What is the difference between severe and attenuated NKH?

Doctors describe two main forms of nonketotic hyperglycinemia. Both usually begin shortly after birth, and both come from the same broken glycine cleavage system. The difference is how much that system still works.

In severe NKH, the system's activity is essentially gone. Babies often have extreme sleepiness, weak muscle tone, and dangerous breathing problems in the first days or weeks of life. Children who survive that period typically have seizures that are very hard to control, feeding difficulties, muscle stiffness, and profound intellectual disability. The foundation has long noted that the severe form makes up close to 85% of diagnoses.

In attenuated NKH, some enzyme activity remains. Symptoms are similar but milder. Many children learn to walk and interact with others, sometimes using sign language. Seizures may be mild or absent, though other challenges such as hyperactivity or movement problems can occur.

Drake and Vivian both live with the severe form. That is why their parents have spent a decade searching for something that could address NKH at its root. Read about the gene therapy they're working toward.

Sources: MedlinePlus Genetics: Nonketotic hyperglycinemia · GeneReviews (NCBI): Nonketotic Hyperglycinemia