A rare inherited disorder where lack of a certain enzyme (3-methylcrotonyl-Coa carboxylase) stops proteins with the amino acid leucine being metabolized normally by the body. The leucine builds up in the body and causes damage to the brain and nervous system. The severity of the condition is variable with some cases being mild enough to be asymptomatic. See also 3-methylcrotonyl-CoA carboxylase deficiency