Nonketotic hyperglycinemia: clinical, biochemical, and therapeutic considerations.

The salient features of nonketotic hyperglycinemia include apnea, feeding difficulties, lethargy, seizures, abnormal muscle tone and reflex activity, significant developmental delay, and, in most instances, early death. The pathogenesis of the biochemical defect leading to increased glycine concentration in blood, urine, and CSF is likely to concern derangements of the glycine cleavage enzyme and/or transport mechanisms of glycine. Our current state of knowledge of this disorder is incomplete. Therapeutic attempts, as described in Table 2, have been largely unsuccessful. Further basic research on the underlying biochemical perturbation, including additional documentation of the glycine cleavage enzyme deficiency patterns, of substrate inhibition of key metabolic pathways, and of glycine transport aberrations, as well as investigations of new pharmacologic approaches, will be a challenge for investigators in this field. It is hoped that new knowledge in these areas will eventually lead to reduction of morbidity and mortality in children with nonketotic hyperglycinemia.