(1) Histopathologically, acanthosis nigricans affects both the papillary dermis and the epidermis and is characterized by epidermal hyperpigmentation, papillomatosis, melanophages, and thickening of the papillary dermis with an increased number of fibrocytes.
(2) Clinically, the smooth, papillated tan–to-brown plaques of acanthosis nigricans develop in intertriginous regions. Additionally the neck, umbilicus, and mamillae can be affected.
(3) Acanthosis nigricans may be congenital or acquired and has been classified into three types: “benign” acanthosis nigricans, an autosomal dominant disorder that appears in childhood; “pseudo”-acanthosis nigricans as a result of an underlying endocrinologic abnormality, obesity or medications, such as corticosteroids and nicotinic acid; and “malignant” acanthosis nigricans, a paraneoplasia usually in connection with an adenocarcinoma of the gastrointestinal or genital tract, breast, or lung.
(4) Confluent and reticulated papillomatosis, which was originally described by Gougerot and Carteaud in 1927, is a rare skin condition that has many features in common with acanthosis nigricans clinically and histopathologically. Some authors regard them as one pathologic process with the first being a variant of the second.