Rhabdomyomatous mesenchymal hamartoma with lentiginous melanocytic hyperplasia: an inductive phenomenon or component of the hamartoma?

Rhabdomyomatous mesenchymal hamartoma (RMH) is a rare cutaneous hamartoma characterized by abundant bundles of mature skeletal muscle fibers admixed with other mesenchymal elements. We report a case of RMH in a 7-month-old girl with associated lentiginous melanocytic hyperplasia. The melanocytic hyperplasia was not apparent clinically and was only evident on histological examination. Unlike a smooth muscle hamartoma, melanocytic abnormalities have never been described in the context of RMH. Though the exact significance of this observation is not clear, we speculate that the melanocytic hyperplasia could be related to an inductive phenomenon or constitute a part of the overall hamartomatous process. Careful attention to epidermal changes in future cases of RMH may help in understanding the significance of this association and refine our knowledge of the pathogenesis of RMH.