How should I differentiate between serotonin syndrome and other similar conditions, such as neuroleptic malignant syndrome?

To differentiate serotonin syndrome (SS) from neuroleptic malignant syndrome (NMS) and other similar conditions, focus on the clinical features, onset, medication history, and laboratory findings.

Serotonin syndrome typically presents with a rapid onset (within hours) after starting or increasing serotonergic agents, characterized by a triad of cognitive effects (agitation, confusion), autonomic instability (hyperthermia, tachycardia, diaphoresis), and neuromuscular abnormalities such as hyperreflexia and clonus, especially inducible or spontaneous clonus NICE CG91Bhanushali & Tuite 2004.

In contrast, neuroleptic malignant syndrome usually develops over days to weeks after exposure to dopamine antagonists (e.g., antipsychotics) and is marked by severe rigidity (lead-pipe rigidity), bradyreflexia, altered consciousness, autonomic dysfunction, and often elevated creatine kinase due to muscle breakdown NICE CG91Bhanushali & Tuite 2004.

Key differentiators include the neuromuscular findings: SS shows hyperreflexia and clonus, whereas NMS shows hyporeflexia and rigidity NICE CG91Bhanushali & Tuite 2004. Additionally, SS patients often have gastrointestinal symptoms (nausea, diarrhea), which are uncommon in NMS NICE CG91.

Medication history is critical: recent serotonergic drug changes suggest SS, while recent initiation or dose increase of neuroleptics suggests NMS NICE CG91Bhanushali & Tuite 2004.

Laboratory tests may support diagnosis but are not definitive; elevated creatine kinase is more prominent in NMS due to rhabdomyolysis NICE CG91.

In summary, rapid onset, hyperreflexia, clonus, and serotonergic drug exposure point to serotonin syndrome, while slower onset, severe rigidity, hyporeflexia, and neuroleptic exposure indicate neuroleptic malignant syndrome.