Antidepressants & Serotonin Syndrome
A high-yield psych lesson mapping each antidepressant class to its signature board pearls, then drilling the serotonin syndrome triad, its cyproheptadine-based management, and the classic distinctions from NMS and TCA overdose. Emphasis throughout is on the diagnosis and \"next best step\" the boards reward.
The big picture
Every antidepressant works by boosting monoamine signaling — serotonin (5-HT), norepinephrine (NE), and/or dopamine — in the synaptic cleft. SSRIs are first-line for major depression and most anxiety-spectrum disorders because they are effective and safe in overdose. The trade-off is that all serotonergic drugs share one feared toxicity: serotonin syndrome, a state of excess serotonergic activity that typically appears when two serotonin-boosting drugs are combined or a dose is escalated.
Boards test three things over and over: (1) matching each class to its signature side effect, (2) recognizing the serotonin syndrome triad and separating it from neuroleptic malignant syndrome (NMS), and (3) the next best step — which is usually stop the drug + supportive care, with cyproheptadine held in reserve.
Antidepressant classes at a glance
| Class | Prototype drugs | Mechanism | Board-favorite pearls |
|---|---|---|---|
| SSRI | fluoxetine, sertraline, paroxetine, citalopram, escitalopram | block 5-HT reuptake | First-line; 4–6 wk to work; sexual dysfunction, GI upset, SIADH/hyponatremia (elderly); citalopram → QT prolongation; paroxetine → discontinuation syndrome; fluoxetine = longest half-life |
| SNRI | venlafaxine, duloxetine, desvenlafaxine | block 5-HT + NE reuptake | Also treat neuropathic pain / fibromyalgia; venlafaxine → ↑ BP; duloxetine → diabetic neuropathy |
| TCA | amitriptyline, nortriptyline, imipramine, clomipramine | block 5-HT + NE reuptake | Anticholinergic, α1-block (orthostasis), sedation; overdose = wide QRS → Na-bicarb; clomipramine → OCD |
| MAOI | phenelzine, tranylcypromine, selegiline, isocarboxazid | inhibit MAO-A/B | Tyramine → hypertensive crisis; washout required; highest serotonin syndrome risk |
| Atypical | bupropion, mirtazapine, trazodone | varied | Bupropion (NDRI): no sexual SE/weight gain, smoking cessation, ↓ seizure threshold; mirtazapine (α2 antagonist): ↑ appetite + sedation; trazodone: sedation, priapism |
- SSRIs are first-line for MDD, GAD, panic, OCD, PTSD; therapeutic effect takes 4–6 weeks even though side effects begin within days
- Black-box warning: increased suicidality in patients < 25 yrs early in treatment
- Bupropion — no sexual dysfunction, no weight gain, aids smoking cessation; lowers the seizure threshold, so it is contraindicated in seizure disorders and in eating disorders (bulimia/anorexia patients are already seizure-prone from electrolyte disturbances)
- Mirtazapine — weight gain + sedation; ideal for the depressed, cachectic, or insomniac elderly patient
- TCA overdose kills via the 3 C's (Cardiotoxicity, Convulsions, Coma); a wide QRS is the danger sign → IV sodium bicarbonate
- MAOI + tyramine (aged cheese, cured meats, red wine) → hypertensive crisis
- Abrupt stop of a short half-life agent (paroxetine, venlafaxine) → discontinuation syndrome (flu-like, dizziness, "brain zaps"); fluoxetine's long half-life is protective
SHIVERS — serotonin syndrome features:
- Shivering
- Hyperreflexia / myoclonus
- Increased temperature (hyperthermia)
- Vital sign instability (autonomic)
- Encephalopathy (altered mental status)
- Restlessness / agitation
- Sweating (diaphoresis)
TCA overdose = the 3 C's: Cardiotoxicity (wide QRS) · Convulsions · Coma
A 24-year-old on fluoxetine is started on tramadol for back pain (other high-yield culprits: linezolid, triptans, meperidine, dextromethorphan, MDMA, St. John's wort; ondansetron is a weaker, less classic association). Within hours she develops agitation, diaphoresis, dilated pupils, tremor, hyperthermia, tachycardia, hyperactive bowel sounds/diarrhea, and — the key finding — lower-extremity–predominant clonus and hyperreflexia (inducible or spontaneous).
NEXT BEST STEP:
- Diagnose clinically (Hunter Criteria) — there is no confirmatory lab; the tip-off is a recently added/up-titrated serotonergic drug.
- Stop ALL serotonergic agents.
- Supportive care: IV fluids, active cooling, and benzodiazepines for agitation and clonus.
- Moderate–severe or refractory → cyproheptadine (a 5-HT2A antagonist).
- Severe hyperthermia (> 41.1 °C) → sedation, intubation + non-depolarizing paralytic (avoid succinylcholine → hyperkalemia with rhabdo). Antipyretics do NOT work — the heat is from muscle activity, not the hypothalamic set point.
Serotonin syndrome vs. NMS — the classic swap
| Feature | Serotonin syndrome | NMS |
|---|---|---|
| Trigger | Serotonergic drug(s), esp. two combined | Dopamine antagonist (antipsychotic) or dopamine-agonist withdrawal |
| Onset | Rapid (< 24 h) | Slow (days) |
| Neuromuscular | Hyperreflexia, clonus, myoclonus (legs) | "Lead-pipe" rigidity, hypo/bradyreflexia |
| Pupils / gut | Mydriasis, hyperactive bowel sounds | Normal |
| Labs | Usually mild (CK can rise if severe → rhabdo) | ↑↑ CK, leukocytosis (rhabdo → AKI) |
| Antidote | Cyproheptadine + benzos | Dantrolene ± bromocriptine + benzos |
- Triad: (1) neuromuscular hyperactivity (clonus, hyperreflexia, myoclonus, tremor) · (2) autonomic instability (hyperthermia, tachycardia, diaphoresis, mydriasis) · (3) altered mental status (agitation, delirium)
- Clonus + hyperreflexia are the single most useful features separating it from NMS's rigidity
- Onset is fast (usually < 24 h) after adding or increasing a serotonergic drug
- Culprit combos: SSRI/SNRI + MAOI, + tramadol/meperidine, + linezolid (has MAO-A activity), + triptans, + dextromethorphan/St. John's wort/MDMA
- Prevention = washout: ~2 weeks between an MAOI and most SSRIs (either direction), but ~5 weeks after fluoxetine (long half-life) before starting an MAOI
- Antidote = cyproheptadine; antipyretics are ineffective
A 19-year-old is brought in after an intentional amitriptyline ingestion: anticholinergic toxidrome (dry, flushed skin; mydriasis; urinary retention; altered mental status), a seizure, and hypotension. ECG shows a widened QRS (> 100 ms) with a terminal R wave in aVR.
NEXT BEST STEP:
- Get an ECG immediately — QRS width predicts arrhythmia and seizure risk.
- Wide QRS, ventricular arrhythmia, or hypotension → IV sodium bicarbonate (overcomes the fast-Na-channel blockade and narrows the QRS).
- Seizures → benzodiazepines.
- Avoid class IA/IC antiarrhythmics (worsen Na-channel block) and avoid physostigmine (can precipitate asystole/seizures in TCA overdose).
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