Serotonin Syndrome and Neuroleptic Malignant Syndrome

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The one-minute consultant answer

Serotonin syndrome is drug-induced excess serotonergic activity in the central and peripheral nervous systems, producing the clinical triad of neuromuscular hyperactivity (clonus — especially inducible or ocular, hyperreflexia, myoclonus, rigidity that is greater in the lower than the upper limbs), autonomic instability (hyperthermia, tachycardia, hypertension, mydriasis, diaphoresis, flushing) and altered mental status (agitation, confusion, delirium) with onset within hours of a dose change or a serotonergic drug interaction ^[1][2]. Neuroleptic malignant syndrome (NMS) is the mirror image: profound dopamine D2 receptor blockade (or sudden withdrawal of dopaminergic therapy in Parkinson disease) producing lead-pipe rigidity (greater in the upper than the lower limbs), hyperthermia often above 40 degrees, altered mental status, autonomic instability, and a gradual onset over days to weeks with elevated creatine kinase and leukocytosis ^[3][4].

The discriminating bedside rule is simple: onset in hours plus clonus and hyperreflexia points to serotonin syndrome; onset over days plus lead-pipe rigidity and bradyreflexia points to NMS ^[5]. Management of both is: stop the offending agent, provide aggressive supportive care (cooling, benzodiazepines for agitation and rigidity, airway protection), and treat the cause pharmacologically — cyproheptadine (a 5-HT2A antagonist) for moderate-to-severe serotonin syndrome, and dantrolene plus bromocriptine or amantadine for severe NMS ^[1][6]. Both are medical emergencies; the febrile, rigid, agitated patient needs the diagnosis made at the bedside, not after the bloods return.

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References11ShowHide

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3. [3]Berman BD Neuroleptic malignant syndrome: a review for neurohospitalists Neurohospitalist, 2011.PMID 23983836
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11. [11]SanFilippo S, Turgeon J, Michaud V, et al. The Association of Serotonin Toxicity with Combination Linezolid-Serotonergic Agent Therapy: A Systematic Review and Meta-Analysis Pharmacy (Basel), 2023.PMID 37987392