Congenital fetal rhabdomyoma in a foal

An Appaloosa filly was born with a ventral midline, approximately 8 x 12 x 15 cm subcutaneous cervical mass. The nonencapsulated mass was composed of interlacing and haphazard bundles of spindle cells on moderate to abundant loose myxomatous stroma. A moderate number of cells showed cross striations with minor nuclear variation and a low mitotic rate. Immunohistochemical staining for myoglobin, desmin, actin, vimentin, and S-100 was positive and negative for glial fibrillar antigen and keratin. Rhabdomyomas are rare benign tumors of striated muscle. Rhabdomyomas described previously in the veterinary literature are analogous to the "adult form" of human rhabdomyoma. This is the first report of a veterinary case that 1) clinically and histologically parallels the "fetal form" in human rhabdomyoma and 2) describes a congenital extracardiac rhabdomyoma.