Hidradenitis suppurativa (HS) was originally classified according to location, and this remains a hallmark of the disease. Shortly after the diagnosis was established, an erroneous association with apocrine glands was made and the name created. A classification according to topography alone obviously does not improve the understanding of pathogenesis and hence is of little help. The erroneous classification according to an incorrect deduction based only on simple co-localization obviously delayed the development of knowledge This mistake comes from a paradox: the lesions of HS are predominantly or exclusively situated in the regions of apocrine sweat glands, yet the histological picture is one of follicular obstruction like that seen in acne lesions, and sweat gland involvement is usually absent from early lesions. The apocrine sweat gland's excretory canal opens into the follicular duct immediately above the sebaceous duct (see Fig. 9.1). This distinctive anatomical characteristic may explain the repercussions of follicular obstruction, with re tention and subsequent infection and inflammation in the apocrine sweat gland. Follicular abnormalities may be a key factor of HS: they are apparent in histological as well as ultrasonographic studies of hair follicles in HS patients (see Chaps. 4, 5). There is also clinical evidence suggesting a relationship between HS and an anatomical anomaly of the pilosebaceous duct in the high prevalence of pilonidal cysts in HS patients. In one series (Faye O, Bastuji-garin S, Poli F, Revuz J. Hidradenitis suppurativa: a clinical study of 164 patients; manuscript in preparation) 30% of 164 patients are reported to have co-existing pilonidal sinus ducts.

HS is clearly a follicular disease located to restricted areas of the body. The pathogenic process in the hair follicle may be elucidated from histology, and appears to be rupture of the deeper parts of the follicle, with spillage of the fol-licular contents into the dermis and subsequent inflammation (see Chap. 4). The exact cause of the rupture is however not established. So even if HS can be classified as a folliculitis, just as acne vulgaris, this classification does not aid our understanding significantly, and additional aspects of the diseases must therefore be considered.

Hidradenitis Suppurativa The Head
Fig. 9.1. The anatomy of the hair follicle
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