What is (are) Pelizaeus-Merzbacher disease ?
Pelizaeus-Merzbacher disease is an inherited condition involving the brain and spinal cord (central nervous system). This disease is one of a group of genetic disorders called leukodystrophies. Leukodystrophies are characterized by degeneration of myelin, which is the covering that protects nerves and promotes the efficient transmission of nerve impulses. Pelizaeus-Merzbacher disease is caused by an inability to form myelin (dysmyelination). As a result, individuals with this condition have impaired intellectual functions, such as language and memory, and delayed motor skills, such as coordination and walking. Typically, motor skills are more severely affected than intellectual function; motor skills development tends to occur more slowly and usually stops in a person's teens, followed by gradual deterioration.  Pelizaeus-Merzbacher disease is divided into classic and connatal 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 crawling or walking. As the child gets older, nystagmus usually stops but other movement disorders develop, including muscle stiffness (spasticity), problems with movement and balance (ataxia), and involuntary jerking (choreiform movements).  Connatal Pelizaeus-Merzbacher disease is the more severe of the two types. Symptoms can begin in infancy and include problems feeding, a whistling sound when breathing, progressive spasticity leading to joint deformities (contractures) that restrict movement, speech difficulties (dysarthria), ataxia, and seizures. Those affected with connatal Pelizaeus-Merzbacher disease show little development of motor skills and intellectual function.