The failure to find significant group by time interaction effects suggests that there was little differential movement among the four groups over time. What this suggests is that there is relative stability, at least among the basic standings, on these critical measures of disability, fatigue, support, optimism and coping over time. However, there was considerable movement in the categories over time. For example by Wave 2, of the original group of 32 individuals diagnosed with CFS, 4 had died, and 24 were found and agreed to be re-evaluated, and of this group, only 16 continued to have CFS (5 developed exclusionary illnesses, 2 were classified as ICF, and one had remitted). What this suggests is that the CFS group for the most part remained rather ill, with a variety of different conditions over time.
Still, there were important differences among the conditions both at Waves 1 and 2. There were clearly more significant differences among the groups at Wave 1 than Wave 2. For example, the CFS, ICF and Exclusionary groups had significantly worse physical composite, mental composite, and fatigue scores at Wave 1, but by Wave 2, only the CFS and Exclusionary groups had significantly worse scores on the physical composite, and there were no significant differences across groups for the mental composite scores. For fatigue scores by Wave 2, only the CFS and ICF groups were significantly worse than the Controls. For stress, support and optimism, several significant Wave 1 differences emerged, but by Wave 2, the CFS group was no longer significantly different from any of the other groups. For coping, there were few significant differences among the groups at Wave 1 and Wave 2 and the CFS group only differed from the Controls at Wave 2 for behavioral disengagement.
Regarding sociodemographic findings, the ICF group a decade earlier had significantly more women and psychiatric co-morbidity than in the Control group. Our findings also indicated that when compared to the Control group, the Exclusionary group had significantly more women, lower SES and education level, but higher psychiatric co-morbidity. In general, the Exclusionary group appeared to be at the highest risk on these sociodemographic variables. The CFS group was only significantly different than the Exclusionary group on psychiatric comorbidity, which was expected. Because race was not significantly different, it is important to note that this was a diverse population, with only the control group having more than 50% of participants reporting themselves as white (50% of CFS, 38.1% of ICF, and 39% of Exclusions).
Our study found significant differences for 6 of the 8 core Fukuda et al. (1994)
symptoms. However, among all the variables in this study, only for post-exertional malaise did the CFS group significantly differ from the other three conditions. This reaffirms the importance of this being a cardinal and critical symptom for CFS, and all of the individuals in the CFS group had this symptom either at Waves 1 or 2. For unrefreshing sleep and impaired memory and concentration, 100% of the CFS group had these symptoms at Wave 1 or Wave 2. Similar to post-exertional malaise, these results support the idea that unrefreshing sleep and impaired memory and concentration are core symptoms of CFS. It is of interest that these two symptoms are also considered core symptoms of the Canadian ME/CFS Clinical Case definition (Carruthers et al., 2003
). For unrefreshing sleep and impaired memory and concentration, it is important to note that the CFS, ICF and Exclusionary groups were significantly different from the Control, so these symptoms might not be as unique in differentiating CFS from other conditions as it post-exertional malaise. For headaches and lymph node pain, the Exclusionary group was significantly different from the Controls, and the CFS group was significantly different than the Controls on headaches. Of particular interest is that the CFS and ICF groups were only significantly different on post-exertional malaise and impaired memory and concentration, and the CFS group had more participants exhibiting the symptoms than in the ICF group. This suggests that these two central features may distinguish CFS and ICF participants.
For Other Diagnoses, significant differences only emerged for muscle weakness, multiple chemical sensitivities and insomnia. For muscle weakness and multiple chemical sensitivities, the CFS and ICF groups had significantly higher percentages than controls (the Exclusionary group also was higher than Controls for multiple chemical sensitivities). For the insomnia condition, the CFS and Exclusionary groups were significantly different from controls. It was of interest that from 19% to 27.3% of individuals within groups went on to develop insomnia over time. Moreover, the controls had the highest percentage of participants developing insomnia, but still the highest percentage without insomnia (only 14.3% of the CFS group did not have insomnia at Wave 1 or Wave 2). Although there were no significant differences for blood transfusions, those in the CFS group had 29.4% which was the highest percentage among the groups. Also, though not significant, 22.2% of those in the CFS group had mononucleosis, which was also the highest percentage. Although the percentage of those with fibromyalgia was highest in the CFS and ICF groups, in general these percentages are relatively low compared to findings from other tertiary care settings.
There is a dearth of studies that have followed representative samples of participants with CFS over time, and the present study adds to the literature on the natural history of CFS over time. This study supports other findings when risk factors at Wave 1 were used to predict status 10 years later, and in that study, the CFS group only had significantly more impairment than the Controls for the physical composite index and the fatigue scores, but for none of the measures of stress, support or coping. Moreover, after examining the difference scores from Wave 1 to Wave 2, it is clear that there are many differences between the CFS and Control groups regarding illness progression and recovery. Also of interest, the symptom post-exertional malaise appears to be unique in differentiating CFS from the other groups, and this symptom is required for a diagnosis of ME/CFS based on the Canadian ME/CFS criteria (Carruthers et al., 2003
). However, post-exertional malaise is not required for the Fukuda et al. (1994)
case definition, as with this CFS criteria, individuals only need to meet 4 out of 8 symptoms for a diagnosis, of which only one of these 8 symptoms is post-exertional malaise. Limitations in this study include the small sample sizes, and the focus on psychological and self-report measures. Future papers will incorporate findings from more biological measures.