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- SUBJECTS AND METHODS
Fibromyalgia syndrome (FMS) has been defined by the American College of Rheumatology (ACR) as consisting of widespread pain of at least 3 months' duration in combination with tenderness at 11 or more of 18 specific tender point sites (1). In addition, patients diagnosed with FMS report a range of physical, cognitive, affective, stress-related, and behavioral symptoms.
Maladaptive learning processes have been viewed as essential for the development of pain behaviors and disability in many chronic pain syndromes (2–4). Pain behaviors are overt expressions of pain, distress, and suffering, such as slowed movement, bracing, limping, and grimacing (5, 6). Pain behaviors have a communicative function (5–7) and signal the presence of pain to others. A central feature of pain behaviors is that they are observable, and therefore capable of eliciting a response from significant others. From an operant conditioning perspective, increased pain behaviors result from reinforcing responses by significant others. For example, solicitous responses by significant others have been found to be positively associated with higher ratings of pain severity, more pain behaviors, greater disability, and decreased activity levels (8–10). However, pain behaviors may also be related to factors other than operant conditioning variables, for example cognitive, affective, and physical variables.
Several studies have specifically examined the presence of pain behaviors and their predictors in FMS patients (8, 11, 12). Romano et al (11) showed that spouses' attentive responses to nonverbal pain behaviors were significant predictors of physical disability in the more depressed patients, and were significant predictors of the rate of nonverbal pain behavior in patients who reported greater pain. Turk and Okifuji (12) observed that the combination of physical, affective, and cognitive predictors explained 53% of the variance of pain behaviors in a sample of FMS patients. Contrary to the results of other studies (9–11, 13), however, operant variables did not predict pain behaviors in this sample.
Several investigators have implicated the role of the hypothalamic-pituitary-adrenal (HPA) axis in FMS (14–16) as one physical factor in the chronicity of FMS. FMS patients show a dysregulation of the HPA axis with reduced growth hormone and hypocortisolism (17–19) that may influence pain thresholds (20). Low levels of cortisol may lead to heightened sensitivity to pain and other symptoms. This increased attentional focus might facilitate the opportunity for operant conditioning to occur.
To date studies on pain behaviors have focused on FMS as if the syndrome consisted of a homogeneous group. There have been suggestions that FMS patients may differ on important variables, such as responses to medication treatment, differences in biologic variables, association of depression with specific cytokine abnormalities, the presence of antipolymer antibody (21), and psychosocial status (22). The failure to consider subgroup differences may contribute to the inconsistency in predictors of pain behaviors, a lack of understanding of FMS, and ultimately inadequate and inappropriate treatment.
Turk and his colleagues (22) used cluster analytic procedures to identify subgroups in chronic pain patient populations based on their responses to the Multidimensional Pain Inventory (MPI) (23). One group, labeled dysfunctional (DYS), exhibited the highest level of pain, emotional distress, and disability. A second group, labeled interpersonally distressed (ID), reported significantly lower levels of pain, disability, and marital satisfaction than the other 2 subgroups. The significant others of ID patients showed a higher level of negative responses to the patients' expressions of pain. The third group, adaptive copers (AC), showed low pain intensity, emotional distress, and interference of pain with daily life and activities.
The primary objective of the present study was to confirm the role of significant others in the presence of pain behaviors for FMS patients. In addition, the role of physical and psychosocial variables in predicting pain behaviors was evaluated. Finally, differences in the prevalence of pain behaviors among subgroups of patients were of interest. Specifically, we examined the following hypotheses: 1) FMS patients will demonstrate a greater number of pain behaviors than pain-free controls when they perform a task requiring physical activity; 2) the pain behaviors of FMS patients will increase during a physical task when their spouse is present; 3) FMS patients classified as DYS will demonstrate a larger number of pain behaviors than those classified as AC or ID; 4) operant (spouse-response) variables will be the best predictors of pain behaviors in the DYS but not in the AC and ID groups; and 5) lower cortisol levels may contribute to enhanced pain sensitivity and thereby more pain behaviors.
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- SUBJECTS AND METHODS
In support of the first hypothesis, the FMS group showed significantly more pain behaviors than the pain-free control group. Among the FMS patients, the DYS group had the greatest number of pain behaviors compared with the ID and AC groups. The results of these analyses partially confirm the third hypothesis of higher pain behaviors in the DYS group. This difference was especially observable in the presence of the spouse but not if the spouse was absent, confirming the second hypothesis. In accordance with the operant pain model (4) and studies on FMS and chronic back pain (7, 8, 13), pain behaviors were high in the FMS group as a whole and particularly for the DYS group. The presence of a spouse appears to serve as a discriminative cue for these behaviors.
The physical, cognitive, affective, stress, spouse-related, and pain-related variables had different predictive values for the pain behaviors of the entire sample and for the psychosocial subgroups. The prevalence of pain behaviors for the entire sample was best predicted by high solicitous and low distracting spouse behavior, a reduced cortisol production, and increased stress at work. The results regarding the role of physical variables as significant predictors of pain behaviors are in accordance with previous reports on other pain groups (35–38). Turk and Okifuji (12), for example, explained 16% of the variance of pain behaviors in a sample of FMS patients by physical factors comparable with our results accounting for 17% of the variance. Thus, physical variables are one important determinant of pain behaviors in FMS but not the sole contributor.
The relationship of reduced cortisol production on pain behaviors has not been reported previously. The role of cortisol production in relation to FMS symptoms is controversial. Bennett et al (14) suggested that reduced production of growth hormones leads to intermittent phases of heightened cortisol levels. In contrast, Griep et al (17) showed a mild hypocortisolemia in FMS patients after a dexamethasone suppression test. Heim et al (18) also demonstrated hypocortisolism in patients with FMS, rheumatoid arthritis, and bronchial asthma with comorbid posttraumatic stress disorder (PTSD) compared with nonpain patients suffering from PTSD who showed hypercortisolism (19). Hormone production is known to influence central nervous system activity. For example, release of cortisol increases sensory thresholds in healthy persons (27). The reduced cortisol production in FMS patients might contribute to their higher pain levels and pain behaviors. Higher pain sensitivity might lead to the expression of more pain behaviors. However, the relationship of cortisol and FMS symptoms, especially pain behaviors, needs further investigation.
Pain behaviors for the entire sample, and particularly for the DYS subgroup, were also predicted by operant variables. The predictor accounting for the largest amount of variance (19%) in pain behaviors for the entire sample was both solicitous spousal behavior and reduced distracting spouse responses. This dichotomy is consistent with studies (8, 39) that found solicitous behaviors as predictors for pain behaviors in the sample. The level of distracting behaviors was not the aim of earlier studies. Continuatively, it may be that solicitous spouse behavior combined with distracting behavior characterizes social support. Further behavioral observation studies that vary the spousal behavior are necessary to differ between solicitous spouse behavior reinforcing pain behavior and social support.
In the DYS group, a much higher percentage of pain behaviors was predicted than in the other groups. For this group, more solicitous spouse responses, less growth hormone and cortisol production, and more pain intensity together explained a large percentage (77%) of the variance, confirming the fourth hypothesis. The predictor accounting for the greatest amount of variance (45%) was solicitous spouse responses. This result is consistent with other studies that have shown that pain behaviors and solicitous responses by significant others are positively associated (3, 7, 9). These results replicate previous findings and further underline the relationship between pain behaviors and reduced cortisol production as a physical variable, which influences pain perception. Reduced cortisol production could lead to higher pain perception and pain behaviors, which could, in turn, enhance pain perception and response to pain.
For the ID group, heightened ACTH production was the best single predictor of pain behaviors, accounting for 42% of variance. It has been proposed that FMS should be considered a stress-related syndrome because symptoms often have their onset triggered by stress, whether psychological, infectious, or traumatic (40). Stress exerts its effects by as yet unknown central pathways that stimulate the hypothalamus to release stress hormones, with ACTH, corticotropin-releasing hormone (CRH), and antidiuretic hormone being the most important. Elevated CRH seems to alter the set point of other hormonal axes, such as the hypothalamic-pituitary-adrenal axis associated with enhanced ACTH and reduced cortisol production (7), which might be one contributing factor to pain behaviors. Born et al (41, 42) and Floretal (27) showed that higher ACTH production is related to enhanced processing of sensory stimuli, including pain (42).
The pain behaviors of the AC group were best predicted by stress at work, explaining 23% of the variance. In contrast to the other groups, stress appears to be especially important as a contributing factor to pain behaviors in the AC group. Overall, the AC showed a lower number of pain behaviors than the other 2 groups.
The different predictors of pain behaviors for the FMS subgroups support the observation that FMS is not a homogeneous syndrome (43). Different factors appear to be important in influencing how patients respond to their symptoms. The results suggest that different treatments for FMS subgroups might be useful. The significance of solicitous responses by significant others as a predictor of pain behaviors in DYS patients suggests that an operant behavioral pain treatment might be appropriate (28, 39). This treatment seems to be less warranted for the ID patients and the AC group because of their lower pain behaviors. The ID and AC patients show high pain behaviors in relation to heightened ACTH production or enhanced stress. Because the ID and AC patients show problems in coping with stress (40, 44), a cognitive-behavioral pain treatment plan would be helpful.
Although the overall sample size for the patient group appears reasonable, subdividing the total sample into 3 subgroups produced relatively small samples. Thus, the interpretation of the results of the regression analyses on the subgroups must be considered with caution. Future research is needed to replicate the predictors of pain behaviors observed in the different subgroups using larger patient samples. Furthermore, it would be necessary to compare the pain behaviors in FMS with other chronic pain conditions (e.g., osteoarthritis, back pain).
Although the FMS patients and the healthy controls showed significant differences in pain behaviors both in the presence and the absence of the spouse, and the psychosocial subgroups also showed significant differences of pain behaviors in the presence of the spouse, significant differences for pain behaviors in the absence of the spouse were not found. The absence of the differences in pain behaviors of the different subgroups under the absence of the spouse condition underlines the importance of the spouse as a discriminative cue for pain behaviors and suggests that pain behaviors are closely related to immediate reinforcement.