Late-onset maple syrup urine disease triggered by human herpesvirus-6 mediated encephalopathy in a toddler

Metabolic crisis should always be on the differential diagnosis of a toddler presenting with focal neurologic signs and refractory ketoacidosis. Maple syrup urine disease (MSUD) is an autosomal recessive metabolic disorder caused by an enzyme deficiency in the branched-chain α-ketoacid dehydrogenase (BCKDH) complex that leads to toxic buildup of the branched-chain amino acids (BCAA) leucine, isoleucine, and valine. Classically diagnosed in the neonatal period (especially with the advent of newborn screening), our patient is a rare case of a previously healthy toddler with late-onset or intermittent MSUD triggered by human herpesvirus-6 and a fasting state. Though MSUD as a diagnosis is incredibly rare beyond the neonatal period, prompt recognition and treatment can be life-saving and lead to good outcomes.