Serotonin Syndrome vs NMS on the NCLEX
Cause, onset, clonus vs rigidity, and treatment for the two hyperthermic drug emergencies.
Both are hot, stiff, and confused. The drug history and the reflexes tell them apart.
Cause
Serotonin syndrome: excess serotonergic activity. SSRI plus MAOI is the classic combination; also tramadol, linezolid, triptans, dextromethorphan, St. John's wort, or an SSRI overdose. NMS (neuroleptic malignant syndrome): dopamine blockade. Antipsychotics (highest risk with high-potency typicals), or abrupt withdrawal of dopaminergic drugs in Parkinson disease.
Onset
Serotonin syndrome: fast, usually within 24 hours of the dose change or interaction. NMS: slow, days to 2 weeks after starting or increasing the antipsychotic.
Motor findings: the key differentiator
Serotonin syndrome: neuromuscular excitation. Hyperreflexia, clonus (worst in the lower limbs), tremor, myoclonus, dilated pupils, diarrhea, agitation. NMS: neuromuscular shutdown. Lead-pipe rigidity, hyporeflexia, bradykinesia. Massive CK elevation from muscle breakdown, risk of rhabdomyolysis. Both share hyperthermia, autonomic instability, and altered mental status.
Treatment
Both: stop the offending drug, aggressive cooling, IV fluids, benzodiazepines for agitation, ICU-level monitoring. Serotonin syndrome antidote: cyproheptadine (serotonin antagonist). NMS: dantrolene (muscle relaxant) and bromocriptine (dopamine agonist). Never rechallenge quickly with the same antipsychotic.
Frequently asked
One-line memory hook?
Serotonin syndrome is fast, twitchy, and hyperreflexic. NMS is slow, rigid, and hyporeflexic.
Why does a 2-week MAOI washout matter?
Starting an SSRI before the MAOI clears causes serotonin syndrome. Switching in either direction needs a washout, and 5 weeks after fluoxetine because of its long half-life.
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