The present study confirmed the results from our earlier study [40
] that increased BMI is common in FMS. Approximately the half of our sample was in the obese range, with an additional 30% in the overweight range. According to the recent Behavioral Risk Factor Surveillance Survey, the state of Utah has an obesity rate of 23.1% and overweight rate of 35.1% [37
]. Our sample had over twice the rate of the obesity in the state. National rates are estimated at 32.2% for obesity and 34.1% for overweight [22
In accordance with the previous findings [39
], obesity was related to increased TP pain sensitivity. The results are also consistent with the findings that obesity is a risk factor for chronic pain [6
], is associated with increased severity in visceral pain [5
], and is generally related to high prevalence of pain complaints [54
]. The mechanisms underlying the obesity-pain sensitivity link are not clear at this time. Poor physical conditioning has been considered as one of the potential contributors of pain sensitivity; conversely, aerobic fitness programs reduce TP pain sensitivity in FMS patients [32
]. In out study, obesity was associated with reduced strength and flexibility but there was no group difference in the walking test. As the recent systematic review [12
] indicates, accumulated evidence suggests the benefit of an aerobic program with some suggestive benefit of strengthening. Muscle strengthening of the neck has resulted in reduced pain thresholds of the area in chronic neck pain patients [59
]. Although these findings are suggestive, whether exercise capacity mediates the obesity-TP pain sensitivity link warrants further investigations.
Another possible mechanism may be related to the endogenous opioid system. Animal studies suggest that obesity may affect sensitivity to noxious stimuli via obesity-related alteration in the endocrine and opioid systems [28
]. In our previous study [40
], we have reported that obesity in FMS is related to the greater levels of proinflammatory indices. Results from animal research strongly suggests the involvement of proinflammatory cytokines in central sensitization [26
] and the development of chronic latent hyperalgesia in muscles [18
]. Whether and how these endogenous changes associated with obesity play a role in heightened pain sensitivity in obese FMS patients may be of interest in future research.
Interestingly, however, the heightened pain sensitivity to TPs in obese patients appeared to be more pronounced in the lower body areas. In addition to centrally modulated pain sensitivity, the pain sensitivity of obese patients may also be influenced by the mechanical loads of having to carry extra weight. While it is well known that obesity is a risk factor for pain disorders involving bones and joints [21
], recent evidence also suggests that obesity may also be a significant contributor for soft-tissue related pain [47
]. Furthermore, a study of people with spinal cord injury suggests that obesity may contribute to increased tissue loading, leading to deep tissue injury [20
], Thus, it is likely that multiple factors appear to be involved in the relationship between obesity and pain sensitivity.
Our results suggest that the degree of hyperalgesia can vary across body sites in FMS, and the assessment of pain sensitivity should take different body sites into consideration. Some pain testing methods, such as the TP examination used in this study and thermal pain sensitivity, may be applied to various body sites, whereas others, such as cold presser and ischemic pain tests are not very practical. Using the multiple methods to determine the levels of pain sensitivity is likely desirable. There is one methodological concern related to this result. The TP examination protocol in our study was standardized so that the order of palpation was same for all patients, raising a question of the order effect. The differences were most pronounced in the lower body sites including gluteal, greater trochanter and knee. Without the random order palpation we cannot totally rule out the possibility of order effect. However, given that the patients in the normal weight range did not show the differential pain sensitivity to upper vs lower body TPs, and that there were palpations of upper body sites after the gluteal sites, the greater difference in pain sensitivity for those lower body sites are unlikely due to the order of the palpations.
As discussed earlier, our results indicate that obesity influences the physical fitness ability to some extent in FMS. Given this, failure to find the group differences in the walking ability was surprising; however, this may have been influenced by the fact that all patients were very sedentary and there was a 20 minute cap and self-determined pace in the walking test. The maximum heart rate during the walking test was significantly higher in the obese patients than normal weight patients. Although this seems to have been accounted for by the baseline heart rate levels, which were significantly elevated in the obese patients, we have shown significance with respect to change as well. The increases in heart rate during the walking test were significantly greater for the obese patients, potentially making the test more laboring for these patients. If so, this may have significant implications for rehabilitation of obese FMS patients. Inclusion of perceived exertion may be helpful in future research to clarify this point further.
The obese patients also showed reduced flexibility in the lower body areas (ie, straight leg raise), as well as reduced strengths in general. Anecdotally, the physical performance testing examiner (LLB) observed that many of the obese patients appeared to put less effort to due to increased pain. This observation is consistent with the report that obese women tend to stop their exercise testing due to musculoskeletal pain [24
]. The results suggest that the heightened pain sensitivity in the lower body areas of the obese FMS patients may bring up an additional barrier for activating therapy.
The results also replicated our earlier findings that obesity in FMS is related to the poorer quality of sleep. Obese patients exhibited reduced sleep duration and increased activities during sleep. However, our results failed to show group difference in self-report symptoms of FMS except for stiffness. It is interesting to note that the factors that were associated with obesity were obtained based upon observed or provoked testing process (ie, TP exam, physical performance, actigraphic sleep data), whereas most of self-reported symptoms show little relationship to obesity. These results seem consistent with previous findings by others [39
]. Yunus et al [61
] also found no difference in VAS measured pain, fatigue, and global severity of FMS between normal weight FMS patients and FMS patients with BMI greater than 25. FMS is a multifactorial condition and the results suggest that obesity does not necessarily affect all aspects of FMS. At this time, it seems reasonable to conclude that the adverse impact of obesity in FMS mostly affects hyperalgesia, disability, quality of life and as we replicated in this study with larger sample, sleep.
However, a recent study evaluating a behavioral weight loss program for FMS suggests that weight loss improves FMS symptoms [51
]. Given the absence of significant association between FMS symptoms and obesity, we must speculate that the relationship between weight loss and symptom reduction is not direct but is mediated by other factors. One possibility is changes in lifestyle. Behavioral weight loss programs typically involve some significant changes in eating habits, physical activities and coping. Shifting towards a healthy lifestyle may positively influence how patients manage their symptoms, how they perceive their plight, and overall quality of life.
There are some limitations of the study we must discuss. First, the study design was cross-sectional and thus the results are all correlational. No causality can be ascertained. Second, the definition of obesity we used in this study was solely based upon BMI. BMI is the vital measure for classifying obesity today and perhaps because of the simplicity, it is most used as a proxy measure of body composition. Although BMI is generally correlated with total body fat [38
], BMI fails to take various individual factors into consideration, such as muscle mass, tallness, age, and ethnic factors [45
]. The use of the direct measures of body fat, such as X-ray absorptiometry or densitometry clearly should be ideal but may not be feasible. A large, worldwide INTERHEART study suggests that waist-to-hip ratio may provide the best anthropometric measure to estimate abdominal fat that predicts cardiovascular morbidity [62
]. The waist-to-hip ratio is easily measured and future studies should benefit from including this measure as a surrogate measure of abdominal fat.
In summary, obesity is a common co-morbidity that may complicate the clinical picture of FMS. Obesity in FMS adversely affects both the quality and quantity of sleep, physical strength and flexibility, and pain sensitivity to pressure particularly in the lower body. Future research needs to clarify the mechanisms of how the obesity and its associated specific and relevant factors influence FMS as well as how successful weight management changes the expression of FMS.