Pharmacology & Emergency Medicine & Psychiatry
Serotonin syndrome
Serotonin syndrome (SS), also known as serotonin toxicity, is an iatrogenic medical emergency resulting from an excess of serotonin (5-hydroxytryptamine - 5-HT) at synapses in the central and peripheral nervous systems. It is most often caused by the concomitant administration of two or more serotonergic agents, by an overdose of a serotonergic agent, or by the addition of a second serotonergic agent to an existing treatment. Serotonin is a neurotransmitter involved in the regulation of mood, sleep, appetite, pain, motor function, and thermoregulation. Its excess at the 5-HT1A and especially 5-HT2A receptors in the central and peripheral nervous systems triggers the classic triad of SS: neurological impairment (agitation, confusion, delirium), neuromuscular hyperactivity (tremor, hyperreflexia, clonus – the most specific cardinal sign, muscle rigidity in severe forms), and autonomic instability (hyperthermia, tachycardia, hypertension, diaphoresis, mydriasis). Clonus – spontaneous or stretch-induced rhythmic muscle contractions, particularly at the ankles (ankle clonus) and knees – is the most specific physical sign of SS, and its presence should immediately suggest this diagnosis in any patient receiving serotonergic medications. The severity of SS ranges from mild forms (tremor, tachycardia, diaphoresis) to severe life-threatening forms (hyperthermia > 41°C, severe muscle rigidity, seizures, rhabdomyolysis, disseminated intravascular coagulation, acute renal failure, ARDS, death). Early recognition is crucial, as immediate discontinuation of the responsible agent(s), benzodiazepines, and cyproheptadine generally allow for resolution within 24–72 hours in moderate forms.
Drugs involved — serotonergic mechanisms
| Mechanism | Medicines | Risk of SS |
|---|---|---|
| Serotonin reuptake inhibitors (SRIs) | SSRIs (fluoxetine + sertraline + paroxetine + escitalopram + citalopram) + SNRIs (venlafaxine + duloxetine + desvenlafaxine) + tricyclics (clomipramine + amitriptyline) + tramadol + tapentadol + methylenedioxymethamphetamine (MDMA — ecstasy) + cocaine | High risk in combination + moderate risk alone at therapeutic doses + extreme risk in combination with an MAOI |
| MAO inhibitors (MAOIs) | Non-selective irreversible MAOIs (phenelzine + tranylcypromine) + selective MAO-A inhibitor (moclobemide) + MAO-B inhibitor (selegiline + rasagiline - at high doses) + linezolid (antibiotic with MAOI properties) + IV methylene blue (weak MAOI) | Combination MAOI + any SRI = ABSOLUTE CONTRAINDICATION → risk of rapid and fatal serotonin syndrome + mandatory wash-out period (14 days for MAOI → SSRI + 14 days for SSRI → MAOI + EXCEPTION: fluoxetine → MAOI = 5 weeks due to very long half-life) |
| 5-HT receptor agonists | Triptans (sumatriptan + rizatriptan + almotriptan) + buspirone + LSD + psilocybin + fentanyl (low dose) + methadone (low dose) | Risk mainly in combination with SSRIs/SNRIs; triptans alone rarely implicated; caution with combination of triptan + SSRI/SNRI (low risk but reported). |
| Serotonin releasers | MDMA (ecstasy) + amphetamines + methylphenidate + cocaine | High risk in combination with SSRIs/MAOIs → toxicological emergency |
| Serotonin metabolism inhibitors | Linezolid (Zyvox® — MAO inhibitor) + IV methylene blue (methethylthioninium) + tedizolid | Major risk of Serotonin Syndrome (SS) if combined with an SSRI/SNRI → ideally stop the SSRI at least 2 weeks before linezolid + if urgent: monitor closely + serotonin syndrome possible even with the first dose of linezolid |
| Serotonin precursors | L-tryptophan + 5-HTP (5-hydroxytryptophan) | Risk in combination with SSRIs/MAOIs |
| Opioids with serotonergic effects | Tramadol +++ (serotonin norepinephrine reuptake inhibitor) + meperidine (pethidine) + fentanyl (weak) + oxycodone (weak) | Tramadol = frequent clinical trap → often prescribed in combination with an SSRI → possible serotonin syndrome at normal doses |
Hunter Criteria (2003) — Most Validated
- Use of a serotonergic agent PLUS AT LEAST ONE of the following signs:
- Spontaneous clonus: Spontaneous muscular rhythmic contractions (without external stimulus) + ankle or patella + most specific sign
- Induced clonus + agitation OR diaphoresis Clonus triggered by tendon stretch + agitation OR diaphoresis
- Ocular clonus + agitation OR diaphoresis horizontal rhythmic eye movements (nystagmus) + AND restlessness OR diaphoresis
- Tremors + hyperreflexia
- Hypertension + temperature > 38°C + ocular clonus OR inducible clonus: severe form
- Sensitivity and specificity of the Hunter criteria sensitivity 84 % + specificity 97 % + exceed Sternbach’s criteria + clonus is the key sign to systematically look for
Severity and clinical presentation
- Light form: Tachycardia + diaphoresis + tremor + myoclonus + mild anxiety + without significant hyperthermia + spontaneous resolution upon discontinuation of the medication
- Moderate form: Hyperreflexia + clonus (cardinal sign) + agitation + mydriasis + moderate hyperthermia (38–40 °C) + hypertension + tachycardia + diarrhea + profuse diaphoresis
- Severe form (life-threatening risk): Hyperthermia > 41 °C + severe muscle rigidity (opisthotonos) + convulsions + rhabdomyolysis (CK > 1,000 IU/L) + DIC + ARF (myoglobinuria) + lactic acidosis + ARDS + death if not treated within hours
- Temporal evolution: rapid onset following the introduction or increase of the causative medication (within 6 hours in 60% of cases + within 24 hours in 80% of cases) → rapid progression to severe forms without treatment
Differential diagnosis
- Neuroleptic Malignant Syndrome (NMS): Main differential diagnosis + caused by antipsychotics (haloperidol ++ + risperidone + olanzapine) → lead-pipe rigidity ++ + hyperthermia + dysautonomia + WITHOUT clonus + WITHOUT hyperreflexia + slow progression (24–72 h) + very high CK + CBC: leukocytosis → treatment: bromocriptine + dantrolene
- Anticholinergic intoxication hyperthermia + tachycardia + agitation + mydriasis + cutaneous flushing + xerostomia + urinary retention + NO clonus + NO hyperreflexia + NO diarrhea → treatment: physostigmine
- Sympathomimetic intoxication (cocaine + amphetamines): tachycardia + hypertension + hyperthermia + agitation + mydriasis + WITHOUT clonus. However, cocaine can also trigger serotonin syndrome due to serotonin release.
- Meningitis + encephalitis: fever + confusion + stiff neck → lumbar puncture + CRP + CBC
ℙ️
Tramadol is a frequent and under-recognized cause of serotonin syndrome, particularly when combined with an SSRI (a very common combination in clinical practice). In addition to its opioid effect, tramadol inhibits the reuptake of serotonin and norepinephrine—this dual mechanism makes it particularly risky when combined with antidepressants. Any patient taking an SSRI or SNRI for whom tramadol is being considered should be informed of the signs of SS and closely monitored, or an alternative analgesic (acetaminophen + NSAIDs if tolerated + low-dose pure opioids) should be preferred.
Treatment
- Immediate discontinuation of all serotonergic agents. most important measure + identify all serotonergic medications (prescription + over-the-counter + illicit drugs) + stop immediately
- Benzodiazepines (first-line treatment): diazepam 5–10 mg IV (or lorazepam 2–4 mg IV) + repeat every 5–10 min as needed + reduces agitation + myoclonus + rigidity + improves hyperthermia by reducing muscle rigidity + no dose ceiling if severe agitation → do not underdose out of fear
- Cyproheptadine (antihistamine 5-HT2A antagonist): 12 mg by mouth or via NG tube as a loading dose + then 2 mg every 2 hours until symptom control (max 32 mg/day) → specific pharmacological antidote + efficacy demonstrated in case series + oral only (no IV formulation) + may cause sedation + dry mouth
- Active cooling: If hyperthermia > 39–40 °C → external cooling (wet sheets + fans) + antipyretics (acetaminophen) → if hyperthermia > 41 °C → intubation + sedation + muscle relaxants + intensive cooling + dantrolene (used in malignant hyperthermia associated with anesthesia) is NOT indicated in SIH (different mechanism)
- Severe forms (ICU): oro-tracheal intubation if uncontrolled agitation + severe hyperthermia + respiratory failure + sedation (propofol + benzodiazepines + muscle relaxant if needed) + hemodynamic support + treatment of rhabdomyolysis (massive IV hydration) + hemodialysis if severe ARF
- TO AVOID: bromocriptine (dopamine agonist — risk of worsening SS) + phenergan (promethazine — dopamine antagonist but also antihistamine — may mask without treating) + haloperidol (masks symptoms + risk of NMS)
Medical emergency — dial 911
Call 911 or go to the emergency room immediately if a person taking antidepressants (SSRIs + SNRIs + tricyclics) + or tramadol + or any other serotonergic medication develops: agitation + confusion + tremor + fever + clonus (rhythmic jerking of the ankles or knees) + profuse sweating. These signs suggest serotonin syndrome — a medical emergency that can progress to fatal hyperthermia within hours without treatment. Stop the suspected medication immediately if possible. Do not administer other medications without medical advice. For checking serotonergic drug interactions as an outpatient, Clinique Omicron offers consultations at its service points in Quebec and via telemedicine. To make an appointment, visit cliniqueomicron.ca.
Consult at Clinique Omicron
Clinique Omicron's nurse practitioners (NPs) systematically check for serotonergic drug interactions before prescribing a new agent (tramadol + triptan + linezolid) to a patient already taking a serotonergic antidepressant, observe mandatory wash-out periods between MAOIs and SSRIs, recognize early signs of serotonin syndrome (clonus + hyperreflexia + agitation) and immediately refer to the emergency room, and educate patients on warning signs. Consultations are available at several service points in Quebec and via telemedicine. To book an appointment, visit cliniqueomicron.ca.
The content of this page is for informational purposes only and does not replace the advice of a doctor or emergency physician. Serotonin syndrome is an iatrogenic medical emergency whose prevention relies on systematically checking for serotonergic drug interactions before any prescription. Clonus is the most specific clinical sign of SS and should be actively sought in any suspect patient.
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