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Gulf War and Health, Volume 7: Long-Term Consequences of Traumatic Brain Injury
and studies of acute outcomes that resolved within days to a few months. The committee did not review general studies of “disability” as a gross measure of morbidity but rather evaluated studies that associated TBI with specific health outcomes.
After its assessment of the 30,000 titles and abstracts, the committee members identified about 1,900 studies for further review. Those studies were objectively evaluated without preconceived ideas about health outcomes or the existence or absence of associations. To assist them in their evaluation, the committee members developed inclusion criteria (see below) to determine which of the 1,900 studies would be included in its review.
The committee adopted a policy of using only peer-reviewed published literature or unpublished reports that had undergone rigorous peer review, such as dissertations and some government reports, as the basis of its conclusions. The process of peer review by fellow professionals increases the likelihood of high quality but does not guarantee the validity of a study or the ability to generalize its findings. Accordingly, committee members read each study critically and considered its relevance and quality. They did not collect original data, nor did they perform any secondary data analysis.
In light of that orientation to the committee’s task and approach, the following section briefly discusses types of evidence and the value of epidemiologic or clinical studies in determining whether an association exists. It is followed by a discussion of the committee’s specific inclusion criteria that were developed to help in deciding whether a study would be included and evaluated. 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 epidemiologic studies to draw its conclusions about the strength of evidence of associations between TBI and health effects. However, animal studies play a critical role in clarifying the mechanism of TBI (see Chapter 2) and in providing biologic understanding of many of the effects seen in humans.
Studies of laboratory animals are essential for understanding mechanisms of action and biologic plausibility and for providing information about possible health effects when experimental research in humans is not ethically or practically possible (NRC, 1991). Such studies permit an injury caused by a blast or other mechanism to be introduced under conditions controlled by the researcher. Mechanism-of-action (mechanistic) studies encompass a variety of laboratory approaches with whole animals and in vitro systems that use tissues or cells from humans or animals.
In deciding on associations between TBI and human health effects, the committee used evidence only from human studies; in some cases, however, it examined animal studies as a basis of judgments about biologic mechanism or plausibility.