Clinical and neurophysiological findings in heterozygotes for nonketotic hyperglycinemia

Heterozygotes for nonketotic hyperglycinemia (NKH), a disorder of glycine degradation, have a slightly abnormal metabolism of glycine. As the severe neurological symptoms which are characteristic for the homozygotes are at least partially due to a disturbance of glycine function as a neurotransmitter, minor neuronal dysfunctions might be expected also in heterozygotes. Although their general health was within normal limits in these 13 heterozygotes, slight neurological symptoms and signs were observed. Neurophysiological investigations revealed disturbance of the vestibular function in six subjects, preponderance of beta-wave activity in five, subnormal amplitude of a-wave, and shortening of implicit time of the first oscillatory potential (OPI) in the retinography. Functioning of peripheral nerves appeared normal in measurements of motor conduction velocity, distal latency and amplitude of muscle response. These minor dysfunctions of the central nervous system may well be due to a slightly abnormal degradation of glycine in heterozygotes for nonketotic hyperglycinemia.