Although neuropsychological and neurological symptoms are among the most common of GW health complaints, a specific neuropsychological or brain syndrome associated with GW service remains difficult to define. Neuropsychological studies have revealed an inconsistent pattern of results, with those studies suggestive of GW-related neuropsychological dysfunction indicating only mild levels of impairment that do not reliably converge with symptom reports. Mood disturbance is the most commonly affected domain in neuropsychological studies, but there is otherwise little consistency across neuropsychological studies regarding the domains or tasks most likely to be affected. Neuroimaging research using small, non-population-based samples has yielded preliminary evidence of reductions in neuronal viability in select brain regions among GW veterans with health complaints, but there is insufficient neuroimaging research to draw conclusions at this point. Of the few neuroimaging studies conducted, none revealed evidence of a non-specific effect of GW service on brain function or structure. Furthermore, in veterans with GW-related health complaints, there are no changes in brain structure or function that are visible with conventional radiology examinations. Single studies with MRS have shown findings that are interpretable as indicating reductions in neuronal viability (NAA/Cr ratio) in the hippocampus and basal ganglia in veterans with GW service and health complaints. However, studies using larger, more representative samples are needed to replicate these initial reports. In addition, neuroimaging studies controlling for stress-related psychiatric disorders or symptoms would be a critical methodological advancement.
Within the GW literature addressing objective neuropsychological impairment, researchers have examined various facets of GW participation as potential contributing factors. These include GW deployment as a non-specific factor, neurotoxicant exposures, unexplained illnesses, stress-related psychopathology and motivational factors. Although findings are not fully uniform, several conclusions from this literature can be derived.
First, there is little evidence that GW deployment alone poses significant risk of objective neuropsychological compromise. That is, studies comparing GW-deployed veterans to appropriate non-deployed comparison samples have not provided compelling evidence of deployment-related neuropsychological abnormalities. As summarized above, this finding is consistent with the few neuroimaging studies that have examined the non-specific effects of GW deployment. The lack of a non-specific GW deployment effect is not surprising, however, given the potential diversity of exposures, experiences and individual characteristics that comprise the deployed GW population.
Second, although self-reported exposures to GW environmental hazards have been associated with neuropsychological performance deficits, findings vary considerably across studies, and the validity of self-reported exposures has been questioned. There is only one published GW neuropsychological outcome study (McDiarmid et al. 2000
) that documented exposure (to depleted uranium) objectively. This study serves as a useful model and provides evidence of neuropsychological dysfunction on a single index score. However, results should be considered preliminary and warrant further replication.
Third, self-reported illness appears in some cases to be associated with neuropsychological compromise. Correspondingly, the few neuroimaging studies suggestive of GW-related neural dysfunction grouped participants according to health complaints. However, without baseline information, the possibility that health symptoms and associated neural dysfunction predated the GW cannot be excluded. Nonetheless, such studies are useful in identifying subsets of GW veterans that are at heightened risk for neuropsychological compromise and require enhanced clinical care.
Finally, regarding emotional functioning and motivational factors, a handful of studies have documented links between PTSD neuropsychological deficits (and early childhood trauma and neuroimaging deficits) in GW veterans. However, the prevalence rates of PTSD in the GW population are sufficiently low that PTSD alone cannot explain the neuropsychological symptoms reported by GW veterans (Wolfe et al. 1999
). In contrast, more generalized emotional distress not necessarily meeting criteria for a mental disorder may be more common among GW veterans (e.g. Perconte et al. 1993
; Sutker et al. 1993
; Holmes et al. 1998
; Unwin et al. 1999
), and studies examining associations between continuous measures of emotional distress, in particular depression, have revealed associations between distress and objective neuropsychological performances. Such associations in the context of potential neurotoxicant exposure pose interpretive challenges, as some neurotoxicants affect the limbic system, resulting in emotional dysfunction, whereas emotional disturbances in the absence of neurotoxicant exposures can be associated with both neural compromise and health problems more generally (Wolfe et al. 2002
; Boscarino 2004
; Schnurr & Green 2004
). In contrast to emotional factors, which have shown some association with neuropsychological compromise, there is no evidence that insufficient motivation contributes to neuropsychological performance deficits in GW veterans.
Conclusions regarding etiological factors for brain dysfunction in GW veterans will continue to be hampered by almost entirely unavoidable methodological factors inherent to this research area, including the paucity of objective exposure data, the absence of baseline data, and, in many cases, the significant time that has now elapsed between GW participation and assessment of neural functioning using objective neuropsychological and brain imaging techniques. The divergence between subjective and objective indices of neuropsychological functioning highlights the necessity of using performance-based measures of neuropsychological integrity, as well as self-reported symptoms. To the extent that exposure data become available, the field would benefit from viewing neuropsychological and brain imaging data in the context of specific known, objectively verified exposures. Similarly, if archival records addressing baseline functioning were available, such information would be useful in evaluating the possibility that pre-existing vulnerability factors contributed to health dysfunction following GW participation. Finally, genetic vulnerabilities and their interaction with environmental and stress exposures can be examined in the absence of baseline data through retrospectively collected genotypic data.
Thus, the current methodological challenges do not necessarily imply that research efforts addressing neural functioning in GW veterans should be abandoned, but rather that new approaches should be considered. Specifically, exclusive focus on single aetiologies has led to ‘straw man’ arguments that are divisive and in many cases counterproductive. More fruitful endeavours will likely include research questions that build on more complex models incorporating individual vulnerabilities, environmental factors and their physiological and emotional consequences and immunologic functioning. Such models have been applied to other ‘medically unexplained’ syndromes such as chronic fatigue and multiple chemical sensitivities (Binder & Campbell 2004
). Ultimately, however, the goal must be to address identification and clinical management of those subsets of GW veterans at greatest risk for neural and/or neuropsychological compromise, regardless of the aetiological factors at play.