by David S. Pisetsky, MD, PhD

The paper by Baraliakos and colleagues represents an important endeavor to describe features of axial spondyloarthritis (axSpA) that can be detected by magnetic resonance imaging (MRI). Comprised of a group of 12 rheumatologists and 2 radiologists, the ASAS (Assessment of Spondyloarthritis International Society) MRI working group reviewed the literature and examined MRI images of 62 patients to revise the current definitions of findings in AxSpA. These findings can be divided into two main categories: inflammatory lesions and structural change. For active lesions, bone marrow oedema (BMO) represents a key distinguishing feature. Structural lesions are more diverse and include fat lesions, erosions, sclerosis, syndesmophytes, and ankylosis. 

With these definitions, the working group evaluated a set of MRI images to determine the extent of agreement among the experts. The greatest agreement occurred with monomorphic BMO corner lesions and corner fat lesions; depending on the other lesion types and their location, agreement was more limited. The revised definitions described in this paper provide an excellent framework to advance research on the pathophysiology of axSpA, the relationship of inflammation to structural change, and changes in response to therapy; the definitions should also be valuable clinically for classification, diagnosis and staging. 

MRI imaging is a biomarker and, like all biomarkers, it should be actionable, reproducible, and value added. Clearly, imaging of the spine is complicated since the spine itself is complicated, with many different articulations. Expertise in the anatomy of the spine as well as the ability to recognize and specify the different lesions will be necessary to translate the definitions into routine clinical use. In this regard, the extent of agreement (or disagreement) among experts in this study is notable, suggesting that some type of algorithm may be needed to encompass all of the findings from one MRI study in a simpler number (e.g., probability score) for clinical use. Such translation will also likely involve analysis of a large number of studies of putatively normal individuals to elucidate the range of findings that truly represent pathology or at least pathology of axSpA. Already, studies show that MRI can demonstrate changes in otherwise healthy individuals and it is not unlikely that features such as age, sex, occupation, and exercise will all impact on these findings. Even for the patient with axSpA, some of the observed changes may not directly arise from the disease itself but may have other origins. The time element in disease will also likely be a determinant of the findings, although early axSpA can be difficult to evaluate and years may pass before the diagnosis becomes clear. As the conceptualization of AxSpA has evolved, anchoring the diagnosis has become more uncertain as exemplified by issues such as the frequency of HLA-B27 and occurrence in males and females. The revised definitions of spinal MRI lesions are therefore a valuable set of tools and will help delineate the true nature of a “positive MRI” for both research and clinical care.