Which clinical finding most reliably distinguishes serotonin syndrome from neuroleptic malignant syndrome?

Explanation

## Distinguishing Serotonin Syndrome from NMS ### Comparative Clinical Features | Feature | Serotonin Syndrome | NMS | |---------|-------------------|-----| | **Onset** | Hours to days | Days to weeks | | **Reflexes** | Hyperreflexia, clonus (pathognomonic) | Normal or decreased | | **Muscle tone** | Increased tone, myoclonus | Lead-pipe rigidity, waxy flexibility | | **GI symptoms** | Diarrhea, nausea (common) | Rare | | **Hyperthermia** | May occur | Prominent (>38.5°C typical) | | **Trigger** | Serotonergic drug interaction | Antipsychotic initiation/increase | | **CK elevation** | Mild or absent | Marked (often >1000 U/L) | **Key Point:** Hyperreflexia and clonus (especially ankle clonus) are **pathognomonic for serotonin syndrome** and virtually absent in NMS. This is the single most reliable clinical discriminator. **High-Yield:** The **Hunter Criteria** for serotonin syndrome require serotonergic drug exposure PLUS one of: - Spontaneous clonus - Inducible clonus + agitation or diaphoresis - Ocular clonus + agitation or diaphoresis - Hyperreflexia + tremor - Hyperthermia (>38.5°C) + inducible clonus **Mnemonic for Serotonin Syndrome: SLUDGE + Clonus** - **S**alivation, **L**acrimation, **U**rination, **D**efecation, **G**astrointestinal distress, **E**mesis - Plus hyperreflexia and clonus (the distinguishing feature) **Clinical Pearl:** Clonus is a rhythmic, involuntary muscle contraction that occurs in response to passive stretch. It is virtually pathognomonic for serotonin syndrome and should prompt immediate discontinuation of serotonergic agents and supportive care.