💊 Tricyclic Antidepressants
Drug Class & Overview
Tricyclic Antidepressants (TCAs) are an older class of antidepressant medications named for their three-ring chemical structure. They are broad-spectrum inhibitors of neurotransmitter reuptake.
Mechanism of Action
TCAs primarily block the reuptake of noradrenaline and serotonin into presynaptic neurons. They also have significant antagonistic effects at muscarinic cholinergic, histamine H1, and alpha-1 adrenergic receptors, contributing to their diverse side effect profile.
Key Indications
While largely superseded by newer antidepressants, TCAs are still used for severe depression, particularly when other treatments have failed. Amitriptyline is commonly used at lower doses for neuropathic pain, chronic tension headaches, and migraine prophylaxis. Imipramine is used for nocturnal enuresis in children.
Contraindications
Absolute contraindications include recent myocardial infarction, severe heart block, and concomitant use with MAO inhibitors. Caution is required in patients with prostatic hypertrophy, angle-closure glaucoma, epilepsy, or significant cardiovascular disease. They are generally contraindicated in severe liver disease.
Adverse Effects
Common side effects include anticholinergic effects (dry mouth, constipation, blurred vision, urinary retention), sedation, and orthostatic hypotension. Cardiac effects, such as QT prolongation and arrhythmias, are serious concerns, especially in overdose. Weight gain, sexual dysfunction, and seizures can also occur.
Monitoring
ECG should be considered before starting treatment, especially in older adults or those with pre-existing cardiac conditions, and periodically thereafter. Blood pressure should be monitored due to the risk of orthostatic hypotension. Therapeutic drug monitoring may be useful for some TCAs to optimise efficacy and minimise toxicity. Regular monitoring for adverse effects is essential.
Prescribing Safety (OSCE)
When prescribing TCAs, always check for drug allergies and a comprehensive medication history to identify potential interactions, especially with MAOIs, other CNS depressants, and drugs affecting cardiac conduction. Counsel patients extensively on the significant side effects, particularly sedation and anticholinergic effects, and the importance of adherence. Emphasise the high toxicity in overdose.
MLA High-Yield Notes
TCAs have a narrow therapeutic index, making them dangerous in overdose, often leading to cardiac arrhythmias and seizures. Amitriptyline is frequently encountered in practice for chronic pain management at doses lower than those used for depression. Their anticholinergic and sedative properties are often exploited for specific indications (e.g., insomnia, neuropathic pain).
Common SBA Themes
SBA questions frequently focus on the characteristic side effects of TCAs, particularly anticholinergic effects, cardiac toxicity (especially in overdose), and orthostatic hypotension. Differentiating TCA overdose from other drug toxicities is a common theme. Questions might also explore their use in neuropathic pain or their place in therapy after failure of newer antidepressants.
References
- BNF (British National Formulary)
- NICE (National Institute for Health and Care Excellence) Guidelines
- MHRA (Medicines and Healthcare products Regulatory Agency) Guidance