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Disease Profile

Glycine encephalopathy

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.

1-9 / 1 000 000

US Estimated

Europe Estimated

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.


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.


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.


Not applicable


Other names (AKA)

Hyperglycinemia nonketotic; Nonketotic hyperglycinemia; Glycine synthase deficiency;


Glycine encephalopathy is an inherited metabolic disease characterized by abnormally high levels of an amino acid called glycine. Glycine is a chemical messenger that transmits signals in the brain.[1][2] According to the symptoms the disease onset, glycine encephalopathy may be divided in:[3][2]

  • 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 (mutations) in the AMT, GLDC or GCSH genes which result in a deficiency of the enzyme that break-up the glycine. Diagnosis is based in the symptoms, the high glycine levels and the enzyme deficiency, as well as genetic testing. Inheritance is autosomal recessive.[2][1] Treatment may include sodium benzoate to reduce the levels of glycine, N-methyl D-aspartate (NMDA) receptor site antagonists, anti-seizure drugs and ketogenic diet.[1] About half of the babies with the classic form, die within a few weeks of life and the survivors may have motor delay, very small head, seizures and stiffness. In the transient form symptoms may improve with time.[3]



Most individuals with glycine encephalopathy begin to show signs and symptoms in the first hours or first days of life (the neonatal period). Of these affected individuals, approximately 85% have a neonatal severe form, and 15% have a neonatal mild form. The signs and symptoms often include progressive lack of energy (lethargy), feeding difficulties, poor muscle tone (hypotonia), abnormal jerking movements (myoclonic jerking) and life-threatening breathing problems such as apnea.[4][2] Infants that survive this period typically have severe intellectual disability and seizures that are difficult to control.[4] Affected males are more likely to survive and tend to have more mild developmental problems than affected females, although the reason for this is unclear.[2] In rare instances, the main features of the condition improve with time; in these cases, the condition is known as transient glycine encephalopathy because glycine decreases to normal or near-normal levels after being very high at birth. Many children with the transient form will develop normally and experience few long-term medical problems, but some individuals may continue to have intellectual disability or seizures even after glycine levels decrease.[2]

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.[2][4] 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.[2]

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
Learn More:
80%-99% of people have these symptoms
Abnormal metabolic brain imaging by MRS
Central hypotonia
EEG with burst suppression
Elevated blood glycine levels
Hypoplasia of the corpus callosum
Underdevelopment of part of brain called corpus callosum
Recurrent singultus
Recurrent hiccup
30%-79% of people have these symptoms
Breathing dysregulation
Generalized myoclonic seizure
Poor suck
Poor sucking
Respiratory acidosis
Percent of people who have these symptoms is not available through HPO
Agenesis of corpus callosum
Aggressive behavior
Aggressive behaviour

[ more ]

Autosomal recessive inheritance
Death in infancy
Infantile death
Lethal in infancy

[ more ]

Generalized hypotonia
Decreased muscle tone
Low muscle tone

[ more ]

More active than typical
High urine glycine levels
Increased reflexes
Decreased reflex response
Decreased reflexes

[ more ]

Intellectual disability
Mental deficiency
Mental retardation
Mental retardation, nonspecific

[ more ]

Muscular hypotonia
Low or weak muscle tone


Currently there is not a cure for glycine encephalopathy.[1][6] All but very mildly or atypically affected individuals develop intellectual disability and seizures, even with treatment. Treatment options for people with glycine encephalopathy may vary depending on the severity of their condition. Tests, such as MRI and EEG, as well as evaluations of development and neurological function can help determine the severity of the condition in an infant, child, or adult.[1]

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.[1] Drug dosage must be individually tailored and requires regular and careful monitoring.[1][6] Studies regarding the effectiveness of these treatments are ongoing.[1] Mildly affected individuals may receive the greatest benefit from treatment, particularly if treatment is started early.[1]

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.[1]

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.


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.

Organizations Supporting this Disease

    Learn more

    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.

    Where to Start

    • 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.

      In-Depth Information

      • 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.


        1. Hamosh A, Scharer G, Van Hove J. Glycine encephalopathy. GeneReviews. 2013; https://www.ncbi.nlm.nih.gov/books/NBK1357/.
        2. Glycine encephalopathy. Genetics Home Reference. April 2007; https://ghr.nlm.nih.gov/condition/glycine-encephalopathy.
        3. 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/.
        4. 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.
        5. Glycine encephalopathy. Genetics Home Reference Website. April 2007; https://ghr.nlm.nih.gov/condition=glycineencephalopathy. Accessed 10/7/2008.
        6. 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.

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