by Michelle Rizzo

Last Updated: 2008-06-18 13:00:12 -0400 (Reuters Health)

NEW YORK (Reuters Health) – The results of a study published in the July issue of Neuroepidemiology suggest that the observed excess risk of amyotrophic lateral sclerosis (ALS) among veterans of the 1991 Gulf War was limited to the decade following the war.

"Recent reports indicate that veterans of the first Gulf War experienced a 2-fold elevated risk of ALS during the decade after the war had ended," Dr. Ronnie D. Horner, of the University of Cincinnati Medical Center, Ohio, and colleagues write. "The etiology of the excess risk is unknown but suggested to be attributable to exposures experienced in preparation for or during the deployment."

This finding of excess ALS cases is somewhat controversial, the investigators add. "Some members of the scientific community believe the purported elevated risk is a consequence of methodological biases." Others acknowledge the excess risk, but suggest it reflects military service itself as an ALS risk factor.

To determine if the excess risk of the disease was time limited, the researchers conducted a secondary analysis using data from a population-based series of ALS cases identified between 1991 and 2001 among all active duty military personnel deployed during the 1991 Gulf War. The team calculated the annual standardized incidence ratios for all cases and for subjects with disease onset before age 45 years.

A total of 124 confirmed cases of ALS were identified among the 2.5 million military personnel who were on active duty between 1991 and 2001. Of these 124 cases, 48 occurred among the 696,118 military personnel who served in the Persian Gulf region during that time.

Overall, 33 of the 48 cases (69%) developed the disease prior to age 45 years. Forty-nine of the 76 ALS cases (64%) in personnel who had not been deployed to the Persian Gulf region experienced disease onset prior to age 45 years.

The researchers found that veterans deployed to the Persian Gulf region during the war had an increased risk of ALS until 1996, with the risk tapering off thereafter. Conversely, the ALS risk level remained fairly constant during the decade after the war among military personnel who were not deployed to the Gulf Region.

"If the occurrence of cases over time has a log normal distribution, this suggests that the exposure to the cause or causes occurred at a common point in time or that the cause was at a common source," Dr. Horner said in an interview with Reuters Health.

"That is, a log normal distribution would suggest that the 1991 Gulf War veterans who developed ALS had been exposed during the same time period or were exposed to the same causal agent(s)," he explained.

"The distribution of the epidemic curve was, indeed, consistent with a log normal distribution," Dr. Horner said. "This finding may help to narrow the array of possible causes of the outbreak."

"Our focus is now on identifying the cause of the outbreak," the researcher continued. To this end, Dr. Horner and his colleagues at Duke University and the Durham VA Medical Center (North Carolina) have completed a spatial analysis of the movement through the war zone of the troop units from which the ALS cases subsequently arose.

The investigators are also preparing a study that will add a time dimension to these spatial findings. "The spatial analysis only tells us if there are geographic areas in the war zone where the risk of developing ALS was elevated," Dr. Horner said. "We need to know if the risk in those areas is associated with a specific point in time."

"If we know where to look for the cause and when to look for it, we may be able to discover what happened," Dr. Horner noted. "Once we know what happened, we can then begin to think about ways to prevent future occurrences," he added. "Moreover, as we develop our understanding of why the disease occurs, this knowledge may also lead to therapies."

Neuroepidemiology 2008;268.

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