What is Nonketotic hyperglycinemia (NKH)

Nonketotic hyperglycinemia, NKH for short, is a rare genetic disorder that affects 1 child in every 60,000 born. Children with this condition have a problem breaking down the amino acid glycine. This is caused by a defect in the genetic code for the machinery that is responsible for the glycine breakdown (called glycine cleavage enzyme). Amino acids are natural components of our body. Because glycine is not broken down, it accumulates in the body. Glycine is an important molecule in the brain where it has various functions such as transmitting signals from one brain cell to another. Excessive glycine disrupts the function of the brain. Children with NKH usually present as newborns, but 1 in 5 children presents in infancy. Typical symptoms include seizures, low tone, and severe problems with learning and development. This can profoundly affect a child’s ability to learn and to do normal things such as eating, sitting, and walking. The seizures can be so severe that they are hardly controlled despite the use of several medications for seizures. Not all children are equally affected. Most commonly children are severely affected and do not make developmental progress and have difficult to control seizures. Some children, about 1 in 6, are more mildly affected. They make progress in their development, and have seizures that can be controlled more easily. Current treatment is limited.


As described by Dr J Van Hove.

Wikipedia

There are several different forms of glycine encephalopathy, which can be distinguished by the age of onset, as well as the types and severity of symptoms. All forms of glycine encephalopathy present with only neurological symptoms, including mental retardation, hypotonia, seizures and brain malformations.[1] With the classical, or neonatal presentation of glycine encephalopathy, the infant is born after an unremarkable pregnancy, but presents with lethargy, hypotonia, seizures and myoclonic jerks, which can progress to apnea requiring artificial ventilation, and often death. Apneic patients can regain spontaneous respiration in their second to third week of life. After recovery from the initial episode, patients have intractable seizures and profound mental retardation, remaining developmentally delayed. Some mothers comment retrospectively that they noticed rhythmic hiccuping during pregnancy.[2] Patients with the infantile form of glycine encephalopathy do not show lethargy and coma in the neonatal period, but often have a history of hypotonia. They often have seizures, which can range in severity and responsiveness to treatment, and are typically developmentally delayed.[5] Glycine encephalopathy can also present as a milder form with episodic seizures, ataxia, movement disorders, and gaze palsy during febrile illness. These patients are also developmentally delayed, to varying degrees. In the later onset form, patients typically have normal intellectual function, but present with spastic diplegia and optic atrophy.[5] Transient neonatal hyperglycinemia has been described in a very small number of cases. Initially, these patients present with the same symptoms and laboratory results that are seen in the classical presentation. The levels of glycine in plasma and cerebrospinal fluid normalize within eight weeks, and in five of six cases there were no neurological issues at follow-up times up to thirteen years. A single patient was severely retarded at nine months. An immature glycine cleavage system in the brain and liver is suspected as the cause of transient neonatal hyperglicinemia.

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