Fibromyalgia syndrome: a pain modulation disorder related to altered limbic function?

Pain in fibromyalgia

Just as our caveman forebears were frail in the face of predatory animals, we are frail in today's society of childhood neglect or abuse, bumper-to-bumper traffic, frustration at work, and multiple daily hassles. The same neuroendocrine systems and pain regulatory mechanisms that protected early man during acute stress are still encoded in our genome, but may be maladaptive in psychologically and physiologically vulnerable people faced with chronic stress. Many patients with fibromyalgia become vulnerable because of the long-lasting psychological and neurophysiological effects of negative experiences in childhood. Ill-equipped with positive cognitive, emotional, and behavioral skills as adults, they display maladaptive coping strategies, low self-efficacy, and negative mood when confronted with the inevitable stressors of life. Psychological distress ensues, which reduces thresholds for pain perception and tolerance (already relatively low in women) even further. Converging lines of psychological and neurobiological evidence strongly suggest that chronic stress-related blunting of the HPA, sympathetic, and other axes of the stress response together with associated alterations in pain regulatory mechanisms may finally explain the pain and fatigue of fibromyalgia. Vulnerable people who can be classified by the ACR criteria as having fibromyalgia do not have a discrete disease. They are simply the most ill in a continuum of distress, chronic pain, and painful tender points in the general population.

Glucocorticoid receptors, fibromyalgia and low back pain

Recently, fibromyalgia (FMS) was shown to be a disorder associated with an altered functioning of the stress response system. FMS patients display a hyperreactive pituitary adrenocorticotropic hormone (ACTH) release in response to corticotropin-releasing hormone (CRH) and to insulin-induced hypoglycemia. We suggested that negative feedback of cortisol could be deranged. Therefore we investigated the properties and function of the glucocorticoid receptors (GR) in FMS patients and compared the results with those of healthy persons and patients with chronic low back pain (LBP a localized pain condition). Forty primary FMS patients (F:M = 36:4), 28 LBP patients (25:3) and 14 (12:2) healthy, sedentary control persons were recruited for the study. Urinary free cortisol excretion in FMS and LBP patients was lower compared to controls. Only FMS patients displayed lower CBG and basal serum cortisol concentrations when compared to controls. However, plasma free cortisol concentrations were similar in the three groups. There was no difference in the number of GR per cell among the three groups (FMS: 6498 +/- 252, LBP: 6625 +/- 284, controls: 6576 +/- 304), but the dissociation constant (Kd) of the FMS (14.5 +/- 0.9 nmol/l) and LBP (14.7 +/- 1.3 nmol/l) subjects was significantly higher than that of the controls (10.9 +/- 0.8 nmol/l) (p < .05). The maximal stimulation of the lymphocytes, as measured by the maximal thymidine incorporation (in the absence of cortisol) in the FMS group was approximately 1.5 times higher (p < .05) than in the control or LBP group. The ED50 (the cortisol concentration giving 50% inhibition of the thymidine incorporation), however, was identical in all three groups. We conclude that FMS patients have a mild hypocortisolemia, increased cortisol feedback resistance in combination probably with a reduced CRH synthesis or release in the hypothalamus. The role of the GR and mineralocorticoid receptor (MR) in the CRH regulation in the FMS patients remains to be solved.

The relationship between fibromyalgia and major depressive disorder

Looking at the results of the seven types of studies discussed previously, it appears that there is strong evidence for an association between fibromyalgia and major depressive disorder on the basis of (1) overlapping symptomatology, (2) similar pattern of comorbid disorders, and (3) high rates of major depressive disorder among relatives of patients with fibromyalgia. There is additional support for an association on the basis of responses to psychological tests and rating scales and the high lifetime rates of mood disorders in fibromyalgia. Two lines of evidence, (1) response to antidepressant medications and (2) response to biologic tests, offer little evidence either for or against an association. On balance, then the weight of the evidence favors an association between fibromyalgia and major depressive disorder. We therefore turn to an analysis of the nature of the association.