A 38-year-old man with schizophrenia on risperidone 6 mg/day for 8 months presents with progressive muscle rigidity, fever (38.5°C), altered mental status, and elevated creatine kinase (2400 U/L). Which investigation is most appropriate to confirm the suspected diagnosis?

Explanation

## Clinical Presentation Analysis The patient presents with the classic tetrad of **neuroleptic malignant syndrome (NMS)**: 1. Fever 2. Muscle rigidity 3. Altered mental status 4. Elevated creatine kinase (CK) ## Investigation of Choice for NMS **Key Point:** Serum electrolytes and renal function tests are the investigation of choice to confirm NMS and assess severity because: - **Elevated CK** (>1000 U/L, often >10,000 U/L in severe cases) is the hallmark biochemical finding - **Hyperkalemia** from muscle breakdown (rhabdomyolysis) - **Elevated creatinine and BUN** indicating acute kidney injury (AKI) from myoglobinuria - **Myoglobinuria** visible on urinalysis (cola-colored urine) ## Diagnostic Criteria for NMS | Feature | Details | |---------|----------| | **Onset** | Usually 24–72 hours after antipsychotic initiation or dose increase | | **Temperature** | >38.5°C (typically 39–40°C) | | **Rigidity** | "Lead pipe" or "waxy" rigidity | | **CK elevation** | Diagnostic threshold: >1000 U/L; often >10,000 U/L | | **Renal involvement** | Creatinine rise, myoglobinuria, oliguria | **High-Yield:** The combination of **elevated CK + hyperkalemia + rising creatinine** in the setting of fever and rigidity after antipsychotic exposure is pathognomonic for NMS. **Clinical Pearl:** Early recognition and discontinuation of the offending antipsychotic, aggressive fluid resuscitation, and supportive care are critical. Dantrolene (2.5 mg/kg IV) and bromocriptine (5 mg TDS) are specific treatments. **Mnemonic:** **FEVER** = Fever, Elevated CK, Vascular collapse risk, Encephalopathy, Rigidity [cite:Stahl's Essential Psychopharmacology 6e]