Biotinidase deficiency deterioration in the second decade, presenting as treatable cerebellar ataxia and encephalopathy masquerading as demyelination

Biotinidase deficiency (BTD) is an inherited metabolic disorder with neurological, dermatological and metabolic manifestations. Clinical deterioration in adequately treated older children is uncommon and raises concerns regarding drug non-compliance, dosage adequacy or, rarely, evolving disease mechanisms. Here we report a middle childhood female with <i>BTD</i> diagnosed based on low biotinidase levels from infancy, and on supplementation with biotin. The patient presented with ataxia and encephalopathy following a febrile illness. The work-up for autoimmune, demyelinating and infectious aetiologies was negative, and later she had one episode of acute metabolic crisis. The child's non-compliance with biotin for the last 3 months, and on restarting the child on biotin, along with symptomatic management of acute metabolic crisis, the child recovered. Drug non-compliance and management of acute metabolic crisis during intercurrent illness need to be emphasised. Knowledge of late-onset manifestations that can mimic demyelinating disorders prevents unnecessary workup.