Diabetes insipidus - cranial
Description
- Deficient hypothalamic AVP (ADH) secretion -> failure to concentrate urine
- Polyuria (>3 L/day, or >50 mL/kg/day), nocturia, polydipsia, dilute urine
Renamed
| Old | Current |
|---|---|
| Cranial/central diabetes insipidus | Arginine vasopressin deficiency (AVP-D) |
| Nephrogenic diabetes insipidus | Arginine vasopressin resistance (AVP-R) |
- Renamed in 2022 to stop lethal confusion with diabetes mellitus - patients nil-by-mouth or on insulin protocols have died from withheld desmopressin
Degree
- Complete - AVP essentially absent, urine osmolality <300
- Partial - some residual AVP, intermediate concentrating ability
The three-way problem
- Every polyuria-polydipsia workup is AVP-D vs AVP-R vs primary polydipsia - and the last is the commonest and most often misdiagnosed
Epidemiology
- ~1 in 25,000; no sex predilection
- Post-neurosurgical is the commonest cause in practice
- Transient AVP-D after transsphenoidal surgery ~20%; permanent 2-7%
- Familial forms present in the first years of life
Aetiopathogenesis
Any lesion must destroy >80-90% of magnocellular neurons before polyuria appears - hence the long latency
Acquired
- Neurosurgical / traumatic - transsphenoidal or transcranial surgery, head injury
- Stalk section higher up = more permanent, because it kills more of the neuronal cell body
- Neoplastic - craniopharyngioma, germinoma (classic in an adolescent with AVP-D + visual field loss), pituitary macroadenoma with suprasellar extension, metastases (breast, lung), lymphoma
- AVP-D from a pituitary adenoma itself is rare - suspect an alternative pathology
- Infiltrative/inflammatory - neurosarcoidosis, Langerhans cell histiocytosis, IgG4, granulomatosis with polyangiitis, TB, lymphocytic infundibuloneurohypophysitis (incl. checkpoint-inhibitor hypophysitis)
- Vascular - Sheehan syndrome, aneurysm, infarction, sickle cell
- Autoimmune anti-vasopressin-neuron antibodies; Guillain-Barre
Congenital/genetic
- Autosomal dominant AVP-neurophysin II (AVP) mutation - misfolded precursor, progressive neuronal death, onset in childhood after a normal infancy
- Wolfram syndrome (DIDMOAD) - WFS1, autosomal recessive: Diabetes Insipidus, Diabetes Mellitus, Optic Atrophy, Deafness
- Septo-optic dysplasia, holoprosencephaly, midline defects
Diagnosis
Step 1 - confirm true hypotonic polyuria
- 24h urine volume >50 mL/kg with urine osmolality <300 mOsm/kg
- Exclude osmotic diuresis: glucose (check BSL), urea, mannitol, high-solute feeds
- Paired serum Na, osmolality, glucose, K, Ca, urea/creatinine
Step 2 - the giveaways before any test
- Serum Na >145 with dilute urine -> AVP-D or AVP-R (a thirsty, drinking patient with primary polydipsia sits low-normal or hyponatraemic)
- Na <135, or baseline copeptin >21.4 pmol/L -> not AVP-D
- Abrupt onset, marked nocturia, preference for iced water, pituitary surgery or anterior hypopituitarism -> AVP-D
- Childhood onset with lifelong daytime-only drinking -> primary polydipsia
Step 3 - stimulation testing
### Copeptin - now the preferred approach
- Copeptin = C-terminal fragment of the AVP precursor, co-secreted equimolar with AVP but stable and easily assayed
- Baseline copeptin >21.4 pmol/L (no fluid restriction) = AVP-R - no further testing
- Hypertonic saline-stimulated copeptin (3% saline to Na >=150):
- >=4.9 pmol/L = primary polydipsia; <4.9 pmol/L = AVP-D
- Diagnostic accuracy ~96%
- Requires close Na monitoring - the reason it is done in a specialist unit
- Arginine-stimulated copeptin (cutoff ~3.8 pmol/L) is safer but significantly less accurate in head-to-head comparison
### Water deprivation test - the fallback
- Still widely used; poor at separating partial AVP-D from primary polydipsia (accuracy ~70%)
- Fluid restriction with 2-hourly weight, obs and paired osmolalities
- Stop for: 3% weight loss, orthostatic symptoms, or Na >=150
- Then give desmopressin:
| Response to desmopressin | Diagnosis |
|---|---|
| Urine osmolality more than doubles (rise >100%) | Complete AVP-D |
| Intermediate rise (~10-50%) | Partial AVP-D or primary polydipsia |
| No rise (<10%) | AVP-R (kidney cannot respond) |
| Concentrates to >700-800 without desmopressin | Normal / primary polydipsia |
- Chronic polyuria of any cause washes out the medullary concentration gradient -> blunted maximal concentration and a falsely 'partial' picture
Step 4 - find the cause
- MRI pituitary/hypothalamus with contrast in every case of confirmed AVP-D
- Loss of the posterior pituitary bright spot (non-specific - also lost with age and in AVP-R)
- Stalk thickening -> infiltrative disease; repeat imaging if initially normal - germinoma may only declare itself later
- Anterior pituitary axes: 9am cortisol, TSH/fT4, IGF-1, gonadotrophins, prolactin
- Beta-hCG and AFP (serum +/- CSF) if stalk thickening in a young patient
- ACE, calcium, CXR/CT chest if sarcoid suspected
Management
Acute / inpatient
- *Never restrict fluid in an unconscious or nil-by-mouth patient with AVP-D*
- Match urine output with hypotonic fluid; strict fluid balance, daily to twice-daily sodium
- Correct hypernatraemia slowly - max 10-12 mmol/L per 24 h (cerebral oedema)
- If ACTH deficiency coexists, give hydrocortisone first - cortisol deficiency masks AVP-D by impairing free water excretion, and replacing it unmasks florid polyuria
Chronic - desmopressin
- Selective V2 agonist; no V1 pressor effect; longer acting than AVP
| Route | Notes |
|---|---|
| Oral tablet / sublingual wafer | Usual choice; typically 2-3 times daily |
| Intranasal spray | More potent, absorption erratic with rhinitis |
| SC/IV | Inpatient use; ~10x more potent than intranasal |
- Titrate to symptoms, not to abolishing all urine output
- *Build in a regular break* (delay a dose until thirst and dilute urine return) - this is what prevents hyponatraemia
- Hyponatraemia is the main hazard - check Na after initiation and dose change, then periodically
- Free access to water; caution with thiazides, SSRIs, carbamazepine, NSAIDs
Special situations
- Adipsic AVP-D (hypothalamic thirst centre also destroyed) - the dangerous variant: fixed desmopressin dose + prescribed daily fluid volume + daily weights
- Triple-phase response after pituitary surgery: polyuria (days 0-5) -> SIADH-like antidiuresis (days 5-10) -> permanent AVP-D
- The middle phase is when desmopressin continued blindly causes severe hyponatraemia
- Pregnancy - placental vasopressinase degrades AVP but not desmopressin; gestational AVP-D may need desmopressin, and pre-existing disease usually needs a higher dose
- Treat the underlying lesion (radiotherapy/chemotherapy for germinoma; steroids for sarcoid/hypophysitis) - AVP-D itself rarely recovers
Associations
- Wolfram syndrome (DIDMOAD) - diabetes insipidus + diabetes mellitus + optic atrophy + deafness
- Anterior hypopituitarism (look for it in every case)
- Germinoma, craniopharyngioma, Langerhans cell histiocytosis
- Neurosarcoidosis, IgG4-related hypophysitis
- Checkpoint inhibitor hypophysitis
- Sheehan syndrome, traumatic brain injury
- Breast and lung metastases to the pituitary stalk
Natural history & complications
- Post-surgical AVP-D is transient in most; permanent in 2-7%
- Idiopathic AVP-D in a child or young adult: up to 30-50% eventually declare an underlying lesion - repeat MRI at 6-12 months and continue surveillance
- With intact thirst and free water access, life expectancy is normal
- Deaths occur from denied water access or withheld desmopressin - unconscious, perioperative, restrained, or nursing-home patients
- Chronic complications: hydronephrosis and bladder dysfunction from very high urine volumes; hyponatraemia from overtreatment
- Flag prominently in the medical record and on discharge summaries - this is a medication-safety diagnosis
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