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The dermis contains smooth muscle fibers in the arrector pili muscles, in the walls of dermal blood vessels, and in the dartos muscle of the scrotum, vulva, nipple, and areola. There are three types of cutaneous smooth muscle lesions: (1) leiomyomas, (2) leiomyosarcomas, and (3) hamartomas. Smooth muscle is recognized histologically by spindle cell shape, eosinophilic, fibrillary cytoplasm, and blunt-ended, oval, “cigar-shaped” nuclei. On immunohistochemical staining, cells express smooth muscle actin and the muscle-specific intermediate filament desmin, but are negative for S100 protein and epithelial membrane antigen (EMA).
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The exact incidence of leiomyomas is unknown. In one study, the 10-year incidence was 0.04% with the majority of lesions occurring in women.1 Although leiomyomas are thought to be relatively uncommon neoplasms, the actual incidence may be higher than previously believed due to failure to recognize and biopsy the lesion.
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Etiology and Pathogenesis
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Cutaneous leiomyomas are divided into three subsets: (1) solitary or multiple piloleiomyomas originating from the arrector pili muscles; (2) genital leiomyomas originating from the mammillary, dartoic, or labial/vulvar muscles; and (3) angioleiomyomas, originating from vascular smooth muscle. Most leiomyomas are acquired; however, familial inheritance patterns have been described. Especially noteworthy are patients with multiple piloleiomyomas. Starting in early adulthood, these patients present with increasing numbers of tumors, developing as many as 100–1,000 lesions. Transmission appears to be autosomal dominant with variable penetrance.2 Women with multiple piloleiomyomas may also develop uterine leiomyomas, an entity termed multiple cutaneous and uterine leiomyomatosis (also known as familial leiomyomatosis cutis et uteri or Reed syndrome).2 In addition, some families with multiple cutaneous and uterine leiomyomatosis have been shown to cluster renal cell cancer, and this has been termed hereditary leiomyomatosis and renal cell cancer. Recently, a loss of function mutation in the gene encoding fumarate hydratase on chromosome 1q42.3–43 has been shown to predispose individuals to these conditions. Fumarate hydratase catalyses the conversion of fumarate to malate in the Krebs cycle but is also considered to be a tumor-suppressor gene.3,4
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The most common presentation is that of multiple piloleiomyomas.5 Individual lesions range in size from several millimeters to 1 cm and are usually reddish-brown firm papulonodules that are fixed to the skin but freely moveable over underling deeper structures (Fig. 127-1). They can coalesce to form plaques or linear, grouped, or dermatomal patterns.5 The extensor extremities, trunk, and sides of the face and neck are the most common locations. Patients ...