Toxicology – Serotonin Syndrome

Definition

Serotonin syndrome is a potentially serious condition caused by excessive serotonergic activity in the central nervous system. It can occur after increasing the dose of a serotonergic medication, taking an overdose, or combining multiple drugs that increase serotonin.

Common Causes

Drugs associated with serotonin syndrome include:

• SSRIs
• SNRIs
• MAO inhibitors
• Tricyclic antidepressants
• Triptans
• Lithium
• Certain opioids such as meperidine or tramadol
• Cocaine and other stimulants
• Cyclobenzaprine
• Dextromethorphan
• Linezolid
• St. John’s wort

Combinations involving MAO inhibitors plus other serotonergic agents are particularly dangerous.

Clinical Features

The syndrome is classically recognized by a combination of:

• Mental-status changes: agitation, confusion, anxiety, lethargy, or coma
• Autonomic hyperactivity: hyperthermia, sweating, tachycardia, hypertension, nausea, vomiting, and dilated pupils
• Neuromuscular hyperactivity: tremor, hyperreflexia, clonus, myoclonus, and increased muscle tone

Clonus and hyperreflexia, especially in the lower extremities, are particularly helpful diagnostic clues.

Symptoms usually develop rapidly, often within hours of a medication change or interacting drug exposure.

Mechanism of Action

Excess serotonin overstimulates central and peripheral serotonin receptors, particularly 5-HT₁A and 5-HT₂A receptors.

This produces the characteristic combination of altered mental status, autonomic instability, and neuromuscular excitation.

Management

Treatment includes:

• Immediately stopping all serotonergic medications
• Airway and cardiovascular support when required
• IV fluids
• Benzodiazepines for agitation, tremor, and excessive sympathetic activity
• Active external cooling for significant hyperthermia

Severe cases with extreme hyperthermia may require sedation, paralysis, and mechanical ventilation.

Cyproheptadine, a serotonin receptor antagonist, may be considered when significant symptoms persist despite supportive treatment.

Differential Diagnosis

Important conditions that can resemble serotonin syndrome include:

• Neuroleptic malignant syndrome
• Anticholinergic toxicity
• Malignant hyperthermia
• Sympathomimetic poisoning

Compared with neuroleptic malignant syndrome, serotonin syndrome generally develops more rapidly and is more strongly associated with clonus and hyperreflexia.

Key Points

• Think of the triad: mental-status changes + autonomic instability + neuromuscular hyperactivity.
• Clonus is one of the most useful bedside findings.
• Symptoms often appear within hours rather than days.
• Stop serotonergic drugs promptly and provide supportive care.
• Severe hyperthermia is caused largely by excessive muscle activity and requires aggressive supportive management.