💊 MAO Inhibitors

Monoamine Oxidase Inhibitors (MAOIs) are an older class of antidepressants that inhibit the activity of monoamine oxidase enzymes, leading to increased levels of neurotransmitters.

MAOIs inhibit the enzyme monoamine oxidase, which is responsible for metabolising monoamine neurotransmitters (noradrenaline, serotonin, dopamine) in the presynaptic neuron. This inhibition leads to increased concentrations of these neurotransmitters in the synaptic cleft.

MAOIs are generally reserved for atypical depression, treatment-resistant depression, or when other antidepressant treatments have failed due to their significant drug and food interactions. Phenelzine and tranylcypromine are examples of irreversible MAOIs. Moclobemide is a reversible inhibitor of MAO-A (RIMA) with a more favourable interaction profile.

Absolute contraindications include concomitant use with SSRIs, SNRIs, TCAs, triptans, pethidine, and other serotonergic drugs due to the high risk of serotonin syndrome. They are also contraindicated with tyramine-rich foods (e.g., aged cheeses, cured meats, some alcoholic beverages) due to the risk of hypertensive crisis. Severe cardiovascular disease, phaeochromocytoma, and severe liver impairment are also contraindications.

Common side effects include orthostatic hypotension, dizziness, insomnia, and sexual dysfunction. The most serious adverse effects are hypertensive crisis (due to tyramine interaction) and serotonin syndrome (due to drug interactions). Liver toxicity and peripheral neuropathy can also occur with some MAOIs.

Blood pressure should be monitored regularly, especially during the initial weeks of treatment, due to the risk of orthostatic hypotension and hypertensive crisis. Liver function tests may be warranted. Patients require extensive education on dietary restrictions and drug interactions. Monitoring for clinical response and adverse effects is crucial.

Prescribing MAOIs requires extreme caution. Always check for drug allergies and a comprehensive medication history to identify all potential interactions. Extensive patient counselling is paramount, covering strict dietary restrictions (tyramine-free diet) and a long list of contraindicated medications. Emphasise the importance of carrying an alert card and seeking immediate medical attention for severe headaches or other symptoms of hypertensive crisis.

MAOIs are rarely used in current UK practice due to their dangerous interactions and side effect profile, but their principles are high-yield for exams. The 'cheese reaction' (hypertensive crisis from tyramine) is a classic exam topic. A washout period is required when switching between MAOIs and other serotonergic antidepressants. Moclobemide (RIMA) has fewer dietary restrictions but still requires caution.

SBA questions almost invariably focus on the severe food and drug interactions of MAOIs, particularly the risk of hypertensive crisis with tyramine-rich foods and serotonin syndrome with other serotonergic drugs. Questions might involve identifying contraindicated food items or drugs, or managing these adverse events. Their use in treatment-resistant depression is also a common theme.