## Distinguishing Serotonin Syndrome from NMS ### Key Clinical Discriminator **Key Point:** Hyperreflexia and clonus (especially lower limb) are hallmark features of serotonin syndrome and are notably ABSENT or minimal in NMS. This is the single best discriminating feature. ### Comparative Table | Feature | Serotonin Syndrome | Neuroleptic Malignant Syndrome | |---------|-------------------|--------------------------------| | **Hyperreflexia & Clonus** | ✓ Present (hallmark) | ✗ Absent or minimal | | **Onset** | Rapid (hours to 24 h) | Gradual (24–72 h) | | **Rigidity pattern** | Predominantly lower limbs, lower > upper | Uniform, "lead pipe" | | **GI symptoms** | Diarrhea common | Rare | | **CK elevation** | Mild to moderate | Marked (often > 1000 IU/L) | | **Trigger** | Serotonergic drugs (SSRIs, MAOIs, tramadol) | Antipsychotics (D2 antagonists) | | **Mydriasis** | Common | Uncommon | | **Tremor** | Fine, rapid | Coarse | ### Pathophysiological Basis **High-Yield:** Serotonin syndrome results from excessive serotonergic activity in the CNS, leading to hyperexcitability of motor neurons → hyperreflexia and clonus. NMS arises from dopamine D2 blockade in the basal ganglia and brainstem, causing loss of inhibitory control → uniform rigidity without hyperreflexia. ### Clinical Pearl **Clinical Pearl:** The presence of clonus (especially inducible ankle clonus) in a patient with agitation and tremor on a serotonergic drug is virtually pathognomonic for serotonin syndrome and should immediately prompt discontinuation of the offending agent. ### Hunter Criteria (Serotonin Syndrome) Diagnosis requires a serotonergic agent PLUS one of: 1. Spontaneous clonus 2. Inducible or ocular clonus + agitation or diaphoresis 3. Hyperreflexia + tremor 4. Hypertonia + temperature > 38.5°C + ocular clonus or inducible clonus [cite:Harrison 21e Ch 398] 
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