Unilateral Nevoid Acanthosis Nigricans Masquerading Verrucous Epidermal Nevus: A Rare Case Report

Unilateral nevoid acanthosis nigricans (UNAN) is a rare, benign, autosomal dominant form of acanthosis nigricans (AN) that can manifest at birth or during childhood or puberty. A 6-year-old boy presented with asymptomatic, brownish-black raised lesions on the left side of the back extending to the left arm 4 years ago. It started as a small dark-colored asymptomatic papule over the left side of the chest. It progressed over 4 years to the present size and remained stable. On examination, there were multiple well-to-ill-defined brownish-black velvety plaques with soft ridges and ill-defined irregular margins, with scaly surface. Dermoscopy showed multiple crista cutis and sulcus cutis with hyperpigmented dots in crista cutis. The clinical differentials considered were linear epidermal nevus (EN) and linear AN. Histopathology revealed hyperkeratosis, mild papillomatosis, and lymphocytic infiltrate. The interpapillary spaces were filled with keratotic material. In accordance with the clinical, dermoscopic, and histopathological findings, the final diagnosis of UNAN was made. UNAN is a rare and benign form of AN inherited as an autosomal dominant. It is not associated with any endocrinopathy, drugs, or internal malignancy. Linear verrucous EN can resemble nevoid AN clinically and histopathologically. The differential diagnoses which can be kept include the EN, confluent and reticulated papillomatosis, Dowling–Degos disease, and seborrheic keratosis.