A 35-year-old man on haloperidol and lithium develops fever (40.2°C), generalised rigidity, diaphoresis, and confusion over 48 hours. CK is 8400 U/L. Which single feature MOST reliably distinguishes neuroleptic malignant syndrome (NMS) from serotonin syndrome?
- A Presence of hyperthermia
- B Lead-pipe rigidity rather than hyperreflexia and clonus ✓
- C Diaphoresis
- D Altered consciousness
Correct answer: B. Lead-pipe rigidity rather than hyperreflexia and clonus
Explanation
Both NMS and serotonin syndrome share hyperthermia, altered sensorium, and autonomic instability. The key differentiator is the neuromuscular pattern: NMS produces 'lead-pipe' rigidity with bradyreflexia, whereas serotonin syndrome characteristically causes hyperreflexia, clonus (especially ankle), tremor, and myoclonus. CK elevation and slower onset over days also favour NMS, but the neuromuscular examination is the most reliable discriminator used in clinical decision-making.
Reference: Kaplan & Sadock's Synopsis of Psychiatry, 11th ed.
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