Radiologic Diagnosis of Ankylosing Spondylitis
Radiographs (plain X-ray films) are the single most important imaging technique for detection, diagnosis, and follow-up monitoring of patients with ankylosing spondylitis. Overall, X-ray films can well depict bony features, subtle deposits of calcium in tissue, and areas of tissue that are hardening into bone. The doctor can reliably diagnose ankylosing spondylitis if its typical radiographic features are present.
Radiographic findings are as follows:
- Sacroiliitis (inflammation of the sacroiliac joints at the base of the spine) occurs early in the course of ankylosing spondylitis and is regarded as a hallmark of the disease. Radiographically, the earliest sign is indistinctness of the joint. The joints initially widen before they narrow. Bony erosions on sides of the joint develop, with eventual bony fusion. Sacroiliitis occurs typically in a symmetric pattern.
- In the spine, the early stages of spondylitis develop as small erosions at the corners of the vertebral bodies. This is followed by syndesmophyte formation (ossification [bone formation] of the outer fibers of the annulus fibrosis [fibrocartilaginous material that surrounds the intervertebral disk]). This causes the corners of one vertebra to bridge to another. The complete fusion of the vertebral bodies by syndesmophytes and other related ossified soft tissues produces the so-called bamboo spine.
- Fractures in established ankylosing spondylitis usually occur at the thoracolumbar and cervicothoracic junctions. Fractures typically extend front to back and frequently pass through the ossified disk. These fractures have been termed chalk stick fractures.
- On the X-ray film, pseudoarthrosis (an abnormal union formed by fibrous tissue within a fracture) appears as areas of diskovertebral destruction and adjacent hardening. Pseudoarthrosis usually develops secondarily to a previously undetected fracture or at an unfused segment but may be mistaken for a disk infection. An important distinguishing imaging feature is the involvement of the posterior elements.
- On the X-ray film, enthesopathy (inflammation where ligaments, tendons, and joint capsules attach to bone) appears as erosions at the sites of attachments. With healing, new bone proliferation occurs. Lesions typically develop bilaterally (on both sides) and are symmetric in distribution. Enthesopathic changes are particularly prominent at certain sites around the pelvis.
- Hip joint involvement is typically bilateral and symmetric. The hip joint space is narrowed uniformly, and the head of the femur (thigh bone) moves inward. Subsequently, the head of the femur protrudes into the pelvis or bony ankylosis.
- Ankylosing spondylitis can affect the lung in the form of progressive fibrosis (fibrous degeneration) and lesion changes at the tops of the lungs. On X-ray films, chest lesions may resemble tuberculous infection. Infections involving Aspergillus species and other opportunistic infections may complicate lung bullae (lesions). Ankylosing spondylitis usually affects the lungs several years after the disease affects the joints.
Computed tomography (CT) may be useful in selected patients in whom ankylosing spondylitis is suspected and in whom initial sacroiliac joint X-ray film findings are normal or inconclusive. Features such as joint erosions and bony ankylosis are easier to see on CT scans than on X-ray films.
CT supplements a diagnostic procedure called bone scintigraphy, which involves injecting a radioactive material into the body and tracking the activity of the material. CT helps the doctor evaluate areas of increased uptake of the radioactive material, particularly in the spine. Bony lesions, such as pseudoarthrosis, fractures, spinal canal narrowing, and facet inflammatory disease can be detected using CT.
Magnetic Resonance Imaging
Advantages of magnetic resonance imaging (MRI) include direct visualization of cartilage abnormalities, detection of bone marrow edema (an abnormal buildup of fluid), improved detection of erosions, and safety from possible radiation hazards.
MRI may have a role in the early diagnosis of sacroiliitis. The detection of synovial enhancement at MRI has been found to correlate with disease activity as measured by laboratory tests. MRI has been found to be superior to CT in the detection of cartilage, bone erosions, and bone changes beneath the cartilage. MRI is also sensitive for assessment of activity early in the course of ankylosing spondylitis and may have a role in monitoring the treatment of patients with active ankylosing spondylitis.
In long-standing ankylosing spondylitis, MRI detects pseudoarthrosis, diverticula associated with cauda equina syndrome (severe compression of nerves at the bottom of the spinal cord), and spinal canal stenosis (narrowing or constriction). In patients with fracture complications or pseudoarthrosis, MRI is useful for assessment of spinal canal narrowing and cord injury. MRI is considered to be mandatory in patients with neurologic symptoms, especially in those with neurologic deterioration after established spinal cord injury.
Scintigraphy has been used to detect early sacroiliitis, but conflicting results have been reported concerning its accuracy. An increase in the uptake of radioactive material by bone based on bone scintigraphy findings may also be used to evaluate active ankylosing spondylitis. Sites affected include the limb joints and entheses. An important application is the evaluation of patients with long-standing ankylosing spondylitis who develop new pain with or without a recent history of trauma. Focal areas of radioactive material uptake may indicate a fracture or pseudoarthrosis.