Pharmacology
Summary
Tricyclic antidepressants (TCAs) represent a group of antidepressant medications which include imipramine and its derivatives desipramine and clomipramine, as well as amitriptyline and nortriptyline. These drugs function by inhibiting the presynaptic uptake of neurotransmitters serotonin and norepinephrine. Their therapeutic applications extend beyond depression, as they are effective in treating conditions like diabetic neuropathy, chronic neuropathic pain, and migraine prevention. Notably, clomipramine, is utilized to manage obsessive-compulsive disorder (OCD), although SSRIs remain the first-line treatment.
However, TCAs come with a suite of side effects. They are known for causing sexual dysfunction, and their antagonistic effects on muscarinic acetylcholine receptors can lead to symptoms like dry mouth, constipation, blurred vision, and urinary retention. While secondary amines, such as nortriptyline and desipramine, present with fewer cholinergic effects, the anticholinergic and antihistamine actions of TCAs make them a less preferable choice for elderly patients. By antagonizing H1 histamine receptors, TCAs can spur increased appetite and consequent weight gain, as well as sedation. They can also cause orthostatic hypotension due to their alpha-1 receptors antagonism. TCAs are also associated with cardiotoxicity and can result in fatal cardiac arrhythmias--the leading cause of death in TCA overdose. TCAs block cardiac fast Na+ channels and can prolong the QRS complex, which can lead to torsades de pointes. Overdose symptoms, such as a widened QRS complex and ventricular arrhythmia, can be managed with sodium bicarbonate.
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FAQs
TCAs are employed for the management of depression, especially when other treatments have failed (treatment-resistant depression). Additionally, they are beneficial for chronic pain, diabetic neuropathy, and migraine prevention. While SSRIs are the primary choice for treating OCD, clomipramine, a TCA, is an alternative option.
TCAs have a range of side effects, some of which include sexual dysfunction, dry mouth, constipation, blurred vision, and urinary retention, owing to their inhibitory effects on muscarinic acetylcholine receptors. Other include increased appetite, weight gain, sedation, and orthostatic hypotension. TCAs can also widen the QRS complex and can lead to fatal cardiac arrhythmias in the context of an overdose.
Nortriptyline and desipramine are classified as secondary amines, resulting in fewer cholinergic side effects, such as dry mouth and urinary retention, compared to other TCAs.
In elderly patients, the use of TCAs is typically approached with caution due to the pronounced anticholinergic and antihistamine effects they elicit. These can lead to confusion, cognitive impairments, and heightened sedation. Additionally, older patients faces a more substantial risk of cardiotoxicity and orthostatic hypotension with TCA usage.
Overdosing on TCAs can manifest as a widened QRS complex on an ECG and potentially life-threatening cardiac arrhythmias like torsades. The standard intervention for these cardiac effects is the administration of sodium bicarbonate. Other critical symptoms, such as seizures and serotonin syndrome, also necessitate immediate medical intervention.
