By Saundra Young
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WASHINGTON (CNN) -- Military service may slightly increase the risk of developing amyotrophic lateral sclerosis (ALS), or Lou Gehrig's disease, but more research is needed, according to a new report from the National Academy of Sciences.
ALS is a progressive disease that affects the nervous system, causing nerve cells in the brain and spinal cord to degenerate. It eventually results in paralysis and is usually fatal. Five to 10 percent of the cases are inherited, but the cause of the remaining 90 to 95 percent is unknown.
The committee looked at the only five studies that have been done assessing the relationship between military service and the disease, and determined flaws in three of them that put their results in question. A fourth study done by the Veterans Administration found no increased risk. The fifth study -- looking at veterans who served between 1910 and 1982 -- found military service was associated with a 1.5 times increased risk of developing the condition.
The chair of the committee, Dr. Richard T. Johnson, said there are more questions than answers, especially when it comes to service in the Gulf War.
Many cases of ALS that could be linked to that war would not have developed yet, he said, because the Gulf War was so recent and there are likely to be many years between exposure to a risk factor and development of disease.
The best study -- the one that found veterans were 1.5 times more likely to develop the disease -- looked at veterans from World War I, Korea and Vietnam.
"The evidence base to answer the question of whether military service increases a person's chances of developing ALS later in life is rather sparse, so we could not reach more definitive conclusions at this time," said Johnson, a professor of neurology at Johns Hopkins University. "Because ALS occurs so rarely, any individual veteran's chances of developing the disease are still low."
The report says the research is too preliminary to draw any firm conclusions. But the committee recommends two steps: better research assessing the prevalence of ALS in veterans, and research into what factors of military service might be increasing the risk.
In particular, researchers are concerned about environmental exposures such as chemicals or aerosolized lead; intense physical activity, which has been shown to increase the risk in the general public; or traumatic events.