A 48-year-old woman with bipolar I disorder is on lithium monotherapy. Her serum lithium level is 1.1 mEq/L (therapeutic range 0.6–1.2 mEq/L), and she has normal renal function. She complains of polyuria, polydipsia, and nocturia. Urine osmolality is 150 mOsm/kg (normal >800 mOsm/kg). What is the drug of choice to manage her lithium-induced nephrogenic diabetes insipidus?

## Lithium-Induced Nephrogenic Diabetes Insipidus (NDI) **Key Point:** Amiloride (a potassium-sparing diuretic) is the drug of choice for lithium-induced NDI because it blocks lithium entry into collecting duct cells via ENaC (epithelial sodium channel), preventing intracellular lithium accumulation and restoring aquaporin-2 expression. ### Pathophysiology of Lithium-Induced NDI 1. **Lithium entry:** Enters collecting duct cells via ENaC (uses Na^+^ gradient) 2. **Intracellular accumulation:** Inhibits phosphoinositol pathway → depletes IP~3~-mediated signaling 3. **Result:** Downregulation of aquaporin-2 water channels → inability to concentrate urine 4. **Incidence:** 20–40% of patients on chronic lithium; usually reversible if caught early ### Mechanism of Amiloride ```mermaid flowchart TD A[Lithium in serum]:::outcome --> B[ENaC in collecting duct]:::outcome B --> C[Lithium enters cell via ENaC]:::outcome C --> D[Intracellular Li+ accumulation]:::urgent D --> E[Phosphoinositol depletion]:::urgent E --> F[Aquaporin-2 downregulation]:::urgent F --> G[Polyuria & NDI]:::urgent H[AMILORIDE blocks ENaC]:::action --> I[Prevents Li+ entry]:::action I --> J[Restores aquaporin-2]:::action J --> K[Urine osmolality normalizes]:::outcome ``` ### Comparison of Diuretics in Lithium-Induced NDI | Drug | Mechanism | Effect on Lithium | Use in NDI | Rationale | |------|-----------|-------------------|------------|----------| | **Amiloride** | Blocks ENaC | Reduces entry into cells | ✓ **FIRST-LINE** | Prevents intracellular Li+ accumulation; restores aquaporin-2 | | Hydrochlorothiazide | Thiazide diuretic | **INCREASES** serum Li+ | ✗ **CONTRAINDICATED** | Reduces renal clearance; increases toxicity risk | | Furosemide | Loop diuretic | Minimal effect | ✗ Ineffective | Does not address mechanism; may worsen dehydration | | Desmopressin (DDAVP) | ADH analog | No direct effect | ✗ Ineffective | Does not work in NDI because kidneys are unresponsive to ADH | **High-Yield:** Thiazide diuretics are **contraindicated** in lithium-treated patients because they reduce lithium clearance and increase serum levels, risking toxicity. ### Clinical Management of Lithium-Induced NDI 1. **First-line:** Amiloride 5–10 mg/day - Effective in 50–70% of patients - Onset: 3–7 days - Monitor K^+^ (may rise due to potassium-sparing effect) 2. **Adjunctive measures:** - Adequate hydration (low-sodium diet to reduce lithium reabsorption) - NSAIDs (indomethacin 25–50 mg/day) — inhibit prostaglandin-mediated aquaporin-2 expression; second-line - Consider lithium discontinuation if NDI is severe and symptomatic 3. **Monitoring:** - Serum creatinine, K^+^, Na^+^ at baseline and 2 weeks - Urine osmolality to assess response - Lithium levels (unchanged by amiloride) **Clinical Pearl:** Lithium-induced NDI is one of the most common chronic side effects of lithium therapy. Early recognition and amiloride therapy can prevent progression to chronic renal insufficiency. **Warning:** Do NOT prescribe thiazide diuretics to lithium-treated patients — this is a common exam trap and a dangerous prescribing error.