Hidradenitis suppurativa/acne inversa: bilocated epithelial hyperplasia with very different sequelae

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Abstract
Hidradenitis suppurativa/acne inversa is a chronic, inflammatory, scarring disease in the terminal hair follicle and apocrine gland-bearing areas (skin folds). There is considerable histological evidence that perifolliculitis and follicular hyperkeratosis precede the rupture of the follicle. The timing of the epithelial hyperplasia at the infundibula of inflamed terminal follicles has not yet been clarified. To clarify the early histopathological life of lesions ('chronology') in hidradenitis suppurativa/acne inversa, focusing on the terminal follicle structure and its surrounding tissue (hyperkeratosis, hyperplasia of follicular epithelium, perifolliculitis and rupture). In total, 485 operative specimens obtained from 128 patients with diagnosed hidradenitis suppurativa/acne inversa (all surgically treated by wide excision) were examined histologically. Two to five histological preparations (total 485) per operation area (total 196) were prepared by multiple slicing. Hidradenitis suppurativa/acne inversa showed a heterogeneous histological pattern: hyperkeratosis of the terminal follicles (89%), hyperplasia of follicular epithelium (80%), pronounced perifolliculitis (68%) and follicle rupture (24%). Perifolliculitis, follicular hyperkeratosis and hyperplasia occurred prior to the rupture of the follicle. Other histological criteria were: subepidermal cellular inflammatory infiltrate (82%), epidermal psoriasiform hyperplasia (56%), pronounced acute dermal inflammation (28%), pronounced chronic dermal inflammation (49%), and involvement of apocrine glands (52%) and subcutis (31%). Infundibular hyperkeratosis, hyperplasia of the follicular epithelium and perifolliculitis are major histopathological characteristics of hidradenitis suppurativa/acne inversa. These apparently precede rupture of the follicle. In particular, hyperplasia of the follicular epithelium probably marks the beginning of sinus formation, which usually spreads horizontally. Psoriasiform hyperplasia of the interfollicular epidermis with subepidermal inflammatory infiltrate might be interpreted as an inflammation-driven process basically identical to that which is evident at the terminal follicle. However, it does not lead to harmful and progressive sequelae like those (rupture, sinus tracts) seen at the terminal follicles.

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