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Genetic and Rare Diseases Information Center (GARD)

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Glycine encephalopathy

Other Names for this Disease
  • Glycine synthase deficiency
  • Hyperglycinemia nonketotic
  • Nonketotic hyperglycinemia
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What are the signs and symptoms of glycine encephalopathy?

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.[1][2] Infants that survive this period typically have severe intellectual disability and seizures that are difficult to control.[1] 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][1] 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]
Last updated: 10/31/2011

  1. Ada Hamosh, Gunter Scharer, Johan Van Hove. Glycine Encephalopathy. GeneReviews. November 24, 2009; Accessed 10/28/2011.
  2. Glycine encephalopathy. Genetics Home Reference. April 2007; Accessed 10/28/2011.