prophylactic measures, although anticipation of exposure to such agents may be a deployment-related stressor, as indicated in Chapter 3. Those stressors have already been thoroughly evaluated by other IOM committees.
In general, for each health effect presented in this chapter, the committee identifies first the primary studies and then the secondary or supporting studies, as defined by the criteria in Chapter 2. A primary study had to include information about the putative exposure (deployment) and specific health effects, demonstrate rigorous methods, include details of its methods, include an appropriate control or reference group, have adequate statistical power to detect effects, and provide appropriate adjustment for confounders. Many of the large cohort studies examined multiple effects and so might be referred to in more than one place. A given study might be deemed a primary study for one or more health effects and be a secondary study for another effect, as determined by how the particular health effects were defined and measured. For example, a study that was well designed for assessing a neurobehavioral effect might not be well designed for assessing a psychiatric disorder. In general, only primary studies appear in the evidence tables that accompany the discussions of health effects.
A secondary study was typically a study that had methodologic limitations, such as not including a rigorous or well-defined measure of exposure, that is, deployment to a war zone or posttraumatic stress disorder (PTSD) as a marker of trauma (see Chapter 1). Some studies assessed past trauma that was not necessarily peculiar to war-zone deployment, so they were included as secondary studies. The secondary studies were reviewed and included in the discussion because they evaluated the same health effects and in some cases provided useful information on veteran populations from the same conflicts as the primary studies; they add information that might increase or decrease confidence in the conclusions based on the primary studies. Confidence in a secondary study is substantially reduced if the statistical analysis did not adjust for confounders, if the data were obtained from self-reported cross-sectional surveys or from screening instruments that relied solely on self-reported answers, or if response rates were unacceptably low. Without evidence from primary studies, the potential for unreliable findings due to bias, chance, or multiple comparisons may outweigh the extent to which secondary studies may contribute, even collectively, to the overall conclusion of the committee about an association between deployment-related stress and any specific health effect. Understanding the relationship between a health effect and deployment-related stress may be hampered by the exposure not being explicitly defined as deployment or, more commonly, by many potentially harmful exposures being compared with multiple effects.
With rare exceptions, the chapter excludes studies of participants in Gulf War registries established by the Department of Veterans Affairs (VA) or the Department of Defense (DoD), which were not intended to be representative of the entire group of Gulf War veterans. Registry participants can not be considered representative of all Gulf War veterans in that they are self-selected subjects, many of whom have joined the registries because they believe that they have symptoms of a new medical syndrome; they were not randomly selected from all Gulf War military personnel, and there is no nondeployed control group. One main exception to the use of registries occurs for studies in which the groups of interest are veterans with and without PTSD. As discussed in Chapter 5, PTSD can be diagnosed only after exposure to a traumatic event, thus, the committee agreed that studies of veterans with deployment-related PTSD compared with deployed veterans who had not developed PTSD were appropriate comparison populations when determining whether health effects were associated with deployment-related PTSD.