A 42-year-old woman with treatment-resistant schizophrenia on clozapine 400 mg/day for 8 months presents with fever (38.5°C), severe muscle rigidity, altered mental status, and a serum creatinine of 2.1 mg/dL (baseline 0.9). Her CPK is 2800 U/L. Which investigation is most appropriate to confirm the suspected diagnosis of neuroleptic malignant syndrome (NMS)?

## Diagnostic Investigation of Neuroleptic Malignant Syndrome (NMS) **Key Point:** Serum CPK level and urinalysis for myoglobin are the investigations of choice for confirming NMS. CPK elevation (often > 1000 U/L, sometimes > 10,000 U/L) reflects severe rhabdomyolysis, and myoglobinuria indicates acute kidney injury risk. ### Clinical Triad of NMS ```mermaid flowchart TD A[Antipsychotic exposure]:::action --> B[Dopamine D2 blockade in hypothalamus & basal ganglia]:::action B --> C[Hyperthermia]:::outcome B --> D[Severe muscle rigidity]:::outcome B --> E[Autonomic instability]:::outcome C --> F[Rhabdomyolysis]:::outcome D --> F F --> G[Elevated CPK + Myoglobinuria]:::outcome G --> H[Acute kidney injury]:::urgent ``` **High-Yield:** NMS is a medical emergency with mortality 10–20% if untreated. The classic tetrad is fever, rigidity, altered mental status, and autonomic instability (tachycardia, hypertension, diaphoresis). ### Diagnostic Criteria for NMS (DSM-5) | Criterion | Finding | | --- | --- | | **Exposure** | Antipsychotic within 72 hours (or recent dose increase) | | **Hyperthermia** | Temperature ≥ 38.5°C (101.3°F) | | **Muscle Rigidity** | "Lead pipe" or "waxy flexibility" rigidity | | **Mental Status** | Altered consciousness, confusion, mutism | | **Autonomic Instability** | Tachycardia, hypertension, diaphoresis, tachypnea | | **Laboratory** | **Elevated CPK (often > 1000 U/L)** | **Clinical Pearl:** CPK elevation in NMS is due to muscle breakdown (rhabdomyolysis) from sustained contraction and hyperthermia. Myoglobinuria causes the characteristic dark, cola-colored urine and precipitates acute tubular necrosis. ### Why CPK & Myoglobin Are Diagnostic 1. **CPK elevation** (often 10–100× normal) is nearly universal in NMS 2. **Myoglobinuria** (positive urine dipstick for blood without RBCs on microscopy) confirms rhabdomyolysis 3. **Elevated creatinine** (as in this case: 2.1 mg/dL) indicates myoglobin-induced acute kidney injury 4. **Electrolyte abnormalities** (hyperkalemia, hypocalcemia) accompany muscle breakdown ### Why Other Options Are Incorrect - **Lumbar puncture with CSF analysis**: CSF is typically normal in NMS; LP is contraindicated if increased intracranial pressure is suspected and does not confirm NMS - **Brain MRI with contrast**: Imaging is normal in NMS; used to exclude other causes of altered mental status (stroke, tumor) only if diagnosis is uncertain - **EEG**: May show nonspecific slowing but is not diagnostic; used to exclude seizures or encephalitis if differential diagnosis is broad **Mnemonic:** **FARM** = **F**ever, **A**ntipsychotic exposure, **R**igidity (lead pipe), **M**yoglobinuria (and elevated CPK). This helps recall the core diagnostic features and the key lab finding. ### Management Implications Once NMS is suspected: 1. **Stop antipsychotic immediately** 2. **Aggressive IV hydration** (target urine output > 200 mL/hr to prevent myoglobin precipitation) 3. **Dantrolene** (2.5 mg/kg IV q 5–10 min, max 10 mg/kg/day) to reduce muscle rigidity 4. **Bromocriptine** (dopamine agonist, 5 mg PO TID) to restore dopaminergic tone 5. **Monitor CPK, electrolytes, renal function** until CPK normalizes and creatinine improves **Warning:** Do not rechallenge with the same antipsychotic for at least 2 weeks after recovery; if rechallenge is necessary, use a different class (e.g., switch from typical to atypical, or to clozapine if not the culprit).