Ankylosing Spondylitis

01Pathophysiology

Ankylosing spondylitis is the prototypical axial spondyloarthropathy, driven by a chronic inflammatory process that targets the entheses, the sites where tendons, ligaments, and joint capsules insert into bone. The disease has a profound genetic underpinning: the HLA-B27 allele is present in approximately 90% of affected patients, though it is important to remember that most HLA-B27 positive individuals never develop the disease. The current understanding is that HLA-B27 may misfold within the endoplasmic reticulum, triggering an unfolded protein response that activates IL-23/IL-17 inflammatory pathways, or it may present arthritogenic peptides that provoke a cytotoxic T-cell response.

The primary battleground is the sacroiliac joint, which is why sacroiliitis is the earliest and most characteristic radiographic finding. From the sacroiliac joints, inflammation ascends along the axial skeleton. At the entheses, chronic inflammation leads to erosion of the underlying bone, followed by a reparative process involving reactive new bone formation. This is the critical concept: destruction followed by inappropriate ossification. Over time, bridging bony outgrowths called syndesmophytes form between adjacent vertebral bodies. These syndesmophytes are thin, vertical, and marginal (arising from the vertebral rim), distinguishing them from the bulky, horizontal osteophytes of degenerative disease. When syndesmophytes fuse across multiple segments, the result is the classic "bamboo spine" appearance on X-ray, a rigid, immobile spinal column.

This pathophysiology directly explains the clinical presentation. Early inflammation at the sacroiliac joints and lumbar spine causes pain and stiffness that is worst after periods of inactivity (e.g., upon waking) because inflammatory cytokines and edema accumulate when the joints are not moving. Physical activity improves symptoms because movement disperses the inflammatory exudate and increases blood flow. This is the hallmark of inflammatory back pain and the single most important clinical feature that separates this condition from mechanical back pain, which follows the opposite pattern.

The extra-articular manifestations also connect to the underlying immune dysregulation. Anterior uveitis (acute, unilateral, painful red eye) is the most common extra-articular feature, occurring in up to 40% of patients. Aortitis and aortic regurgitation result from inflammation of the aortic root. Apical pulmonary fibrosis (upper lobe fibrosis) is a rare but classic association. These three "A" complications (Anterior uveitis, Aortitis, Apical fibrosis) are high-yield associations tested in vignettes.

02Classification and Clinical Manifestation

03Diagnostic Workup

The diagnostic approach begins with clinical recognition of inflammatory back pain. The distinguishing features are: onset before age 40, insidious onset, duration greater than 3 months, morning stiffness lasting more than 30 minutes, and improvement with exercise but not with rest. When a vignette describes a young patient with these features, the next step is plain X-ray of the sacroiliac joints and lumbar spine.

If X-ray shows bilateral sacroiliitis, the diagnosis is essentially confirmed in the correct clinical context using the modified New York criteria. However, radiographic changes may take years to develop. When the clinical picture is strongly suggestive but X-rays are normal or equivocal, the next best step is MRI of the sacroiliac joints. MRI can detect bone marrow edema (osteitis) at the sacroiliac joints, which represents active inflammation before erosive or sclerotic changes become visible on plain film. This is the concept of non-radiographic axial spondyloarthropathy, and MRI is the most sensitive imaging modality for detecting it.

HLA-B27 is supportive but never diagnostic on its own. Approximately 6 to 8% of the general population carries HLA-B27 without disease. It should be ordered when clinical suspicion exists and imaging is inconclusive, but the diagnosis should never rest on HLA-B27 positivity alone. Exam questions that present HLA-B27 as the definitive answer to "what confirms the diagnosis?" are designed as traps.

Rheumatoid factor and anti-CCP antibodies are expected to be negative. Ankylosing spondylitis belongs to the seronegative spondyloarthropathy family. If these markers are positive, you should reconsider the diagnosis and think about rheumatoid arthritis or overlap syndromes.

04Management and Treatment

Acute management centers on NSAIDs, which remain the first-line pharmacotherapy. Unlike their use in other rheumatic conditions where they serve only as symptom relief, in ankylosing spondylitis NSAIDs may have a disease-modifying effect on radiographic progression when used continuously. Indomethacin has the longest track record, but naproxen or other NSAIDs are acceptable alternatives. The key exam point is that patients should trial at least two different NSAIDs, each for a minimum of 2 weeks (total 4 weeks), before treatment is considered to have failed.

Long-term management for patients who do not respond to NSAIDs involves escalation to biologic therapy. TNF-alpha inhibitors are the preferred second-line agents. Before starting any TNF-alpha inhibitor, you must screen for latent tuberculosis (tuberculin skin test or interferon-gamma release assay) and hepatitis B (HBsAg, anti-HBc, anti-HBs), because TNF blockade can reactivate these infections. This screening step is a common exam question.

An important principle tested on exams is knowing what does not work for axial disease. Methotrexate, sulfasalazine, and systemic corticosteroids are ineffective for spinal (axial) symptoms. Sulfasalazine has a limited role restricted to peripheral joint involvement only. Systemic glucocorticoids are not recommended. These are common wrong answer choices designed to test whether students inappropriately generalize rheumatoid arthritis treatment to ankylosing spondylitis.

Physical therapy is not optional adjunctive care; it is a core component of management. Regular exercise and postural training help maintain spinal range of motion, prevent fusion in a kyphotic position, and reduce long-term disability.

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The most common way students lose points on ankylosing spondylitis questions is by failing to recognize the pattern of inflammatory back pain. The vignette will describe a young male with chronic low back pain, and the distinguishing detail will be buried in one line: "stiffness is worst in the morning and improves as the day goes on" or "symptoms are relieved by exercise." Students who do not immediately flag this as inflammatory rather than mechanical will be steered toward degenerative disease or disc herniation and select the wrong workup.

The second major trap involves HLA-B27. Test-writers frequently offer HLA-B27 as the "confirmatory test" or "best next step." It is neither. HLA-B27 is supportive evidence, but the diagnosis is confirmed by imaging (X-ray showing sacroiliitis, or MRI showing bone marrow edema when X-rays are normal). Choosing HLA-B27 as the most accurate test is a reliably wrong answer.

A third pitfall is applying rheumatoid arthritis treatment logic to ankylosing spondylitis. Students who select methotrexate or systemic corticosteroids for axial disease are making a category error. These agents have no proven benefit for spinal inflammation in ankylosing spondylitis. The correct escalation pathway is NSAIDs followed by TNF-alpha inhibitors, not NSAIDs followed by methotrexate.

Finally, remember the extra-articular associations. A vignette may present a patient with known ankylosing spondylitis who develops an acute red, painful eye. The answer is anterior uveitis, and the next step is slit-lamp examination and ophthalmology referral, not a new rheumatologic workup. Similarly, if a patient with long-standing bamboo spine presents after minor trauma with sudden neurological deficits, think unstable spinal fracture through a fused, osteoporotic spine. The fused spine is paradoxically more fragile, not more protected.

The core competency being tested across all ankylosing spondylitis questions is your ability to distinguish inflammatory from mechanical back pain, sequence imaging correctly (X-ray first, MRI for early disease), and apply the correct treatment algorithm without borrowing from rheumatoid arthritis management.