Serotonin Syndrome
Serotonin syndrome is a potentially life-threatening condition caused by excess serotonergic activity. It presents with a triad of neuromuscular excitability, autonomic dysfunction, and altered mental status.
Key Facts
- Triad: Neuromuscular excitability (clonus, hyperreflexia, myoclonus), autonomic dysfunction (hyperthermia, diaphoresis, tachycardia), altered mental status (agitation, confusion)
- Clonus (especially lower limbs) is the most specific clinical sign
- Onset typically within hours of starting or increasing a serotonergic drug, or combining serotonergic agents
- Hunter criteria: Decision rule for diagnosis - requires serotonergic agent + at least one of: spontaneous clonus, inducible clonus + agitation/diaphoresis, ocular clonus, tremor + hyperreflexia, or hypertonia + temperature >38°C + clonus
- Common causes: SSRI + MAOI, SSRI + tramadol, SSRI + triptans, SSRI + St John's wort, SSRI + linezolid
- Treatment: Stop serotonergic drugs, supportive care, benzodiazepines for agitation, cyproheptadine 12mg PO then 2mg q2h (5-HT2A antagonist)
- Usually resolves within 24-72 hours after drug discontinuation
- Mild cases may go unrecognised - tremor, myoclonus, agitation attributed to underlying psychiatric condition
Overview
Key Facts
Serotonin syndrome results from excessive 5-HT receptor stimulation, usually from drug combinations or overdose. Recognition is critical as it can progress to life-threatening hyperthermia and multiorgan failure.
Epidemiology
Exact incidence unknown - likely underdiagnosed, especially mild cases. Estimated ~15% of SSRI overdoses develop some features. Increasing incidence with rising serotonergic drug prescribing.
Aetiology
Common drug combinations causing serotonin syndrome:
- SSRI + MAOI (most dangerous combination)
- SSRI + tramadol
- SSRI + triptans (5-HT1 agonists)
- SSRI + linezolid (weak MAOI)
- SSRI + St John's wort
- SSRI + lithium
- Combining multiple serotonergic antidepressants
- MDMA (ecstasy) + SSRI
Pathophysiology
- Excess 5-HT stimulation at 5-HT1A and 5-HT2A receptors in brainstem and spinal cord
- 5-HT2A receptor stimulation → hyperthermia, agitation, neuromuscular excitability
- 5-HT1A receptor stimulation → autonomic dysfunction
- Severity depends on degree of serotonergic excess - spectrum from mild tremor to life-threatening hyperthermia
Clinical Presentation
Mild
- Tremor, myoclonus (especially in lower limbs)
- Anxiety, restlessness, insomnia
- Diarrhoea, mydriasis
Moderate
- Agitation, hyperreflexia, inducible clonus
- Diaphoresis, tachycardia
- Temperature 38-40°C
Severe/Life-threatening
- Temperature >41°C, severe clonus/rigidity
- Seizures, delirium, coma
- Rhabdomyolysis, DIC, multiorgan failure
Red Flags
- Temperature >41°C - life-threatening, aggressive cooling needed
- Sustained clonus with rising temperature
- Seizures or loss of consciousness
- Combination of SSRI + MAOI - highest risk combination
Differential Diagnosis
| Diagnosis | Key Distinguishing Features |
|---|---|
| NMS | Lead-pipe rigidity (not clonus), onset days-weeks, D2 blocker cause |
| Malignant hyperthermia | Intraoperative, volatile anaesthetic agents |
| Anticholinergic toxicity | Dry skin/mouth (not diaphoresis), urinary retention, dilated pupils |
| Meningitis | Neck stiffness, CSF abnormalities |
| Sympathomimetic intoxication | Similar features, recreational drug history |
Diagnosis / Investigation
Bedside
- Temperature: Core temperature - may be markedly elevated
- Observations: HR, BP, RR, SpO2 - continuous monitoring
- Neurological exam: Clonus (spontaneous and inducible), hyperreflexia, tremor, myoclonus
Bloods
- CK: Elevated in moderate-severe cases (rhabdomyolysis)
- U&Es: AKI from rhabdomyolysis
- LFTs, coagulation: Hepatic injury, DIC
- FBC: WCC may be elevated
- ABG/lactate: Metabolic acidosis
Special Tests
- Drug levels: Serotonergic drug levels may guide management
- ECG: Sinus tachycardia, QTc monitoring
- No specific diagnostic test - diagnosis is clinical (Hunter criteria)
Management
Immediate
- STOP all serotonergic drugs
- Supportive care: IV fluids, continuous monitoring
- Benzodiazepines: Diazepam 5-10mg IV for agitation, myoclonus, seizures
Moderate-Severe
- Cyproheptadine: 12mg PO/NG loading dose, then 2mg every 2 hours (max 32mg/day) - 5-HT2A antagonist
- Active cooling: Evaporative cooling, ice packs; avoid antipyretics (ineffective - hyperthermia is muscular, not hypothalamic)
- Intubation and paralysis: If temperature >41°C with severe rigidity - non-depolarising neuromuscular blocker (e.g., rocuronium)
Post-Recovery
- Symptoms typically resolve within 24-72 hours of drug discontinuation
- Review medication - avoid serotonergic combinations
- If antidepressant needed, choose drug with lower serotonergic potential or different mechanism
- Ensure adequate washout period before starting MAOI (2 weeks for most SSRIs; 5 weeks for fluoxetine)
Referral Criteria
- Moderate-severe serotonin syndrome - ICU/HDU
- Temperature >39°C with neuromuscular symptoms - urgent medical management
- Persistent symptoms >72 hours - investigate for alternative diagnosis
Prognosis
- Mild cases: Resolve within 24 hours with drug withdrawal
- Moderate cases: Resolve within 24-72 hours with supportive care + cyproheptadine
- Severe cases: Mortality <5% with modern management; usually from multiorgan failure
- Most patients recover fully with no long-term sequelae
- Prevention through careful prescribing and awareness of drug interactions is key
Other Relevant Information
Hunter Criteria for Serotonin Syndrome
Requires a serotonergic agent PLUS at least one of:
| Criterion |
|---|
| Spontaneous clonus |
| Inducible clonus + agitation OR diaphoresis |
| Ocular clonus + agitation OR diaphoresis |
| Tremor + hyperreflexia |
| Hypertonia + temperature >38°C + ocular clonus OR inducible clonus |
Key Serotonergic Drug Interactions
| Combination | Risk Level |
|---|---|
| SSRI + MAOI | Highest risk - CONTRAINDICATED |
| SSRI + tramadol | High |
| SSRI + triptans | Moderate |
| SSRI + linezolid | Moderate-high |
| SSRI + St John's wort | Moderate |
| SSRI + lithium | Low-moderate |