Prevalence estimates on Rare Medical Network websites are calculated based on data available from numerous sources, including US and European government statistics, the NIH, Orphanet, and published epidemiologic studies. Rare disease population data is recognized to be highly variable, and based on a wide variety of source data and methodologies, so the prevalence data on this site should be assumed to be estimated and cannot be considered to be absolutely correct.
Age of Onset
Autosomal dominant ?A pathogenic variant in only one gene copy in each cell is sufficient to cause an autosomal dominant disease
Autosomal recessive ?Pathogenic variants in both copies of each gene of the chromosome are needed to cause an autosomal recessive disease and observe the mutant phenotype
X-linked dominant ?X-linked dominant inheritance, sometimes referred to as X-linked dominance, is a mode of genetic inheritance by which a dominant gene is carried on the X chromosome.
X-linked recessive ?Pathogenic variants in both copies of a gene on the X chromosome cause an X-linked recessive disorder
Mitochondrial or multigenic ?Mitochondrial genetic disorders can be caused by changes (mutations) in either the mitochondrial DNA or nuclear DNA that lead to dysfunction of the mitochondria and inadequate production of energy.
Multigenic or multifactor ?Inheritance involving many factors, of which at least one is genetic but none is of overwhelming importance, as in the causation of a disease by multiple genetic and environmental factors.
Other Names (AKA)
Hyperglycinemia nonketotic; Nonketotic hyperglycinemia; Glycine synthase deficiency;
- Classical neonatal form (most common): Symptoms start within a few days of life and may include poor feeding, lack of energy (lethargy), weak muscle tone (
hypotonia), hiccups, breathing problems, seizures, hiccups, and coma.
- Infantile form: Symptoms start only after 6 months of age, as
intellectual disability, abnormal movements, and behavioral problems
- Late onset: Symptoms include tightness or stiffness of the legs or arms (spastic diplegia), and vision loss due to a damage of the eye nerve (optic atrophy).
- Transient form: Symptoms are similar to the classic form, but glycine levels decrease and the symptoms may improve within time.
Glycine encephalopathy is caused by changes (
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,
This table lists symptoms that people with this disease may have. For most diseases, symptoms will vary from person to person. People with the same disease may not have all the symptoms listed. This information comes from a database called the Human Phenotype Ontology (HPO) . The HPO collects information on symptoms that have been described in medical resources. The HPO is updated regularly. Use the HPO ID to access more in-depth information about a symptom.
|Medical Terms||Other Names||
|80%-99% of people have these symptoms|
|Abnormal metabolic brain
Elevated blood glycine levels
|Hypoplasia of the
Underdevelopment of part of brain called corpus callosum
|30%-79% of people have these symptoms|
|Generalized myoclonic seizure||0002123|
|Percent of people who have these symptoms is not available through HPO|
|Agenesis of corpus callosum||0001274|
[ more ]
|Death in infancy||
Lethal in infancy
[ more ]
Decreased muscle tone
Low muscle tone
[ more ]
More active than typical
High urine glycine levels
Decreased reflex response
[ more ]
Mental retardation, nonspecific
[ more ]
Low or weak muscle tone
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);
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
Related diseases are conditions that have similar signs and symptoms. A health care provider may consider these conditions in the table below when making a diagnosis. Please note that the table may not include all the possible conditions related to this disease.
Conditions with similar signs and symptoms from Orphanet
Differential diagnosis includes organic acidemias that may present hyperglycinemia such as D-glyceric acidemia, propionic acidemia, methylmalonic acidemia, isovaleric acidemia, and ketoacidosis due to beta-ketothiolase deficiency (see these terms). Conditions characterized by neonatal seizures should also be considered. Valproate treatment may cause hyperglycinemia.
Visit the Orphanet disease page for more information.
Support and advocacy groups can help you connect with other patients and families, and they can provide valuable services. Many develop patient-centered information and are the driving force behind research for better treatments and possible cures. They can direct you to research, resources, and services. Many organizations also have experts who serve as medical advisors or provide lists of doctors/clinics. Visit the group’s website or contact them to learn about the services they offer. Inclusion on this list is not an endorsement by GARD.
Nonketotic Hyperglycinemia International Family Network
1401 Ridgefield Avenue
Ocoee, FL 34761
These resources provide more information about this condition or associated symptoms. The in-depth resources contain medical and scientific language that may be hard to understand. You may want to review these resources with a medical professional.
- Genetics Home Reference (GHR) contains information on Glycine encephalopathy. This website is maintained by the National Library of Medicine.
- The National Organization for Rare Disorders (NORD) has a report for patients and families about this condition. NORD is a patient advocacy organization for individuals with rare diseases and the organizations that serve them.
- 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.
- The Monarch Initiative brings together data about this condition from humans and other species to help physicians and biomedical researchers. Monarch’s tools are designed to make it easier to compare the signs and symptoms (phenotypes) of different diseases and discover common features. This initiative is a collaboration between several academic institutions across the world and is funded by the National Institutes of Health. Visit the website to explore the biology of this condition.
- Online Mendelian Inheritance in Man (OMIM) is a catalog of human genes and genetic disorders. Each entry has a summary of related medical articles. It is meant for health care professionals and researchers. OMIM is maintained by Johns Hopkins University School of Medicine.
- Orphanet is a European reference portal for information on rare diseases and orphan drugs. Access to this database is free of charge.
- PubMed is a searchable database of medical literature and lists journal articles that discuss Glycine encephalopathy. Click on the link to view a sample search on this topic.
- Hamosh A, Scharer G, Van Hove J. Glycine encephalopathy. GeneReviews. 2013; https://www.ncbi.nlm.nih.gov/books/NBK1357/.
- Glycine encephalopathy. Genetics Home Reference. April 2007; https://ghr.nlm.nih.gov/condition/glycine-encephalopathy.
- Iqbal M, Prasad M & Mordekar SR. Nonketotic hyperglycinemia case series. Journal of Pediatric Neurosciences. 2015; 10(4):355-358. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4770648/.
- Ada Hamosh, Gunter Scharer, Johan Van Hove. Glycine Encephalopathy. GeneReviews. July 11, 2013; https://www.ncbi.nlm.nih.gov/books/NBK1357/. Accessed 6/9/2015.
- Glycine encephalopathy. Genetics Home Reference Website. April 2007; https://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; https://www.ncbi.nlm.nih.gov/pubmed/16151895. Accessed 8/26/2011.