 Introduction/Background Axial spondyloarthritis or axial spondyloarthropathy (axSpA) describes a heterogeneous group of inflammatory disorders affecting the axial skeleton that were historically classified separately as ankylosing spondylitis (AS), reactive arthritis, psoriatic spondyloarthritis, enteropathic spondyloarthritis, juvenile spondyloarthritis, and undifferentiated spondyloarthritis [1]. The prevalence of axSpA is estimated to be between 0.9% to 1.4% in the United States adult population [2]. There is a genetic component to axSpA, including a strong association with HLA-B27, which is positive in 74% to 89% of patients [1]. Patients with axSpA often present before age 45 with chronic back pain and stiffness and may have elevated inflammatory markers [3]. A clinical hallmark is the presence of inflammatory back pain, which is present in 70% to 80% of patients [4]. There are varying definitions for inflammatory back pain, although characteristically this pain includes the following features: insidious onset, improvement with exercise, no improvement with rest, occurring at night, and age of onset <40 years of age [4]. Inflammatory back pain symptoms, depending on the criteria used, have been reportedly been present in 5% to 6% of the general adult population [5], and in up to 15% of patients in the primary care setting [6]. Although recognition of axSpA is improving, a mean delay of 4.9 years from onset of symptoms to diagnosis was recently reported, highlighting the challenge of establishing this diagnosis early in the disease course [7]. Imaging plays a critical role in the diagnosis of axSpA. Historically, imaging diagnosis was based on radiographs using the modified New York criteria [15]; however, the radiographic changes of axSpA were subsequently found to evolve slowly over the course of years [16,17].
Inflammatory Back Pain Known or Suspected Axial Spondyloarthropathy PCAs.  Introduction/Background Axial spondyloarthritis or axial spondyloarthropathy (axSpA) describes a heterogeneous group of inflammatory disorders affecting the axial skeleton that were historically classified separately as ankylosing spondylitis (AS), reactive arthritis, psoriatic spondyloarthritis, enteropathic spondyloarthritis, juvenile spondyloarthritis, and undifferentiated spondyloarthritis [1]. The prevalence of axSpA is estimated to be between 0.9% to 1.4% in the United States adult population [2]. There is a genetic component to axSpA, including a strong association with HLA-B27, which is positive in 74% to 89% of patients [1]. Patients with axSpA often present before age 45 with chronic back pain and stiffness and may have elevated inflammatory markers [3]. A clinical hallmark is the presence of inflammatory back pain, which is present in 70% to 80% of patients [4]. There are varying definitions for inflammatory back pain, although characteristically this pain includes the following features: insidious onset, improvement with exercise, no improvement with rest, occurring at night, and age of onset <40 years of age [4]. Inflammatory back pain symptoms, depending on the criteria used, have been reportedly been present in 5% to 6% of the general adult population [5], and in up to 15% of patients in the primary care setting [6]. Although recognition of axSpA is improving, a mean delay of 4.9 years from onset of symptoms to diagnosis was recently reported, highlighting the challenge of establishing this diagnosis early in the disease course [7]. Imaging plays a critical role in the diagnosis of axSpA. Historically, imaging diagnosis was based on radiographs using the modified New York criteria [15]; however, the radiographic changes of axSpA were subsequently found to evolve slowly over the course of years [16,17].