Histopathological progression of hidradenitis suppurativa/acne inversa: A morphological study with a closer look on the early changes of the folliculosebaceous apocrine apparatus

Background: It is generally acknowledged that the first morphological change of hidradenitis suppurativa/acne inversa (HS/AI) consists of infundibular plugging of the folliculosebaceous apocrine apparatus, which is followed by acute and chronic inflammation, cysts with sinus formation, and fibrosis. Alternatively, it has been hypothesized that HS/AI is primarily a neutrophilic autoinflammatory disease and that the follicular plugging typical of this disease is secondary to inflammation. Objective: To review the sequence of the changes that mark the disease development, we have performed a histopathologic study on the surgical material from a series of axillary and inguinal/perineal cases. Methods: The histologic material from surgery on Hurley’s second and third stage HS/AI was retrieved and collected with the patients’ clinical images. The virtually uninvolved skin peripheral to the lesions was studied together with the main inflammatory foci on vertical sections stained with hematoxylin-eosin and immunohistochemistry for the follicle sheaths. Results: The fully developed lesions showed acute and chronic, suppurative and granulomatous inflammation overlapping fibrosis, cysts, and sinuses. Instead, the skin adjacent to florid inflammation showed plugging and dysmorphic alterations of the hair follicles associated with immunopathological changes of the inner root sheath keratin expression. Conclusion: Our observations coincide with the classical pathological studies on the progressive changes of HS/AI; however, in our specimens, the virtually normal skin peripheral to the fully developed lesions show seemingly initial follicular changes that suggest development error. This finding would support the hypothesis of combined mutation-induced epithelial differentiative defects and immunological derangement in HS/AI pathogenesis.