Ankylosing Spondylitis Characteristics and Manifestations

Pathological Characteristics

The American College of Rheumatology notes that ankylosing spondylitis (AS) is a chronic inflammatory disease primarily affecting the spine and sacroiliac joints, characterized by sacroiliitis, enthesitis, and a strong tendency for spinal and sacroiliac joint fusion 1
Inflammation at the entheses, the sites of tendon and ligament attachment to bone, is a primary location of inflammation in AS 1
The involvement of the sacroiliac joints is a key criterion for the modified New York classification, which is used to diagnose AS 1, 2

AS is a prototype and subtype of the spondyloarthritis family of diseases, and can be the end result of other spondyloarthritis subtypes 3, 4
AS is included in the spectrum of axial spondyloarthritis, which encompasses patients with chronic back pain who have AS (with definitive structural changes on radiographs of the sacroiliac joints) and patients with early or abortive forms of spondyloarthritis 3, 4

The European League Against Rheumatism notes that AS is characterized by loss of spinal flexibility due to progressive ankylosis, which eliminates normal spinal movement 2
The disease can result in a classic "bamboo spine" appearance due to extensive ossification of ligaments and fusion of vertebral joints 2
Peripheral arthritis affecting large joints (particularly the knees) in an oligoarticular asymmetric pattern occurs in 30-50% of patients 5

The American College of Rheumatology notes that characteristic radiographic features of AS include squaring of the vertebral bodies due to erosion and new bone formation at the anterior corners 2
"Shiny corners" representing erosion at the vertebral body corners are also a characteristic feature of AS 2
Erosion at the vertebral endplates and facet joints is a destructive change that can occur in AS 2

Uveitis (ocular inflammation) is the most common extra-articular manifestation of AS, and can precede or accompany the disease 5
Inflammatory bowel disease is associated with an increased risk of developing AS 5

The presence of HLA-B27 is found in 74-89% of patients with AS, although only 1% of individuals with HLA-B27 develop AS 5