrine investigations, after we realize 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]

For paediatric patients, consider genetic testing early — particularly for X-linked AVPR2 mutations (AVP-R) or autosomal dominant pre-pro-vasopressin mutations (AVP-D). ^[12]

These water deprivation tests are definitely not good for paediatric patients — they subject them to long periods of water deprivation → copeptin is a better test. ^[12]

In one teaching case, a paediatric patient with AVP-R was diagnosed by array CGH showing a homozygous deletion of the AVPR2 gene on the X chromosome, confirming nephrogenic DI without needing a water deprivation test ^[12].

The core of AVP-D treatment is hormone replacement — just like giving levothyroxine for hypothyroidism or hydrocortisone for adrenal insufficiency. Since the patient lacks AVP, you give a synthetic analogue.

AVP-D is often a consequence of another process (tumour, infiltration, surgery). Treating the cause may restore AVP production, or at least prevent progression.

The same lesion that destroys AVP-producing neurons often damages the anterior pituitary too. The order of hormone replacement matters — cortisol first, then thyroid, then DDAVP.

Both under-treatment (hypernatraemia) and over-treatment (hyponatraemia from too much DDAVP + excess water intake) are dangerous. The goal is a careful balance.

DDAVP = 1-Deamino-8-D-Arginine Vasopressin (desmopressin). Breaking down the name:

• "Deamino" = the amino group at position 1 of native AVP has been removed → this makes the molecule resistant to enzymatic degradation (including placental vasopressinase), giving it a much longer half-life (~8–20 hours vs. ~10–20 minutes for native AVP)
• "D-Arginine" = the L-arginine at position 8 has been replaced with the D-isomer → this selectively enhances V2 receptor activity (antidiuretic) while drastically reducing V1a activity (vasopressor) — so DDAVP controls polyuria without causing dangerous vasoconstriction

DDAVP is the mainstay of treatment for AVP-D (Cranial DI). ^[4][5]

Routes of administration: intranasal, PO, sublingual, SC, IV. ^[4][5]

Dosing: adjusted based on clinical symptoms → minimal effective dose to control polyuria. ^[5]

Should prevent nocturia but allow a degree of polyuria from time to time before the next dose. ^[5]

Why this approach? Because the biggest risk of DDAVP therapy is over-treatment causing dilutional hyponatraemia:

• If you give too much DDAVP, the patient retains all their water intake → serum Na falls → water intoxication → seizures, cerebral oedema
• By deliberately allowing mild breakthrough polyuria between doses, you create a "safety valve" — the patient passes some dilute urine, preventing dangerous water retention
• Teach patients: "If your urine is always concentrated and you never have clear urine, your dose may be too high"

Side effects: excessive treatment → water intoxication, hyponatraemia; inadequate treatment → hypernatraemia. ^[5]

Triphasic pattern: transient DI (hours to days) → antidiuresis (2–14 days) → return of DI (may be permanent). ^[4]

Three main categories of hypernatraemia: hypervolaemic (Conn's), isovolaemic or hypovolaemic (fluid loss or DI) — separate based on serum vs. urine osmolality. ^[15]

AVP-D patients are typically euvolaemic or mildly hypovolaemic (free water loss, not saline loss).

Free water deficit = Body weight (kg) × 0.6 × (measured Na − 140) / 140 ^[4]

Example: 60 kg woman with Na 160 mmol/L:

• Free water deficit = 60 × 0.6 × (160 − 140) / 140 = 60 × 0.6 × 20/140 = 5.1 litres
• This is the minimum amount of free water needed to correct the sodium back to 140 — but don't give it all at once!

Rate of correction: < 8–10 mmol/24h to avoid cerebral oedema. ^[4]

Why? In chronic hypernatraemia, the brain has adapted by generating idiogenic osmoles (organic osmolytes like taurine, myo-inositol, betaine) to prevent cell shrinkage. If you correct sodium too rapidly, these osmoles are still present → the brain cells are now hyperosmolar relative to the ECF → water rushes INTO brain cells → cerebral oedema → raised ICP → herniation.

This is the mirror image of the central pontine myelinolysis risk from over-rapid correction of hyponatraemia.

• The most dangerous form — the patient lacks both AVP and thirst.
• Cannot rely on thirst-driven fluid intake to prevent hypernatraemia.
• Management: fixed daily fluid intake prescription (e.g., 1.5–2 L/day, adjusted by weight and Na monitoring) + scheduled DDAVP doses.
• Requires very close monitoring — these patients are at high risk of both severe hypernatraemia (forgot to drink) and hyponatraemia (drank too much with DDAVP on board).

• Placental vasopressinase degrades native AVP → transient DI in pregnancy.
• DDAVP is the treatment of choice — it is resistant to vasopressinase (the deamination protects it).
• Resolves post-partum; dose can be tapered after delivery.

• Children are particularly vulnerable to dehydration and electrolyte disturbances.
• DDAVP oral melt/sublingual preferred over intranasal in young children (more reliable absorption).
• Intranasal formulation associated with increased risk of seizures — avoid in enuresis context ^[11].
• Careful dose titration — children's fluid intake is variable and unpredictable.
• If genetic cause suspected → early genetic testing ^[12].

This is the central, life-threatening complication of AVP-D.

Why does it happen?

• No AVP → no AQP-2 insertion → collecting duct is impermeable to water → copious dilute urine → free water is lost from the body while sodium is relatively preserved → serum sodium rises → hyperosmolality.
• As long as the patient has intact thirst and access to water, they compensate by drinking — so hypernatraemia remains mild or absent. Hypernatraemia is usually high-normal except in impaired thirst mechanism (e.g., unconscious, hypothalamic lesion). ^[2][4]
• It becomes dangerous when the patient cannot drink: post-operative, sedated/intubated, cognitively impaired (dementia), infantile, or in adipsic AVP-D (hypothalamic thirst centre also damaged).

Consequences of hypernatraemia — explained from first principles:

Clinical features of hypernatraemia: headache, irritability, seizures, ICH/SAH (rare). ^[4]

• Severe, prolonged free water loss without adequate replacement → hypovolaemia → tachycardia → hypotension → shock.
• While AVP-D classically causes euvolaemic hypernatraemia (losing pure water, not saline), at extremes the total body water depletion is so great that the patient becomes haemodynamically compromised.
• Particularly dangerous in infants (high BSA-to-volume ratio → rapid dehydration) and in adipsic AVP-D.

Prolonged DI → the urine buildup will dilate the entire pathway. ^[3]

This is an often-overlooked but clinically important complication, especially in patients with untreated or poorly controlled AVP-D for years (e.g., children with undiagnosed hereditary forms). Regular renal ultrasound is warranted.

• In infants and children with undiagnosed AVP-D, chronic dehydration and electrolyte disturbance impairs growth.
• Constant thirst → the child prefers water over calorie-dense food → failure to thrive.
• If the underlying cause (e.g., craniopharyngioma, LCH) also affects GH secretion → compounded growth failure.

• Nocturia (often waking 3–6 times per night to urinate and drink) → chronic sleep deprivation → fatigue, poor concentration, mood disturbance, reduced quality of life.
• Children: nocturnal enuresis → psychosocial distress.
• Adults: impaired work performance, relationship strain.

Side effects: excessive treatment → water intoxication, hyponatraemia; inadequate treatment → hypernatraemia. ^[5]

Why does this happen?

• DDAVP makes the collecting duct permeable to water (inserts AQP-2). If the patient takes more fluid than needed while on DDAVP, the kidneys cannot excrete the excess water → plasma becomes diluted → Na falls.
• This is essentially iatrogenic SIADH — you've created a state of inappropriate antidiuresis.

Clinical features of hyponatraemia (severity-dependent):

If DDAVP-induced hyponatraemia develops and is corrected too aggressively:

If correction occurs too rapidly, may result in a complication called central pontine myelinolysis. ^[15]

Central pontine myelinolysis is very severe → patients become tetraplegic. ^[15]

Mechanism from first principles:

1. In chronic hyponatraemia, brain cells adapt by losing osmolytes → brain cells are now "iso-osmolar" with the diluted ECF.
2. If you rapidly raise serum Na (using hypertonic saline or by suddenly stopping excess water intake), the ECF becomes hyperosmolar relative to the adapted brain cells.
3. Water rushes out of the brain cells → rapid cellular dehydration → myelin sheaths in the pons (and extrapontine areas) are damaged → demyelination.
4. Clinical features: dysarthria, dysphagia, quadriparesis/quadriplegia, "locked-in syndrome" in severe cases.

Prevention:

• Correct chronic hyponatraemia at < 0.5 mmol/L/hour, or < 8–12 mmol/L/day ^[15]
• Hypertonic saline should only be used in very experienced hands ^[15]

Rate of correction for chronic hyponatraemia should be slow — over 2 days; less than 0.5 mmol/L/hour, or < 12 mmol/L/day. ^[15]

If you correct the hypernatraemia of AVP-D too quickly:

• In chronic hypernatraemia, brain cells have generated idiogenic osmoles (taurine, myo-inositol, betaine) to maintain volume.
• If Na is corrected rapidly → ECF becomes hypotonic relative to brain cells → water rushes INTO brain cells → cerebral oedema → raised ICP → herniation.
• Rate of correction for hypernatraemia: < 8–10 mmol/24h to avoid cerebral oedema. ^[4]

The same lesion causing AVP-D often damages the anterior pituitary too:

• Suprasellar lesions (craniopharyngioma, large adenoma, germinoma) compress the optic chiasm → bitemporal hemianopia (loss of temporal visual fields bilaterally).
• The optic chiasm sits immediately superior to the pituitary fossa — suprasellar extension of any mass compresses the crossing nasal retinal fibres.

• Large tumours or hydrocephalus from a third ventricle mass → headache, papilloedema, vomiting, reduced consciousness.

• Tumour recurrence after surgery (craniopharyngioma has a high recurrence rate, ~20–50%).
• LCH can progress to involve other organ systems (bone, skin, liver, lungs).
• Sarcoidosis may progress; TB requires full treatment course.
• Idiopathic AVP-D: serial MRI is recommended — some cases evolve into identifiable pathology (germinoma or autoimmune hypophysitis may initially present with a normal MRI and only become apparent on follow-up imaging).