The most common cause of childhood-onset spasticity with hyperglycinemia is a genetic disorder called Glycine encephalopathy, also known as Non-Ketotic Hyperglycinemia (NKH). This disorder is caused by a mutation in the GLDC gene, which is responsible for the production of an enzyme called glycine decarboxylase. This enzyme is responsible for breaking down the amino acid glycine, which is found in many foods. When the enzyme is not functioning properly, glycine builds up in the body, leading to spasticity and other neurological symptoms. Other causes of childhood-onset spasticity with hyperglycinemia include metabolic disorders, such as urea cycle disorders, and certain genetic disorders, such as Lesch-Nyhan syndrome.