Efficacy of Low-dose Dextromethorphan in the Treatment of Nonketotic Hyperglycinemia

¹ Department of Pediatrics, University of Tennessee Graduate School of Medicine, Knoxville, TN 37920
² Developmental & Genetic Center, University of Tennessee, Graduate School of Medicine, Knoxville, TN 37920

Nonketotic hyperglycinemia (NKH) is an inborn error of glycine degradation causing muscular hypotonia, seizures, apnea, and lethargy; it has a poor prognosis. Accumulation of glycine in the brain is thought to cause excessive stimulation of the N-methyl-D-aspartate receptor. Dextromethorphan (DM), an N-methyl-D-aspartate receptor antagonist, in doses of 5 to 35 mg/kg per day has been shown to have beneficial therapeutic effects in some patients with NKH. We report the case of a 1-year-old infant with NKH, seizure disorder, and psychomotor delay who was clinically seizure free during treatment with sodium benzoate, arginine, benzodiazepam, and phenobarbital. Although sodium benzoate normalized serum glycine levels (103 to 125 µmol/L), cerebrospinal fluid glycine levels remained elevated (42 to 47 µmol/L), with epileptiform activity on electroencephalography. The addition of low-dose DM (0.25 mg/kg per day) to the treatment led to improvement of electroencephalographic activity, resolution of nystagmus with increased eye contact, and modest progression of developmental milestones. These data suggest that DM at doses significantly lower than previously reported may be beneficial in some patients with NKH. Treatment with low-dose DM needs further evaluation.