Blue Nevus-Smooth Muscle Hamartoma: A Rarely Reported Entity.

To the Editor: The presence of smooth muscle hyperplasia in association with a blue nevus is rarely reported. Tieche first reported a case of blue nevus with fibromatous and myogenic elements. Park et al later described another case of plaque-type blue nevus combined with nevus spilus and hyperplastic arrector pili muscles. An example of congenital melanocytic nevus combined with smooth muscle hamartoma, arising in association with a Becker’s nevus was subsequently described by Patrizi et al. More recently, Tzu et al reviewed the clinicopathological features of a series of combined blue nevi (comprising 2 or more cytologically different populations of melanocytes) with smooth muscle hyperplasia. In the latter study, all cases contained thickened, elongated bundles of smooth muscle interspersed with the melanocytes and arranged parallel to the epidermis, with no relationship to follicular structures. In addition, all lesions were noted to have lentiginous melanocytic proliferation and varying degrees of epidermal change, neither of which correlated with the degree of smooth muscle hyperplasia. We report 2 additional cases of this unusual cutaneous lesion. Similar to most previously reported examples, our cases arose on the back of middle-aged to older patients, were less than 6 mm in diameter, and clinically interpreted as melanocytic lesions. The first example was diagnosed on the left upper back of a 54-year-old woman, demonstrating features of a combined nevus (blue nevus and common-type nevus) admixed with hyperplastic smooth muscle bundles (Figs. 1A, B). The second case was from the right back of a 76-year-old man and comprised a common blue nevus, also occurring in conjunction with a hyperplastic myomatous component (Figs. 1C, D). Immunohistochemical studies for actin confirmed the presence of thickened smooth muscle bundles admixed with dermal melanocytes in both cases (Fig. 1, inserts). Like previously described examples, case 1 demonstrated epidermal acanthosis, with elongation of rete ridges and basilar hyperpigmentation. Case 2 did not exhibit significant overlying epidermal changes. An associated overlying intraepidermal melanocytic proliferation was not identified in either case. Per discussion with referring physicians, both cases did not demonstrate any specific macroscopic features to allow clinical distinction from other subtypes of melanocytic nevi. The 2 nevi arose on clinically normal skin, with no adjacent nevi or cutaneous pigmentation. A number of theories have been proposed to explain the presence of admixed blue nevus cells and hyperplastic smooth muscle bundles in these rarely described cases. Tzu et al suggest that these neoplasms exist on a spectrum of lesions derived from aberrant migration and development of pluripotent neural crest cells, with evidence of phenotypic heterogeneity as a result of focal nonneural crest cellular differentiation. Other putative explanations include the presence of dermal stem cells and/or melanoblasts with the potential to differentiate into divergent cell lineages, including melanocytes and smooth muscle cells. Another possible theory is segmental, postzygotic mosaicism, which is when an individual loses a wild-type allele at early developmental stage leading to loss of heterozygosity in a mosaic patch. FIGURE 1. A and B, Combined nevus (blue nevus and common-type nevus) admixed with hyperplastic smooth muscle bundles on the left upper back of a 54-year-old woman. C and D, Common blue nevus, occurring in conjunction with a hyperplastic myomatous component on the right back of a 76-year-old man. Immunohistochemical studies for actin confirmed the presence of thickened smooth muscle bundles admixed with dermal melanocytes in both cases (inserts).