A 28-year-old man with bipolar I disorder started on lithium 600 mg daily 1 week ago is brought to the clinic by his wife with complaints of polyuria (8–10 L/day), polydipsia, and nocturia ×5. Serum lithium level is 0.8 mEq/L (therapeutic). Serum sodium is 145 mEq/L, osmolality is 310 mOsm/kg, and urine osmolality is 200 mOsm/kg despite high serum osmolality. What is the most likely diagnosis and the best management?

Explanation

## Clinical Diagnosis: Lithium-Induced Nephrogenic Diabetes Insipidus (NDI) ### Clinical Presentation Analysis **Key Point:** The patient has: - **Polyuria** (8–10 L/day) + **polydipsia** + **nocturia** (classic DI triad) - **Hypernatremia** (145 mEq/L, normal 135–145) and **elevated serum osmolality** (310 mOsm/kg) - **Inappropriately dilute urine** (osmolality 200 mOsm/kg) despite high serum osmolality - **Therapeutic lithium level** (0.8 mEq/L) — toxicity is NOT the cause - **Recent lithium initiation** (1 week ago) — timing fits This diagnostic pattern is pathognomonic for **nephrogenic diabetes insipidus (NDI)**. ### Differential Diagnosis: DI vs. Psychogenic Polydipsia | Feature | Central DI | Nephrogenic DI | Psychogenic Polydipsia | |---|---|---|---| | **Serum osmolality** | ↑ (>295) | ↑ (>295) | Normal or ↓ | | **Urine osmolality** | ↓ (<200) | ↓ (<200) | ↓ (<200) | | **Response to desmopressin** | Urine osmolality ↑ | No response | No response | | **Cause** | Vasopressin deficiency | Kidney resistance to vasopressin | Excessive water intake | | **Lithium role** | None | **Direct cause** | Psychiatric disorder | **High-Yield:** In this patient, serum Na^+^ is NORMAL-HIGH (145), not low. Psychogenic polydipsia causes **hyponatremia** from excessive water intake. This rules out psychogenic polydipsia. ### Mechanism of Lithium-Induced NDI ```mermaid flowchart TD A["Lithium enters collecting duct cells<br/>via ENaC channel"]:::outcome A --> B["Inhibits adenylyl cyclase<br/>& depletes cAMP"]:::outcome B --> C["Aquaporin-2 channels NOT inserted<br/>into apical membrane"]:::outcome C --> D["Collecting duct becomes<br/>impermeable to water"]:::outcome D --> E["Nephrogenic DI:<br/>Polyuria + Polydipsia"]:::urgent ``` **Clinical Pearl:** Lithium-induced NDI occurs in 20–40% of patients on chronic lithium therapy. It can develop within days of initiation (as in this case) or after years. The mechanism is direct damage to the collecting duct's ability to respond to vasopressin (ADH). ### Management of Lithium-Induced NDI **Key Point:** The standard treatment is a **thiazide diuretic** (e.g., hydrochlorothiazide) or **amiloride** (a potassium-sparing diuretic). **Why amiloride is preferred:** - **Amiloride blocks ENaC** (epithelial sodium channel) in the collecting duct → blocks lithium entry into collecting duct cells - Prevents lithium accumulation and preserves the kidney's concentrating ability - Does NOT cause hypokalemia (unlike thiazides) - Dose: 5–10 mg daily **Why thiazides work (alternative):** - Cause mild volume depletion → ↑ proximal tubular reabsorption of sodium and water - Paradoxically reduces urine output in NDI (despite being diuretics) - Dose: Hydrochlorothiazide 25 mg daily **Lithium continuation:** Lithium is NOT discontinued if the patient is stable on it for mood control and NDI is manageable. The goal is to treat NDI while maintaining mood stabilization. ### Why NOT the Other Options? **Central DI + desmopressin:** Central DI is caused by vasopressin deficiency. The kidney CAN respond to exogenous desmopressin. In this patient, the kidney is RESISTANT to vasopressin (the collecting duct is impermeable to water regardless of ADH). Desmopressin would be ineffective and could worsen hyponatremia if given with free water intake. **Psychogenic polydipsia + fluid restriction:** Psychogenic polydipsia causes hyponatremia (low Na^+^) from excessive water intake, not hypernatremia. This patient has Na^+^ 145, ruling out psychogenic polydipsia. Fluid restriction would not address the underlying nephrogenic defect. **Acute kidney injury + discontinue lithium:** While lithium can cause chronic kidney disease, acute kidney injury is NOT the diagnosis here. Serum creatinine is not mentioned as elevated. The clinical picture (polyuria, high urine output) is inconsistent with AKI. Discontinuing lithium is premature without first attempting NDI management. **High-Yield:** Amiloride is the FIRST-LINE agent for lithium-induced NDI because it blocks lithium entry into collecting duct cells at the source, preventing further accumulation.