AS affects up to 1% of the population. It presents classically with insidious inflammatory back pain and morning stiffness that improves with activity and deteriorates with rest. Typically the patients are young (<40 years) and male (ratio 3 male: 1 female), and a family history of spinal disease may be available.

Symptoms are caused by fibrosis and ossification of ligaments, tendons and insertions, mainly in the region of the intervertebral discs and the sacroiliac joints, and eventually spinal fusion may occur as a result of syndesmophyte formation (bridging spurs of bone at the corner of adjacent vertebral bodies). Spinal fusion is surprisingly painless until microfractures occur.

Chest expansion is reduced by disease at the costovertebral and costochondral junctions.

Insertional tendonitis (enthesopathy) and peripheral synovitis (mono- or oligoarticular) occur in a significant proportion of patients.

Advanced AS produces a ‘question-mark’ posture, characterised by:

• Loss of lumbar lordosis.
  
• Exaggerated thoracic kyphosis.
  
• Cervical hyperextension.
  
• Compensatory flexion at the knee.

One brief assessment of AS relies on assessment of chest expansion and spinal mobility. The latter can be determined by measuring both lateral and forward flexion (Schober’s test, see opposite). A number of validated scores called the Bath Ankylosing Indices cover disease activity (BASDAI), metrology (BASMI) and function (BASFI). The BASDAI is calculated from visual analogue scores regarding: fatigue, spinal pain, joint pain/swelling, areas of local tenderness (enthesopathy), morning stiffness and severity. Decisions regarding biologic therapy for patients with severe AS are based on the BASDAI (see below).

AS has a number of extra-articular associations:

• Anterior uveitis occurs in up to 40% of patients with AS, but its occurrence bears no relation to disease activity in the spine.
  
• Apical pulmonary fibrosis, pleuritis and fusion of thoracic chest wall cause restrictive lung disease and all patients must be advised against smoking.
  
• Aortic incompetence.

Diagnosis

Diagnosis hinges on a combination of radiological evidence of sacro-iliitis and either a typical history or examination findings. Additional radiological features in AS include:

• Romanus lesions ‘shiny corners’ of localised oedema at entheses.
  
• Squaring of vertebrae.
  
• Ossification.
  
• Syndesmophytes.
  
• Facet joint involvement.

The inflammatory markers CRP (C-reactive protein) and ESR (erythrocyte sedimentation rate) are not always elevated. HLA-B27 is not helpful diagnostically as it has such a high prevalence in the normal population; a negative HLA-B27 however is very reassuring.

Treatment

The mainstay of therapy is patient education, physiotherapy to maintain mobility and non-steroidal anti-inflammatory drugs (NSAIDs) for spinal disease. Peripheral disease may be controlled by sulfasalazine.

For those patients with severe disease (BASDAI > 4 and at least 4cm on a visual analogue score of spinal pain) that have failed to respond to two NSAIDs, biologic therapy in the form of anti-TNF agents have proven to be very effective.