Biotin-responsive encephalopathy with myoclonus, ataxia, and seizures.

Prominent neurological abnormalities, including myoclonus, seizures, ataxia, and hearing loss, have been noted in juvenile-onset biotin-responsive MCD. The underlying defect in many of these patients, who generally present in the first year of life, appears to be a deficiency of biotinidase. We have presented a young woman with adult-onset myoclonus, ataxia, hearing loss, seizures, hemianopia, and hemiparesis who responded to pharmacologic dosages of biotin. Although she displayed many of the clinical and biochemical features of juvenile-onset MCD, she did not have a biotinidase deficiency, and the underlying defect remains to be determined. Because of her response to biotin, we have advocated that other patients with unexplained myoclonus syndromes be evaluated for biotin-dependent carboxylase deficiencies and undergo a therapeutic trial with biotin.