Ankylosing Spondylitis, Radiologic Perspective

Life expectancy for people with ankylosing spondylitis is the same as that of the general population, except for patients with severe symptoms and complications.

What is the medical definition of ankylosing spondylitis?

• Ankylosing spondylitis is a type of arthritis that is due to inflammation of multiple joints, characteristically the spinal facet joints and the sacroiliac joints at the base of the spine.

Why is ankylosing spondylitis so painful?

• While it tends to affect these joints and the soft tissues around the spine, other joints may also be affected as well as tissues around the joints (entheses, where tendons and ligaments attach to bone).
• This disorder frequently results in bony ankylosis (or fusion).
• The term ankylosing is derived from the Greek word ankylos, which means stiffening of a joint. Spondylos means vertebra (or spine). Spondylitis refers to inflammation of one or more vertebrae.
• Ankylosing spondylitis is usually a chronic and progressive form of arthritis.
• Ankylosing spondylitis may also involve areas of the body other than the joints, such as the eyes, heart, and lungs.

Who is at risk for ankylosing spondylitis?

• Ankylosing spondylitis is very rare. The frequency in the United States is similar to that of the rest of the world. Ankylosing spondylitis primarily affects young males. Males are more likely to have ankylosing spondylitis than females.
• Most people with the disease develop it between 15-35 years of age, with an average age of 26 years at onset.

Although the exact cause is unknown, ankylosing spondylitis is believed to be due to combination of a genetic influence and a triggering environmental factor. Most patients with ankylosing spondylitis have the tissue antigen human leukocyte antigen B27 (HLA-B27). People with ankylosing spondylitis often have a family history of the disease.

Patients with ankylosing spondylitis most often have lower back pain. The pain is located over the sacrum (the bottom of the spinal column) and may radiate to the groin and buttocks and down the legs. The typical patient is a young man who experiences repeated episodes of back pain that wake him at night along with spinal stiffness in the morning. The low back pain persists even while at rest. This pain pattern is characteristic of bilateral sacroiliitis (inflammation of the sacroiliac joints).

With time, the back pain progresses up the spine and affects the rib cage. Chest expansion then becomes restricted. The patient must practice breathing using the diaphragm. The neck part of the spine (cervical spine) stiffens late in the course of the disease, leading to restriction in neck movement and head rotation. Eventually, the spine is completely rigid and loses its normal curvatures and movement.

The earliest objective sign of spinal involvement is the loss of side-to-side movement of the lower part of the spine (called the lumbar spine). The doctor may detect sacroiliitis in the patient if (1) tapping on the area over the sacroiliac joints causes tenderness or if (2) pushing on the pelvis with the patient lying face down causes pain. Several tests have been designed to measure the spinal restriction that occurs as the disease progresses. The doctor may encounter synovitis (inflammation of a membrane of a joint) and joint motion restriction while examining joints in the patient's limbs.

Criteria for the diagnosis of ankylosing spondylitis were developed at rheumatic disease conferences in Rome and New York and subsequently have been referred to as the Rome criteria (1963) and the New York criteria (1968), respectively. Although these criteria are not perfect, they have been generally accepted to be useful. Sacroiliitis is the distinctive characteristic of ankylosing spondylitis, and its presence is required for the diagnosis under both sets of criteria.

• Rome criteria (1963): Ankylosing spondylitis is present if bilateral sacroiliitis is associated with any of the following criteria:
  • Lower back pain and stiffness for more than three months
  • Pain and stiffness in the thoracic region
  • Limited motion in the lumbar region
  • Limited chest expansion
  • History of evidence of iritis (inflammation of the iris) or conditions that result from iritis
• New York criteria (1968): Definite ankylosing spondylitis is present if advanced-to-severe bilateral sacroiliitis is associated with at least one of the clinical criteria below or if advanced-to-severe unilateral sacroiliitis or moderate bilateral sacroiliitis is associated with clinical criterion 1 or with both clinical criteria 2 and 3 (see below). Probable ankylosing spondylitis is present if advanced-to-severe bilateral sacroiliitis is associated with none of the criteria. The criteria are as follows:
  1. Limited movement ability of the lumbar spine in forward movement, side-to-side movement, and extension
  2. History or presence of pain at the thoracolumbar junction or in the lumbar spine
  3. Limitation of chest expansion to 1 inch or less

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

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.

Bone Scintigraphy

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.

Once a definitive diagnosis is established, a detailed explanation of the disease, including its implications, should be provided to the patient. Regular lifelong exercises comprise the mainstay of the treatment program. Adequate analgesics (painkillers) consisting of nonsteroidal antiinflammatory drugs, such as ibuprofen (Motrin, Advil) or naproxen (Aleve), should be administered under supervision to control pain and stiffness and to allow the patient to continue exercising through pain. Other drugs, such as the tumor necrosis factor alpha antagonist group may also be used, as well as methotrexate and sulfasalazine. For more information, see Understanding Ankylosing Spondylitis Medications. Severe hip involvement may require hip replacement surgery. Spinal surgery may be required to treat complications of long-standing spinal disease.