Monoamine oxidase inhibitors (MAOIs) bind (through an irreversible covalent bond) and inactivate monoamine oxidase (MAO). MAO is primarily found on mitochondrial membranes and is responsible for the metabolism of endogenous and exogenous biogenic amines in the presynaptic nerve terminal, liver, and intestinal mucosa. Two forms of this enzyme exist, MAO-A and MAO-B. MAO-A is responsible for the degradation of norepinephrine, epinephrine, and serotonin. MAO-B degrades phenylethylamine. Dopamine and tyramine are substrates of both MAO-A and MAO-B. By blocking the metabolism of these biogenic amines, MAOIs increase the intraneuronal levels of amine neurotransmitters. MAOIs are not often used in current practice because safer options are available, but they are still used in some patients with depression, Parkinson disease, and panic attacks whose symptoms do not respond or who become refractory to other therapies. Anesthesia providers should be aware of the side effects and potential life-threatening interactions that may occur when patients undergoing an anesthetic have been taking an MAOI.

MAOIs are classified as either hydrazide or nonhydrazide derivatives. Additionally, they are either selective or nonselective inhibitors of MAO. The MAOIs available for use in the United States include phenelzine, tranylcypromine, isocarboxazid, selegiline, and rasagiline (Table 96-1). Phenelzine, tranylcypromine, and isocarboxazid inhibit the activity of both MAO-A and MAO-B. Selegiline and rasagiline block MAO-B and are used for the treatment of Parkinson disease. Although selegiline and rasagiline are selective inhibitors of MAO-B, they lose their selectivity in a dose-dependent fashion. The lowest strength (6-mg) selegiline patch is efficacious in the treatment of depression without the need for dietary modification. This mode of drug delivery allows selegiline to bypass first-pass hepatic metabolism and be delivered to the brain in a concentration that inhibits both MAO-A and MAO-B.