Background
Pathophysiology
- Failure of antidiuretic hormone (ADH) – either central (ADH deficiency) or nephrogenic (ADH resistance) – leading to polyuria with dilute urine.
- Normal serum Na+ often maintained through ↑fluid intake, and condition may be unmasked when fluids are restricted.
Causes
- Tumours – craniopharyngioma, stalk lesion, metastases – or pituitary tumour surgery.
- Acute trauma, subarachnoid haemorrhage.
- Inflammatory: CNS infection or autoimmune damage, the latter often associated with Hashimoto's thyroiditis or T1 diabetes.
- Infiltration: histiocytosis, sarcoidosis.
- Congenital: Wolfram syndrome (aka DIDMOAD)
- Phenytoin
Nephrogenic DI:
- Altered electrolytes: ↑Ca2+,↓K+.
- Medication: lithium, demeclocycline.
- Congenital: AVP-V2 mutation.
- Secondary to other conditions: sickle cell, sarcoidosis, amyloidosis, myeloma.
Gestational DI:
- Can occur during pregnancy and postpartum period.
Signs and symptoms
- Polyuria and nocturia.
- Polydipsia, which may be severe, and dehydration.
- CNS symptoms of hypernatraemia: often non-specific, including irritability and hypertonia.
Differential diagnosis
Primary polydipsia
- Aka psychogenic polydipsia, dipsogenic DI.
- Polyuria+dipsia without dilute urine.
- ↓Na+
- Sometimes associated with schizophrenia.
Investigations
- ↑Urine output: >3L/day, or in children >100 ml/kg/h.
- ↑/normal plasma osmolality, ↓urine osmolality.
- ↑N+, ↓K+, ↑Ca2+.
Water deprivation test:
- Dehydrate patient for 7 hours. In DI, will lead to ↑plasma osmolality but failure to concentrate urine. In primary polydipsia, urine will concentrate as normal.
- Then give ADH (desmopressin stimulation test). In central DI, the kidney will respond and osmolality will normalize. In nephrogenic DI, it won't.
ADH itself is hard to measure and so is not part of the diagnostic process.
Management
- Desmopressin (synthetic ADH).
Nephrogenic DI:
- Treat cause.
- Low Na+ diet.
- Bendroflumethiazide
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