Medically Unexplained Illnesses

There are numerous illnesses with a symptom profile resembling that seen in Gulf War veterans. The broad category often used to label these syndromes is variously called “medically unexplained illnesses,” “medically unexplained symptom syndromes,” “functional somatic syndromes,” “chronic multisystem illness,” or “symptom-based conditions.” These labels refer to conditions marked by somatic complaints unaccompanied by objective laboratory findings or established causation. For simplicity, the remainder of this appendix refers to these conditions as medically unexplained illnesses, but any one of these labels could apply. That the nomenclature is still so variable is but one indication of the uncertainty enveloping these conditions.

In ongoing research, the medically unexplained illnesses most frequently compared with illnesses in Gulf War veterans are fibromyalgia, chronic fatigue syndrome, and multiple chemical sensitivity (for summary descriptions, see Boxes D.1D.3). All three are characterized by multisystem somatic complaints, usually pain, headache, and fatigue (Table D.1, Table D.2). For this reason, patients diagnosed with one of these conditions frequently meet case criteria for one or more of the others (Buchwald and Garrity, 1994; Slotkoff et al, 1997; Donnay and Ziem, 1999). Chronic fatigue syndrome and fibromyalgia have such overlapping presentations that the two conditions may possibly be different presentations of the same underlying condition (Buchwald and Garrity, 1994; Buchwald, 1996). On the other hand, a factor analysis study has offered evidence that chronic fatigue syndrome and fibromyalgia are distinct clinical entities (Robbins et al., 1997). The research question of whether the three conditions are separate conditions or variants of the same underlying condition is likely to remain unresolved until more is known about the etiology and pathogenesis11 of each. For the present, they are considered discrete conditions, as reflected by separate case criteria and/or separate listings in ICD-10 (WHO, 1992; see Table D.1).

Other medically unexplained illnesses, documented in military populations from past conflicts tracing back to the U.S. Civil War, include DaCosta syndrome, effort syndrome, and combat stress reaction (Hyams et al., 1996). Several other medically unexplained illnesses, investigated mostly in civilian populations, have been labeled sick building syndrome, irritable bowel syndrome, and silicone-associated atypical rheumatic disease (Hyams, 1998; Wessely et al., 1999). The names of some of these conditions are often disputed, as is their very existence as distinct clinical entities (AMA, 1992; AAAAI, 1999). Some regard unexplained illnesses as manifestations of depression, anxiety, or somatization


Pathogenesis refers to the series of biological events that give rise to the manifestation of symptoms or disease states.

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