The adult type generally affects those greater than 40 years of age with a marked male predominance while the fetal type generally affects those under the age of three years. The genital type occurs in the vulvar and vaginal regions of the female genital tract. Histologically, adult type rhabdomyomas consist of well-demarcated, unencapsulated nodules with densely packed, uniform, polygonal cells containing granular, eosinophilic cytoplasm. The nuclei tend to be small and round, and located both peripherally and centrally. Nuclear pleomorphism, hyperchromatism and mitotic figures are generally absent.
Immunohistochemical stains confirm the skeletal muscle differentiation.
Most tumors of this type stain positively for myoglobin, desmin and muscle-specific actin. Reactivity for cytokeratin, epithelial membrane antigen, glial fibrillary acidic protein and CD68 are generally not found. Immunohistochemical staining helps to distinguish adult type rhabdomyoma from granular cell tumors, hibernomas, oncocytomas and paragangliomas with which they are frequently confused. Debate as to whether these extracardiac rhabdomyomas are hamartomas, true neoplasms or lesions of disorderly skeletal muscle degeneration and regeneration continues. Recent studies demonstrating clonal cytogenetic abnormalities involving reciprocal translocations of chromosomes 15 and 17, and various abnormalities in chromosome 10 suggest a true neoplastic origin.