by Dr. David Borenstein M.D. updated 06/2020

Ankylosing spondylitis (AS) is an inflammatory spinal disease which is associated with damage to the axial skeleton including the sacroiliac joints, the lumbar, thoracic, and cervical spine. Inflammation in these areas of the spine results in changes in the tissues that is identifiable with ordinary xray studies. For example, plain radiographs of the sacroiliac joints (SI) in AS patients may show whitening of the bone (sclerosis), joint erosion, or in its final stages, joint fusion. In the very earliest stages of the illness, inflammation is present, but has not caused enough damage to be noticeable with regular radiographs. At this early stage of disease, joint inflammation is characterized by swelling in the bone marrow located near the edges of the joint and is identified by MRI. The question exists to whether these MRI changes are specific to AS or can be seen in other medical conditions.

Pregnancy and delivery has a remarkable effect on SI joints. A study was completed examining post-partum women at 10 days, 6 and 12 months after delivery. Thirty-five women underwent MRI investigations at those time intervals. Seventy-seven% of subjects (27/35) had sacroiliac bone marrow edema (BME) within 2 weeks of delivery. At 6 months, 46% (15/33) had BME. At 1 year, 12% (4/33) had BME. However, few structural lesions like joint erosions were noted in the post-partum group.

Back pain is a common complaint in a majority of women during pregnancy. Pain was not correlated with the presence of BMO.

This study suggests that MRI scans should be done 6 months or longer after delivery if sacroiliitis is suspected. Criteria for the diagnosis of sacroiliitis may need to be modified to include joint damage such as erosion to be considered an inflammatory lesion associated with AS. Without this change in definition, women with mechanical changes in the pelvis may be inaccurately diagnosed with an inflammatory arthritis like AS. The result will be the use of expensive therapy like biologics which would have no benefit.

References:

1 Renson T et al. High prevalence of spondyloarthritris-like MRI lesions in postpartum women: a prospective analysis in relation to maternal, child, and birth characteristics http://dx.doi.org/10.1136/annrheumdis-2020-217095