Classification can also be made along purely practical lines, i.e., from the therapy. To classify diseases according to their response to standardized therapies may appear non-academic but is useful in practice and allows more specific speculations to be made about the etiology and pathogenesis when the therapeutic principle of the drug is known. In HS, clindamycin-rifampicin, anti-TNF biologics, sometimes cor-ticosteroids and even immunosuppressive drugs may be helpful, while they are not useful in acne. In contrast, the retinoids, which are the most effective drugs in the treatment of acne, appear generally ineffective in HS (see Chap. 17). Thus the terminology acne inversa may lead to an erroneous management. The lack of efficiency of retinoids is in good agreement with the absence of local seborrhea and supports the classification of HS as a follicular disease different from the acnes.
The exact cause of the rupture of the follicle is not established, although a lymphocytic inflammatory infiltrate appears to be present in early lesions (see Chap. 4). There is some evidence of infundibular epithelial hyperproliferation as well. In older lesions, sinus tract formation predominates the histopathology. It is speculated that the introduction of follicular material into the dermis as well as secondary colonization of sinus tracts cause flares of HS. These mechanisms suggest that HS can be classified as a folliculitis of unknown origin affecting the deeper end of the hair follicle and not involving the sebaceous glands. The polymicrobial colonization
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