Hidradenitis suppurativa: Rheumatologic comorbidities, classification, categorization, and mechanical stress

Hidradenitis suppurativa (HS) is a common, unpleasant auto-inflammatory skin disease. It is considered a follicular occlusion disease. Painful inflammatory nodules, abscesses and/or sinus tracts (tunnels) present in the body folds, such as the armpits and groin. The exact cause and pathophysiologic process remain unknown. A cure does not exist yet.

In this thesis, we have shown that HS is more prevalent in patients with the rheumatologic disease axial spondyloarthritis. Vice versa, we found that typical features of axial and peripheral spondyloarthritis are more prevalent in HS. Both HS and spondyloarthritis are auto-inflammatory diseases and share pathophysiologic processes and treatment options. Furthermore, we investigated several steps (construct validity, interrater and intrarater reliability, and face validity) within the validation process of the refined Hurley classification for staging HS patients. Results suggest that the refined Hurley classification is valid. We also developed an HS patient symptom self-screening questionnaire which shows the current refined Hurley stage can be derived accordingly. Further, as a contribution to the description of sound phenotypes of HS, we identified five clinical patient categories within the HS disease spectrum based on the clinical variables sex, smoking history, body mass index, and presence of other associated follicular occlusion diseases. Interestingly, the ‘limited HS’ category describes HS patients without associated risk factors smoking, overweight and other follicular occlusion diseases. Lastly, we present the case of a patient with HS located on the instep of his foot, an atypical location for HS, which supports the theory of mechanical stress as a risk or contributing factor for the development of HS(-like) lesions. This thesis contributes to the epidemiological and clinical knowledge of HS.