Background/Purpose: Several recent reports have implicated peripheral small fiber neuropathy (SFN) as a significant contributing factor to the pain seen in fibromyalgia (FM).  Large fiber neuropathy (LFN) in this disorder has not been extensively described, however. Therefore, we retrospectively examined electromyographic (EMG) and nerve conduction study (NCS) data (collectively, “EDX”) on a cohort of FM patients and controls for LFN, and clinically correlated our findings. 

    Methods:   We reviewed clinical and EDX data from 100 consecutive patients, meeting 1990 ACR FM criteria, for study inclusion / exclusion.  Subjects with a clinical disorder known to produce LFN (e.g., diabetes, Vitamin B-12 deficiency, familial neuropathy, Sjögren’s syndrome, etc.) were excluded.  Those with FM + RA were allowed.  EDX findings from 30 “FM Only” (27 Caucasian; 23 female; mean age 59, range 21 – 90 yrs.), 25 FM + RA (20 Caucasian; 23 female; mean age 57, range 26 – 84 yrs.), and 14 control subjects with No FM & No RA (mostly incidental evaluation for carpal tunnel syndrome) were compared.  

     Results:  All 55 FM subjects in this cohort had EDX findings of LFN (Figure), with most having a sensorimotor polyneuropathy (90%), which was demyelinating only, in 22%, axonal only, in 12%, and mixed demyelinating / axonal, in 66%.  A sensorimotor polyneuropathy was seen in only 7% of control patients (P < 0.0001).  NCS features in FM Only included temporal dispersion, prolonged duration, abnormal F-wave, or carpal tunnel syndrome in 28%, 10%, 21%, and 24% respectively.  EMG in FM Only showed findings of lower extremity axonal denervation, thought most likely due to a polyneuropathy, in 61%.  Clinical correlation in the FM Only group showed a significant association between EMG denervation and proximal muscle weakness (P < 0.005).  EDX findings meeting published criteria for, at least, “possible” chronic inflammatory demyelinating polyneuropathy (CIDP) were seen in 41% of FM Only subjects, but in no control subjects (P<0.003).  Calf epidermal nerve fiber density (ENFD) was reduced to ≤ 6.5 fibers / mm in 53% and ≤ 7.0 fibers / mm in 63% of FM Only subjects.

     Conclusion:  All FM subjects in our cohort had evidence of LFN, suggesting that these lesions participate in FM symptomatology.  “FM Only” and FM + RA participants did not demonstrate any major between-group differences in EDX findings.  EDX appeared to compliment ENFD determinations in defining the peripheral neuropathology of FM.  These EDX findings probably hold important implications for the better understanding and treatment of this painful disorder.

LFN in FM: A vs. C (P value) <0.0001, B vs. C <0.0001, D vs. F <0.0001, E vs. F = 0.0001, G vs. I = 0.02, H vs. I = 0.01, J vs. L = 0.003, K vs. L = 0.002, M vs. O = 0.03, N vs. O = 0.01, S vs. U = 0.08, T vs. U = 0.01.  (Fischer’s Exact Test, 2 tailed) Other comparisons were NS.  * Indicates 1% for illustration purposes; actual value = 0%.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/is-there-peripheral-large-nerve-involvement-in-fibromyalgia-a-systematic-emg-nerve-conduction-study-evaluation-of-55-consecutive-fm-patients/