Excerpt from article used with permission from the author, Dr. Amy Waldman, of Children's Hospital of Philadelphia.  

Alexander Disease is a regressive disease that essentially destroys white brain matter; practically speaking, this means those affected by the disease gradually lose the ability to walk, talk, eat, and eventually give way to the disease. Because it is very rare, with only hundreds of diagnoses since the 1940s, there is no "norm" or time frame for how quickly the disease progresses. No treatment is available yet, which means therapy and trying to maintain quality of life is the only option. 

For a more technical definition, consider the details below from leading expert on the topic, Dr. Amy Waldman. 

Alexander disease (MIM #203450) is one of a group of neurologic disorders, collectively referred to as leukodystrophies, which predominantly affect the central nervous system white matter. These disorders are caused by defects in the synthesis (ie, dysmyelination) or maintenance of the myelin sheath that insulates the nerves. While leukodystrophy is the predominant abnormality in the neonatal and infantile forms of Alexander disease, advances in genetics and imaging have revealed a broad phenotypic variability that includes juvenile and adult forms of the disorder without obvious leukodystrophy [1].

Prior to the identification of the genetic features, other names were used to describe the clinical features and pathology of Alexander disease, such as hyaline panneuropathy and dysmyelinogenic leukodystrophy [2]. These terms are no longer used.