Evidence of peripheral large nerve involvement in fibromyalgia: a retrospective review of EMG and nerve conduction findings in 55 FM subjects

Abstract

Objective: Whereas small fiber neuropathy (SFN) is now a recognized part of fibromyalgia (FM), sur-prisingly little attention has been paid to any findings of large fiber neuropathy (LFN) in this disorder.Since 90% to 95% of FM subjects seen in our outpatient facility routinely undergo EMG and nerveconduction studies (NCS) we elected to retrospectively review the EMG/NCS results garnered from alarge cohort of unselected subjects in order to describe the electrodiagnostic features of LFN in FM.Methods: Records from 100 consecutive, unselected clinic patients meeting the 1990 ACR criteria forFM, who had undergone EMG/NCS, were reviewed. The same electromyographer tested all subjects.After exclusion of FM patients with any other clinically relevant condition that might influence EMGresults (e.g., familial neural degenerative conditions, diabetes mellitus, Vitamin B-12 deficiency, etc.)fifty-five FM subjects remained: 29 subjects with “FM Only,” and 26 subjects with FM+RheumatoidArthritis (“FM+RA”). All subjects had also undergone ankle area skin biopsy for determination of epi-dermal nerve fiber density (ENFD). Fourteen other subjects, without FM or RA, examined by the sameelectromyographer, were used as an EMG/NCS comparison group.Results: Ninety percent of the “FM Only” subjects demonstrated a demyelinating and/or axonal, sen-sorimotor polyneuropathy, and 63% had findings of SFN (ENFD ≤7 fibers/mm), suggesting a mixed fi-ber neuropathy in most. Furthermore, 61% of the “FM Only” subjects showed EMG findings suggestiveof non-myotomal lower extremity axonal motor denervation, most likely due to a polyneuropathy,and 41% satisfied published criteria for “possible” chronic inflammatory demyelinating polyneuropa-thy (CIDP). There was surprisingly little difference in the EMG/NCS findings between the “FM Only” andthe “FM+RA” groups. With the exception of carpal tunnel syndrome, our EMG/NCS comparison groupshowed few to none of these findings.Conclusion: Our review of the EMG / NCS results, gleaned from the largest FM cohort yet studied withthese modalities, shows that electrodiagnostic features of polyneuropathy, muscle denervation, andCIDP are common in FM. Furthermore these electrodiagnostic findings are often seen coincident withSFN, and are not significantly influenced by the presence of RA. These results, particularly when takenas a whole, suggest that EMG/NCS may be clinically useful in detecting LFN in FM and help in betterunderstanding the etiopathogenesis of this painful disorder.