A novel missense mutation in a neonate with nonketotic hyperglycinemia.

Nonketotic hyperglycinemia (OMIM #605899), also known as glycine encephalopathy, is an autosomal recessive disorder of glycine metabolism caused by a defect in the glycine cleavage system. A term neonate developed progressive lethargy, muscular hypotonia, and respiratory insufficiency on day 2 after birth, but no overt clinical seizures. Amplitude-integrated electroencephalography indicated a continuous burst-suppression pattern. The diagnosis of nonketotic hyperglycinemia was made biochemically and was confirmed by genetic studies, which revealed two missense mutations (one not previously described) within the glycine decarboxylase gene, GLDC. Nonketotic hyperglycinemia should be incorporated into the differential diagnosis of neonatal hypotonia, to avoid an erroneous diagnosis of sepsis or hypoxic ischemic injury. Amplitude-integrated electroencephalography may be helpful in the initial assessment of severely sick and hypotonic neonates without overt clinical seizures, and may direct further diagnostic evaluation.