Review: Enthesitis: New Insights Into Pathogenesis, Diagnostic Modalities, and Treatment

Enthesitis is a central feature of spondyloarthritis (SpA). Although enthesitis has traditionally been considered to be a focal insertional disorder, advanced imaging and pathologic findings suggest that enthesitis is a diffuse process with effects on adjacent bone and soft tissue. As a result of repeated biomechanical stress, it appears that microdamage at the enthesis triggers an inflammatory response in the synovium, leading to synovitis. Along with mechanical stress, exogenous bacteria may play a role in activating the immune response, especially in genetically predisposed individuals whose major histocompatibility locus encodes the class I molecule HLA–B27. Recent studies in animal models suggest that autoimmunity against versican and fibrocartilage proteins, and bone morphogenetic protein (BMP) signaling play roles in enthesitis development. Finally, interleukin-23 (IL-23) has been implicated in enthesitis with inflammatory effects mediated through IL-17 and tumor necrosis factor (TNF), and new bone formation driven by IL-22. Although prior therapeutic choices were limited to nonsteroidal antiinflammatory drugs (NSAIDs) and activity modification, in recent years TNF inhibitors have proven to be useful. Further research on the effects of IL-22 and IL-23 blockade is needed to understand the effects on the treated patient. While enthesitis is underdiagnosed by physical examination alone, the use of ultrasound has proven to be highly sensitive for the detection of enthesitis, with utility in monitoring response to therapy, and will be an invaluable tool for assessing the efficacy of newer treatments. This review summarizes the substantial progress that has been made in addressing the pathophysiology, molecular mechanisms, genetic associations, clinical features, diagnostic modalities, and treatment of enthesitis.