• Musculoskeletal (symptoms of joint pain, joint stiffness, or muscle pain).
Because the study was funded and conducted by the Centers for Disease Control and Prevention (CDC), that characterization is often referred to as CDC’s case definition of CMI.
Before 1998, the terms Gulf War syndrome, Gulf War veterans’ illness, unexplained illness, and undiagnosed illness were used interchangeably to describe chronic unexplained symptoms in veterans of the 1991 Gulf War. Earlier committees of the Institute of Medicine (IOM) found no evidence of a specific symptom complex (or syndrome) that was peculiar to deployed Gulf War veterans (IOM, 2006, 2010).
Reports of chronic unexplained symptoms are not peculiar to Gulf War veterans; this phenomenon has been documented in military personnel throughout modern history (Hyams et al., 1996; IOM, 2010; Jones, 2006). Before World War I, such chronic unexplained symptoms as fatigue, shortness of breath, and chest pain were referred to as irritable heart, soldier’s heart, Da Costa’s syndrome, and others. Other terms associated with the adverse effects of combat experience on health and well-being include shell shock (World War I), psychoneurosis (World War II and the Korean War), and post-Vietnam syndrome, later identified as posttraumatic stress disorder (PTSD) (Jones, 2006). A cluster analysis of common symptoms in veterans from 1900 to the 1991 Gulf War did not reveal a unique set of symptoms that were associated with each war (Jones and Wessely, 2005; Jones et al., 2002). However, veterans of the 1991 Gulf War have reported more cases of chronic medically unexplained symptoms than veterans of prior conflicts (Hunt, 2012).
Chronic unexplained symptoms are common in civilians. Such terms as medically unexplained symptoms, medically unexplained physical symptoms, somatoform disorders (for example, somatization disorder, undifferentiated somatoform disorder, and pain disorder), and functional somatic syndromes are often used to describe the disorders of civilians who have chronic unexplained symptoms. The common thread among the terms is that symptoms experienced by patients cannot be explained as pathologically defined, or organic, disease (Sharpe and Carson, 2001). Such syndromes as irritable bowel syndrome (IBS), chronic fatigue syndrome (CFS, also called myalgic encephalomyelitis), and fibromyalgia often are included in this group of unexplained illnesses, as are chronic unexplained symptoms that do not meet case definitions for IBS, CFS, fibromyalgia, and other functional somatic syndromes that have specified diagnostic criteria.