Relationship between sympathetic activity and pain intensity in fibromyalgia

Objective. Fibromyalgia (FM) is a syndrome characterised by chronic musculoskeletal pain, hyperalgesia on specific areas of tenderness (tender points) and by an autonomic nervous system dysfunction consistent with sympathetic overactivity. It is not known whether there is any relationship between the amount of cardiovascular sympathetic activity and the magnitude of pain. Our objective was to assess this potential relationship in patients with FM. Methods. Electrocardiogram, finger blood pressure, respiration and postganglionic sympathetic discharge activity (muscle sympathetic nerve activity, MSNA) were continuously recorded at rest in 25 patients with primary FMS. The autonomic profile was assessed by MSNA and spectral indices of cardiac sympathetic (LF) and vagal (HFRR) modulation and of sympathetic vasomotor control (LF-(SAP)) computed by spectrum analysis of RR and systolic arterial pressure (SAP) variability. Cardiac baroreflex function was evaluated by the index a (a). Baroreceptor modulation of the sympathetic vasomotor control (sBRS) was assessed by the MSNA/diastolic pressure relationship. Results. Pain intensity was linearly correlated with LFRR/HFRR (r(2)=0.21; p=0.03), LFSAP (r(2)=0.26; p=0.02) and MSNA (burst rate) (r(2)=0.45; p=0.003). Pain intensity was inversely correlated with the alpha(LF) index (r(2)=0.24; p=0.02) and the sBRS (r(2)=0.28; p=0.03). Thus, the higher the sympathetic drive to the heart and vessels, the higher the magnitude of chronic pain. Also, the gains of both the cardiac and MSNA baroreceptor control were inversely related to the pain intensity. Conclusion. These findings raise the theoretical possibility that in FM patients the use of anti-adrenergic agents might lessen chronic pain intensity by reducing the underlying excessive sympathetic activity.