A 48-year-old woman with bipolar I disorder has been stable on lithium 750 mg daily for 3 years. She now presents for routine follow-up. Her serum lithium level is 0.8 mEq/L (therapeutic range 0.6–1.2 mEq/L), and she has normal renal and thyroid function. She reports new-onset polyuria and polydipsia. What is the drug of choice to manage her lithium-induced nephrogenic diabetes insipidus?

## Lithium-Induced Nephrogenic Diabetes Insipidus (NDI) ### Pathophysiology **Key Point:** Lithium causes NDI through: 1. **Collecting duct damage:** Lithium inhibits adenylyl cyclase → reduced cAMP → impaired aquaporin-2 (AQP2) water channel expression 2. **Chronic tubulointerstitial injury:** Leads to irreversible structural changes in the collecting duct 3. **Incidence:** Occurs in 20–40% of patients on chronic lithium therapy **Clinical Pearl:** Lithium-induced NDI is often **partially reversible** if caught early, but becomes **irreversible** with prolonged exposure. ### Management Strategy for Lithium-Induced NDI ```mermaid flowchart TD A[Lithium-Induced NDI Diagnosed]:::outcome --> B{Can lithium be discontinued?}:::decision B -->|Yes| C[Discontinue lithium]:::action B -->|No - Essential for mood control| D[Continue lithium]:::action C --> E[Monitor urine output & serum Na]:::action D --> F{Choose agent}:::decision F -->|First-line: Amiloride| G[Potassium-sparing diuretic]:::action F -->|Alternative: HCTZ| H[Thiazide diuretic]:::action G --> I[Blocks lithium entry into collecting duct cells]:::outcome H --> J[Mild volume depletion enhances proximal reabsorption]:::outcome ``` ### Why Amiloride Is the Drug of Choice **High-Yield:** Amiloride (potassium-sparing diuretic) is preferred because: | Feature | Amiloride | Hydrochlorothiazide | Desmopressin | Furosemide | | --- | --- | --- | --- | --- | | **Mechanism** | Blocks ENaC in collecting duct → prevents lithium entry into cells | Mild volume depletion → enhanced proximal reabsorption | Exogenous ADH replacement | Loop diuretic | | **Efficacy in NDI** | Highly effective (60–80% response) | Effective (paradoxical effect) | Ineffective (ADH-resistant) | Worsens polyuria | | **Lithium levels** | May increase (monitor) | May increase (monitor) | No effect | No effect | | **Potassium** | Increases (risk of hyperkalemia) | Decreases (risk of hypokalemia) | No effect | Decreases | | **First-line status** | **YES** | Alternative | No | No | **Key Point:** Amiloride works by blocking the epithelial sodium channel (ENaC) in the collecting duct principal cells. Since lithium enters cells via the same channel as sodium, blocking ENaC prevents lithium accumulation in the collecting duct, thereby reducing lithium-induced NDI. ### Dosing & Monitoring **Clinical Pearl:** - **Amiloride:** 5–10 mg daily (usual dose 5 mg) - **Onset:** 3–5 days for symptom improvement - **Monitoring:** Serum potassium, creatinine, and lithium levels (amiloride may increase lithium levels by 20–30%) - **Contraindications:** Baseline hyperkalemia, renal impairment (eGFR <30), concurrent ACE inhibitor/ARB use ### Why NOT the Other Options **Desmopressin:** - Lithium-induced NDI is **ADH-resistant** — the collecting duct cannot respond to ADH (endogenous or exogenous) because of structural damage - Desmopressin is ineffective and may cause hyponatremia if used **Furosemide:** - Loop diuretics **worsen polyuria** and increase urine output - Causes volume depletion and hypokalemia - No role in NDI management **Hydrochlorothiazide:** - While thiazides can reduce urine output in NDI (paradoxical effect via mild volume depletion), amiloride is superior because it directly blocks lithium entry - Thiazides cause hypokalemia and may increase lithium levels unpredictably - Reserved as second-line if amiloride is contraindicated ### Clinical Approach 1. **Confirm diagnosis:** Urine osmolality <300 mOsm/kg despite serum osmolality >295 mOsm/kg and elevated AVP 2. **Assess reversibility:** Check if lithium can be discontinued (best option) 3. **If lithium must continue:** Start amiloride 5 mg daily 4. **Monitor:** K^+^, Cr, lithium level at baseline, 1 week, then q3 months 5. **Educate:** Ensure adequate hydration and sodium intake **Warning:** Do NOT use desmopressin alone in lithium-induced NDI — it is ineffective and delays appropriate therapy. [cite:KD Tripathi 8e Ch 12; Harrison 21e Ch 397]