Diabetes Insipidus
Definition | Aetiology | Pathophysiology | Risk Factors | Signs and Symptoms | Investigations | Management
Definition
Diabetes insipidus is a disorder of water balance caused by impaired secretion or response to antidiuretic hormone (ADH), leading to excessive urinary output and dehydration.
Aetiology
- Central diabetes insipidus: due to inadequate ADH production from the hypothalamus or pituitary (e.g., traumatic brain injury, pituitary tumours, neurosurgery, meningitis).
- Nephrogenic diabetes insipidus: renal resistance to ADH (e.g., chronic kidney disease, lithium toxicity, hypercalcaemia, hypokalaemia).
- Gestational diabetes insipidus: increased metabolism of ADH by placental enzymes.
- Primary polydipsia: excessive fluid intake leading to suppression of ADH secretion.
Pathophysiology
- Inadequate ADH secretion (central DI) or renal resistance to ADH (nephrogenic DI) leads to reduced water reabsorption in the collecting ducts.
- This results in excessive urine output (polyuria), dehydration, and compensatory thirst (polydipsia).
- In chronic cases, hypernatraemia may develop due to excessive free water loss.
Risk Factors
- Head trauma or neurosurgery.
- Brain tumours affecting the hypothalamus or pituitary.
- Chronic lithium therapy.
- Electrolyte imbalances (hypercalcaemia, hypokalaemia).
- Pregnancy (gestational DI).
Signs and Symptoms
- Polyuria: excessive urine output (>3L/day).
- Polydipsia: excessive thirst and fluid intake.
- Nocturia: frequent urination at night.
- Dehydration: dry mucous membranes, hypotension, tachycardia.
- Hypernatraemia: in severe or untreated cases.
Investigations
- Serum and urine osmolality: low urine osmolality (<300 mOsm/kg) with high serum osmolality (>295 mOsm/kg) suggests DI.
- Water deprivation test: failure to concentrate urine suggests DI; response to desmopressin differentiates central from nephrogenic DI.
- Serum sodium: may be elevated in dehydration.
- Magnesium, calcium, and potassium: assess for electrolyte imbalances contributing to nephrogenic DI.
- Pituitary MRI: indicated if central DI is suspected.
Management
1. Central Diabetes Insipidus:
- Desmopressin (DDAVP): synthetic ADH analogue to reduce polyuria and polydipsia.
- Fluid management: maintain adequate hydration.
- Identify and treat underlying cause: e.g., surgical intervention for pituitary tumours.
2. Nephrogenic Diabetes Insipidus:
- Thiazide diuretics: paradoxically reduce urine output by inducing mild volume depletion.
- NSAIDs (e.g., indomethacin): enhance renal response to ADH.
- Correct underlying cause: discontinue lithium, correct hypercalcaemia or hypokalaemia.
3. General Measures:
- Encourage appropriate fluid intake to prevent dehydration.
- Monitor serum sodium and osmolality regularly.
4. Referral:
- Endocrinology: for specialist input in persistent or complex cases.
- Nephrology: if nephrogenic DI is resistant to treatment.
- Neurosurgery: if DI is secondary to pituitary or hypothalamic pathology.