Biotinidase deficiency: A treatable cause of opticospinal syndrome in young adults

•Biotinidase deficiency is a rare autosomal metabolic disorder.•It is easily tested by measuring biotinidase activity in serum or through whole blood spotted on filter paper in newborns.•Newborn screening tests are available since 1984, but not globally implemented.•Symptoms usually present in childhood, but onset in early adulthood as an opticospinal syndrome is possible.•Biotinidase deficiency is a treatable disorder. Diagnosis of biotinidase deficiency is rare and usually made in infancy, through newborn screening or after presenting symptoms. We present the case of 19-year old male with progressive optic atrophy and in a second phase spinal cord syndrome unresponsive to immunosuppressive therapies. After diagnosis of profound biotinidase deficiency, oral biotin substitution was started with partial visual improvement and normalization of gait. This case highlights the possibility of late-onset biotinidase deficiency and its treatable character.