Comparative radiologic characteristics of isotretinoin-induced sacroiliitis and axial spondyloarthritis

BACKGROUND: Sacroiliitis is a major cause of inflammatory low back pain in young adults and a key feature of spondyloarthritis (SpA), but it may also occur with infection, pregnancy, or drug exposure. Isotretinoin, widely used for severe acne, can rarely induce sacroiliitis. AIM: This study aimed to compare the MRI characteristics of isotretinoin-induced sacroiliitis with those of axial spondyloarthritis (axSpA). METHODS: The study population consisted of 68 patients, comprising 28 with isotretinoin-induced sacroiliitis (median age 24 years; 5 males) and 40 with axSpA (median age 30 years; 26 males). MRI scans were assessed for active inflammatory lesions (bone marrow edema, synovitis, capsulitis, enthesitis) and early structural lesions (erosion, fat metaplasia). SPARCC scoring was performed using the standardized 0-72 system. Ankylosis and sclerosis were not included as it was absent in both groups. Group comparisons were conducted using the Mann-Whitney U test, chi-square test, or Fisher's exact test. RESULTS: While isotretinoin-induced sacroiliitis showed a purely inflammatory pattern without structural lesions, patients with axSpA exhibited both active inflammatory and structural lesions. Erosions and fat metaplasia were observed exclusively in axSpA patients (p < 0.001). Bilateral and symmetric involvement, as well as higher SPARCC scores, were significantly more frequent in axSpA (p < 0.05). CONCLUSION: Isotretinoin-induced and axSpA sacroiliitis exhibit distinct MRI characteristics. Erosions and fat metaplasia are specific markers of axSpA, whereas isotretinoin-induced sacroiliitis represents a reversible, non-destructive inflammatory process. The present results may assist clinicians and radiologists in distinguishing between these two conditions in clinical practice.