There has been significant debate around the pathological features of hidradenitis suppurati-va (HS) over the years. This debate focuses on whether the primary event relates to an inflammatory process of the apocrine duct or whether follicular occlusion is integral to the initiating process.

HS was first described as a distinct clinical entity in 1839 by Velpeau [1]. In 1854 Verneuil suggested that the inflammatory changes which affected skin of the axillae, sub-mammary/ mammary regions (Fig. 4.1) and perianal areas were linked directly to a disease of the sweat glands [2]. In 1922 a direct association was made between HS and the apocrine glands [3]. As a result of the anatomical distribution and the inflammatory changes noted, the term "apo-crinitis" was used as a synonym for HS. This terminology was supported when Brunsting demonstrated the presence of distended apo-crine glands containing polymorph neutrophils in the subcutis sections from 16 cases of established HS. He concluded that the disease was an infection that entered the hair follicle duct and expressed its full inflammatory effect within apocrine glands, with further progression occurring via the subcutaneous lymphatic channels [4]. This explanation of pathogenesis was considered the explanation for the increased frequency of HS in African Americans, who have more apocrine glands per unit area of skin.

Fig. 4.1. Chronic hidradenitis suppurativa of the axilla

However, in 1955 Shelley and Cahn applied belladonna-impregnated occlusive tape to depilated axillary skin in 12 healthy male volunteers. They produced typical lesions of HS in 3 out of the 12 cases at the sites of application of the adhesive tape. The histological inflammation was confined to the apocrine glands [5]. This work introduced the concept that the initiating event in HS relates to follicular occlusion followed by involvement of the apocrine gland. In 1990, Yu and Cook retrospectively examined axillary skin from 12 patients with established HS [6]. Of the 12 cases, 10 showed squamous-epithelium-lined cysts or sinuses in the dermis all containing keratin and half contained hair shafts, suggesting they were derived from hair follicles. Only 4 of the cases had apocrine inflammation and when apparent this was evident around eccrine glands, hair follicles and epithelium-lined structures. This work suggested that follicular occlusion was a more constant diagnostic feature than inflammation around the apocrine glands.

A further retrospective pathological study of 118 skin specimens from 110 patients suffering from HS demonstrated that follicular occlusion was evident in all of the specimens regardless of disease duration, which ranged from 1 month to 18 years. In contrast, control specimens from axillary and inguinal regions did not demonstrate any follicular occlusion [7]. In the same study, active folliculitis was associated with apocrinitis and apocrine destruction whereas apoeccrine glands, which drain directly onto the epidermal surface, remained intact and showed no evidence of inflammation. This work provided clear evidence that follicular occlusion by keratinous material, with subsequent active folliculitis and secondary destruction of the skin adnexae and subcutis, occurs as an integral step in the pathogenesis of HS.

A further study examining early lesions has confirmed that keratin plugging of follicles and sinuses and inflammation around the hair follicle are frequent features in HS [8]. Clinical support for follicular occlusion includes typical, large, multiple and grouped comedones evident in HS in apocrine sites [9, 10].

Hence, it is now believed that HS conforms to a disorder of terminal follicular epithelium within apocrine-gland-bearing skin but that apocrine involvement does not appear to be a primary event in the majority of cases [11].

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