Clinical diagnosis and imaging of sacroiliitis, Innsbruck, Austria, October 9, 2003

As reported by Calin, many patients with sacroiliitis are misdiagnosed and may be treated inappropriately. During the past decade, detection of sacroiliitis has been improved, and the time to get a diagnosis could be shortened from 10 to 7 years. However, it still takes years to establish the diagnosis. Diagnosis is improved by simple, cheap, readily reproducible screening tests and improved imaging techniques. Earlier diagnosis and assessment of disease activity are still a challenge for clinicians and radiologists as scoring tools are limited in their sensitivity to change. At present the following clinical indications for imaging sacroiliitis can be summarized: 1. To establish the diagnosis of sacroiliitis (especially in AS): MRI should be performed in early disease when the diagnosis is suspected but the radiograph is normal. In later disease, a pelvic radiograph (or preferably CT) can be used to confirm the diagnosis. 2. To determine disease activity: Contrast enhanced MRI is the method of choice in the termination of early, mild, and high active disease. 3. To determine the amount of actual destruction in chronic disease: CT accurately depicts the bony destruction. 4. To document sacroiliitis (according to the European Spondylarthropathy Study Group and the modified New York criteria) and to justify treatment of AS with TNF blockers: pelvic radiograph (or preferably CT). If disease activity assessment is taken into account for followup, MRI is necessary. 5. To exclude septic sacroiliitis; early stages can be assessed by MRI, later stages by CT. 6. To evaluate patients with widespread pain and oligoarticular manifestations: scintigraphy allows the assessment of inflammatory manifestations at different sites and can be used in patients with contraindications to MRI. Overall, imaging techniques may be differentiated into structural techniques like radiography or CT and functional techniques such as contrast enhanced MRI, color-Doppler sonography/contrast enhanced color-Doppler sonography, or scintigraphy. While structural techniques are capable of detecting bony changes during later stage disease, functional techniques help to assess disease activity during both early and chronic sacroiliitis. CT effectively displays chronic disease as cortical and subchondral alterations and is superior to MRI in detecting those bony abnormalities in particular at the enthesis and ligamentous portion. CT is fast and readily available, but radiation exposure should limit the use in children and young women. Scintigraphy is sensitive to inflammatory changes and reveals disease activity, but it lacks specificity and is reserved to cases where imaging of general active inflammation is suspected in patients who cannot go for MRI. MRI can be considered the most sensitive imaging modality, with high specificity, detecting active inflammatory changes of sacroiliac joints; and this before any changes are visible by CT and radiography. Thus, when available, MRI should be the first choice in suspected early SpA, in women and children, and when imaging more sensitive than conventional radiography is required.