: 793   Glycine concentrations in the CSF of affected patients are typically more markedly elevated than in plasma, leading to a corresponding elevation in the ratio of glycine concentrations in the cerebral spinal fluid to that in the plasma. This ratio can also be slightly elevated in patients receiving valproic acid. : 811 Glycine encephalopathy (nonketotic hyperglycinemia, or NKH) should not be confused with other metabolic disorders that can produce elevated glycine levels. For example, certain inherited organic acidurias (aka organic acidemias) can produce elevated glycine in plasma and urine, although these disorders are not caused by defects in the glycine cleavage system, and they are not typically accompanied by corresponding elevations of glycine in cerebrospinal fluid (CSF). Glycine encephalopathy is unique in the fact that levels of glycine are disproportionately elevated in CSF (in addition to elevations in blood and urine), whereas CSF glycine levels are normal or near-normal in patients with inherited organic acidurias. Glycine metabolism
Glycine is metabolized in the bodys cells to end products of carbon dioxide and ammonia. The glycine cleavage system, which is responsible for glycine metabolism in the mitochondria is made up of four protein subunits, the P-protein, H-protein, T-protein and L-protein. : 793

Diagnosis
Classification
There are several different forms of glycine encephalopathy, which can be distinguished by the age of onset, as well as the types and severity of symptoms. All forms of glycine encephalopathy present with only neurological symptoms, including intellectual disability (IQ scores below 20 are common), hypotonia, apneic seizures, and brain malformations.
However, levels of glycine in plasma and cerebrospinal fluid typically normalize within eight weeks, and in five of six cases there were no neurological issues detected at follow-up times up to thirteen years. A single patient was severely retarded at nine months. The suspected cause of transient neonatal hyperglicinemia is attributed to low activity of the glycine cleavage system in the immature brain and liver of the neonate. Treatment
A treatment of sodium benzoate, which binds to glycine and forms hippurate, and dextromethorphan, which weakly inhibits the N-methyl-D-aspartate receptors that glycine acts on has been shown to improve outcomes in select cases where the disorder is present in attenuated form. Prognosis
The prognosis is very poor. Two studies reported typical age of deaths in infancy or early childhood, with the first reporting a median age of death of 2.6 for boys and less than 1 month for girls. Research
Response to treatment is variable and the long-term and functional outcome is unknown. To provide a basis for improving the understanding of the epidemiology, genotype/phenotype correlation and outcome of these diseases their impact on the quality of life of patients, and for evaluating diagnostic and therapeutic strategies a patient registry was established by the noncommercial International Working Group on Neurotransmitter Related Disorders (iNTD). See also
List of amino acid metabolism disorders
inborn errors of metabolism

References


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