Pelizaeus-Merzbacher disease

Pelizaeus-Merzbacher disease is an inherited condition involving the brain and spinal cord (central nervous system) that primarily affects males. This disease is one of a group of genetic disorders called leukodystrophies. Leukodystrophies are conditions that involve abnormalities of the nervous system's white matter, which consists of nerve fibers covered by a fatty substance called myelin. Myelin insulates nerve fibers and promotes the rapid transmission of nerve impulses. In particular, Pelizaeus-Merzbacher disease involves hypomyelination, which means that the nervous system has a reduced ability to form myelin. As a result, overall neurological function is reduced.

Pelizaeus-Merzbacher disease is divided into classic and connatal (present from birth) types. Although these two types differ in severity, their features can overlap.

Classic Pelizaeus-Merzbacher disease is the more common type. Within the first year of life, those affected with classic Pelizaeus-Merzbacher disease typically experience weak muscle tone (hypotonia), involuntary movements of the eyes (nystagmus), and delayed development of motor skills, such as sitting or grasping objects. Some individuals are able to walk with assistance. Despite these neurological problems, intellectual and motor skills develop throughout childhood, but development usually stops around adolescence, and these skills are slowly lost (developmental regression). As the condition worsens, nystagmus usually goes away but other movement disorders develop, including muscle stiffness (spasticity), problems with movement and balance (ataxia), head and neck tremors (titubation), involuntary tensing of the muscles (dystonia), and jerking (choreiform) movements.

Connatal Pelizaeus-Merzbacher disease is the more severe of the two types. Symptoms can begin in infancy and include problems with feeding, poor weight gain and slow growth, high-pitched breathing caused by an obstructed airway (stridor), nystagmus, progressive speech difficulties (dysarthria), severe ataxia, hypotonia, and seizures. As the condition worsens, affected children develop spasticity leading to joint deformities (contractures) that restrict movement. Individuals with connatal Pelizaeus-Merzbacher disease are never able to walk, and many are not able to purposefully use their arms. They also have problems producing speech (expressive language) but can generally understand speech (receptive language).