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  • Updated 05.24.2023
  • Released 03.30.1995
  • Expires For CME 05.24.2026

Monoamine oxidase deficiency



Monoamine oxidase-A deficiency is an X-linked disorder affecting the catabolism of serotonin and the catecholamines. Main symptoms are mild intellectual disability and behavioral abnormalities consisting of excessive, sometimes violent aggression. Diagnosis may be inferred from a finding of elevated urinary concentrations of the monoamine oxidase-A substrates, normetanephrine, 3-methoxytyramine, and tyramine, in combination with reduced amounts of the monoamine oxidase products, vanillylmandelic acid, homovanillic acid, 3-methoxy-4-hydroxyphenolglycol, and 5-hydroxyindoleacetic acid. Monoamine oxidase deficiency has been found as an isolated defect affecting monoamine oxidase A and, in combination with a deletion of the Norrie disease gene, as a combined deficiency of monoamine oxidase A and B or an isolated deficiency of monoamine oxidase B. Confirmation of monoamine oxidase-A deficiency is obtained by measurement of the activity of this enzyme in dexamethasone-stimulated fibroblasts. Combined monoamine oxidase-A and -B deficiency has similar biochemical consequences. Presentation is with intermittent hypotonia, stereotypic hand movements, and developmental delay.

Key points

• Monoamine oxidase deficiency is inherited as an X-linked trait.

• Monoamine oxidase exists in two distinct forms encoded by separate genes, monoamine oxidase-A and monoamine oxidase-B, and each has preferential affinities for biogenic amine and other amine substrates.

• Monoamine oxidase deficiency has been described as isolated monoamine oxidase-A deficiency, as a combined monoamine oxidase-A and B deficiency, as a combined monoamine oxidase-A and B deficiency in association with Norrie disease, and as an isolated monoamine oxidase-B deficiency in association with Norrie disease.

• Individuals with monoamine oxidase deficiency should avoid foods or drugs containing amines.

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