Rhabdomyoma is a benign tumor that originates in striated muscle tissue as opposed to smooth involuntary muscles. In dogs, it is primarily a heart tumor, but it is found also on the larynx. This type of neoplasm is quite rare. Primary tumors originating in the heart are uncommon in dogs, and 46% of these are hemangiosarcoma. The actual instance of rhabdomyoma is unknown, since heart tumors can be difficult to diagnose until after death, and many are not specified other than “tumor-heart.” Most tumors in dogs are found on the right side of the heart and can involve either the atrium or the ventricle. Unlike its malignant counterpart, rhabdomyosarcoma, rhabdomyoma is benign and will not grow or metastasize to other places in the body; however depending on the placement of the tumor it can still limit heart function. Most symptoms related to rhabdomyoma resemble other forms of congestive heart failure. Tumors located inside the heart cause fluid accumulation, pericardial effusion, and can affect the normal passage of blood between the atrium and the ventricle. Many tumors may not be symptomatic, but they will weaken the heart so that heart failure develops at a young age. In humans, rhabdomyoma in the heart is primarily a pediatric cancer, and it is also found more often in younger dogs. Many veterinarians believe there is an inherited component. Rhabdomyoma is sometimes called a hamartoma, a benign tumor in which there is no change in the tissue at the cellular level, although it grows into an abnormal mass.

Benign heart tumors that originate in the striated muscle tissue of the heart are called rhabdomyoma. These are rare tumors in dogs. Even though they are benign, rhabdomyoma can still disrupt heart function.

These are some of the signs that might indicate your dog has a rhabdomyoma tumor in the heart.

Striated muscle tumors can be either malignant or benign.

Other types of tumors that could be found in the heart include hemangiosarcoma, fibroma, fibrosarcoma, myxoma, teratoma, lipofibroma, mesothelioma, and granular cell tumors.

There is no known cause for rhabdomyoma. There may be a genetic component, but instances are too rare to be adequately studied in dogs. Rhabdomyoma is found more commonly in other animals, especially pigs, and it seems to behave similarly in dogs. Unlike most other forms of cancer, it is more common in younger animals.

Many rhabdomyoma tumors may be asymptomatic and you may not know your dog has the condition. This is often the case with domestic animals like cattle or sheep where the tumor is only discovered during the slaughtering process. It may also be found on an x-ray taken for another purpose. If the tumor is disrupting heart function enough to cause a murmur, the veterinarian may hear this through a stethoscope on a routine exam, or it may be diagnosed symptomatically because your dog is showing typical symptoms related to heart dysfunction, like poor exercise tolerance, lethargy, or fluid accumulation in the abdomen. If the veterinarian suspects a tumor or another heart problem, x-rays, electrocardiography, and echocardiography will be ordered. Depending on the size and placement, the tumor may or may not be visible on an x-ray. Electrocardiography will diagnose an abnormal rhythm, while echocardiography (an ultrasound of the heart) can help to locate which part of the heart is affected by the tumor. Definitive identification of the tumor may only be possible with a heart biopsy, which is invasive and rarely done.

Congestive heart failure is usually treated symptomatically with medical management. ACE inhibitors can help to support blood flow, while diuretics and diets low in sodium encourage fluid elimination. If pericardial effusion or ascites is present, a tube may be inserted periodically to help drain the fluid. This is called pericardiocentesis or abdominocentesis. Pericardial effusion will eventually cause cardiac tamponade, compression of the heart, which can lead to sudden cardiac arrest. Pericardiectomy is a surgical procedure that removes some of the heart lining to help relieve the compression. In dogs with pericardial effusion and tamponade from heart tumors, this was found to increase survival time considerably. Surgical removal may be possible, depending on the placement of the tumor. Removal is usually more successful with tumors located on the right atrial appendage. Since this is a benign cancer, removal will usually be curative, but many tumors may be located in places where they cannot be reached without difficult and costly surgery. If the tumor is discovered accidently and is not causing a problem for your dog, no treatment is needed. Many rhabdomyomas are asymptomatic and don’t affect the length of a dog’s life.

Your dog will only make a complete recovery if the tumor is surgically removable, which is rare. However mild symptoms of congestive heart failure may be manageable for a considerable period of time. Many rhabdomyomas that are asymptomatic may not cause a problem or even go unnoticed. The general outlook for symptomatic rhabdomyoma is guarded, but your dog’s exact prognosis will depend on the diagnosis of a veterinarian.