SN1
SHORT NOTE
5M
Define Diabetes Insipidus. Describe its causes, clinical features, investigations and diagnosis.
Definition
Diabetes insipidus (DI) is the passage of large volumes of dilute urine due to either deficiency of antidiuretic hormone (ADH/vasopressin) — central DI — or renal resistance to ADH — nephrogenic DI.
Types & Causes
| Type | Mechanism | Causes |
|---|---|---|
| Central | ↓ ADH secretion | Tumours, trauma, surgery, infiltration, idiopathic |
| Nephrogenic | Renal resistance to ADH | X-linked/genetic, hypokalaemia, hypercalcaemia, drugs, obstruction |
Clinical Features
- Polyuria and polydipsia with a craving for water; nocturia/enuresis.
- Risk of hypernatraemic dehydration if fluid access is limited (especially in infants).
Investigations & Management
- Dilute urine (low osmolality) despite raised/normal serum osmolality; water deprivation test with a desmopressin response distinguishes central from nephrogenic.
- Central DI — desmopressin (DDAVP).
- Nephrogenic DI — treat the cause, ensure water access, low-solute diet, and thiazide diuretics (± amiloride/indomethacin).
Differential Diagnosis
- Primary polydipsia (psychogenic water drinking) — dilute urine but low/normal serum sodium.
- Osmotic diuresis (diabetes mellitus), hypercalcaemia, hypokalaemia.
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