A 48-year-old man with bipolar I disorder on lithium carbonate 1200 mg daily (serum level 0.9 mEq/L, therapeutic) presents for routine follow-up. He reports a 6-week history of progressive polyuria (4–5 L/day), polydipsia, and mild cognitive slowing. His fasting glucose is 95 mg/dL, serum creatinine 1.1 mg/dL (baseline 0.9 mg/dL), serum sodium 142 mEq/L, and serum osmolality 310 mOsm/kg. Urine osmolality during fluid restriction is 180 mOsm/kg. Which of the following is the most likely diagnosis and the most appropriate next step?

Explanation

## Diagnosis: Lithium-Induced Nephrogenic Diabetes Insipidus (NDI) **Key Point:** This patient has classic nephrogenic diabetes insipidus (NDI) caused by chronic lithium exposure. The diagnostic triad is polyuria (4–5 L/day), polydipsia, and inability to concentrate urine despite adequate ADH (evidenced by low urine osmolality 180 mOsm/kg even during fluid restriction). ## Pathophysiology of Lithium-Induced NDI 1. **Lithium accumulation** in collecting duct principal cells 2. **Inhibition of aquaporin-2 (AQP2) channels** → reduced water reabsorption 3. **Impaired responsiveness to ADH** (nephrogenic, not central) 4. **Chronic changes**: interstitial fibrosis, chronic kidney disease over years ## Diagnostic Criteria Met in This Case | Finding | Patient Value | Interpretation | |---------|---------------|----------------| | **Polyuria** | 4–5 L/day | Exceeds 3 L/day threshold | | **Serum osmolality** | 310 mOsm/kg | Elevated (normal 280–295) | | **Urine osmolality (fluid restricted)** | 180 mOsm/kg | **Inappropriately low** — hallmark of NDI | | **Serum sodium** | 142 mEq/L | Mild hypernatremia | | **Lithium level** | 0.9 mEq/L | Therapeutic but chronic exposure | | **Renal function** | Creatinine 1.1 (↑ from 0.9) | Early decline | **High-Yield:** The low urine osmolality despite high serum osmolality and fluid restriction definitively rules out primary polydipsia (in which urine would concentrate normally during restriction) and central diabetes insipidus (which responds to desmopressin). ## Management Algorithm ```mermaid flowchart TD A[Polyuria + low urine osmolality]:::outcome --> B{Lithium on board?}:::decision B -->|Yes| C[Lithium-induced NDI]:::outcome C --> D{Mood control adequate?}:::decision D -->|Yes, consider switching| E[Discontinue lithium]:::action D -->|Yes, but want to continue| F[Add amiloride 5 mg daily]:::action D -->|No| G[Switch to alternative mood stabilizer]:::action E --> H[Monitor urine output & osmolality]:::action F --> H G --> H ``` ## Why Amiloride Is the Preferred Agent If Lithium Continuation Is Desired **Clinical Pearl:** Amiloride, a potassium-sparing diuretic, blocks lithium entry into collecting duct cells via the epithelial sodium channel (ENaC). This reduces intracellular lithium accumulation and restores aquaporin-2 function, improving urine concentration without requiring lithium discontinuation. - **Dose:** 5–10 mg daily - **Mechanism:** Blocks ENaC → reduces lithium influx → restores water reabsorption - **Efficacy:** Reduces polyuria by 30–50% in ~50% of patients - **Advantage:** Allows continuation of effective mood stabilizer **Warning:** NSAIDs and thiazide diuretics increase lithium levels and worsen NDI; avoid them. ## Why Discontinuation Is the Alternative If mood control is not optimal or amiloride fails: - Discontinue lithium and switch to valproate, lamotrigine, or an atypical antipsychotic - NDI may partially reverse over months, but chronic interstitial fibrosis may persist - Monitor renal function annually even after discontinuation