A 46-year-old woman on lithium carbonate 900 mg/day for bipolar I disorder reports polyuria and polydipsia for several months. Serum lithium is 0.8 mEq/L. Urine osmolality is 110 mOsm/kg after overnight water deprivation, and it does not rise after desmopressin. The most appropriate next step is:
- A Start amiloride ✓
- B Start hydrochlorothiazide
- C Start desmopressin nasal spray
- D Stop lithium immediately and initiate valproate the same day
Explanation
Chronic lithium causes nephrogenic diabetes insipidus by downregulating aquaporin-2 expression in the collecting duct. The dilute urine unresponsive to desmopressin confirms the nephrogenic type. Amiloride blocks the epithelial sodium channel in the collecting duct, blunting lithium entry into principal cells and partially restoring concentrating ability, so it is the drug of choice. Desmopressin is ineffective in nephrogenic disease, thiazides paradoxically reduce urine volume but worsen lithium retention through volume contraction, and abrupt discontinuation risks manic relapse.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.