Clinical Features

The clinical characteristics of HS, i.e., deep-seated lesions and topography, are very specific and the hallmark of the disease; however, they are not explained by the histological pictures which form the main evidence for establishing a connection with acne and the so-called follicular obstruction diseases. Exceptional case reports of an association of HS with dissecting folliculitis of the scalp, acne conglobata, large epithelial cysts and pilonidal cysts have focused attention on a possible common mechanism shared by these diseases and their grouping together under the term "follicular obstruction diseases." Some case reports of an association with Dowling-Degos pigmentation of the flexure also point to a follicular obstruction. In spite of these anecdotal reports, the prevalence of acne in HS patients is identical to the prevalence in controls. The rarity of these reports and the

Table 9.1. Similarities and differences between acne vulgaris, acne conglobata, hidradenitis suppurativa (HS), and folliculitis. Etiology reflects known mechanisms such as inflection in simple folliculitis, morphology describes similarities in clinical morphology, pathogenesis describes similarities in known pathogenesis, e.g., seborrhea, and treatment describes response to similar treatments, e.g., response to isotretinoin

Table 9.1. Similarities and differences between acne vulgaris, acne conglobata, hidradenitis suppurativa (HS), and folliculitis. Etiology reflects known mechanisms such as inflection in simple folliculitis, morphology describes similarities in clinical morphology, pathogenesis describes similarities in known pathogenesis, e.g., seborrhea, and treatment describes response to similar treatments, e.g., response to isotretinoin

Similarities between follicular diseases

Acne conglobata

Acne vulgaris

Acne conglobata

HS

Etiology - ? Morphology - no Pathogenesis - ? Treatment - yes

HS

Etiology - no Morphology - no Pathogenesis - no Treatment - ?

Etiology - ? Morphology - ? Pathogenesis - ? Treatment - ?

Folliculitis

Etiology - no Morphology - no Pathogenesis - no Treatment - yes

Etiology - no Morphology - yes Pathogenesis - no Treatment - yes

Etiology - no Morphology - yes Pathogenesis - no Treatment - yes

potential for positive reporting bias therefore raise questions about the validity of this assumption.

As for individual lesions the differences between acne and HS are significant: the deep-seated nodules and the absence of closed comedones - hallmark of acne - are characteristics of HS. Open comedones - black heads - are regularly observed in old lesions of HS, frequently as double or multiple comedones, but these are secondary lesions, i.e., tombstone comedos. Scarring is also more prominent in HS than in acne. In particular, the hypertrophic cicatrizing process, which leads to the formation of highly specific rope-like scars, is another characteristic of HS, very rarely seen in acne. Finally the timespan of the diseases differ. The long-lasting evolution of HS over decades is in sharp contrast with the usually self-healing nature of acne. The reclassification of the disease as acne inversa does not adequately reflect the unique features of HS and carries a serious risk of drawing incorrect analogies to acne.

Looking at four key factors of clinical relevance which may be used for classification of diseases (etiology, morphology, pathogenesis, and treatment) a comparison between acne vulgaris, acne conglobata, HS and folliculitis is made in Table 9.1. As can be seen from this, all these diseases have similarities and differences, which can reasonably be said to influence their classification.

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