What is spondylitis?

What is Spondylitis?

Spondylitis, specifically ankylosing spondylitis (AS), is a chronic inflammatory arthritis that primarily attacks the spine and sacroiliac joints, causing inflammatory back pain, progressive spinal stiffness, and potentially leading to spinal fusion (ankylosis) over time. 1

Core Disease Characteristics

Ankylosing spondylitis is distinguished by universal sacroiliac joint inflammation or fusion and a marked propensity for spinal ankylosis, with radiographic sacroiliitis forming the diagnostic cornerstone. 1 The disease affects 0.1-0.5% of the population and shows a high prevalence of HLA-B27 positivity (74-89% of patients). 1, 2

Pathophysiology

• AS involves immune-mediated mechanisms with HLA-B27, inflammatory cellular infiltrates, and cytokines (particularly tumor necrosis factor alpha and interleukin-10) playing key roles in disease development. 3
• The disease is characterized by enthesitis (inflammation at tendon and ligament insertion sites), excess spinal bone formation, and progressive structural damage. 1, 4

Clinical Presentation

Inflammatory Back Pain Features

Inflammatory back pain is present in 70-80% of AS patients and has distinct characteristics that separate it from mechanical back pain: 2

• Insidious onset before age 40-45 years with chronic duration (≥3 months). 2
• Prolonged morning stiffness that is a distinguishing feature. 2
• Improvement with exercise but no improvement with rest (in contrast to mechanical back pain which improves with rest). 2
• Night pain, particularly awakening in the second half of the night. 2
• Alternating buttock pain indicating sacroiliac joint involvement. 2

Disease Progression Pattern

• Sacroiliac joint pain is typically the initial site of involvement, presenting as lower back or buttock pain. 2
• Spinal involvement progresses to affect the thoracic spine and thoracolumbar junction most commonly. 2
• Chronic back pain and stiffness lead to progressive loss of spinal mobility and can result in severe kyphotic deformity in advanced untreated cases. 2, 5

Associated Manifestations

Peripheral and extra-articular features include: 2

• Peripheral arthritis affecting large joints (most commonly knees) in an oligoarticular, asymmetric pattern occurs in 30-50% of patients. 2
• Uveitis (inflammatory eye disease) is a common extra-articular manifestation. 2, 3
• Psoriasis occurs in association with AS. 2
• Inflammatory bowel disease may be present. 2

Diagnostic Considerations

Clinical Recognition Challenges

The diagnosis of AS is frequently delayed by 4.9 to 8 years from symptom onset, highlighting the substantial challenge of early recognition. 2 This delay occurs because inflammatory back pain symptoms are present in 5-6% of the general adult population and up to 15% in primary care settings, requiring additional features beyond back pain alone to establish the diagnosis. 2

Diagnostic Approach

• Detection of sacroiliitis by radiography, magnetic resonance imaging, or computed tomography in the presence of clinical manifestations is diagnostic for AS. 3
• The presence of inflammatory back pain plus at least two other typical features of spondyloarthropathy (such as enthesitis and uveitis) is highly predictive of early AS. 3
• HLA-B27 positivity supports the diagnosis but is not diagnostic alone, as it is present in the general population. 2
• Elevated inflammatory markers (C-reactive protein) may be present but are not always elevated in active disease. 2

Non-Radiographic Axial Spondyloarthritis

The term "non-radiographic axial spondyloarthritis" (nr-axSpA) encompasses patients who have chronic back pain and features suggestive of spondyloarthropathy but do not meet classification criteria for AS due to absence of radiographic sacroiliitis. 1 This distinction is important because radiographic features may take years to develop, potentially excluding patients early in the disease course. 1

Disease Spectrum and Classification

AS is part of the spondyloarthritis (SpA) family of diseases, which share clinical, genetic, and immunologic features. 1 The umbrella term "axial spondyloarthritis" includes both AS (with radiographic sacroiliitis) and nr-axSpA (without radiographic changes). 1

Clinical Impact

AS imposes substantial physical and social burdens on patients, interfering with work productivity, causing work disability and unemployment, substantial healthcare resource utilization, and significantly reduced quality of life. 1, 3 The skeletal disease manifestations vary widely among patients and over the disease course. 1

Common Pitfalls in Recognition

• Do not dismiss chronic back pain in young adults (under age 45) as mechanical, especially when accompanied by morning stiffness lasting more than 30 minutes. 2
• Do not expect all patients to have elevated inflammatory markers—normal C-reactive protein does not exclude active AS. 2
• Do not wait for radiographic changes to consider the diagnosis—early disease may show inflammation on MRI before radiographic sacroiliitis develops. 1, 3
• Recognize that 75% of AS patients show good or very good response to full-dose NSAIDs within 48 hours, in contrast with only 15% of patients with mechanical back pain—this dramatic difference can aid early recognition. 1