THE AUTHOR REPLIES

Lea Steele

Kansas Health Institute, 212 SW Eighth Avenue, Suite 300, Topeka, KS 66603.

I thank Dr. Wartenberg and his colleagues for their comments (1). I appreciate their interest, but the concerns they raise dispute recommendations that were neither made nor implied in my commentary (2). Specifically, I did not advocate the use of undefined "epidemiologic methods" in lieu of exploratory factor analysis (EFA) to identify a unique syndrome in Gulf War veterans, nor did I suggest comparisons among Gulf War veterans and other veterans for that purpose.

I did comment that, since wartime exposure data are, for the most part, unavailable for the Persian Gulf conflict, epidemiologic methods traditionally used in the absence of individual exposure data could be useful in generating etiologic hypotheses regarding the unexplained illnesses reported by Gulf War veterans. Differences in illness prevalence and profiles associated with Gulf veteran subgroups defined by, for example, place and time of deployment have the potential to identify associations between illness and experiences in theater. This suggested use of traditional epidemiologic methods was separate from, and unrelated to, observations made regarding the use of EFA.

My comments questioning the use of EFA related only to its suitability for determining whether there is or is not a unique "Gulf War syndrome." Wartenberg et al. appear to have mistaken that point for a broader one regarding the use of EFA in general. The distinction between the use of EFA for statistically defining symptom domains and its use to address the question of the presence/absence of a unique "Gulf War syndrome" is not a minor one.

In a growing number of studies (39), first-order factors obtained from general lists of symptoms endorsed by heterogeneous groups of Gulf veterans have been found to be similar to symptom factors observed in comparison veteran populations. Does this general similarity in symptom factors mean that there is no "Gulf War syndrome"? The answer is, it doesn’t really address the issue. EFA identifies latent constructs that underlie groups of correlated variables. In factor analyses of symptoms, these constructs tend to represent symptom domains associated with difficulties in particular organs or systems. Neurologic difficulties, for example, are often represented by factors that include symptoms such as difficulty concentrating, memory problems, and mood changes—regardless of the etiology or pathophysiology of the problems being expressed. Respiratory conditions often manifest symptoms such as coughing, wheezing, and shortness of breath, which are highly correlated in a population, independently of the diversity of conditions giving rise to those symptoms. Thus, even if subgroups of Gulf veterans experienced, for example, unique combinations of neurotoxic and respiratory insults, the symptoms and first-order factors expressed would probably be similar to those of populations that included persons experiencing other types of neurologic and respiratory difficulties.

While it is interesting that one group of investigators has identified a unique symptom factor among a minority of symptomatic Gulf veterans (8), the absence of unique first-order factors in other studies indicates only that symptoms associated with dysfunction in specific organs or systems tend to be correlated in similar ways in different populations, regardless of the specific diseases in those populations.

A more straightforward observation relates to familiar medical conditions found in any population group, including veterans. When symptoms from these populations are subjected to EFA, "unique" symptom factors are not generally identified for common chronic conditions such as diabetes or hypothyroidism. It is unclear why investigators expect that putative "new" conditions associated with Gulf War service should emerge when comparing the results of EFA between Gulf veterans and other populations.

REFERENCES

  1. Wartenberg D, Kipen H, Hallman W, et al. Re: "Invited commentary: unexplained health problems after Gulf War service—finding answers to complex questions." (Letter). Am J Epidemiol 2003;157:567–8.[Free Full Text]
  2. Steele L. Invited commentary: unexplained health problems after Gulf War service—finding answers to complex questions. Am J Epidemiol 2001;154:406–9.[Abstract/Free Full Text]
  3. Fukuda K, Nisenbaum R, Stewart G, et al. Chronic multisymptom illness affecting Air Force veterans of the Gulf War. JAMA 1998;280:981–8.[Abstract/Free Full Text]
  4. Ismail K, Everitt B, Blatchley N, et al. Is there a Gulf War syndrome? Lancet 1999;353:179–82.[CrossRef][ISI][Medline]
  5. Doebbeling BN, Clark WR, Watson D, et al. Is there a Persian Gulf War syndrome? Evidence from a large population-based survey of veterans and nondeployed controls. Am J Med 2000;108:695–704.[CrossRef][ISI][Medline]
  6. Knoke JD, Smith TC, Gray GC, et al. Factor analysis of self-reported symptoms: does it identify a Gulf War syndrome? Am J Epidemiol 2000;152:379–88.[Abstract/Free Full Text]
  7. Cherry N, Creed F, Silman G, et al. Health and exposures of United Kingdom Gulf War veterans. Part I: the pattern and extent of ill health. Occup Environ Med 2001;58:291–8.[Abstract/Free Full Text]
  8. Kang HK, Mahan CM, Lee KY, et al. Evidence for a deployment-related Gulf War syndrome by factor analysis. Arch Environ Health 2002;57:61–8.[ISI][Medline]
  9. Shapiro SE, Lasarev MR, McCauley L. Factor analysis of Gulf War illness: what does it add to our understanding of possible health effects of deployment? Am J Epidemiol 2002;156:578–85.[Abstract/Free Full Text]




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