Nonketotic hyperglycinemia (NKH) is a rare autosomal- recessive trait involving an error of glycine metabolism. Large amounts of the amino acid glycine accumulate in the body fluids, particularly in the cerebrospinal fluid. Two subtypes of this disorder exist: nonclassic NKH and neonatal NKH. The former appears later in life and presents with milder neurological deficits and varying degrees of mental retardation.