Background/Purpose: Both patients with fibromyalgia (FM) and with axial spondyloarthritis (SpA) are suffering from pain.  The patients with axial SpA are mainly manifested as inflammatory back pain, while widespread pain is found in the patients with FM.  However for those FM patients, the commonly-seen sleep disturbance due to anxiety/depression usually amplifies the sensation of back pain during the night time, which is likely to be recognized as inflammatory back pain; thus some FM patients had been misdiagnosed as axial SpA.  This study intends to further understand the modified American College of Rheumatology (ACR) 2010 diagnostic criteria for FM, increase diagnostic accuracy of axial SpA and reduce misdiagnosis of FM as axial SpA.

Methods: Clinical data of 26 patients with primary FM were collected, who had previously misdiagnosed as axial SpA by other tertiary hospitals in China. The re-diagnosis of those 26 FM patients was established based on the modified ACR 2010 criteria. Each patient was assessed by Widespread Pain Index (WPI) and Symptom Severity (SS) scale adopted by the modified ACR 2010 criteria, as well as parameters including serous C-reactive protein, erythrocyte sedimentation rate (ESR), HLA-B27 and sacroiliac joint images.

Results: Among the subjects, 8 males (30.77%) and 18 females (69.23%) complained widespread pain, most notably in the spine. None of peripheral arthritis, enthesitis, dactylitis, and limited spine motion was found.  WPI median (P25-P75) was 5.0 (4.0-8.0).  SS score was 9.269 ±1.589.  Serous C-reactive protein (normal range: 0 – 0.8 mg/dl) was 0 in 5 cases, less than 0.348 mg/dl in 15 cases and between 0.37-0.74 mg/dl in 6 cases.  ESR median (P25-P75) was 5.0 (3.0-9.0) mm/1h.  HLA-B27 positivity was detected in 73.08% cases.  4 patients (15.38%, all females) were found with osteitis condensans ilii by CT and MRI, 5 patients with normal sacroiliac CT scan, 6 with normal sacroiliac MRI result, and the rest 11 with normal sacroiliac CT and MRI.

Conclusion: The modified ACR 2010 criteria for FM has adopted the criteria of WPI and SS scale replacing the tender point count and enhanced the diagnostic weight of anxiety/depression.  However, in medical practice in China, it isn’t uncommon to see the misdiagnosis of FM as axial SpA, which was caused by two reasons: (i) over-relying on the tender point counts indicated in ACR 1990 criteria for FM and (ii) failure to take inflammations as the priority factor of diagnosis of axial SpA.  In order to increase diagnostic accuracy of FM and axial SpA, it is necessary to (i) fully investigate and evaluate somatization of anxiety/depression, instead of using tender point counts as the exclusive criterion of FM; and (ii) emphasize inflammations in diagnosis of axial SpA, i.e., at least one of the following factors should be identified: peripheral arthritis, enthesitis, dactylitis, definite sacroiliitis on imaging and elevated inflammatory biomarkers such as serous C-reactive protein and/or ESR.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/misdiagnosis-of-fibromyalgia-fm-as-axial-spondylarthritis-spa-lessons-from-analysis-of-26-cases/