A wasting of tissues, organs, or the entire body.

Muscle atrophy can be caused by lack of physical exercise (Disuse atrophy), and can be reversed by exercise. But the second and most severe type of muscle atrophy is neurogenic atrophy. It occurs when there is injury or disease to a nerve. This type of muscle atrophy tends to occur more suddenly than disuse atrophy. Diseases affecting the nerves that control muscles are poliomyelitis (polio), amyotrophic lateral sclerosis (ALS or Lou Gehrig’s disease), and Guillain-Barre syndrome.