Gulf War and Health Volume 8: Update of Health Effects of Serving in the Gulf War (2010) / Chapter Skim
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2 CONSIDERATIONS IN IDENTIFYING AND EVALUATING THE LITERATURE
2 CONSIDERATIONS IN IDENTIFYING AND EVALUATING THE LITERATURE
Pages 23-38
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From page 23... ...
The chapter also includes a discussion of the committee's evaluation criteria, the limitations of the studies reviewed, and the categories of association that the committee used in drawing conclusions about the possible health effects that might result from being deployed in the Gulf War. Because the committee was tasked with determining the prevalence of diseases and symptoms in Gulf War veterans, the committee reviewed primarily observational studies that compared health outcomes seen in or reported by veterans deployed to the Gulf War with their nondeployed counterparts.
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From page 24... ...
For each health outcome, one or more committee member with expertise or knowledge of a particular health outcome volunteered to screen all 400 studies in the database to identify all the epidemiologic studies that appeared to include information on that health outcome. The responsible committee member then conducted a preliminary review of the studies, including those cited in Volume 4, to determine what, if any, information the study had on the health outcome of interest and if an individual study met the inclusion criteria for a primary or secondary study (see below)
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From page 25... ...
The committee also notes the numerous factors that it considered in evaluating the evidence in a study and, finally, presents the categories of association used in drawing conclusions about the strength of associations. TYPES OF EVIDENCE The committee relied entirely on clinical and human epidemiologic studies to draw its conclusions about the strength of evidence regarding associations between deployment to the Gulf War and health outcomes seen in Gulf War veterans.
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From page 26... ...
Reverse causality bias may occur when the outcome actually precedes the exposure; for example, a study might suggest that a particular psychiatric outcome is a result of a traumatic brain injury. However, in reality, the psychiatric condition actually preceded the injury and the presence of the psychiatric condition placed individuals at increased risk of being injured.
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From page 27... ...
The remaining eight aspects are neither necessary nor sufficient requirements for causation but do present a framework for consideration. While the committee was mindful of the Bradford Hill aspects when assigning the categories of association discussed later in the chapter, it did not use them as rigid criteria but rather guidelines to inform its conclusions about the association between deployment to the Gulf War and a particular health outcome.
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From page 28... ...
Using data from a cohort study, investigators can test hypotheses about whether exposure to a specific agent is related to the development of disease and can examine multiple health outcomes that might be associated with exposure to a given agent (for example, to deployment)
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From page 29... ...
• Rare or unique exposures (such as Gulf War exposures) can be studied, and the investigators can study multiple health outcomes.
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From page 30... ...
In such studies disease or symptom prevalence between groups with and without exposure to the specific agent are compared. Several health studies of Gulf War veterans are cross-sectional studies that compare a sample of veterans who were deployed to the Gulf War with a sample of veterans who served during the same period but were not deployed to the Gulf War.
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From page 31... ...
Since military personnel are at overall lower risk of adverse health outcomes compared to the general population, any excess risk associated with an exposure they experience must be large enough to overcome their inherent advantage in order to be detectable by such methods as SMR. INCLUSION CRITERIA The Update committee included studies that would answer the question, "What does the literature tell us about the health status of Gulf War veterans?
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From page 32... ...
The committee did consider studies that compared health outcomes seen in deployed veterans who may or may not have been exposed to nerve agents as a result of the Khamisiyah detonation and to oil-well fire smoke; some of these studies also included nondeployed control groups. Health Outcome Assessment For medical conditions that have no morphological features, the use of validated symptom criteria, such as those of the Rome Foundation for irritable bowel syndrome, are
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From page 33... ...
The evidence tables were refined to include study limitations for the primary studies and to present the pertinent results; secondary studies were not included in the evidence tables. It should be noted that some of the larger cohort studies used a variety of methods and instruments to assess the health status of Gulf War veterans and it is for this reason that the committee discussed at some length the diagnostic approaches and use of self-reports for each paper.
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From page 34... ...
Sufficient Evidence of a Causal Relationship Evidence is sufficient to conclude that a causal relationship exists between being deployed to the Gulf War and a health outcome. The evidence fulfills the criteria for sufficient evidence of a causal association in which chance, bias, and confounding can be ruled out with reasonable confidence.
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From page 35... ...
Failure to account for such differences can lead to biased estimates of an effect. These factors were all considered by the committee in evaluating the quality of data and individual studies, in determining the primary and secondary literature that would be used to draw conclusions, and in evaluating how those studies contribute to the body of evidence concerning health effects seen in Gulf War veterans.
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From page 36... ...
The limitations include the possibility that study samples do not represent the entire Gulf War population, the relatively young age of the exposed population, low rates of participation in studies, reinforcement of self-reporting of symptoms and exposures, insensitivity of instruments for detecting abnormalities in deployed veterans, and a period of investigation that is too brief to detect health outcomes that have long latency, such as cancer. In addition, many of the US studies are cross-sectional and this limits the opportunity to learn about symptom duration and chronicity, latency of onset, and prognosis.
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From page 37... ...
1977. Judgment and causal inference: Criteria in epidemiologic studies.
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