Acanthosis nigricans (AN) is a localized skin disorder typically characterized by symmetric hyperpigmented velvety plaques in skin folds. It is due to overstimulation of fibroblasts and keratinocytes via insulin-like growth factor receptors and tyrosine kinase receptors. AN is most commonly associated with obesity and is a marker of insulin resistance. A 2007 study found that up to 90% of youth with AN also carried a diagnosis of diabetes mellitus.

AN is classified into 8 types:

I. Obesity-associated – Most common; linked to diabetes, insulin resistance, high body mass index (BMI), metabolic syndrome, etc.

II. Malignancy-associated / cutaneous paraneoplastic syndrome – Very rare in children, but has been documented with juvenile gastric adenocarcinoma (most common), Wilms tumor, and osteogenic sarcomas.

III. Drug-induced – Niacin (nicotinic acid) is the most closely associated medication, but AN can also be caused by oral contraceptives, corticosteroids, diethylstilbestrol, heroin, fusidic acid, methyltestosterone, protease inhibitors, and folate.

IV. Syndromic – Related to a specific syndrome, eg, HAIR-AN (hyperandrogenism, insulin resistance, and acanthosis nigricans) syndrome, polycystic ovarian syndrome, or autoimmune-disease, among others.

V. Acral – Affects elbows, knees, and knuckles. Typically seen in healthy sub-Saharan African patients.

VI. Unilateral – Also called nevoid. Usually represents an epidermal nevus in childhood.

VII. Benign – Rare autosomal dominant form developing at birth or during childhood. Typically stabilizes or regresses during puberty.

VIII. Mixed – Any 2 or more of the above types combined.