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INTRODUCTION

The spondyloarthritides are a group of overlapping disorders that share certain clinical features, genetic associations, and pathogenic mechanisms. The classic designations include ankylosing spondylitis (AS), reactive arthritis (ReA), psoriatic arthritis (PsA) and spondylitis, enteropathic arthritis and spondylitis, juvenile-onset spondyloarthritis (JSpA), and undifferentiated SpA. More recently, these disorders have been broadly classified as predominantly axial SpA, affecting the spine, pelvis, and thoracic cage, or predominantly peripheral SpA, affecting the extremities.

ANKYLOSING SPONDYLITIS AND AXIAL SPONDYLOARTHRITIS

AS is an inflammatory disorder of unknown cause that primarily affects the axial skeleton; peripheral joints and extraarticular structures are also frequently involved. The disease usually begins in the second or third decade. The term axial spondyloarthritis (ax-SpA) is now in common use, supported by criteria formulated in 2009 (Table 355-1). This classification includes definite AS, early stages that will progress to meet classical criteria for AS, and one or more nonprogressing phenotypes. The estimated prevalence of ax-SpA in the US adult population is from 0.9 to 1.4%, similar to that of rheumatoid arthritis (RA).

TABLE 355-1ASAS Criteria for Classification of Axial Spondyloarthritis (to be applied for patients with back pain ≥3 months and age of onset <45 years)a

EPIDEMIOLOGY

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