Diabetes Insipidus
Diabetes insipidus (DI) is a disorder of water balance caused by either a deficiency of antidiuretic hormone (ADH) -- central DI -- or resistance to ADH in the kidneys -- nephrogenic DI. Despite the name, it is unrelated to diabetes mellitus. The cardinal features are polyuria and polydipsia that persist even during water restriction.
Symptoms
Markedly increased urination (3 -- 20 liters per day), intense thirst and craving for cold water, nocturia disrupting sleep, and dehydration if access to water is restricted. Neonates and young children may present with irritability, failure to thrive, and fever.
Causes and risk
Central DI is caused by damage to the hypothalamus or posterior pituitary -- most commonly by surgery, trauma, tumor, or infiltrative disease (sarcoidosis, histiocytosis). Nephrogenic DI is caused by chronic lithium use, hypercalcemia, hypokalemia, certain medications, or inherited mutations in the ADH receptor. Gestational DI occurs transiently in late pregnancy due to placental vasopressinase.
How it is evaluated
Urine osmolality and sodium distinguish DI from primary polydipsia. The water deprivation test with subsequent DDAVP administration differentiates central from nephrogenic DI. Pituitary MRI with gadolinium evaluates structural causes of central DI.
Treatment
Central DI is treated with desmopressin (DDAVP), a synthetic ADH analogue available as nasal spray, oral tablet, or injection. Nephrogenic DI is managed by eliminating the offending cause (e.g., lithium) and using thiazide diuretics plus low-sodium diet. Fluid intake is adjusted to match output.