Amyotrophic Lateral Sclerosis (ALS, sometimes called Maladie de Charcot, or, in the United States, Lou Gehrig's Disease) is a progressive, usually fatal, neurodegenerative disease caused by the degeneration of motor neurons, the nerve cells in the central nervous system that control voluntary muscle movement. As a motor neuron disease, the disorder causes muscle weakness and atrophy throughout the body as both the upper and lower motor neurons degenerate, ceasing to send messages to muscles. Unable to function, the muscles gradually weaken, develop fasciculations (twitches) because of denervation, and eventually atrophy because of that denervation. The patient may ultimately lose the ability to initiate and control all voluntary movement except for the eyes.

Cognitive function is generally spared except in certain situations such as when ALS is associated with frontotemporal dementia. However, there are reports of more subtle cognitive changes of the frontotemporal type in many patients when detailed neuropsychological testing is employed. Sensory nerves and the autonomic nervous system, which controls functions like sweating, generally remain functional.

Epidemiology, causes and risk factors

ALS is one of the most common neuromuscular diseases worldwide, and people of all races and ethnic backgrounds are affected. One to 2 people per 100,000 develop ALS each year. ALS most commonly strikes people between 40 and 60 years of age, but younger and older people can also develop the disease. Men are affected slightly more often than women.

"Familial ALS" accounts for approximately 5%–10% of all ALS cases and is caused by genetic factors. Of these, approximately 1 in 10 are linked to a mutation in copper/zinc superoxide dismutase (SOD1), an enzyme responsible for scavenging free radicals. An antisense drug trial is underway using a small interfering RNA molecule (siRNA) that inhibits the production of this mutant SOD1 protein.

Although the incidence of ALS is thought to be regionally uniform, there are three regions in the West Pacific where there has in the past been an elevated occurrence of ALS. This seems to be declining in recent decades. The largest is the area of Guam inhabited by the Chamorro people, who have historically had a high incidence (as much as 143 cases per 100,000 people per year) of a condition called Lytico-Bodig disease which is a combination of ALS, Parkinsonism, and dementia. Two more areas of increased incidence are the Kii peninsula of Japan and West Papua.

Although there have been reports of several "clusters" including three American football players from the San Francisco 49ers, more than fifty soccer players in Italy , three soccer-playing friends in the south of England, and reports of conjugal (husband and wife) cases in the south of France, these are statistically plausible chance events. Although many authors consider ALS to be caused by a combination of genetic and environmental risk factors, so far the latter have not been firmly identified, other than a higher risk with increasing age.