Sudden colour enhancement of congenital speckled lentiginous naevus after partial resection.

A 17-year-old girl was referred to our hospital for excision of a congenital pale brown macule with multiple dark brown spots on her left buttock (Fig. 1a). Serial resections were planned, and one-third of the macule was excised along its long axis. Histopathological examination revealed hyperpigmentation and scattered melanocytes in the basal layer without nest formation (Fig. 2a). A diagnosis of speckled lentiginous naevus was established. About 2 months after the first resection, she noticed marked darkening of the entire macule. When she revisited us for additional resection 3 months after the first surgery, the entire macule had become homogeneously black and slightly elevated (Fig. 1b). Dermatoscopy showed starburst-like streaks of pigment around the edge. The whole lesion was completely resected to exclude malignancy. Histopathologically, melanocytic cells were proliferating with forming nests in the basal layer of the epidermis and appendage epithelium. The cells contained large amounts of melanin, and the nuclei were large and uniform (Fig. 2b). There were no malignant findings. We diagnosed the lesion as a recurrent melanocytic naevus occurring on speckled lentiginous naevus. Speckled lentiginous naevus generally refers to a congenital round tan patch containing multiple small dark brown spots. The area of the tan patch contains scattered lentigo-like melanocytes in the basal layer, while dark brown spots are junctional or compound naevi; however, some reports have stated that naevus cells were also present in the tan patch. A recurrent melanocytic naevus is induced by proliferation of remaining melanocytes after partial incomplete resection of a melanocytic naevus. A recurrent naevus has uncommon features such as starburst pattern by dermatoscopy, and the histopathological examination sometimes reveals mild to moderate atypia known as ‘pseudomelanoma’, which make the naevus difficult to differentiate from malignant melanoma. In a recurrent naevus, melanocytes are activated by several growth factors possibly released from fibroblasts and keratinocytes during wound healing. The same phenomenon may have occurred in our case; however, our case is distinctive in that the whole pigmented area, not only the area around the scar, became blackish and formed numerous nests. We speculate that growing nests near the scar can influence the adjacent melanocytes and gradually spread to neighbouring cells, which eventually results in the whole macule turning black. A retrospective study of recurrent pigmentation after resection showed that 61.3% of lesions were recurrent nevi and that 38.8% were melanomas. In our case, malignant melanoma was ruled out because the pigmentation was very homogenous and did not extend beyond the original lesion, no atypical cells were found in the histopathologic examination, and the time to recurrence was short. Our case indicates that the tan patch of a congenital speckled lentiginous naevus can potentially form nests induced by a surgical procedure.