A 42-year-old man with bipolar II disorder has been on lithium 750 mg daily for 18 months with good mood stabilization. He presents for routine follow-up. Laboratory investigations show: serum lithium 0.9 mEq/L (therapeutic), serum creatinine 1.4 mg/dL (baseline 0.9 mg/dL), eGFR 52 mL/min/1.73m² (baseline 85), and fasting glucose 128 mg/dL (baseline 95 mg/dL). Urinalysis shows specific gravity 1.008 (low). He reports polyuria and polydipsia for the past 3 months. What is the most likely diagnosis, and what is the next step in management?

Explanation

## Lithium-Induced Nephrogenic Diabetes Insipidus (NDI) ### Clinical Diagnosis The patient presents with the classic triad of **lithium-induced nephrogenic diabetes insipidus**: 1. **Polyuria and polydipsia** (3 months duration) 2. **Low urine specific gravity** (1.008 — normal >1.010) 3. **Chronic lithium exposure** (18 months at therapeutic levels) **Key Point:** Lithium-induced NDI is the most common renal complication of chronic lithium therapy, occurring in 20–40% of patients on long-term lithium. ### Mechanism of Lithium-Induced NDI ```mermaid flowchart TD A[Lithium enters collecting duct cells]:::action --> B[Inhibits adenylyl cyclase via G-protein coupling]:::action B --> C[↓ cAMP production]:::action C --> D[↓ Aquaporin-2 water channel insertion]:::action D --> E[Impaired water reabsorption in collecting duct]:::action E --> F[Polyuria + Polydipsia]:::outcome F --> G[Low urine osmolality & specific gravity]:::outcome A --> H[Chronic lithium: chronic interstitial fibrosis]:::action H --> I[Progressive renal insufficiency]:::outcome ``` ### Why This Patient Has Chronic Kidney Disease | Finding | Interpretation | |---------|----------------| | Serum creatinine ↑ (0.9 → 1.4 mg/dL) | Progressive renal function decline | | eGFR ↓ (85 → 52 mL/min/1.73m²) | Stage 3b CKD — likely lithium-induced chronic interstitial nephritis | | Serum lithium 0.9 mEq/L (therapeutic) | Therapeutic level, but cumulative renal toxicity has occurred | | Urine specific gravity 1.008 (low) | **Pathognomonic for NDI** | **High-Yield:** Lithium causes **two distinct renal syndromes**: - **Acute:** reversible if lithium is stopped early - **Chronic:** irreversible interstitial fibrosis and CKD (this patient) ### Diagnostic Approach: Water Deprivation Test ```mermaid flowchart TD A[Suspected NDI]:::outcome --> B[Baseline: urine osmolality, serum osmolality]:::action B --> C[Fluid restriction 8-12 hours]:::action C --> D{Urine osmolality increases?}:::decision D -->|Yes, >600 mOsm/kg| E[Central DI]:::outcome D -->|No, <300 mOsm/kg| F[Nephrogenic DI]:::outcome F --> G[Administer desmopressin]:::action G --> H{Urine osmolality increases?}:::decision H -->|Yes| I[Desmopressin-responsive NDI]:::outcome H -->|No| J[Lithium-induced NDI - desmopressin resistant]:::outcome ``` ### Management of Lithium-Induced NDI **Option 1: Lithium Discontinuation** (if mood stability permits) - Most definitive - Polyuria resolves in weeks to months - CKD may stabilize but is often irreversible **Option 2: Amiloride (Preferred if lithium must continue)** - **Mechanism:** ENaC blocker in collecting duct → reduces lithium entry into cells - **Dose:** 5–10 mg daily - **Efficacy:** Reduces polyuria by 50–70% in 1–2 weeks - **Advantage:** Allows continuation of lithium for mood stabilization - **Monitoring:** Check serum potassium (risk of hyperkalemia) **Option 3: NSAIDs (e.g., indomethacin)** - Inhibit prostaglandin synthesis → reduce urine flow - Less effective than amiloride - Risk of worsening renal function (avoid in this patient with CKD) **Option 4: Thiazide Diuretics** (paradoxically effective) - Cause mild volume depletion → enhanced proximal reabsorption - Reduces distal delivery of fluid - Less commonly used now **Clinical Pearl:** Amiloride is the **first-line pharmacological agent** if lithium continuation is necessary, as it specifically blocks lithium entry into collecting duct cells. ### Why the Glucose is Elevated **Incidental finding:** Lithium impairs glucose tolerance and increases diabetes risk (mechanism: ↓ insulin secretion, ↑ insulin resistance). This patient should be screened for diabetes. ### Summary: Next Steps for This Patient 1. **Confirm NDI:** Water deprivation test (urine osmolality <300 mOsm/kg, no response to desmopressin) 2. **Assess mood stability:** Can lithium be discontinued? - If yes → D/C lithium; polyuria resolves in weeks - If no → Start amiloride 5–10 mg daily 3. **Monitor renal function:** eGFR, serum creatinine every 3–6 months 4. **Counsel on hydration:** Ensure adequate free water intake to prevent dehydration 5. **Screen for diabetes:** Fasting glucose, HbA1c **Warning:** Do NOT start levothyroxine for hypothyroidism (option C) without TSH confirmation. Lithium-induced hypothyroidism is common but is a separate issue from NDI and requires thyroid function tests.