- Glycine synthase deficiency
- Hyperglycinemia nonketotic
- Nonketotic hyperglycinemia
Your QuestionMy granddaughter has just been diagnosed with glycine encephalopathy. She is 3 months old. All the articles we have read are very complicated. What is glycine encephalopathy about? What treatment is there and what are her survival chances?
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About 85% of individuals affected in the neonatal period (first hours to days of life) have a severe form, and 15% have a mild form. Of the individuals who begin to have signs and symptoms in infancy (the infantile form), about 50% have a severe form and 50% have a mild form. Overall, about 20% of all infants with the neonatal or infant form have a mild outcome. Occasionally, affected individuals have an intermediate outcome between mild and severe.
Individuals with a mild form of the disease can have variable degrees of developmental progress; they may learn to walk, interact with others and attend special education classes. One study found that up to 20% of surviving children learn to walk and say or sign words. These individuals may have little spasticity and they often develop a treatable seizure disorder.
Individuals with a severe form typically do not make developmental progress. At most, they may learn to sit and have very limited interaction with their environment. During the first year of life, they typically develop seizures that become increasingly difficult to treat, usually requiring multiple anticonvulsant medications. They typically have progressive spasticity early on, they have a tendency to develop scoliosis, and they often have swallowing dysfunction that requires tube feeding.
There have been affected individuals with "atypical" forms of the condition with variable signs and symptoms; these forms have ranged from milder disease with onset from late infancy to adulthood, to rapidly progressing and severe disease with late onset. The most common "atypical" form is known as the infantile form and is characterized by hypotonia, developmental delay and seizures. Individuals with this form may develop normally until signs and symptoms begin at approximately 6 months of age. As they age, many of these individuals develop intellectual disability, abnormal movements and behavioral problems. Other atypical forms of glycine encephalopathy can appear later in childhood or adulthood and cause a variety of medical problems that primarily affect the nervous system.
The goal of treatment is to reduce the amount of glycine in the plasma (blood). Treatment may involve a medication called sodium benzoate, which binds with glycine allowing it to be passed out in the urine, and dextromethorphan, ketamine, or felbamate, which block some of the harmful effects of excessive glycine. These treatments may help control seizures, increase alertness, and in mildly affected individuals, improve behavior. Drug dosage must be individually tailored and requires regular and careful monitoring. Studies regarding the effectiveness of these treatments are ongoing. Mildly affected individuals may receive the greatest benefit from treatment, particularly if treatment is started early.
Other treatments include drugs to control seizures (anti-epileptic drugs); assistive devices or surgeries to aid with feeding and swallowing (e.g., gastrostomy tube); physical therapy; and scoliosis management. Parents and family members may benefit from genetic counseling. Click here to learn more about genetic consultations.
For further details on treatment, please visit the following link to GeneReviews. GeneReviews provides current, expert-authored, peer-reviewed, full-text articles describing the application of genetic testing to the diagnosis, management, and genetic counseling of patients with specific inherited conditions. Because of the complexity of the information in the article, we recommend that you review it with a health care provider.
- Ada Hamosh, Gunter Scharer, Johan Van Hove. Glycine Encephalopathy. GeneReviews. November 24, 2009; http://www.ncbi.nlm.nih.gov/books/NBK1357/. Accessed 10/28/2011.
- Glycine encephalopathy. Genetics Home Reference. April 2007; http://ghr.nlm.nih.gov/condition/glycine-encephalopathy. Accessed 10/28/2011.
- Hamosh A, Scharer G, Van Hove J. Glycine encephalopathy. GeneReviews. November 2009. ; http://www.ncbi.nlm.nih.gov/books/NBK1357/. Accessed 11/10/2011.
- Glycine encephalopathy. Genetics Home Reference Website. April 2007; http://ghr.nlm.nih.gov/condition=glycineencephalopathy. Accessed 10/7/2008.
- Van Hove JL, Vande Kerckhove K, Hennermann JB, Mahieu V, Declercq P, Mertens S, De Becker M, Kishnani PS, Jaeken J. Benzoate treatment and the glycine index in nonketotic hyperglycinaemia. J Inherit Metab Dis. 2005;28(5):651-63; http://www.ncbi.nlm.nih.gov/pubmed/16151895. Accessed 8/26/2011.