The serotonin syndrome is BEST distinguished from neuroleptic malignant syndrome (NMS) by which clinical feature?

• A Serotonin syndrome presents with clonus, hyperreflexia, and tremor (due to spinal cord 5-HT2A activation), whereas NMS features lead-pipe rigidity without hyperreflexia ✓
• B Hyperthermia is present in serotonin syndrome but absent in NMS
• C NMS has a rapid onset within hours whereas serotonin syndrome develops over weeks
• D Raised CK is exclusively seen in NMS and not in serotonin syndrome

Explanation

The key clinical distinction is neuromuscular: Serotonin syndrome (excess 5-HT at spinal cord 5-HT2A/1A receptors) causes hyperreflexia, clonus (especially at ankle), tremor, and myoclonus. NMS (dopamine D2 blockade) causes 'lead-pipe' rigidity without hyperreflexia (alpha motor neuron mechanism). Both can cause hyperthermia, autonomic instability, and elevated CK, so these features do not differentiate them. NMS develops more gradually (days) while serotonin syndrome often develops within 24 hours of drug change. Hunter's criteria (clonus + serotonergic drug) are used to diagnose serotonin syndrome.

Reference: Kaplan & Sadock's Synopsis of Psychiatry, 11th ed.

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Written and medically reviewed by the StethoPrep medical team.