caused by the degeneration of motor neurons, the nerve cells in the central nervous system that control voluntary
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. Between 1 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.
ALS is classified into three general groups, familial ALS, sporadic ALS and Guamanian ALS.
- "Familial ALS" accounts for approximately 5%-10% of all ALS cases and is caused by genetic factors. Of these approximately 10% are linked to a mutation in Superoxide Dismutase (SOD1), a copper/zinc dependant dismutase that is responsible for scavenging free radicals.
- Most of the remaining 90-95% of cases are classified as "sporadic ALS" and have no known hereditary component.
- A third type, called "Guamanian ALS", represents a small cluster of cases concentrated on the Pacific island of Guam.
Although there have been reports of several "clusters" including three American football players from the San Francisco 49ers, three soccer-playing friends in the south of England, and reports of conjugal (i.e., 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.
Cause and risk factors
Scientists have not found a definitive cause for ALS and the onset of the disease can be linked to a variety of risk factors. It is believed that one or more of the following factors are responsible for the majority of ALS cases. Researchers suspect a virus, exposure to neurotoxins or heavy metals, DNA defects (especially in familial ALS), immune system abnormalities, and enzyme abnormalities as the leading causes of the disease. There is a hereditary factor in familial ALS (FALS) however there is no known hereditary component in the 90-95% cases diagnosed as sporadic ALS.
A few causative factors have been discovered. Prolonged exposure to a dietary neurotoxin is the suspected risk factor in Guamanian ALS. The neurotoxin is a compound (a sterol beta-D-glucoside) found in the seed of the cycad Cycas circinalis, a tropical plant found in Guam, which was used in the human food supply during the 1950s and early 1960s. An inherited genetic defect linked to a defect on chromosome 21 is believed to cause approximately 40% of familial ALS cases. This mutation is believed to be autosomal dominant.
The children of those diagnosed with familial ALS have a higher risk factor for developing the disease, however those who have close family members diagnosed with sporadic ALS have no greater a risk factor than the general population .
According to The ALS Association, military veterans are at an increased risk of contracting ALS. In its report ALS in the Military, the group pointed to an almost 60% greater chance of the disease in military veterans than the general population.