Interspinous ligamentitis in patients with psoriatic arthritis

Objectives Axial involvement in psoriatic arthritis (axPsA) is common but is more heterogeneous than classical axial spondyloarthritis (axSpA). Posterior soft-tissue inflammation is a potential cause of back pain in axPsA. Here, we report a case series illustrating this presentation. Methods All cases were newly presented with a diagnosis of psoriatic arthritis (PsA) and inflammatory back pain. Clinical and demographic details were extracted from medical notes and imaging of the spine and sacro-iliac joints was performed according to local protocols. Results All five patients presented with peripheral symptoms and had inflammatory back pain. All patients had some soft-tissue inflammation of the posterior elements of the spine, usually interspinous ligamentitis. Three patients also had imaging evidence of sacroiliitis and two patients imaging evidence of enthesitis in the vertebral bodies. Conclusions This report highlights interspinous ligamentitis as a possible cause of inflammatory back pain in axPsA that may be overlooked in MRI reports that focus on the sacroiliac joints and vertebral bodies. This feature may also have implications for treatment options in axPsA.