Alexander disease is a rare disorder of the nervous system. It is one of a group of disorders, called leukodystrophies, that involve the destruction of myelin. Myelin is the fatty covering that insulates nerve fibers and promotes the rapid transmission of nerve impulses. If myelin is not properly maintained, the transmission of nerve impulses could be disrupted. As myelin deteriorates in leukodystrophies such as Alexander disease, nervous system functions are impaired.