Boxer cardiomyopathy (arrhythmogenic right ventricular cardiomyopathy (ARVC))

Boxer cardiomyopathy is an inherited heart disease. The disease affects the heart muscle and can cause the development of arrhythmias, heart failure and sudden death. It would appear there is a marked geographical pattern, with the majority of cases reported in the US and in the UK, but this disease appears to be seen much less frequently in mainland Europe. Screening programmes have been introduced to try to identify affected dogs and remove them from the breeding population.

Boxer cardiomyopathy is an inherited heart disease of Boxer dogs. As a genetic disease almost all affected dogs will have a family history of relatives affected with the same condition. A gene mutation has been identified in affected dogs in the US. Every dog has 2 copies of all the genes. If both copies of the “cardiomyopathy gene” are abnormal then that individual will develop a more severe form of the disease.

Affected dogs develop serious disturbances of heart rhythm, a weakened heart muscle or both. Outward signs of disease include fainting, sudden death and heart failure. The disease has many features in common with arrhythmogenic right ventricular cardiomyopathy (ARVC) in humans, hence why it is common to use the same name to refer to Boxer cardiomyopathy.

Although dogs as young as 6 months old have been reported with ARVC, it is usually a disease of adult dogs.

There are three forms of the disease and dogs with each type have different signs:

Type I – Occult cardiomyopathy (asymptomatic)
Type I dogs, by definition, have no outward signs of disease. This form can only be identified by specific screening tests or it may be picked up when investigations are performed for unrelated problems.

Type II – Arrhythmogenic form (rhythm disturbances)
Most dogs with Type II disease are diagnosed because they have intermittent or continuous signs of poor cardiac output. Signs will include fainting, weakness, exercise intolerance, or collapse. Occasionally dogs with Type II disease may die suddenly with no apparent cause.

Type III – Dilative form (weakened heart muscle)
Signs in dogs with Type III disease most closely resemble those in dogs with dilated cardiomyopathy (DCM). Presenting signs often include either low cardiac output signs (weakness, collapse, exercise intolerance, or fainting) or signs due to congestive heart failure (coughing, difficulty breathing or distended abdomen).

Despite this academic classification, it is not uncommon for affected Boxer dogs to present with a combination of type II and type III signs (this is: arrhythmias and poor heart muscle contraction).

ARVC is diagnosed by finding a specific type of rhythm disturbance on ECG recordings. Sometimes these are detected on a single ECG recording made in the hospital. However, to quantify the severity of the disease dogs can be fitted with an ambulatory ECG recording device or Holter monitor which can measure heart rhythm over a 24-hour period at home. This is much more sensitive and can also detect changes in the heart rhythm that may only occur when your dog is exercising or asleep. Doppler echocardiography (ultrasound scan of the heart) will allow for the detection of a weakened heart muscle and rule out other causes of heart arrhythmias. Your veterinarian may also want to preform blood testing to look for signs of damage to the heart (cardiac biomarkers). A genetic test for the disease is available but is more likely to be used as a screening test in dogs that have not developed signs yet. Other tests, including radiographs of the chest and abdominal ultrasound, are often needed to exclude other causes of heart arrhythmias.

Not all animals with ARVC have obvious signs – silent carriers on the stage I of the disease are easily missed. This is problematic if animals carrying the disease are allowed to breed. For this reason a screening programme has been introduced. Many screening programmes will begin screening at the age of 2 years, and continue on an annual basis. If your dog has a normal Holter monitor recording it does not mean that the disease is completely ruled out – just that it does not show electrocardiographic signs at this particular moment in time. It is important that annual screening is continued as the average age for detection of abnormalities is 6 years.

There is now a genetic test for the disease. This simple blood test can detect whether your dog is carrying the “abnormal gene” and whether one or both copies of the relevant gene are affected. Dogs with 2 copies of the gene will definitely pass the condition to all their offspring, so breeding from these individuals is unwise. Dogs with one abnormal gene and one normal gene will pass the condition, on average, to half of their offspring; these dogs should be screened with Holter monitoring and those without clinical signs can eventually be bred with dogs with no gene mutations, as the resulting puppies are unlikely to develop disease, although may be silent carriers of it.

Owners commonly worry about whether exercise is bad for their dog. Excitement, anxiety, illness and any other forms of stress can make signs worse. While exercise certainly can increase the frequency of the arrhythmia, no one really knows whether restricting exercise actually protects dogs. You really have to use your own judgement as to how important exercise and fun is to your pet’s quality or life, with the understanding that exercise may increase the risk for sudden death or may make clinical signs worse in some cases.

Drugs which reduce rhythm disturbance (anti-arrhythmics) are available. The most common medication combinations include sotalol alone or in combination with mexiletine; mexiletine alone or in combination with atenolol, procainamide, or amiodarone. Antiarrhythmics can be given to dogs with no clinical signs to try to reduce the frequency of ventricular arrhythmia and prevent fainting episodes or reduce the risk of sudden death. Your pet’s cardiologist may prescribe antiarrhythmic therapy if they feel the risk of sudden death is very high. However, unfortunately, there is little evidence that treatment can actually prevent sudden death.

Dogs that present with heart failure may need hospitalization and emergency treatment to stabilise their condition. In dogs with congestive heart failure quality of life can be improved by therapy with diuretics and ACE inhibitors. However ARVC is a progressive disease and although treatment will usually improve the clinical condition at least initially, it will not affect the underlying disease and this may progress over time despite treatment.

All drugs have potential side effects and therefore, part of the monitoring process is to check for any side effects of medication. A bizarre side effect of antiarrhythmic medication is that it can sometimes make heart rhythm disturbances worse. It is therefore really important to keep monitoring 24-hour ECGs once antiarrhythmic medication is started and every so often thereafter; every three to six months depending on the severity of the disease may be recommended by your pet’s cardiologist.

Beta blockers and potassium channel blockers (eg atenolol and sotalol), in addition to the effects on the heart rhythm, may also reduce the blood pressure. Any animal on beta blockers should have regular checks of their blood pressure.

Periodic evaluation with thoracic radiographs and cardiac ultrasound is appropriate for dogs with Type III disease. For dogs with congestive heart failure regular monitoring is more intense: in addition to the blood pressure monitoring and X-rays and ultrasound, blood tests will be needed for dogs receiving diuretics and ACE inhibitors to check the kidney function and electrolytes.

The outlook for dogs with Type I and II disease is extremely variable. Some dogs will die suddenly and others will continue to have regular episodes of collapse. Other dogs will live the rest of their life without symptoms and finally succumb to an unrelated disease.

Some dogs will develop congestive heart disease. Sadly the outlook for dogs affected with Type III disease is not good. Life expectancy may be in the order of weeks to months from diagnosis for this type.