Which feature helps differentiate serotonin syndrome from NMS?

Which feature helps differentiate serotonin syndrome from NMS?

• A. Myoclonus is commonly seen in serotonin syndrome
• B. Fever is only seen in NMS
• C. Rigidity is absent in serotonin syndrome
• D. Serotonin syndrome always presents with coma

Answer: (A)

The distinguishing idea is the neuromuscular pattern: serotonin syndrome causes neuromuscular hyperactivity, while NMS centers on severe rigidity. Serotonin excess leads to clonus, tremor, and especially myoclonus with hyperreflexia, reflecting heightened reflex activity in the nervous system. This myoclonus is a common and characteristic feature of serotonin syndrome and helps set it apart from NMS, where the hallmark is lead-pipe rigidity with hyporeflexia or normal reflexes and a slower onset. Fever can occur in both conditions, and rigidity is more typical of NMS rather than serotonin syndrome, so those findings are less useful for differentiation. In short, myoclonus signals serotonin syndrome rather than NMS.

The distinguishing idea is the neuromuscular pattern: serotonin syndrome causes neuromuscular hyperactivity, while NMS centers on severe rigidity. Serotonin excess leads to clonus, tremor, and especially myoclonus with hyperreflexia, reflecting heightened reflex activity in the nervous system. This myoclonus is a common and characteristic feature of serotonin syndrome and helps set it apart from NMS, where the hallmark is lead-pipe rigidity with hyporeflexia or normal reflexes and a slower onset. Fever can occur in both conditions, and rigidity is more typical of NMS rather than serotonin syndrome, so those findings are less useful for differentiation. In short, myoclonus signals serotonin syndrome rather than NMS.