Tetralogy of Fallot

What is ... ?: 

Tetralogy of Fallot is a rare but potentially very serious combination of defects in the heart that arise when the puppy is still growing as a fetus in the dam (mother).  As the "tetra-" component of the name implies, tetralogy of Fallot consists of 4 defects inside the heart. These are: pulmonic stenosis, ventricular septal defect, dextroaorta/overriding aorta, and right ventricular concentric hypertrophy secondary to the pulmonic stenosis. Evidence suggests that these defects are the result of varying degrees of abnormality in a single developmental process - the growth and fusion of the conotruncal septum, a key region in the heart of a growing fetus.

In pulmonic stenosis, there is partial obstruction of blood flow from the right side of the heart through the pulmonic valve. Because of the obstruction, the right side of the heart has to work harder to pump blood to the lungs. This causes an increase in the mass of the heart muscle, or right ventricular hypertrophy, one of the hallmarks of this disorder.
A ventricular septal defect is a defect or hole in the muscular wall of the heart (the septum) that separates the right and left ventricles.  This communication between two parts of the heart that are supposed to be separate is inefficient, and it causes an increase in workload of the heart.

Dextroaorta, or overriding aorta, diverts unoxygenated blood away from the lungs, where it should go to be oxygenated, and out to the body's tissues.  Consequently, a dog with tetralogy of Fallot has tissues that exist in a state of constant oxygen deprivation, which explains the exercise intolerance and poor stamina commonly seen with this heart condition.
The result of the defects that make up tetralogy of Fallot is that the heart is forced to work harder than normal during every heartbeat, and poorly oxygenated blood is delivered to the tissues of the body.

How is ... inherited?: 

While it can occur in any breed of dog, the keeshond is most commonly affected.  Based on studies with affected keeshonden, the mode of inheritance is believed to be autosomal recessive with variable expression.  This means that the problem can be transmitted from either the dam or the sire (or both) to a puppy, and that the degree of severity is unpredictable: in the same litter, some puipies may be gravely affected, some not at all, and some in between, in varying proportions.

What does ... mean to your dog & you?: 

As with other heart defects, the presence of symptoms and overall impact of tetralogy of Fallot depends on the severity of the defect. If a dog has tetralogy of Fallot with a very mild degree of pulmonic stenosis and a small ventricular septal defect, for example, then he or she may only have a heart murmur and a normal life with no associated problems.

More serious cases are common, where puppies with this combination of defects experience weakness, failure to thrive and grow, a reduced tolerance for exercise, and general cyanosis (blue-grey coloration of the mucous membranes of the mouth and eyes, instead of the normal pale pink). These signs are the result of the delivery of poorly oxygenated blood to the different parts of the body.
Mildly affected dogs may enjoy a normal life, but dogs with symptoms such as those described above often have a shortened lifespan.  Tetralogy of Fallot does not cause suffering (there is no pain associated with it) but in its most severe forms it can confine a dog to a quiet, easygoing lifestyle.
 

How is ... diagnosed?: 

Puppies with severe cases of this disorder may be weak and may grow poorly, and such symptoms are initial clues to a veterinarian that tetralogy of Fallot may be present.  On closer physical examination, a veterinarian may identify cyanosis, and a heart murmur is apparent on listening to the heart with a stethoscope in all cases of tetralogy of Fallot. Definitive confirmation comes with thoracic radiographs (X-rays of the chest) and an echocardiogram (ultrasound of the heart; cardiac sonogram), both of which are noninvasive tests that can be performed without sedation in a majority of dogs.

How is ... treated?: 

Complex open-heart surgery is required to correct the condition, and this is routinely performed successfully in children. In dogs, however, surgery has a high mortality rate and is not considered a viable treatment option at this time.

In the absence of definitive correction through surgery, tetralogy of Fallot is managed by giving daily medication at home to reduce the impact of sympathetic activity ("adrenaline rushes") on the heart, which can be especially strenuous on sick hearts like those with tetralogy of Fallot.  These medications are called beta-blockers and include such prescription medications as atenolol, metoprolol, or carvedilol.  Some dogs with tetralogy of Fallot develop a very high red blood cell count, which further hampers adequate tissue oxygenation.  In these dogs, periodic phlebotomy (bloodletting) may restore a more normal viscosity or thickness to the blood, and thus improve circulation, or a medication to reduce red blood cell formation in the body (hydroxyurea) may be prescribed for this purpose.  Regardless, dogs with tetralogy of Fallot that require medications need to have regular medical checkups, often every few months.

 

For the veterinarian: 

 

  1.      MURMUR: depends on the relative degree of pulmonic stenosis and size of the ventricular septal defect.  Classically, a systolic ejection murmur, loudest over pulmonic valve area, is heard, with or without a concurrent right-sided apical systolic murmur due to turbulent flow through the ventricular septal defect.  The net effect may simply be of one loud murmur ausculted widely over both sides of the chest.
  2.      ELECTROCARDIOGRAM: may suggest right ventricular hypertrophy (right axis deviation).
  3.      RADIOGRAPHS: classcally, evidence of right ventricular enlargement, diminished pulmonary vasculature, malpositioned aorta; post-stenotic dilation of main pulmonary artery possible.
  4.      ECHOCARDIOGRAPHY: conclusive diagnostic test. Right ventricular concentric hypertrophy, pulmonic stenosis, ventricular septal defect, dextroaorta. Contrast echocardiography may show right-to-left shunting across the ventricular septal defect, and in some cases may reveal a right-to-left shunting patent foramen ovale.
  5.      OTHER: cyanosis at rest or after exercise, polycythemia, low arterial oxygen concentration and saturation are possible.

 

Breeding advice: 

Affected pups and their parents (assumed to be carriers) should not be bred. Siblings that appear normal after careful physical examination may be used for breeding, with meticulous follow-up of offspring. The offspring should be examined with echocardiography at age 8-12 weeks and, if affected, the breeding of the parents should be discontinued.
 

FOR MORE INFORMATION ABOUT THIS DISORDER, PLEASE SEE YOUR VETERINARIAN.

Resources: 

Oyama MA, Sisson DD, Thomas WP, Bonagura JD.  Congenital heart disease.  In Ettinger SJ, Feldman EC, eds.  Textbook of Veterinary Internal Medicine, 7th ed (St. Louis, MO: Saunders Elsevier, 2010) pp. 1250-1298.
Bonagura JD.  Tetralogy of Fallot.  In Cote E, ed.  Clinical Veterinary Advisor: Dogs and Cats, 2nd ed (St. Louis, MO: Mosby Elsevier, 2011) pp. 1084-1086.


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