Suspected Serotonin Syndrome
Patient with serotonergic drug exposure and concerning symptoms
All Pathways
PsychiatryEmergency
Serotonin Syndrome (Hunter Criteria)
Serotonin Syndrome (Hunter Criteria): Suspected Serotonin Syndrome → Identify Serotonergic Agent(s) → Apply Hunter Criteria → Does Not Meet Hunter Crite...
Pathway Overview
14 steps
Algorithm Steps
14 total
01Start 02Action Identify Serotonergic Agent(s)
Recent addition or dose increase within 24 hours
- SSRIs: fluoxetine, sertraline, paroxetine, citalopram
- SNRIs: venlafaxine, duloxetine
- TCAs: clomipramine, imipramine
- MAOIs: phenelzine, tranylcypromine, selegiline
- Other: tramadol, fentanyl, meperidine, linezolid
- Illicit: MDMA, cocaine, LSD
- OTC: dextromethorphan, St. John's Wort
03Decision Apply Hunter Criteria
In presence of serotonergic agent, any ONE of:
- 1. Spontaneous clonus
- 2. Inducible clonus + agitation OR diaphoresis
- 3. Ocular clonus + agitation OR diaphoresis
- 4. Tremor + hyperreflexia
- 5. Hypertonia + temperature >38°C + (ocular OR inducible clonus)
04Action Does Not Meet Hunter Criteria
Consider alternative diagnosis
- NMS (slower onset, lead-pipe rigidity)
- Anticholinergic toxicity (dry, no clonus)
- Malignant hyperthermia (anesthesia exposure)
- Sympathomimetic toxicity
- Infection/sepsis
05Warning ⚠️ Serotonin Syndrome Confirmed
Hunter Criteria positive
06Warning ⚠️ STOP All Serotonergic Agents
Immediate discontinuation essential
- Discontinue ALL serotonergic medications
- Review all medications including OTC
- Most cases resolve 24-72 hours after stopping
07Decision Assess Severity
Based on clinical findings
08Action Mild Serotonin Syndrome
Tremor, hyperreflexia, mild agitation
- Stop serotonergic agents
- Supportive care
- Benzodiazepines PRN: Lorazepam 1-2mg
- Observation 6-8 hours
- Most resolve spontaneously
09Outcome Recovery & Future Considerations
Most resolve within 24-72 hours
- Avoid re-exposure to combination
- If serotonergic needed: wait 2 weeks (5 weeks fluoxetine)
- Educate patient on drug interactions
- Consider alternative classes if possible
10Action Moderate Serotonin Syndrome
Significant vital sign abnormalities, clonus
- Hospital admission
- IV fluids
- Benzodiazepines: Lorazepam 2mg IV q2h PRN
- Consider cyproheptadine (see below)
- Continuous monitoring
11Action Cyproheptadine (5-HT Antagonist)
For moderate-severe cases
- Loading dose: 12mg PO/NG
- Maintenance: 2mg q2h until improvement
- Maximum: 32mg/24hr
- Only available PO/NG (no IV form)
- Evidence limited but commonly used
12Warning ⚠️ Medications to AVOID
Can worsen serotonin syndrome
- Avoid antipyretics (ineffective for drug-induced hyperthermia)
- Avoid succinylcholine (rhabdomyolysis risk)
- Avoid restraints without sedation (worsens hyperthermia)
- Avoid bromocriptine (different mechanism than NMS)
13Action Monitoring & Complications
Watch for complications
- Serial temperature
- CK for rhabdomyolysis
- Renal function
- Metabolic panel
- Cardiac monitoring (serotonergic agents prolong QT)
- Path rejoins step 09Shared downstream outcome
14Warning Severe Serotonin Syndrome
Hyperthermia >41°C, muscle rigidity, rhabdomyolysis
- ICU admission
- Aggressive cooling (target <38.5°C)
- Intubation and paralysis if needed
- IV fluids for rhabdomyolysis
- Consider cyproheptadine
- Path rejoins step 11Shared downstream outcome
Guideline Source
The Hunter Serotonin Toxicity Criteria: Simple and Accurate Diagnostic Decision Rules for Serotonin Toxicity
Clinical Safety Information
Clinical Decision Support — Not a Substitute for Clinical Judgment
Individual patient factors may require deviation from these recommendations.
Known Limitations
- Hunter Criteria validated in overdose patients - may miss mild cases
- Distinguishing from NMS can be challenging
- Onset typically within 24 hours of exposure (vs. NMS days-weeks)
- Mild cases resolve within 24-72 hours with supportive care
Applicable Regions
USEUGlobal
Version 1Next review: 2027-01-11
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Frequently Asked Questions
What is the Serotonin Syndrome (Hunter Criteria)?
The Serotonin Syndrome (Hunter Criteria) is a emergency clinical algorithm for Psychiatry. It provides a structured decision tree to guide clinical decision-making, based on The Hunter Serotonin Toxicity Criteria: Simple and Accurate Diagnostic Decision Rules for Serotonin Toxicity.
What guideline is the Serotonin Syndrome (Hunter Criteria) based on?
This algorithm is based on The Hunter Serotonin Toxicity Criteria: Simple and Accurate Diagnostic Decision Rules for Serotonin Toxicity (DOI: 10.1093/qjmed/hcg109).
What are the limitations of the Serotonin Syndrome (Hunter Criteria)?
Known limitations include: Hunter Criteria validated in overdose patients - may miss mild cases; Distinguishing from NMS can be challenging; Onset typically within 24 hours of exposure (vs. NMS days-weeks); Mild cases resolve within 24-72 hours with supportive care. Individual patient factors may require deviation from these recommendations.
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