Axial spondyloarthritis (ankylosing spondylitis)

Definition

Axial spondyloarthritis is a chronic inflammatory disease mainly of the axial skeleton (sacroiliac joints and spine). Its prototype is ankylosing spondylitis, a systemic disorder with nocturnal back pain, morning stiffness, increasing kyphosis, involvement of large peripheral joints and digits, constitutional symptoms, and possible aortitis, cardiac conduction abnormalities, and anterior uveitis.

Classification

• Radiographic axial spondyloarthritis (ankylosing spondylitis): sacroiliitis on radiographs, typically bilateral
• Nonradiographic axial spondyloarthritis: clinically similar, without typical sacroiliitis on radiographs
• Peripheral form: predominantly peripheral joint involvement

ASAS classification criteria (back pain for at least 3 months, onset before age 45):

• Imaging arm: sacroiliitis on radiographs or MRI plus at least 1 SpA feature
• Clinical arm: HLA-B27 plus at least 2 other SpA features
• SpA features: inflammatory back pain, arthritis, heel enthesitis, uveitis, dactylitis, psoriasis, inflammatory bowel disease, good response to NSAIDs, family history, HLA-B27, elevated CRP

Occurrence & epidemiology

Men are affected about three times more often than women. The disease usually begins between ages 20 and 40. It is 10 to 20 times more common among first-degree relatives than in the general population.

Aetiopathogenesis

A strong genetic component is shown by the association with HLA-B27: the allele is present in about 90 % of White patients but, depending on ethnicity, also in up to 10 % of the general population. Inflammation affects the sacroiliac joints, spine, and tendon insertions; over time, subchondral erosions, sclerosis, syndesmophytes, ligamentous calcification, and finally bony fusion (ankylosis) develop.

Clinical features

• Inflammatory back pain: the most common symptom, often nocturnal; onset by age 40, gradual, morning stiffness, improvement with activity, duration of at least 3 months.
• Reduced chest expansion due to costovertebral involvement; occasionally fatigue, weight loss, anemia.
• Increasing kyphosis, loss of lumbar lordosis, in late stages fixed bent-forward posture.
• Arthritis of hips and shoulders, dactylitis, Achilles and patellar tendinitis; onset in peripheral joints especially in children and women.
• Systemic manifestations in about 25 %: recurrent acute anterior uveitis, aortic insufficiency, aortitis, pericarditis, conduction abnormalities, rarely apical pulmonary fibrosis.
• Nonspecific colitis in about 30 %; neurologic complications from vertebral fractures or cauda equina syndrome.

Diagnosis

• Laboratory tests: ESR and CRP inconsistently elevated; rheumatoid factor and antinuclear antibodies negative; HLA-B27 supports the diagnosis but has low predictive value.
• Radiographs of the sacroiliac joints and lumbar spine: initially pseudo-widening due to subchondral erosions, then sclerosis, joint space narrowing, and fusion, symmetric. In the spine, vertebral squaring with corner sclerosis ("shiny corner"), syndesmophytes, in late stages bamboo spine.
• Pelvic MRI: shows early inflammation (bone marrow edema) and structural changes before radiographs become abnormal.
• Typical constellation: young man with nocturnal back pain, prolonged morning stiffness, reduced chest expansion, enthesitis, or unexplained anterior uveitis.

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Further reading (open access)

1. MSD Manual Professional: Ankylosing Spondylitis
2. Ther Clin Risk Manag 2014: Active axial spondyloarthritis (PMC-Volltext, Tabelle ASAS-Kriterien)

Cross-references