Serotonin Syndrome
Toxicology
Illness script · Toxicology
Serotonin Syndrome
Life-threatening excess serotonergic activity at CNS/PNS receptors, classically from drug combination, causing neuromuscular, autonomic, and mental status changes.
This illness script for Serotonin Syndrome covers predisposing factors, classic presentation, mechanism, workup, management, and the clinical pivots that separate it from look-alikes—written for USMLE Step 1 and clerkship reasoning.
01
Predisposing factors
- Most common trigger: SSRI + another serotonergic agent (tramadol, linezolid, triptans, fentanyl, MDMA, dextromethorphan)
- MAOIs carry highest risk — especially combined with SSRIs or meperidine
- Recreational drug use (MDMA, cocaine) is a common precipitant in young patients
- Therapeutic dosing errors or intentional overdose with serotonergic drugs
- Any new serotonergic drug addition increases risk even at standard doses
02
Presentation
- Classic triad: altered mental status + autonomic instability + neuromuscular abnormalities
- Clonus (inducible, spontaneous, or ocular) is the most specific finding
- Hyperreflexia and clonus GREATER in lower extremities than upper — key exam pearl
- Autonomic features: hyperthermia, tachycardia, diaphoresis, hypertension, mydriasis
- Onset rapid: usually <24 hours (often within 6 hours) of offending agent
- Severe cases: hyperthermia >41°C, rhabdomyolysis, seizures, cardiovascular collapse
03
Pathophysiology
- Excess stimulation of 5-HT1A and 5-HT2A receptors in brainstem and spinal cord
- 5-HT2A activation → neuromuscular hyperactivity (clonus, rigidity) and autonomic instability
- Spinal cord interneuron hyperexcitability drives clonus, the hallmark finding
- Onset typically within minutes to hours of drug exposure or dose change
04
Diagnostics
- Clinical diagnosis using Hunter Criteria (most sensitive/specific; no lab required)
- Hunter Criteria: serotonergic drug + clonus, agitation, diaphoresis, tremor, hyperreflexia combinations
- No specific serum serotonin level is diagnostic or clinically useful
- Workup to assess severity: CK (rhabdomyolysis), BMP (renal function, electrolytes), CBC
- ECG to evaluate QTc; lactate if hemodynamically unstable
05
Management
- Immediately discontinue ALL serotonergic agents
- Benzodiazepines (first-line) for agitation, neuromuscular hyperactivity, and seizures
- Cyproheptadine (5-HT2A antagonist) — antidote; give orally or via NG tube
- Aggressive cooling for hyperthermia >41°C; avoid antipyretics (not prostaglandin-mediated)
- ICU admission for severe cases; avoid succinylcholine (hyperkalemia risk with rigidity); avoid physical restraints alone
- Symptoms typically resolve within 24 hours once offending agents stopped
06
Clinical pivots
How to separate this script from the look-alikes that show up on exams and on the wards.
Neuroleptic Malignant Syndrome
NMS: onset over days–weeks with bradykinesia/lead-pipe rigidity, NOT clonus/hyperreflexia; caused by dopamine antagonists not serotonergic drugs
Anticholinergic Toxidrome
Anticholinergic: hot/dry/flushed skin, urinary retention, absent bowel sounds; NO clonus or diaphoresis
Malignant Hyperthermia
MH: triggered by inhaled anesthetics or succinylcholine intraoperatively; muscle rigidity with masseter spasm; RYR1 mutation
Sympathomimetic Toxidrome
Sympathomimetic: shares tachycardia/hypertension/hyperthermia but lacks clonus and hyperreflexia; no serotonergic drug exposure
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Educational use only. This illness script is a study framework, not medical advice. Confirm decisions with current guidelines and your clinical supervisors.