Cheung and Earl1 describe a family consisting of a mother and 2 sons exhibiting similar clinical and biochemical manifestations, which these authors suggest constitute a syndrome due to an inherited monoamine oxidase (MAO) deficiency. Elevated blood serotonin and normal or low-normal urinary 5-hydroxyindoleacetic acid (5-HIAA) levels are noted in all 3 subjects who manifest symptoms reminiscent of carcinoid syndrome, presumably largely due to their excess of circulating serotonin. These clinical features are long-term recurrent episodic facial flushing, headache, diarrhea, and psychiatric symptoms. The sons in particular are described as exhibiting symptoms strongly suggestive of attention-deficit/hyperactivity disorder.
Warner RRP. Monoamine Oxidase Deficiency: A Cause of Symptomatic Hyperserotoninemia in the Absence of Carcinoid. Arch Intern Med. 2002;162(14):1647–1648. doi: