Which clinical finding is most characteristic of serotonin syndrome but NOT seen in neuroleptic malignant syndrome?

Explanation

## Distinguishing Serotonin Syndrome from NMS **Key Point:** Hyperreflexia and clonus (especially lower limb) are pathognomonic for serotonin syndrome and are absent in NMS. ### Serotonin Syndrome: Hyperreflexia & Clonus Excessive serotonergic activity in the spinal cord and brainstem produces: - **Hyperreflexia** (brisk deep tendon reflexes) - **Clonus** (rhythmic involuntary muscle contractions)—both spontaneous and inducible - Myoclonus (sudden jerking movements) - GI hyperactivity (diarrhea, nausea) - Tremor These reflect **lower motor neuron hyperexcitability** due to serotonin excess. ### NMS: Rigidity Without Hyperreflexia Dopamine blockade produces: - **Lead-pipe or waxy rigidity** (uniform resistance throughout range of motion) - **Normal or decreased reflexes** (no clonus) - Bradykinesia - Dystonia These reflect **basal ganglia dysfunction** due to dopamine deficiency. **High-Yield:** Presence of clonus = serotonin syndrome until proven otherwise. Absence of clonus in a hyperthermic, rigid patient = NMS. ### Comparison Table | Feature | Serotonin Syndrome | NMS | |---------|-------------------|-----| | **Reflexes** | Hyperactive + clonus | Normal or decreased | | **Rigidity type** | Absent or mild | Lead-pipe/waxy | | **GI symptoms** | Diarrhea, nausea | Absent | | **Tremor** | Fine, prominent | Absent | | **Onset** | Hours to days | Days to weeks | | **Trigger** | Serotonergic drugs | Dopamine antagonists | **Mnemonic:** **SEROTONIN** = **S**pinal hyperreflexia, **E**xcessive reflexes, **R**hythmic clonus. **NMS** = **N**o clonus, **M**uscle rigidity, **S**evere (but reflexes normal). [cite:Harrison 21e Ch 387]