Researchers are looking for aspects of lifestyle that can interact with genes to cause or contribute to ALS. The notion that ALS could be linked to viral infection is a concept that reappears periodically when researchers focus on the issue. Exposure to toxins, or the influence of intense exertion, are ideas researchers consider as possible reasons for the finding that some veterans and some athletes have increased incidence of ALS. ALSA will continue to support investigations into the way that environmental factors may interact with genetics to produce ALS.
After finding the mutated gene for SOD1 (see also SOD1) in some inherited cases of ALS, researchers continue to search for additional factors that can cause or contribute to the disease. Toxins encountered in the environment have been examined as possible factors in producing the disease. As yet no conclusive proof exists for any toxin as a causative factor in ALS. Suspects that scientists have researched include heavy metals, solvents, radiation, and electromagnetic fields.
As no one would propose to give a known or potentially toxic substance to people on purpose, any study of the potential role of environmental or dietary factors in ALS must be by the methods of epidemiology. These studies look at populations of people and use answers to questionnaires or information collected from patients and stored in databases to investigate what common factors among people with ALS could provide a link to the disease. Such studies can only reveal a potential association and can never prove cause.
One of the first clues that ALS might involve an environmental factor was obtained on the island of Guam in the Pacific, where an unusually high proportion of people over the past century have developed symptoms similar to ALS as they age. Suspects have included trace metals or their lack in the soil, and dietary factors such as the indigenous preference for eating entire bats cooked in coconut milk. No proof has yet nailed down any of these suspects as a cause of the ALS. Dietary studies and observations of metal content in tissue from ALS patients do not lend strong support to a role for trace elements, either deficiency or excess in diet.
Many investigations of heavy metal exposure, particularly lead, and including mercury and manganese, have looked at risk factors for ALS. A positive association between past exposure to heavy metals and risk of ALS has not consistently appeared across studies. These studies include occupational exposure that epidemiologists often use as a surrogate to assess potentially toxic exposures. Many of the studies relied on self-reporting by questionnaire. Self reports can allow a recall bias--I have this disease, what did I do to produce it-- that has the potential to artificially inflate the data. Some support for an association between ALS and exposure to organic solvents appears in epidemiologic studies yet these have found only weak association and a mix of positive and negative results. As well, no consistent associations with ALS have emerged from investigation into the potential role of radiation and electromagnetic fields. For more information on these topics please see http://www.atsdr.cdc.gov/NEWS/alsreport.html
As well as the Chamorro of Guam, U.S. veterans are another group of people who appear to develop ALS more often than the general population. A study found that the relative risk of dying from ALS for veterans was 1.5 times that seen for the men who did not serve. The increased risk was apparent for veterans of World War II, the Korean War, and the War in Vietnam . A prior study had found that the rate of ALS in young Gulf War veterans was more than two times greater than expected for the general population.
Scientists have expressed concern that the degree of excess risk is not convincing because of the small number of total ALS cases documented in these studies of veterans. Statistical theory says that such small numbers may not be able to provide reliable answers to questions about associations between diseases and their causes. Larger sample sizes are needed for better certainty.
One idea that researchers offer is that soldiers on active duty are engaged in strenuous physical labor. Or, they are exposed to toxins which could play a role in ALS. The Association is helping to fund an effort by the Veterans Administration to collect information about veterans diagnosed with ALS, to shed light on possible environmental factors associated with the disease. This VA registry will also serve as a stepping stone to clinical trials. Both possibilities, of toxin exposure or the influence of intense exertion, are advanced when researchers consider the finding that Italian soccer players appear to have increased incidence of ALS. It remains unclear whether exercise is indeed a risk factor and what types of exercise may be of concern. Indeed some studies seem to suggest that a degree of exertion may be beneficial in ALS. Other ideas are that pesticides or some other chemical encountered on maintained playing fields might be involved.
A recurring idea about environmental influences on ALS is that a virus is responsible for the condition. Polio virus, for instance, infects motor neurons and can at times lead to a weakening condition called the post-polio syndrome years after the initial infection. Scientists have considered viral infection for many other disorders of the nervous system, from schizophrenia to multiple sclerosis, as well as for ALS. A concrete link to viral infection has never been documented in any of these diseases.
The ALS Association has recognized the debate over a viral role in ALS, and has funded study of the issue, but so far findings have failed to confirm any link with ALS and viral infection. The Association will continue to support investigations into the way that environmental factors may interact with genetics to produce ALS.
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