THU0276 Mri lesion definitions in axial spondyloarthritis: a consensus reappraisal from the assessments in spondyloarthritis international society (ASAS)

Background There has been substantial progress in the characterisation of MRI lesions in the sacroiliac joints (SIJ) and spine in axial spondyloarthritis (axSpA) since the last consensus-based descriptive reports from ASAS1,2. There is as yet a lack of international consensus on standardised definitions of all the lesions reported to date. Consequently, the ASAS MRI group was convened to generate a consensus update. Objectives To evaluate the literature describing the spectrum of MRI lesions in axSpA and to generate a consensus update on standardised definitions. Methods The literature pertaining to MRI lesion definitions in axSpA was discussed at 3 meetings of the ASAS MRI group attended by 26 investigators. The group reviewed the literature for MRI lesion definitions and decided by consensus which definitions would be retained, which required modification, and which required a new definition. Results For definitions denoting signs of activity in the SIJ, there are no revisions to the most current ASAS definition of a positive MRI1. Definitions for capsulitis and enthesitis are revised. A new definition, joint space enhancement, denotes increased signal on contrast-enhanced images in the joint space of the cartilaginous portion of the SIJ. This replaces the term ‘synovitis’ and a separate definition describes what constitutes joint space fluid. For structural change in the SIJ, the definition for sclerosis is unchanged. Revised definition for a fatty lesion incorporates characteristics typical of axSpA, and for erosion requires both loss of cortical bone as well as adjacent marrow matrix. A new definition, fat metaplasia in the joint space (‘backfill’), denotes the reparative change on a T1W image at the site of erosion when signs of activity recede. The new definition for ankylosis stresses the continuity of bright marrow signal across the joint space. Spinal lesion definitions are divided into those that occur in defined central and lateral sagittal slices. The revised definition of a vertebral corner inflammatory lesion divides this into a regular (type A) and dimorphic (type B) lesion. A new definition for corner erosion requires both loss of cortical bone as well as adjacent marrow matrix. New definitions for new bone growth require bright signal on T1W images extending from the vertebral corner marrow or endplate, which may (ankylosis) or may not (bone spur) be continuous with the adjacent vertebra. Conclusions The ASAS MRI group has generated a consensus based update on MRI lesions in axSpA. References [1] Lambert, et al. Ann Rheum Dis2016;75:1958–1963. [2] Hermann, et al. Ann Rheum Dis2012;71:1278–1288. Disclosure of Interest None declared