The pain experienced by patients with fibromyalgia syndrome (FMS) may not be caused by central sensitization, or abnormal processing of sensation by neural brain activity, but linked to elevated pressure in the muscles affected.
In an evaluation of muscle tenderness and intramuscular pressure among patients with this condition, researchers found that pressure in the trapezius muscle is significantly elevated and may be an intrinsic feature of FMS that should be monitored. Full results are published in the Journal of Rheumatology.
Participants were recruited from a pool of patients enrolled at a rheumatologic clinical practice at Rush University Medical Center. Participants who comprised the FMS group completed the Fibromyalgia Survey Questionnaire to determine if they satisfied the modified American College of Rheumatology (ACR) criteria for FMS inclusion. The 108 patients in the FMS group were also required to have a confirmed diagnosis of FMS by a rheumatologist and pain over the trapezius muscle. The control group (n=30) included patients who did not have a diagnosis of FMS but fulfilled criteria for other rheumatic diseases and scored above the ACR criteria for fibromyalgia.
The primary measures included pain, muscle tenderness, and intramuscular pressure. Pain was measured on a 0 to 10 visual analogue scale (VAS) of intensity with anchors of 0 (no pain) and 10 (extreme pain). Muscle tenderness was graded according to both digital palpation and dolorimetry. Digital palpation was calculated by applying pressure to the trapezius muscle for a period of 4 seconds; discomfort was graded on a 4-point scale (0=no discomfort or pain; 3=severe discomfort and withdrawal to palpation).
Additionally, a 30-lb dolorimeter was placed over the trapezius muscle, and pressure was applied at a slow rate. The point at which applied pressure transitioned to pain was recorded in pounds. Finally, intramuscular pressure was measured with a Stryker pressure gauge in which 0.3 cc of saline was injected into the mid portion of the trapezius muscle, and pressure was recorded in mm Hg.
A total of 84% of patients in the FMS group were female, and the mean age of all participants was 48.3 years (compared with 47.9 years in the control group). Pain was moderately high in the FMS group (mean score, 6.68) compared with an average pain score of 1.43 in the control group. The mean pressure/pain threshold score was 14.5 pounds less in the FMS group compared to the control group (8.03 lbs vs 22.54 lbs). On digital palpation, the FMS group had an average score of 2.09±0.73 whereas controls had an average score of 0.47±0.78.
Pressure in the trapezius muscle was elevated in the FMS group with an average of 33.48 ±5.90 mm HG; in the control group, the average muscle pressure was 12.23±3.75 mm Hg. Only 2 values of intramuscular pressure in the FMS group were below 23 mmHg.
No morbidity was associated with the determination of muscle pressure with a pressure gauge and needle. No significant differences were identified in muscle pressure between males and females, and muscle pressure did not vary by age.
These findings may significantly change the way clinicians treat FM. “Currently the first-line medications for pain in FM are centrally acting, including pregabalin, duloxetine, and milnaciprin. They either decrease glutamate or increase serotonin and norepinephrine to try to ease the presumed central nervous system effect on widespread musculoskeletal pain,” noted the authors
“Appreciation of elevated trapezius muscle pressure as a mechanism for hypoperfusion and diffuse muscle pain in FMS is important and suggests a target for both evaluation and intervention,” concluded the authors.
Katz RS, Leavitt F, Small AK, Small BJ. Intramuscular pressure is almost three times higher in fibromyalgia patients: a possible mechanism for understanding the muscle pain and tenderness. J Rheumatol. Published online September 15, 2020. doi:10.3899/jrheum.191068