Magnetic resonance imaging detects inflammation in the sacroiliac joints in affected patients early in the course of ankylosing spondylitis when no chronic changes are detectable on radiograph, said Prof. Jürgen Braun, medical director of Rheumazentrum Ruhrgebiet, Ruhr-Universität Bochum, Herne (Germany).
The spinal stiffness and loss of spinal mobility that are often the presenting symptoms of ankylosing spondylitis result from spinal inflammation and/or structural damage that is in turn triggered primarily by osteoproliferation rather than osteodestruction. Inflammation is assumed to trigger new bone formation. However, no close correlation between inflammation and osteoproliferation has been found so far.
Sacroiliitis is a hallmark of the disease, especially in early stages. However, radiographs of the sacroiliac joints can appear normal in the early phase of the disease, meaning that structural changes might not be apparent for several years. The inability of radiographs to detect chronic changes can lead to diagnostic delay in ankylosing spondylitis. MRI has proved to be useful in the detection of axial inflammation in very early stages of the disease, commented Dr. Braun.
Used with an imaging technique called short tau inversion recovery (STIR) that does not require contrast agents, MRI also can identify sacroiliitis and spondylitis in patients with other spondyloarthritides including the undifferentiated form that progresses into ankylosing spondylitis in more than 50% of the cases. In one study, MRI of sacroiliac joints was able to predict the development of structural radiographic changes in these joints with a positive predictive value of 60%, 3 years before the changes occurred (J. Rheumatol. 1999;26:1953–8).
While the use of MRI to detect chronic changes continues to be under investigation, radiographs offer a more sensitive way to detect existing structural changes. “Therefore, a radiograph of the sacroiliac joints is always needed, especially at early disease stages,” he said, noting that 20%–30% of patients will already have developed structural changes within the first 2 years of inflammatory back pain.
MRI also appears to be useful for the detection of enthesitis and synovitis, not only in the axial skeleton but also in the peripheral joints and entheses.
Several medications have been available for the treatment of ankylosing spondylitis, with varying degrees of efficacy. But the introduction of the tumor necrosis factor (TNF) blockers has been a substantial development for patients with the disease. Earlier diagnosis could translate into earlier use of these effective medications and preservation of joint architecture.
Three agents are currently approved for ankylosing spondylitis: the monoclonal chimeric antibody infliximab (Remicade), the fully humanized monoclonal adalimumab (Humira), and the recombinant human soluble TNF-α receptor fusion protein etanercept (Enbrel).
MRI may have a role in assessing response to therapy. “Clinical disease activity and spinal inflammation as detected by MRI are substantially reduced by TNF blockers, as shown after short-term and long-term anti-TNF therapy,” said Dr. Braun. On the basis of two recent studies it appears unlikely that the treatment with TNF blockers is able to completely halt radiographic progression. However, recent long-term data on anti-TNF-α therapy in ankylosing spondylitis suggested that function and mobility of the patients who were consistently treated over 5 years is preserved in an improved state, compared with baseline.
Inflammatory lesions of the sacroiliac joint are seen with STIR technique MRI.
Inflammatory lesions can also be seen in the spine using STIR technique MRI. Photos courtesy Dr. Xenofon Baraliakos