M06.15.004 Tricyclic Antidepressants (TCAs)

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Learning Objectives

By the end of this session, the learner will be able to identify the primary TCAs, explain their nonspecific reuptake inhibition mechanism, and recognize the clinical hallmark of their toxicity—the “3 C’s” (Coma, Convulsions, Cardiotoxicity).

1. Mechanism and Drugs

TCAs are older antidepressants that are highly effective but have a narrow therapeutic index due to their lack of selectivity.

Mechanism Common TCA Drugs
Nonspecific blockade of 5-HT and NE reuptake. Amitriptyline, Imipramine, Clomipramine.

2. Clinical Indications

Because of their side-effect profile, TCAs are rarely first-line for depression today but remain vital for specific “niche” indications.

Indication Specific Drug / Note
Major Depression Typically reserved for treatment-resistant cases.
OCD Clomipramine is the gold standard (though SSRIs are used first).
Neuropathic Pain Amitriptyline is frequently used at low doses for chronic pain/migraines.
Enuresis (Bedwetting) Imipramine (decreases Stage 4 sleep).

3. Side Effects: The “Off-Target” Blockade

The toxicity of TCAs comes from their blockade of receptors other than the reuptake transporters.

Receptor Blocked Clinical Side Effect
Muscarinic (M_1) Anticholinergic: Dry mouth, blurred vision, constipation, urinary retention.
Alpha-1 (\alpha_1) Orthostatic hypotension (risk of falls in the elderly).
Histamine (H_1) Sedation, somnolence, and weight gain.
Fast Na^+ Channels Cardiotoxicity: Arrhythmias and QRS prolongation.

Toxicity: The 3 C’s of TCA Overdose

A TCA overdose is a medical emergency. The classic triad includes: Coma (due to H_1 and M_1 blockade).Convulsions (seizures).Cardiotoxicity (due to Na^+ channel blockade leading to Wide QRS and fatal arrhythmias).
Antidote: Sodium Bicarbonate (NaHCO_3)—it helps overcome the $Na^+$ channel blockade.

Clinical Pearls:

  • Alpha-2 Agonist Interaction: TCAs block the reuptake of NE. Therefore, they prevent the antihypertensive effect of drugs like Clonidine (which rely on sympathetic inhibition).
  • Serotonin Syndrome: Similar to SNRIs, TCAs must never be combined with MAOIs or other serotonergic drugs.
  • Elderly Caution: Due to alpha_1 and M_1 blockade, TCAs are listed on the Beers Criteria as potentially inappropriate for the elderly (risk of delirium and fractures).

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