DISCUSSION

Clinically, AN commonly presents as hyperpigmented, papillomatous, velvety-like plaques in any location, but most typically in the body folds such as the axillae, the back and sides of the neck, the anogenital region, the groin and the umbilicus (3–6). Based on the most common associations, AN is classified into 5 clinical subtypes: insulin resistance-related, malignancy-related, drug-induced, idiopathic and others, which include syndromic, acral and naevoid (4). The histology of AN is consistent regardless of its clinical type. The characteristic features include hyperkeratosis and slight acanthosis undulating with dermal papillomatosis (4). Naevoid AN is an exceedingly rare true benign form of AN with an irregular autosomal-dominant inheritance, manifestating at birth, childhood or puberty (1–3). It tends to enlarge for a time and then remains stable or regresses (1–3). Thus far, naevoid AN has been reported as a unilateral presentation, which was called a unilateral AN (1–3). An associated neural element in the pathogenesis of the disease has been suggested.

Our case is unique in that the naevoid AN occurred on the midline of the abdomen, which is an unusual site. Clinically, our case may simulate a hyperpigmented naevus (Becker’s, epidermal or melanocytic), a confluent and reticulated papillomatosis (Gougerot-Carteaud syndrome), or a reticulate pigmented flexural anomaly (Dowling-Degos disease).