Toxicology – Tricyclic Antidepressant (TCA) Toxicity

Source

Tricyclic antidepressants were once widely used for depression but are now also prescribed for conditions such as neuropathic pain, migraine prevention, OCD, and nocturnal enuresis. Examples include amitriptyline, clomipramine, desipramine, doxepin, imipramine, and nortriptyline.

Typical Presentation

A patient with TCA overdose may present with altered mental status, anticholinergic findings, seizures, hypotension, or ECG abnormalities, especially a widened QRS complex.

Clinical Features

Toxicity commonly affects the nervous and cardiovascular systems.

Anticholinergic findings:

• Dilated pupils
• Dry skin and mucous membranes
• Flushing
• Hyperthermia
• Tachycardia
• Urinary retention
• Confusion

Other important findings include:

• Sedation progressing to coma
• Seizures
• Orthostatic hypotension
• Cardiac arrhythmias

ECG abnormalities may include:

• Sinus tachycardia
• PR prolongation
• QRS widening
• QT prolongation
• Potentially fatal ventricular dysrhythmias

Mechanism of Action

TCAs have several pharmacologic actions, including:

• Anticholinergic activity
• Antihistamine effects
• α-adrenergic blockade
• Inhibition of serotonin and norepinephrine reuptake

Their most dangerous cardiac effect results from voltage-gated sodium channel blockade, which slows cardiac conduction and widens the QRS. Potassium-channel effects may also contribute to QT prolongation.

Management

Treatment requires rapid supportive care:

• Airway and respiratory support as needed
• Continuous cardiac monitoring and serial ECGs
• Benzodiazepines for seizures
• IV fluids for hypotension
• Sodium bicarbonate is the key therapy when significant QRS widening, ventricular dysrhythmias, or persistent hypotension is present
• Vasopressor support, typically norepinephrine, may be required if hypotension persists
• IV lipid emulsion may be considered in severe refractory toxicity with specialist guidance

Key Points

• QRS widening is an important marker of severe TCA toxicity.
• Increasing QRS duration is associated with a greater risk of seizures and ventricular arrhythmias.
• Cyclobenzaprine is structurally similar to TCAs and may produce a similar overdose pattern.
• Some TCAs, particularly desipramine, may produce prominent cardiotoxicity even without obvious anticholinergic findings.