What causes ALS?
Although the cause of ALS is unknown, genetic inheritance plays a role in 5% to 10% of cases. A fraction of all cases of familial ALS (that is, inherited ALS) is believed to be caused by a defective gene that prevents the body from producing a normal amount of an enzyme called superoxide dismutase. This enzyme helps neutralize free radicals — highly reactive oxygen molecules produced during metabolism and capable of damaging body tissues. Researchers speculate that defects in protective enzymes may also account for noninherited ALS and that environmental toxins may be a factor. Some evidence suggests that the disease may be triggered by exposure to heavy metals, animal hides, or fertilizers, although this is by no means proven. In addition, viral infection and severe physical trauma have been implicated as possible contributors. Other theories link ALS to a phenomenon called excitotoxicity, in which the nerve cells that control movement are so relentlessly stimulated by a neurotransmi
