Amyotrophic lateral sclerosis (ALS) – commonly known as Lou Gehrig's disease – is a motor neuron disease. It's caused when the motor neurons that send electric "messages" from the brain to the spinal cord (upper motor neurons) or from the spinal cord (lower motor neurons) to the muscles are slowly being destroyed. These neurons are responsible for muscle movement.
When the messages don't get through from the spinal cord, the muscles eventually lose strength and waste away. This process is called atrophy or amyotrophy. When signals from the brain to the spinal cord are blocked, the muscles become stiff and slow. This is called spasticity.
Approximately 2 out of 100,000 people each year are newly diagnosed with ALS. It's more common in men than women, with symptoms usually appearing between the ages of 50 and 75. About 10% of people with ALS have a family history of the disease. The average life expectancy is 2 to 5 years after diagnosis.