It typically presents as a severe encephalopathy with myoclonic seizures, is rapidly progressive and eventually results in respiratory arrest.

Standard evaluation for inborn errors of metabolism and other causes of this presentation does not reveal any abnormality (no acidosis, no hypoglycaemia, or hyperammonaemia and no other organ affected).

[citation needed] Glycine encephalopathy has an estimated incidence of 1 in 60,000, making it the second most common disorder of amino acid metabolism, after phenylketonuria.

[citation needed] A small percentage of affected individuals do not have detectable mutations in any of the three genes (listed above) that are typically associated with the disease.

Damage caused by elevated levels of glycine in the brain and cerebrospinal fluid is responsible for the characteristic seizures, breathing difficulties, movement disorders, and intellectual disability.

[citation needed] Glycine is metabolized in the body's cells to end products of carbon dioxide and ammonia.

All forms of glycine encephalopathy present with only neurological symptoms, including intellectual disability (IQ scores below 20 are common[5]), hypotonia, apneic seizures, and brain malformations.

After recovery from the initial episode, patients have intractable seizures and profound intellectual disability, remaining developmentally delayed.

[3] 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.

[6] Glycine encephalopathy can also present as a milder form with episodic seizures, ataxia, movement disorders, and gaze palsy during febrile illness.

In the later onset form, patients typically have normal intellectual function, but present with spastic diplegia and optic atrophy.

However, levels of glycine in plasma and cerebrospinal fluid typically normalize within eight weeks, and in five of six cases there were no neurological issues detected at follow-up times up to thirteen years.

[9] NKH is a rare disease but maintains a very active community presence for educating parents with newly diagnosed children and fundraising for genetic therapy.