Is it axSpA or just a simple case of pelvic pain?

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Bone erosion, fat in joint spaces (backfill), and ankylosis visible on MRI could help identify women with axial spondyloarthritis, investigators wrote in a research article published last month in Arthritis & Rheumatology. 

Identifying the specific cause of buttock/pelvic pain requires careful analysis of sacroiliac joint lesions by different types of magnetic resonance imaging. Such an analysis would help in differentiating pain associated with axial spondyloarthritis (axSpA) and pain from other conditions, but this can be tricky.

Writing in the July 16 issue of the journal, Sengul Seven, M.D., of the University of Copenhagen in Denmark, and colleagues investigated whether different types of sacroiliac joint lesions that can be identified on MRI could differentiate between axial spondyloarthritis and other types of buttock or pelvic pain.

“We have found that erosion above a certain threshold and backfill and ankylosis were highly specific for axial spondyloarthritis, whereas fat lesions and low-level erosion and bone marrow edema on sacroiliac joint MRI were not exclusively present in patients with axial spondyloarthritis,” the authors wrote.

This was a cross-sectional study of 204 patients with axial spondyloarthritis and lumbar disc herniation, and women with buttock and pelvic pain who had given birth within the previous four to 16 month. The control group included postpartum women who had no pain and individuals who engage in hard physical activity, such as hospital cleaning staff, long-distance runners, as well as healthy men. Study participants received a physical exam of 44 joints and an MRI scan that was assessed for inflammatory lesions.

Inflammation, bone erosion, and fat lesions were present in all groups. However, at high cut-off points, inflammation and bone erosion occurred almost exclusively in axial spondyloarthritis patients. Fat lesions were present even in high cut-offs in all groups except cleaning staff. Patients with this axial spondyloarthritis experience all of these lesions most often, followed next by postpartum women with pain, 56 percent and 24 percent, respectively.

Backfill and ankylosis were present only on the MRI scans of patients with axial spondyloarthritis. MRIs identified relatively few lesions in study participants who were cleaning staff, long-distance runners, healthy males, and participants with disc herniation. And, when lesions were present, almost all were inflammation and fat lesions.

Postpartum women who had disc herniation or who were cleaning staff or long-distance runners also had much lower levels of inflammation and none had fat lesions, erosion, backfill, or ankylosis. On average, these women had their last pregnancy nearly 10 years prior.

Bone marrow edema detected by MRI is not enough to diagnose axial spondyloarthritis, but, if other types of lesions are present, a potential diagnosis can be made, the authors wrote.

The greatest difficulty in analyzing the MRI results, researchers wrote, was determining the root cause of a woman’s discomfort because axial spondyloarthritis and postpartum pain can present with the same lesions. They posited that recent childbirth could be responsible for the similarities. The women who had given birth within the previous 4-to-16 months were more likely to present with inflammation, erosion, and fat lesions. Only 5 percent of women who had been pregnant years before exhibited any inflammation, and none had erosion. Those results suggest that the length of time since pregnancy could affect MRI findings. 

MRI scans could have a high level of accuracy for identifying both the presence and absence of these lesions. The scans appear to be more accurate in correctly identifying fat lesions and erosions than they are inflammation, except when comparing patients with axial spondyloarthritis to healthy men. And, as cut-off levels increase, the imaging appears to do an even better job with positive pinpointing. But, the accuracy of negative tests for inflammation is a mixed bag. The findings pointed to it being consistently higher than that for erosion, but less than for fat lesions, and as cut-off levels increased the accuracy decreased for all lesion types.

REFERENCE:  Seven S, Ostergaard M, Morsel-Carlson L, et.al. “MRI Lesions in the Sacroiliac Joints for Differentiation of Patients with Axial Spondyloarthritis from Postpartum Women, Patients with Disc Herniation, Cleaning Staff, Long Distance Runners and Healthy Persons – A Prospective Cross-sectional Study of 204 Participants.”Arthritis & Rheumatology (2019), doi: 10.1002/art.41037