Hyperglycinuria and hyperglycinemia in two siblings with mild developmental delays.

Two preschool-age siblings with similar histories of encephalopathy were examined for developmental retardation and found to have elevated levels of urinary and blood glycine. Their inability to convert glycine into serine in the absence of elevated blood and urinary ketone levels was suggestive of a defect in the glycine-cleavage enzyme system (or serine hydroxymethyl transferase). These patients differ significantly from the majority of reported cases of nonketotic hyperglycinemia in that they did not manifest life-threatening neonatal illness, severe mental retardation, or neurological deficits. However, during an oral glycine load, alterations in the electroencephalographic pattern occurred that suggested a relationship between elevated blood glycine levels and pathological involvement of the central nervous system. The ratio of CSF-blood glycine was found to be in the range expected for nonketotic hyperglycinemia.