Supplementary Material for: High Prevalence of Back Pain and Axial Spondyloarthropathy in Patients with Hidradenitis Suppurativa

Background: Hidradenitis suppurativa (HS) is a chronic inflammatory disease, causing fistulating sinuses in the intertriginous skin of axillary, genitofemoral and perianal sites. Objective: As other chronic inflammatory diseases, e.g. psoriasis, are frequently associated with spondyloarthropathies (SpA), the goal of this study was to quantify the prevalence of back pain and SpA in HS patients. Methods: A prospective questionnaire survey in 100 HS patients and a retrospective evaluation of pelvic magnetic resonance imaging (MRI) scans in 46 HS patients were conducted. Results: 71% of HS patients were suffering from back pain. There was no difference between age at onset of HS, disease duration, body mass index (BMI), or disease severity between HS patients with and without back pain. Evaluating pelvic MRI scans, 32.6% of HS patients showed signs of chronic SpA and 39.1% signs of active SpA. Again, no significant differences between patients with/without SpA were found regarding age at time of MRI, age at onset of HS, disease duration, smoking habits, and BMI. Furthermore, there was no correlation between these parameters and the degree of SpA. Limitations: Only patients with moderate/severe HS (Hurley stage II and III) in genitofemoral/perianal sites were analysed via MRI scans. Conclusion: Back pain and SpA are very common among patients with moderate/severe HS. Neither medical history nor clinical parameters provide hints for the presence of SpA.