Solitary congenital neurofibroma with overlying melanocytic hyperplasia.

Abstract

The common embryonic origin of neuroid and melanocytic cells justifies the simultaneous presence of the two cell types in the same neoformation (3). Neurofibroma (NF) can be solitary, diffuse when it has a large extension, and plexiform when it appears as a collection of multiple nodules and cordoniform structures. 10% of diffuse NFs and almost all plexiform NFs are associated with neurofibromatosis (1, 2). Then there is the so-called pigmented or melanotic NF, in which a part of the cells making up the neoplasm is loaded with melanin; this finding is present exclusively in syndromic NF, ie associated with neurofibromatosis. Thus, screening for neurofibromatosis is warranted in the case of plexiform, diffuse, multi-element and melanotic NF (4). In 28% of syndromic NFs and in 4.6% of sporadic NFs there is also a proliferation of epidermal melanocytes, most often in the form of lentiginous melanocytic hyperplasia, but exceptionally in the form of junctional nevus (4). Before assuming that melanocytic hyperplasia is related to NF, it must be considered that melanocytic hyperplasia is also found in café au lait spots and therefore we could face a café au lait spot superimposed on the NF; this hypothesis should be discarded since lentiginous hyperplasia is also found in sporadic NF and because in any case the histological finding does not always correspond to a clinically evident hyperpigmentation. A casual collision between two different pathologies could also be hypothesized, but this hypothesis is discarded because the limits of melanocytic hyperplasia coincide perfectly with those of the underlying NF (4).