Comorbid pain in axial spondyloarthritis, including fibromyalgia

The main symptom in patients with axial spondyloarthritis (axSpA) is inflammatory back pain, caused principally by inflammation of the sacroiliac joints and the spine. However, not all back pain in patients with axSpA is related to active inflammation: other types of pain can occur in these patients, and may be related to structural damage (e.g. ankylosis), degenerative changes, vertebral fractures or comorbid fibromyalgia, which are not uncommon in these patients. Structural damage and ankylosis may lead to a biomechanical stress, which can lead to chronic mechanical pain; and degenerative changes of the spine may also exist in patients with axSpA also leading to mechanical pain. Osteoporosis is more prevalent in axSpA patients than in the general population, and vertebral fractures may result in acute bone pain, which can persist for several months. Fibromyalgia, which is also more prevalent in patients with chronic inflammatory diseases (including axSpA), presents with widespread pain which can mimic entheseal pain. A correct diagnosis of the origin of the pain is crucial, since treatments and management may differ considerably. Recognizing these causes of pain may be a challenge in clinical practice, especially for fibromyalgia, which can coexist with axSpA and may have a significant impact on biologic drug response. In this review, we provide an update of the most common causes of pain other than inflammatory back pain in axSpA patients, and we discuss the latest management options for such causes.