Ankylosing Spondylitis Diagnosis and Medical Therapy
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Direct answer
Ankylosing spondylitis is a chronic inflammatory arthritis of the axial skeleton that begins in the sacroiliac joints and can progress to spinal fusion, with strong association with HLA-B27. Diagnosis rests on inflammatory back pain together with sacroiliitis on imaging, as defined by the modified New York criteria. TNF inhibitors are reserved for patients with persistent disease despite NSAIDs and physiotherapy.
What you must remember
- HLA-B27: Carried by the large majority of patients, but most carriers never develop the disease, so a positive test alone does not establish the diagnosis.
- Inflammatory back pain: Insidious onset before about 40 years, improving with exercise and not with rest, with night pain and morning stiffness lasting more than 30 minutes.
- Modified New York criteria: Radiographic sacroiliitis, plus at least one clinical criterion such as limited lumbar movement or reduced chest expansion.
- Imaging: MRI shows active sacroiliitis, with bone marrow oedema, before radiographs change; the bamboo spine reflects syndesmophytes and fusion and is a late feature.
- First-line therapy: NSAIDs, regular exercise and physiotherapy, with TNF inhibitors or IL-17 inhibitors such as secukinumab for active disease.
- Extra-articular features: Anterior uveitis, which is the commonest, aortic regurgitation, apical lung fibrosis and amyloidosis.
A worked pathway: a young man with night back pain
Consider a 27-year-old man with four months of low back pain that is worse at night and eases after exercise. He has early morning stiffness lasting most of an hour. Plain X-ray of the sacroiliac joints is equivocal, so MRI is requested and shows bone marrow oedema in the sacroiliac joints. HLA-B27 is positive and CRP is raised. Under the ASAS classification, MRI sacroiliitis together with HLA-B27 and back-pain features supports the diagnosis, whereas the modified New York criteria still require radiographic change. An NSAID is started at an adequate dose, with a structured exercise programme. Before any biologic is considered, the team screens for latent tuberculosis and hepatitis B, because TNF inhibitors increase infection risk. If pain persists after an adequate NSAID trial, a TNF inhibitor is added, and if the patient has uveitis, the choice of agent may be guided by ophthalmology review. Spinal mobility and disease activity are followed at regular intervals. Physiotherapy goals are set early, with posture and breathing exercises, because restriction of chest expansion and spinal flexion is the main long-term problem if the disease is left untreated.
Where the exam tries to catch you
Examiners test the difference between mechanical and inflammatory back pain. Mechanical pain worsens with activity and eases with rest, whereas inflammatory pain improves with movement and wakes the patient at night. HLA-B27 is another trap: the allele is common enough in healthy people that a positive result is not diagnostic, and the clinical picture decides. Students also confuse ankylosing spondylitis with reactive arthritis and psoriatic arthritis, which share axial and enthesitis features but differ in their triggers and in skin or urethral findings. The bamboo spine is a late feature and not a precondition for diagnosis, a frequent point in viva questions about early disease.
Frequently asked questions
Which HLA allele is associated with ankylosing spondylitis?
HLA-B27 is the allele strongly associated with ankylosing spondylitis, and the large majority of patients carry it. Most carriers never develop disease, so a positive test is supportive but never diagnostic on its own.
What are the modified New York criteria for ankylosing spondylitis?
The modified New York criteria require radiographic sacroiliitis, graded as bilateral grade two or more or unilateral grade three or four, plus at least one clinical criterion. Clinical criteria include low back pain lasting over three months that improves with exercise, restricted lumbar movement or reduced chest expansion.
Which imaging finding shows early active sacroiliitis before X-ray changes?
MRI of the sacroiliac joints shows bone marrow oedema, which indicates active sacroiliitis before radiographs show erosions or sclerosis. This is why MRI is useful in young patients with suggestive symptoms and equivocal plain films.
When are TNF inhibitors indicated in ankylosing spondylitis?
TNF inhibitors are indicated for patients with persistently active disease despite an adequate trial of NSAIDs and a structured physiotherapy programme. Screening for latent tuberculosis and hepatitis B is required before starting them.