Insulin-Induced Acanthosis Nigricans.

Acanthosis nigricans (AN) is a hyperkeratotic dermatosis characterized by hyperpigmented, velvety skin plaques, most commonly found on intertriginous sites such as the dorsal cervical region and axillae, though it can potentially occur on any skin surface. AN usually presents as a manifestation of systemic disorders associated with high-grade insulin resistance, such as obesity, metabolic syndrome, or type 2 diabetes mellitus (DM2). However, AN occurring at sites of subcutaneous insulin injections is rare. We present a case of AN developing at sites of repetitive, high-dose insulin administration on the anterior abdomen. A 70-year-old male with DM2 was referred for evaluation of a "rash" at his insulin injection sites. He was prescribed a cumulative daily dose of 680 units of U-500 insulin and 80 units of insulin glargine U-300, all of which he reported injecting into the periumbilical area. Examination revealed a discrete, hyperpigmented, verrucous plaque surrounding the umbilicus, consistent with AN, with no other areas of affected skin. Histologic findings from a biopsy of the lesion showed papillomatosis and hyperkeratosis, features characteristic of AN. The patient was advised to avoid injecting insulin into the affected area and to rotate injection sites frequently. Modest improvement in AN was observed over the next three months, with more significant improvement during the subsequent six months following treatment with 0.1% retinoic acid cream.