Definition of Amyotrophic lateral sclerosis:

Amyotrophic lateral sclerosis, or ALS, is a disease of the nerve cells in the brain and spinal cord that are responsible for voluntary muscle movement.

ALS is also known as Lou Gehrig's disease.

Causes, incidence, and risk factors:

In about 10% of cases, ALS is caused by a genetic defect. In other cases, the cause of the nerve deterioration is unknown.

In ALS, nerve cells (neurons) waste away or die, and can no longer send messages to muscles. This eventually leads to muscle weakening, twitching, and an inability to move the arms, legs, and body. The condition slowly gets worse. When the muscles in the chest area stop working, it becomes hard or impossible to breathe on one's own. Persons with ALS eventually need a breathing machine.

ALS affects approximately 1 out of every 100,000 people.

Except for having a family member who has a hereditary form of the disease, there are no known risk factors.