What are ankylosing spondylitis (AS) and undifferentiated spondyloarthropathy (USpA)?

Updated: Feb 02, 2021
  • Author: Lawrence H Brent, MD; Chief Editor: Herbert S Diamond, MD  more...
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Ankylosing spondylitis (AS), a spondyloarthropathy, is a chronic, multisystem inflammatory disorder involving primarily the sacroiliac (SI) joints and the axial skeleton. [1]  See the image below. The outcome in patients with axial spondyloarthritis, including AS, is generally good compared with that in patients with a disease such as rheumatoid arthritis. Nevertheless, AS is a chronic progressive disease that can result in significant morbidity.

Non-radiographic axial spondyloarthropathy (nr-axSpA) is a term used to describe patients with predominantly axial features and includes patients with clinical features of AS but with normal plain radiographs of the sacroiliac joints and spine. This disorder probably represents an earlier phase or milder form of AS.

Undifferentiated spondyloarthropathy (USpA) is used to describe patients with predominantly peripheral features and may represent an early phase or incomplete form of AS or another spondyloarthropathy such as psoriatic arthritis.

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