Letter: Of worms and asthma, or Tullis revisited

Dextromethorphan and high-dose benzoate therapy for nonketotic hyperglycinemia in an infant

To test the hypothesis that nonketotic hyperglycinemia causes overstimulation of the excitatory N-methyl-D-aspartate receptor by allosteric glycine activation, and that reduction of glycine and blocking of the cation channel coupled to the receptor would be beneficial, we administered benzoate and dextromethorphan, a blocker of the N-methyl-D-aspartate channel to an infant with nonketotic hyperglycinemia. Therapy with benzoate, 500 mg/kg per day, was started on day 5, and the dosage was increased to 750 mg/kg per day on day 8, with prompt normalization of the neurologic and electroencephalographic findings. The glycine concentrations in both plasma and cerebrospinal fluid were substantially reduced. Dextromethorphan was added to the regimen on day 12. The electroencephalogram remained normal until the infant was 8 months of age, when diffuse slowing became apparent. Serial brain magnetic resonance imaging showed delayed myelination. At 12 months of age, physical examination findings and growth were normal except for hypotonia. The developmental quotient was approximately 60, and the child was free of seizures. This outcome, although not ideal, is better than that typical for nonketotic hyperglycinemia. Our results suggest that trials with additional patients and other N-methyl-D-aspartate cation channel blockers are warranted.

Nonketotic hyperglycinemia: analyses of glycine cleavage system in typical and atypical cases

The molecular nature of the glycine cleavage system was investigated in eight patients with typical (neonatal) and two patients with atypical (late onset) nonketotic hyperglycinemia (NKH). The overall activity of the glycine cleavage system was found to be decreased in all of the liver and brain tissue studied, but it was undetectable or extremely low in typical NKH, whereas there was some residual activity in atypical NKH. Six patients with typical NKH had a specific defect in the P protein, and one a defect in the T protein; the activity of the T protein was defective in one patient with atypical NKH.

A treatment of non-ketotic hyperglycinaemia

The treatment comprising a special diet (without glycine, serine, and with a reduced amount of threonine), strychnine nitrate and ursodesoxycholic acid (UDCA) led to normoglycinaemia in this form of severe non-ketotic glycine encephalopathy. Diet and treatment were well tolerated but without significant effect upon psychomotor development. This treatment should be more effective if administered before irreversible brain damage occurs, particularly in moderate and chronic forms of NKH.

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.