Glycine Encephalopathy (Nonketotic Hyperglycinemia) - Classification

Classification

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.

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. 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. 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.

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.