Dowling-Degos disease (DDD) or dark dot disease and Kitamura's reticulate acropigmenta-tion are both rare pigmentation genodermatoses that have been described in association with HS.
DDD was first described by Dowling and Freudenthal in 1938 , then by Degos and Os-sipowski in 1954 . It is characterized by a reticulate pigmentation of the flexures with prominent comedo-like lesions and pitted scars [37-56]. The disease runs in families, it has a genetic autosomal dominant transmission [42, 44] but it may be more prominent in women. DDD, usually localized in axillae, neck and groin, may occur during childhood or early adulthood and may extend progressively. Also noteworthy are the pitted acneiform scars around the mouth present in most of the patients described in the literature.
Pathology shows thin branching pigmented epidermal strand-like proliferations arising from the lower border of the epidermis and the walls of the follicles. The principal differential diagnosis is acanthosis nigricans but the presence of comedo-like structures and the pathology can make the difference. Only two therapeutic options have been proposed: topical adapalene  and Erbium YAG laser .
Kitamura's reticulate acropigmentation (KR A) is another rare genodermatosis. This reticulate, slightly depressed pigmentation affects the extensor surfaces of hands and feet, occurs during childhood, and it is often associated with mili-um-sized keratotic papules or simple breaks in the epidermal ridge pattern on palms and fingers. A few families have been described in whom some members show features of KRA and some patients have features of both DDD and KRA diseases [36, 38, 39, 42-45]. Crovato and other authors have therefore suggested that these disorders of pigmentation are two different clinical expressions of the same nosological entity.
Some patients are reported as having only DDD or only KRA while others have an "association" of HS and a pigmentation disorder. In most of these observations, patients' pigmentation phenotype is not reported and one could wonder whether this abnormality is not more common in people with a dark skin colour. This pigmentation may be the result of post-inflammatory pigmentation after bacterial infections or after the frequent inflammation of the folds as seen in HS. The specificity of these signs would therefore appear to be poor. The association is not well supported by epidemiological
studies either. It is fair to ask questions such as: "How often - in our everyday practice - do we see hyperpigmentation in axillary and other folds?" and "How often do we report it?". The answers are most likely "Very often" for the first question and "Never" for the second one for most dermatologists. Data are therefore lacking to establish the likelihood of a causal relationship between these rare diseases and HS. The likelihood of a causal relationship is nevertheless estimated to be low.
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