Tumors showing skeletal muscle differentiation are classified as either benign rhabdomyomas or malignant rhabdomyosarcomas. Rhabdomyomas are rare benign mesenchymal tumors of soft tissue originating from skeletal muscle cells and account for about 2% of all skeletal muscle tumors.¹,²,³ Classification is based on histologic, topographic, clinical, and immunohistochemical criteria. They are usually subdivided into two major subtypes, cardiac and extracardiac, based on their location.⁴,⁵

The cardiac type is more common. It is seen mostly in children and is frequently associated with phacomatoses.⁶ Cardiac rhabdomyomas are considered to be hamartomatous lesions derived from embryonal myoblasts. They exhibit a fetal pattern of atrial natriuretic peptide immune reactivity and are associated with tuberous sclerosis complex in up to 60% to 96% of cases.⁷

The extracardiac type of rhabdomyoma is extremely uncommon, usually affecting the soft tissues of the head and neck with a 2:1 to 3:1 male predominance.⁶,⁸,⁹ The peak age of incidence is in the fifth to sixth decades, but they can be seen in patients ranging from childhood to old age. Tumors occur in diverse anatomical sites and have been divided into subtypes based on clinical and morphological criteria¹,²,¹⁰,¹¹,¹²,¹³: the adult and fetal subtypes. Unlike the cardiac type, extracardiac rhabdomyomas do not have an association with tuberous sclerosis.¹⁰

Adult rhabdomyomas most often derive from the musculature of the third and fourth branchial arches located in the head and neck.¹⁴,¹⁵ They frequently involve the larynx,¹⁶ pharynx,⁶ floor of the mouth,¹⁷ and tongue.¹⁸ Other rare locations include the cheek,¹⁹ lower lip,²⁰ the tunica vaginalis of the testis,²¹ the sternocleidomastoid muscle,¹,²² and the muscular wall of the stomach, even though striated muscle is normally not present in this location.²³ Adult rhabdomyoma has also very rarely been reported in the skin.²⁴,²⁵,²⁶ Only two cases of adult rhabdomyoma have been described involving the eyelid,²⁷,²⁸ and six orbital cases were described involving the rectus muscles.²⁹,³⁰,³¹,³²,³³,³⁴

The fetal type of extracardiac rhabdomyoma is the least common variety. It is seen most often in the head and neck region of young male children, less than 3 years of age.³⁵,³⁶ They are classified histologically into myxoid and cellular subtypes.¹¹,³⁶ The myxoid type has been described in infants during the first year of life as subcutaneous lesions in the head and neck, particularly in the preauricular and postauricular regions, and in the vulvovaginal region of middle-aged women. The cellular type occurs most often in mucosal and soft tissues of the head and neck of children, adolescents, and occasionally older male adults.³⁶,³⁷,³⁸ This type has also been reported in the stomach, retroperitoneum, upper extremity, and anus.¹¹,³⁹ Two cases of fetal rhabdomyoma were described in the eyebrow.⁴⁰,⁴¹