Rhabdomyomas

Rhabdomyoma is a lesion of striated muscle. There are two main types, as follows:

• Neoplastic
• Hamartoma

The neoplastic variety is further divided into the following three subtypes:

• Adult
• Fetal
• Genital

Hamartomas are divided into the following two subtypes:

• Cardiac rhabdomyoma
• Rhabdomyomatous mesenchymal hamartomas of the skin

Rhabdomyoma is an exceedingly rare tumor. Some investigators believe that mature striated muscle is unlikely to develop tumorous tissue. Therefore, they believe that rhabdomyoma may arise from fetal rests.

Rhabdomyoma is diagnosed most often in men aged 25-40 years. However, the so-called fetal rhabdomyoma chiefly affects boys between birth and age 3 years.

Most rhabdomyomas involve the head and neck regions. ^[1] The cardiac rhabdomyoma, which is believed to be a hamartoma, usually is diagnosed in the pediatric age group. However, hamartomas are benign tumorlike growth made up of normal mature cells in abnormal number or distribution. Whereas malignant tumors contain poorly differentiated cells, hamartomas consist of distinct cell types retaining normal functions. Because their growth is limited, hamartomas are not true tumors.

The adult rhabdomyoma is a rare tumor; very few cases have been reported in the literature. ^{[2, 3]} This tumor usually presents as a round or polypoid mass in the region of the neck. The head and neck area harbors 90% of adult rhabdomyomas and should be considered in a differential diagnosis in this region. ^[4]

Studies in immunohistochemistry confirm that the tumors are almost totally matured neoplasms of clonal origin. Although the mass is usually asymptomatic, it may compress or displace the tongue, or it may cause partial obstruction of the pharynx. Consequently, the patient may experience some hoarseness, difficulty breathing, and difficulty swallowing.

The histopathology of adult rhabdomyoma is characterized by the presence of well-differentiated large cells that resemble striated muscle cells. Cross-striation has been demonstrated by phosphotungstic acid hematoxylin (PTAH), muscle specific actin, desmin, and myoglobin; dystrophin is shown to be expressed in the cell membranes. The cells are deeply eosinophilic polygonal cells with small peripherally placed nuclei and occasional intracellular vacuoles. Adult rhabdomyoma usually is localized to the oropharynx, the larynx, and the muscles of the neck.

Fetal rhabdomyoma occurs most often in the subcutaneous tissues of the head and neck in children between birth and age 3 years. The histopathology of fetal rhabdomyoma reveals the presence of a mixture of spindle-shaped cells with indistinct cytoplasm and muscle fibers, which resemble striated muscle tissue observed in intrauterine development at 7-12 weeks. The fetal rhabdomyoma is usually found in the subcutaneous tissues of the head and neck.

Genital rhabdomyoma most often involves the vagina or vulva of young or middle-aged women. ^[5]  Most patients are asymptomatic. However, some patients have dyspareunia. The histopathology of genital rhabdomyoma reveals a mixture of fibroblastlike cells with clusters of mature cells containing distinct cross-striations and a matrix containing varying amounts of collagen and mucoid material. The genital rhabdomyoma usually presents as a polypoid or cystlike mass involving the vulva or vagina.

Cardiac rhabdomyoma is a hamartomatous lesion that occurs most often in the pediatric age group. ^{[6, 7, 8]}  Cardiac rhabdomyomas typically develop in utero and are often detected on prenatal ultrasonography. It usually involves the myocardium of both ventricles and the interventricular septum.

Cardiac rhabdomyoma is considered a hamartomatous proliferation frequently associated with tuberous sclerosis of the brain, sebaceous adenomas, and various hamartomatous lesions of the kidney and other organs. The association of tuberous sclerosis and cardiac rhabdomyoma is important and has usually been explained by strong clinical association. ^[9]  Molecular evidence of this association have now been identified as the TSC2 gene missense mutation (E36; 4672 G>A, 1558 E>K TSC2). ^[10]

Rhabdomyomatous mesenchymal hamartoma is usually diagnosed in male and female newborns and infants. The histopathology of rhabdomyomatous mesenchymal hamartoma of the skin reveals that the lesions are located in the subcutis and contain poorly oriented or perpendicular bundles of well-differentiated skeletal muscle with islands of fat, fibrous tissue, and occasionally proliferating nerves. ^[11]

Malignant transformation of rhabdomyomas is very rare, though there are a few case reports in the literature.

Rhabdomyoma probably represents a genetic variant of striated muscle development. Drugs or environmental factors have not been identified as causes of this neoplasm.

United States statistics

Compared with other soft-tissue tumors, rhabdomyoma is extremely rare in the United States. Specifically, in the category of benign primary tumors of the heart, rhabdomyoma has a relative incidence of 5.8%.

International statistics

Worldwide, rhabdomyoma is rare. Exact data regarding its incidence within particular populations have not been cited. In the world literature, 14 cases of multifocal adult head and neck rhabdomyoma are reported. ^[12]

Age-related demographics

Adult rhabdomyoma occurs in older adults (usually >40 years). Fetal rhabdomyoma occurs between birth and age 3 years. Genital rhabdomyoma is observed in young and middle-aged women. Cardiac rhabdomyomas occur chiefly, but not exclusively, in the pediatric age group; a 45-year single-institution review found rhabdomyoma to account for 58% of cardiac neoplasms in 64 pediatric patients (age < 18 years) who presented for surgical evaluation of a cardiac tumor. ^[13]  Rhabdomyomatous mesenchymal hamartoma of the skin is observed in newborns and infants.

Sex-related demographics

Adult rhabdomyoma has been diagnosed mostly in men. Some reports exist of cases in women. Fetal rhabdomyoma affects boys. Genital rhabdomyoma affects young and middle-aged women. Cardiac rhabdomyoma is observed in men and women. Rhabdomyomatous mesenchymal hamartoma of skin is observed in male and female newborns and infants.

Race-related demographics

Rhabdomyoma has been identified in all racial groups. No predilection for any particular racial group exists.

The prognosis for patients who have undergone surgery for the removal of rhabdomyomas ranges from fair to good, depending on the part of the body involved. Patients with cardiac rhabdomyomas have the highest risk.

The morbidity of rhabdomyoma depends on the type of lesion and its location. This is a benign tumor of striated muscle. Metastases have not been associated with this tumor.

Cardiac rhabdomyomas are associated with the potential for flow abnormalities if they grow to sufficient size to restrict the left ventricular outflow tract. Although many are asymptomatic, some affected patients become symptomatic in the perinatal period. The types of clinical manifestations were illustrated in a report that included 15 children with cardiac rhabdomyoma (12 with tuberous sclerosis): the clinical presentation consisted of heart failure or a cardiac murmur in six patients each and arrhythmia in three patients. ^[14]

Based on clinical and molecular evidence, the diagnosis of fetal cardiac rhabdomyoma should lead to the careful evaluation of other fetal structures, including brain and renal parenchyma, to search for signs of tuberous sclerosis.

Patients with rhabdomyomas should be aware of the nature of their particular tumors and the type of surgical procedures available for treatment.

Any surgical procedure has a risk of a malpractice claim. The patient must sign a detailed informed consent form before any procedure is performed.