The inability of the kidneys to concentrate urine due to deficiency of antidiuretic hormone (ADH/vasopressin) or renal resistance to ADH.
Types
- Central (Neurogenic) DI
- Caused by insufficient production or secretion of ADH from the hypothalamus/posterior pituitary.
- Etiology:
- Head trauma, brain tumors, neurosurgery
- Infections (meningitis, encephalitis)
- Autoimmune destruction
- Idiopathic (most common)
- Nephrogenic DI
- Kidneys fail to respond to ADH.
- Causes:
- Genetic (mutations in V2 receptor or aquaporin)
- Medications (Lithium, demeclocycline)
- Chronic kidney disease
- Electrolyte disturbances (hypercalcemia, hypokalemia)
- Gestational DI
- Occurs during pregnancy due to placental vasopressinase breaking down ADH.
- Primary Polydipsia/Psychogenic Polydipsia (Dipsogenic DI-like)
- Excessive water intake suppresses ADH.
- Seen in psychiatric disorders.
Clinical Presentation
- Polydipsia (intense thirst, prefers cold water)
- Signs of dehydration:
- Dry mucous membranes
- Hypotension, tachycardia
- Hypernatremia (if intake < urine loss)
Investigations
Water deprivation test
- After obtaining baseline lab values, patients stop drinking water for 2–3 hours before the first measurement
- Check serum and urine osmolarities hourly for 8hr, then 4hr after giving DDAVP
- If Ur/serum osmol ratio >2 it is normal, otherwise DI
- If Ur/serum osmol ratio is low and improves after DDAVP then it is due to cranial DI (not nephrogenic)
Imaging
- If CDI is diagnosed, a CT scan or MRI of the head should be conducted to rule out brain tumours (especially craniopharyngioma)
Management
Neurogenic
- Desmospray (nasally) or desmopressin oral tablets
- Desmopressin IM injection - generally reserved for emergency or post pituitary surgery
Nephrogenic
- No current pharmacological treatment
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