Serotonin Syndrome

Topic: Serotonin Syndrome

Key Points:

• A potentially life-threatening condition caused by excessive serotonergic activity in the central nervous system (CNS).
• It is a clinical diagnosis based on a history of exposure to serotonergic agents and the presence of characteristic clinical features.
• The classic triad involves neuromuscular excitation, autonomic hyperactivity, and altered mental status.
• Treatment is supportive. The cornerstone of management is the immediate cessation of the precipitating drug(s) and aggressive supportive care, including sedation with benzodiazepines and control of agitation, hyperthermia, and autonomic instability.
• Severe cases require intensive care and may be treated with the serotonin antagonist cyproheptadine.

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1. Aetiology & Pathophysiology

Serotonin syndrome results from overstimulation of central (primarily 5-HT_2A) and peripheral serotonin receptors.

• Mechanism: Most commonly due to therapeutic use, intentional overdose, or drug interactions involving serotonergic drugs.
• Common Culprits:
  • Antidepressants: SSRIs (e.g., citalopram, sertraline, fluoxetine), SNRIs (e.g., venlafaxine), TCAs (e.g., clomipramine, amitriptyline), MAOIs.
  • Opioids: Tramadol, pethidine, fentanyl, oxycodone. Methadone is a weak inhibitor.
  • Antimigraine drugs: Triptans.
  • Antiemetics: Ondansetron, metoclopramide.
  • Antibiotics: Linezolid (a weak MAOI), Ritonavir (via CYP450 inhibition).
  • Recreational Drugs: MDMA (Ecstasy), cocaine, LSD, amphetamines.
  • Others: Lithium, dextromethorphan, St. John’s Wort.

Important Interactions:

• The combination of an MAOI with an SSRI is classically dangerous and must be avoided (a washout period of 2-5 weeks is recommended when switching, depending on the drugs involved).
• Adding a second serotonergic drug to a stable regimen (e.g., tramadol for pain in a patient on an SSRI) is a common scenario.

2. Clinical Features

Symptoms can develop rapidly, often within hours of a change in dose or ingestion of a new agent. The Hunter Serotonin Toxicity Criteria are widely used for diagnosis as they are more specific than the older Sternbach criteria.

To make the diagnosis using the Hunter Criteria, a patient must have taken a serotonergic agent and have one of the following:

1. Spontaneous clonus.
2. Inducible clonus PLUS agitation or diaphoresis.
3. Ocular clonus PLUS agitation or diaphoresis.
4. Tremor AND hyperreflexia.
5. Hypertonia.
6. Temperature >38°C PLUS ocular clonus or inducible clonus.

Spectrum of Severity:

System	Mild	Moderate	Severe (Life-Threatening)
Mental Status	Anxiety, Restlessness	Agitation, Confusion	Delirium, Coma, Seizures
Neuromuscular	Tremor, Myoclonus, Hyperreflexia	Hyperreflexia, Inducible Clonus	Spontaneous clonus, Rigidity (especially lower limbs), Opsoclonus
Autonomic	Tachycardia, Shivering, Diaphoresis	Hypertension, Tachycardia, Mydriasis, Hyperthermia (<38.5°C)	Hyperthermia (>39°C), Profuse diaphoresis, Labile BP, Tachycardia

Key Differential Diagnoses:

• Neuroleptic Malignant Syndrome (NMS): Onset is slower (days to weeks), features lead-pipe rigidity, bradykinesia, and bradyreflexia. It is associated with dopamine antagonists (e.g., antipsychotics).
• Anticholinergic Toxidrome.
• Malignant Hyperthermia.
• Sympathomimetic Toxidrome.
• Sepsis/Meningoencephalitis.
• Alcohol/Benzodiazepine withdrawal.

3. Investigations

• Bedside: ECG, Capillary Blood Glucose, Core Temperature (rectal).
• Bloods: FBC, U&Es, LFTs, CK (rhabdomyolysis is common in moderate-severe cases), Coagulation screen (DIC can occur in severe hyperthermia).
• Other: Consider paracetamol/salicylate levels (in overdose), blood cultures/lumbar puncture if infection is a concern.
• Imaging: Not diagnostic, but may be needed to exclude other pathologies.

4. Management (RCEMlearning & TOXBASE/NPIS)

1. Immediate Actions & Resuscitation (ABCDE):

• Airway/Breathing: High-flow oxygen. Intubate and ventilate if GCS is low or for severe agitation/hyperthermia.
• Circulation: Secure IV access. Aggressive IV fluid resuscitation for hyperthermia, rhabdomyolysis, and hypotension.
• Disability:
  • Sedation: Benzodiazepines are first-line (e.g., IV Diazepam 5-10mg or IV Lorazepam 2-4mg). They reduce agitation, muscle rigidity, and autonomic instability. Repeat as needed.
  • Hyperthermia: This is a medical emergency.
    • 38.5°C: Active external cooling (e.g., tepid sponging, fans, ice packs).
    • >39°C: This indicates severe disease. Immediate sedation and paralysis with a non-depolarising neuromuscular blocker (e.g., rocuronium) followed by rapid sequence induction and ventilation is often required. Avoid suxamethonium due to the risk of hyperkalaemia in the setting of rhabdomyolysis.
    • Dantrolene may be considered where there is muscular hyperactivity (initially 2-3 mg/kg by intravenous injection, to a maximum of 10 mg/kg.

2. Decontamination:

• Stop all serotonergic drugs immediately.
• Activated Charcoal: Consider within 1 hour of a significant, single ingestion if the patient has a protected airway. It is not useful for therapeutic use or chronic overdose.

3. Specific Antidote:

• Cyproheptadine: A histamine-1 antagonist with non-selective serotonin (5-HT_2A) antagonist properties.
  • Indication: Moderate to severe cases not responding to benzodiazepines.
  • Dosing (TOXBASE/NPIS):
    • Initial dose: 8-12 mg orally (can be crushed and given via NG tube).
    • Maintenance: 4-8 mg every 4-6 hours, up to a maximum of 32 mg in 24 hours.
    • A clinical response should be seen within 1-2 hours.
  • It is only available orally, so the patient must have a safe swallow or NG tube.

4. Supportive Care & Monitoring:

• Admit all patients with moderate or severe symptoms. Severe cases require HDU/ICU.
• Manage autonomic instability supportively (e.g., short-acting agents like esmolol for tachycardia, noradrenaline for hypotension).
• Treat complications aggressively (e.g., IV fluids for rhabdomyolysis to prevent acute kidney injury).

5. Disposition

• Mild Cases: Can often be managed in the ED/Observation Unit with supportive care, cessation of the drug, and discharge with safety-netting advice if symptoms resolve completely. Follow-up with GP to review medication is essential.
• Moderate/Severe Cases: Require hospital admission. Severe cases with hyperthermia, rigidity, or autonomic instability require ICU admission.
• Duration: Symptoms of serotonin syndrome typically resolve within 24-48 hours after the offending agent is discontinued, but this can be longer with drugs with long half-lives (e.g., fluoxetine).

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References:

1. RCEMlearning: Serotonin Syndrome. [RCEM Learning]
2. TOXBASE: Serotonin Syndrome. [Registration required for healthcare professionals].
3. National Poisons Information Service (NPIS): Specialist advice available 24/7 via TOXBASE or telephone.

This topic is intended for educational purposes. Clinical management should be guided by local protocols and, where necessary, discussion with a clinical toxicologist or the NPIS.