studies compared the prevalence of a given medical condition or symptom in the deployed veterans with the prevalence in nondeployed veterans. If the prevalence of a symptom or condition was linked by the study authors to any specific exposures experienced during deployment—such as vaccines, oil-well fire smoke, anti-nerve-gas agents, or combat—the committee reviewed those associations as well. Although for the most part the Update committee considered the same health outcomes as did the Volume 4 committee, there were several differences: the Update committee added two new health outcomes: genitourinary diseases and diseases of the blood and blood-forming organs. The committee also used the term multisymptom illness to refer to a health outcome rather than the International Classification of Diseases, 9th revision, category of “signs, symptoms, and abnormal clinical and laboratory findings.” Chronic fatigue syndrome and multiple chemical sensitivity were included in the section on multisymptom illnesses; hospitalization and mortality studies were discussed in the relevant health-outcome sections; and chronic widespread pain was included in the section on fibromyalgia.
All studies of each health outcome, including those originally cited in Volume 4, were reviewed and categorized as primary or secondary by the entire committee in plenary session before it came to a consensus on the appropriate category of association to be assigned to each health outcome. As in previous volumes of the Gulf War and Health series, the primary studies on which the committee based its conclusions are detailed in the evidence table at the end of each health-outcome section. Using the weight-of-the-evidence approach required that the Update committee be more rigorous in its review of the studies in Volume 4; as a result, some studies considered to be primary in Volume 4 were recategorized as secondary for the present report and vice versa. Thus, the Update committee summarizes de novo the information from both Volume 4 and any new publications to arrive at its conclusions on the strength of associations between deployment to the Gulf War and health outcomes. Box S-1 summaries the health outcomes assigned to each category of association by the Update committee.
Many studies of Gulf War veterans have been conducted, but their quality is varied, and many have substantial limitations. As a result, there is still uncertainty about the relationship between deployment to the Gulf War and health outcomes. The limitations include
Lack of representativeness of the entire Gulf War population in some studies, which affects external validity in such a way that what we learn from the population studied cannot be easily extrapolated to all Gulf War veterans.
Low participation rates and differential participation rates in many studies, which affect internal validity because of selection bias (for example, significantly higher rate of response of deployed veterans than of nondeployed control groups and the possibility that deployed troops participated because they already experienced health problems).
Narrowness of assessment of health status (for example, self-reported outcomes, such as hypertension, diabetes, and cardiovascular disease), or insufficient sensitivity or validity of instruments to detect abnormalities in deployed veterans (for example, death certificates or hospital discharge diagnoses); there is a particular problem with self-reported exposures, especially if respondents are aware of mass-media reports that link outcomes with putative exposures.