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TCA vs SSRI Overdose & Serotonin Syndrome

TL;DR

For MRCP Part 1, differentiating TCA vs SSRI overdose & serotonin syndrome is essential and frequently tested. TCAs cause life-threatening cardiac toxicity (QRS widening, arrhythmias), whereas SSRIs are generally safer but can trigger serotonin syndrome. Recognising key clinical patterns and initiating correct management (e.g., sodium bicarbonate for TCA toxicity) is crucial for scoring well. Focus on ECG findings, toxidromes, and first-line treatments.


Why this matters

Antidepressant overdose is a classic MRCP Part 1 topic, combining pharmacology, acute medicine, and cardiology. Questions often hinge on recognising distinct toxidromes and selecting the correct immediate treatment.

A structured approach using the MRCP Part 1 overview alongside active recall via Free MRCP MCQs can significantly improve retention and exam performance.


Core sections

1) Mechanism of toxicity (high-yield contrast)

  • Tricyclic antidepressants (TCAs):

    • Block fast sodium channels → cardiac conduction delay

    • Anticholinergic effects → delirium, dry skin

    • GABA antagonism → seizures

  • Selective serotonin reuptake inhibitors (SSRIs):

    • Increase serotonin levels

    • Risk mainly from serotonin toxicity, especially in combinations

2) Clinical features comparison

Feature

TCA Overdose

SSRI Overdose

Serotonin Syndrome

CNS

Coma, seizures

Mild drowsiness

Agitation, confusion

CVS

Arrhythmias, hypotension

Usually stable

Tachycardia, hypertension

Pupils

Dilated

Normal/slightly dilated

Dilated

Reflexes

Normal/reduced

Normal

Hyperreflexia, clonus

Temperature

Mild ↑

Normal

Hyperthermia

👉 Exam pearl: Hyperreflexia and inducible clonus strongly suggest serotonin syndrome.

3) ECG findings (very high-yield)

  • TCA overdose hallmark findings:

    • QRS widening >100 ms → seizure risk

    • QRS >160 ms → ventricular arrhythmias

    • Right axis deviation

    • Prominent R wave in aVR

  • SSRIs:

    • Usually normal ECG

    • Citalopram may cause QT prolongation

4) Management essentials

TCA overdose (medical emergency)

  1. Airway, breathing, circulation (ABC)

  2. IV sodium bicarbonate (first-line)

  3. Benzodiazepines for seizures

  4. Continuous cardiac monitoring

SSRI overdose

  • Supportive management

  • Activated charcoal (early presentation)

  • Monitor ECG if citalopram ingestion

Serotonin syndrome

  1. Stop serotonergic drugs

  2. Sedation → benzodiazepines

  3. Cyproheptadine (antidote)

  4. Active cooling

5) Five most tested subtopics

  1. ECG interpretation in TCA toxicity

  2. Sodium bicarbonate indications

  3. Diagnostic triad of serotonin syndrome:

    • Mental status changes

    • Autonomic instability

    • Neuromuscular hyperactivity

  4. Drug interactions (e.g., SSRI + MAOI)

  5. Differentiating toxidromes

6) High-yield checklist (exam revision)

  1. TCA overdose = cardiotoxic + seizures

  2. QRS >100 ms → sodium bicarbonate

  3. SSRIs usually safer than TCAs

  4. Serotonin syndrome = clonus + hyperreflexia

  5. Cyproheptadine treats serotonin syndrome

  6. Avoid physostigmine in TCA overdose

  7. Anticholinergic signs = dry skin, confusion

  8. Always perform ECG in overdose


Practical examples / mini-cases

Case MCQ

A 32-year-old man presents after a deliberate overdose. He is drowsy, hypotensive, and has a QRS duration of 140 ms. Pupils are dilated and skin is dry.

What is the most appropriate next step?

A) CyproheptadineB) IV sodium bicarbonateC) Magnesium sulphateD) Activated charcoal only

Answer: B) IV sodium bicarbonate

Explanation: This is classic TCA toxicity with cardiotoxic features (wide QRS, hypotension). Sodium bicarbonate reverses sodium channel blockade and reduces arrhythmia risk.


MRCP Part 1 revision setup with notes on antidepressant overdose and ECG interpretation

Common pitfalls (5 bullets)

  • Confusing serotonin syndrome with anticholinergic toxicity

  • Missing QRS widening on ECG

  • Using flumazenil in overdose settings

  • Assuming SSRIs are always harmless

  • Forgetting drug interaction triggers


FAQs

1) How can serotonin syndrome be distinguished clinically?

Look for clonus and hyperreflexia, which are not seen in anticholinergic toxicity.

2) What is the hallmark ECG finding in TCA overdose?

QRS widening, particularly >100 ms, is the key finding.

3) Which antidepressant overdose is most dangerous?

TCAs, due to their cardiotoxic and pro-arrhythmic effects.

4) What is the antidote for serotonin syndrome?

Cyproheptadine, along with supportive care.

5) Are SSRIs safe in overdose?

Generally yes, but they can cause serotonin syndrome or QT prolongation in certain cases.


Ready to start?

To consolidate this topic, revise systematically using the MRCP Part 1 overview, practise actively with Free MRCP MCQs, and test readiness via a Start a mock test.


Sources

 
 
 

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