A 48-year-old woman with bipolar I disorder has been stable on lithium 750 mg daily (serum level 0.9 mEq/L) for 18 months. She presents to psychiatry clinic with complaints of polyuria (3–4 L/day), polydipsia, and a 4 kg weight gain over 3 months. Serum sodium is 138 mEq/L (normal 135–145), fasting glucose 95 mg/dL, TSH 2.1 mIU/L (normal), and urine osmolality is 180 mOsm/kg (low). Water deprivation test shows no rise in urine osmolality. What is the most appropriate next step in management?

Explanation

## Clinical Diagnosis: Lithium-Induced Nephrogenic Diabetes Insipidus (NDI) ### Key Findings **Key Point:** The patient has **lithium-induced nephrogenic diabetes insipidus (NDI)**, confirmed by: - Polyuria (3–4 L/day) and polydipsia - Low urine osmolality (180 mOsm/kg) despite normal serum osmolality and sodium - **Failure to concentrate urine on water deprivation test** — this is the diagnostic hallmark of nephrogenic DI (distinguishes it from central DI, where desmopressin would work) - Therapeutic lithium level (0.9 mEq/L), so toxicity is not the issue ### Pathophysiology of Lithium-Induced NDI Lithium inhibits adenylyl cyclase in the collecting duct, blocking the cAMP cascade required for aquaporin-2 water channel insertion. This causes: 1. Inability to concentrate urine despite ADH 2. Polyuria and compensatory polydipsia 3. Risk of hypernatremia if fluid intake is inadequate 4. Chronic lithium use can cause irreversible collecting duct damage (sclerosis, atrophy) ### Management Strategy **High-Yield:** The goal is to **preserve mood stabilization while minimizing polyuria**. Lithium is highly effective for bipolar disorder; abrupt discontinuation risks relapse. #### Why Amiloride? Amiloride is a **potassium-sparing diuretic** that works by: - Blocking sodium reabsorption in the collecting duct - Reducing the positive charge gradient that normally drives lithium reabsorption - Decreasing lithium entry into collecting duct cells by ~50% - **Reducing urine output by 30–50%** without affecting lithium's mood-stabilizing effect **Clinical Pearl:** Amiloride is the **first-line pharmacological intervention** for lithium-induced NDI because it reduces polyuria while allowing continuation of lithium therapy. #### Supportive Measures - **Adequate hydration**: Counsel patient to drink 2.5–3 L of water daily - **Low-sodium diet**: Reduces proximal tubule sodium reabsorption, which indirectly reduces lithium reabsorption - **NSAIDs** (e.g., indomethacin): Inhibit prostaglandin synthesis, reducing urine output (use cautiously; monitor renal function) - **Thiazide diuretics**: Counterintuitively reduce polyuria in NDI by causing mild volume depletion → increased proximal tubule reabsorption of both sodium and water ### Comparison of Management Options | Intervention | Mechanism | Efficacy | Indication | |---|---|---|---| | **Amiloride** | ↓ Lithium entry into collecting duct | 30–50% reduction in urine output | First-line for lithium-induced NDI | | **Thiazide diuretic** | Mild volume depletion → ↑ proximal reabsorption | 30–40% reduction | Alternative if amiloride not tolerated | | **NSAID** (indomethacin) | ↓ Prostaglandin synthesis | 20–30% reduction | Adjunctive; monitor renal function | | **Switch mood stabilizer** | Remove lithium | Eliminates NDI | Only if lithium-induced NDI severe or progressive | | **Discontinue lithium** | Remove lithium | Eliminates NDI | Last resort; high relapse risk | **Mnemonic:** **SALT** for lithium-induced NDI management: - **S**witch to amiloride (or thiazide) - **A**dequate hydration - **L**ow-sodium diet - **T**herapy continuation (keep lithium if effective) ### Decision Algorithm ```mermaid flowchart TD A[Lithium-Induced NDI Confirmed]:::outcome --> B{Severity & Impact?}:::decision B -->|Mild, tolerable polyuria| C[Hydration + Low Na diet]:::action B -->|Moderate, affecting QoL| D[Add Amiloride 5 mg daily]:::action B -->|Severe or Progressive| E{Lithium Essential?}:::decision E -->|Yes, highly effective| F[Amiloride + NSAID + Thiazide]:::action E -->|No, alternatives available| G[Switch to Valproate/Lamotrigine]:::action C --> H[Monitor Urine Output<br/>& Serum Na]:::action D --> H F --> H G --> I[Reassess Mood Stability]:::outcome ``` **High-Yield:** This patient is **clinically stable on lithium** (level 0.9, mood controlled for 18 months). Amiloride preserves this benefit while addressing the NDI complication. Switching or stopping lithium is premature and risks relapse. [cite:Kaplan & Sadock's Synopsis of Psychiatry 12e Ch 31; Harrison 21e Ch 397]