Medicine – Ankylosing Spondylitis (The ‘A’ Disease)

Ankylosing spondylitis (AS) is a chronic inflammatory disease that mainly affects the sacroiliac joints and axial skeleton. It belongs to the seronegative spondyloarthritis group and is strongly associated with HLA-B27. The disease typically begins in younger adults and is characterised by inflammatory back pain, progressive spinal stiffness, and sacroiliitis. It is sometimes remembered as the “A disease” because several important extra-articular complications begin with the letter A.

Clinical Features

1. Arthritis

Arthritis is a major feature of ankylosing spondylitis. The disease primarily affects the sacroiliac joints and spine, producing inflammatory back pain and morning stiffness that tend to improve with activity. Peripheral joints may also be involved, particularly the hips and shoulders, and this may contribute to pain and reduced mobility.

2. Anterior Uveitis

Anterior uveitis is one of the most common extra-articular manifestations of ankylosing spondylitis. It usually presents acutely with painful red eye, photophobia, blurred vision, and excessive tearing. The condition often affects one eye at a time and may recur. Prompt ophthalmological treatment is important to prevent complications.

3. Apical Pulmonary Fibrosis

Longstanding ankylosing spondylitis may rarely cause fibrosis of the upper lobes of the lungs, particularly at the apices. This can lead to reduced respiratory function and may predispose to secondary infection. Restriction of chest wall movement due to involvement of the costovertebral joints may further impair ventilation.

4. Amyloidosis

Chronic inflammation can occasionally result in secondary amyloidosis, in which amyloid proteins are deposited in organs such as the kidneys. This may lead to proteinuria and progressive renal dysfunction, although it is much less common with modern treatment.

5. Aortic Regurgitation

Ankylosing spondylitis can involve the aortic root and aortic valve, leading to aortic regurgitation. Patients may eventually develop exertional breathlessness, palpitations, or other signs of valvular heart disease if the regurgitation becomes significant.

6. Aortitis

Aortitis, particularly inflammation involving the aortic root, is another recognised cardiovascular complication. Chronic inflammation and fibrosis may cause dilation of the aortic root and contribute to the development of aortic regurgitation or conduction abnormalities.

Radiological Features

1. Sacroiliac Joints

Irregular Joint Margins

One of the earliest radiographic abnormalities is irregularity of the sacroiliac joint margins due to inflammatory erosions. These changes may initially appear patchy and can affect both sides.

Sclerosis and Fusion

As the disease progresses, subchondral sclerosis develops around the sacroiliac joints. Continued inflammation and repair can eventually result in partial or complete fusion of the joints, producing ankylosis.

2. Spine

Loss of Lumbar Lordosis

Progressive spinal inflammation and stiffness may lead to loss of the normal lumbar lordosis. The lower back becomes straighter and less flexible, contributing to the characteristic postural changes of advanced disease.

Vertebral Squaring

Inflammation at the vertebral corners may cause erosion followed by new bone formation, producing squaring of the vertebral bodies on radiographs.

Syndesmophyte Formation

Syndesmophytes are thin, vertical bony bridges that form between adjacent vertebral bodies. They develop as a result of ossification at the outer fibres of the annulus fibrosus and related spinal ligament attachments. Progressive syndesmophyte formation gradually limits spinal movement.

Bamboo Spine

In advanced ankylosing spondylitis, extensive syndesmophyte formation together with ossification of spinal ligaments can produce the classic “bamboo spine” appearance. This reflects continuous bony bridging between vertebrae and indicates advanced spinal ankylosis.

3. Peripheral Joints

Erosive Arthropathy

Although axial involvement is dominant, peripheral joints can develop erosive inflammatory arthropathy, especially the hips and shoulders. Severe peripheral joint disease may result in pain, loss of movement, and functional impairment.

Key Clinical Pattern

Ankylosing spondylitis can be remembered by the “A” features: arthritis, anterior uveitis, apical pulmonary fibrosis, amyloidosis, aortic regurgitation, and aortitis. Radiologically, the disease is characterised by sacroiliitis, vertebral squaring, syndesmophyte formation, and ultimately a bamboo spine.