Boxer cardiomyopathy (ARVC) in dogs

Overview

ARVC stands for arrhythmogenic right ventricular cardiomyopathy, a heart muscle disease that causes abnormal rhythms. The main arrhythmias originate in the heart's right ventricle, although they may also arise elsewhere in the heart.

Not every affected dog has visible signs, even when an electrocardiogram identifies ventricular premature beats. In other dogs, the arrhythmia causes fainting without a significant reduction in the heart's pumping strength.

Some later develop enlargement of the left ventricle and reduced contraction strength. ARVC therefore has a different emphasis from conventional dilated cardiomyopathy, in which enlargement and poor contraction are central findings.

Despite its name, tissue changes can also affect the wall between the ventricles and the left ventricular wall.

Causes

Heart muscle changes disrupt electrical activity

Muscle cells in an affected heart are replaced by fat and fibrous tissue, particularly in the right ventricle. Tissue changes and heart enlargement are different findings, so a normal ultrasound does not exclude an arrhythmia.

A ventricular premature beat is an extra electrical signal to contract that originates in a ventricle. A rapid sequence of ventricular beats forms ventricular tachycardia, an abnormally fast rhythm arising from the ventricles.

An excessively fast rhythm reduces blood flow to the body, which may also leave the brain with insufficient circulation. Ventricular tachycardia exceeding 400 beats per minute may cause fainting.

An interruption of blood flow to the brain for 6–8 seconds may be enough to cause loss of consciousness. If the rapid episode stops spontaneously, the dog may recover quickly from the faint.

An inherited predisposition may not be apparent in a young dog

The disease is associated with genetic changes that may disrupt the attachment of heart muscle cells to one another. One known association involves the gene encoding striatin.

The effect of a genetic variant varies: some dogs only develop signs when older, and others never develop apparent disease.

Desmosomes are attachment points between cells that help the heart muscle maintain its structural integrity.

Risk factors

Boxers are the main risk group, although a similar arrhythmic disease is also recognised in Bulldogs. The disease is usually diagnosed in adulthood, so a normal heart examination in a young dog does not describe its lifetime risk.

Other causes of ventricular premature beats are investigated carefully in a Boxer under 3 years old, as ARVC is less likely at that age. A family history of arrhythmias and sudden cardiac deaths is important when an inherited disease is suspected.

Dogs carrying 2 copies of a variant may have more severe arrhythmias than those carrying one copy. However, finding two copies does not automatically mean that signs will begin at a younger age.

A striatin gene variant has been identified in around 90% of Boxers with dilated cardiomyopathy, but this figure does not describe all healthy Boxers.

Diagnosis often occurs at 5–7 years of age, although disease has also been identified in dogs aged 1–3 years.

Typical symptoms

At home, a sudden episode often attracts attention because the arrhythmia may occur only intermittently.

• Your dog faints, briefly falling onto its side or losing the ability to support itself on its legs.

• Brief episodes of weakness occur.

• Exercise tolerance declines, particularly if the disease includes reduced pumping strength.

• Breathing may become laboured if heart failure develops.

• Coughing may be associated with heart failure and fluid accumulating in the lungs.

• In advanced pump failure, abdominal swelling may result from fluid accumulating there.

• Your dog may also lose weight as the disease progresses to heart failure.

Not all dogs with ARVC develop these signs of heart failure, as heart structure and contraction remain normal in many. An irregular rhythm detected at the veterinary practice may be the first finding, before you notice any episodes.

Complications and consequences

Dangerous ventricular tachycardia can become ventricular fibrillation, which prevents effective circulation and may cause death. Sudden death can occur despite medication.

A US clinical estimate suggests that around 10% of Boxers with ARVC develop dilated cardiomyopathy and heart failure. This is not a measured disease rate for Boxer populations in other countries.

Without dilated cardiomyopathy, the outlook can be good and the treatment response may last several years. When DCM and heart failure develop, the prognosis often worsens to a few months.

Dogs with severe DCM generally may survive only a few weeks, so the degree of heart failure affects the outlook.

Diagnosis at the vet

Holter monitoring records the rhythm during an ordinary day

An electrocardiogram, or ECG, shows the heart's electrical activity at the time of examination. With Holter monitoring, the dog wears a recording device for at least 24 hours.

The longer recording helps identify intermittent arrhythmias that do not occur during a short appointment. As well as counting premature beats, the recording is assessed for pairs of beats and longer, rapid runs.

Thresholds of more than 100–300 ventricular premature beats in 24 hours, or documented ventricular tachycardia, have been used when considering ARVC. However, there is no single universally applied premature-beat threshold defining disease.

Myocarditis, tumours and other diseases can also cause ventricular premature beats. A normal Holter result does not rule out ARVC developing later.

Even in a 2-year-old dog, the premature-beat count may be higher or lower the following year.

Ultrasound and genetic testing complement rhythm assessment

Ultrasound assesses the heart's structure, contraction and blood flow. In most Boxers with ARVC, ultrasound and chest radiographs may be normal.

When pumping strength declines, ultrasound helps demonstrate left ventricular enlargement and poor contraction. With heart failure, radiographs may show pulmonary vascular congestion and pulmonary oedema.

A genetic test identifies the variants tested for, but a negative result does not prevent other heart disease from developing. A positive result, meanwhile, does not mean that every dog will develop severe signs.

Possible structural and functional heart problems are therefore always investigated alongside the electrical arrhythmia.

An abnormal heart rhythm can also involve an excessively slow rhythm or a conduction disorder, whose treatment differs from that for a fast ventricular rhythm. Boxers can also develop sick sinus syndrome, in which the heart's natural pacemaker node does not send impulses normally.

If myocarditis is suspected, testing for troponin, a marker of heart muscle injury, may be used alongside rhythm investigations.

Other causes of fainting-like episodes, such as an epileptic seizure, are assessed using the event history and a thorough examination. Blood pressure, a blood count and blood biochemistry may also form part of investigating an unexplained collapse.

Neither troponin nor BNP concentration alone has been shown to be a sufficient independent screening test for ARVC.

Treatment

Arrhythmia treatment follows the investigation findings

Your vet prescribes treatment according to the heart rhythm, signs, pumping strength and other diseases. A dog that faints and has a documented ventricular arrhythmia is treated with antiarrhythmic medication.

Frequent premature beats or dangerous runs in a dog without signs may also lead to a decision to prescribe medication. More than 1,000 ventricular premature beats in 24 hours has been used as one threshold for considering treatment, but the importance of rapid runs is assessed separately.

Sotalol is one possible medicine, and mexiletine or a combination of medicines is sometimes needed. Responses vary between dogs, so a poor response may require a change of medicine or combination.

The response is monitored with ECG and Holter recording, as with the original arrhythmia. After medication starts, a follow-up Holter may be used at around 2–3 weeks, for example.

A follow-up interval of 2–4 weeks is also used, with the precise timing chosen according to the dog's situation. A reduction of more than 85% in premature-beat count has been considered a good response.

The aim is also to assess dangerous runs of arrhythmia, rather than only count individual premature beats.

A Holter recording before medication provides a baseline for assessing the response if the dog's condition allows time for testing. An antiarrhythmic medicine can sometimes worsen an arrhythmia, so monitoring considers effectiveness and safety together.

Sotalol and atenolol can reduce activity and lower blood pressure. Mexiletine can cause nausea or vomiting.

Heart failure needs its own treatment

If the heart enlarges and its pumping strength declines, treatment also follows the principles used for DCM. Diuretics may be needed to reduce fluid accumulating in the lungs or abdomen.

Pimobendan may support pumping function when indicated as part of DCM treatment. Other heart failure medicines may be added to reduce the heart's workload.

Antiarrhythmic medication can keep a dog free of signs for years, but does not remove all risk of sudden death.

Prevention

An inherited predisposition cannot be judged simply from a puppy appearing healthy and moving normally. Annual Holter monitoring from 3 years of age has been recommended for ARVC screening in Boxers without signs.

Breeding from an affected dog, one with fainting episodes or one with ventricular tachycardia is discouraged. Breeding decisions based on genetic results are made alongside clinical examinations so that the breed's genetic diversity is also preserved.

Some Boxer breeding-screening schemes focus particularly on aortic and pulmonary stenosis, congenital narrowings of the heart's outflow tracts. In some such schemes, the minimum age for the auscultation examination is 12 months.

Some schemes consider the auscultation result valid for breeding for 24 months. Under these rules, a murmur or an uncertain result requires cardiac ultrasound before mating.

An official auscultation programme of this kind does not replace long-term rhythm monitoring for ARVC.

Some breeding schemes exclude a Boxer from breeding if official examinations identify any heart disease.

Important for dog owners

Keep a diary of fainting and weakness episodes. Record the time, duration, what your dog was doing immediately beforehand and how it recovered. If an episode can be filmed safely, a short video helps describe what happened. Do not organise an exercise test at home to provoke an episode.

During Holter monitoring, record walks, rest, meals and any signs in the diary according to the instructions supplied with the device. Keep previous Holter reports, ultrasound results and genetic results so changes can be compared between examinations.

Agree with your vet what exercise suits your dog and what to do if another faint occurs. Bring an up-to-date medication list to every appointment. Do not alter antiarrhythmic medication yourself because your dog has had a symptom-free day. If breathing becomes difficult or your dog collapses, contact an emergency vet immediately.

Frequently asked questions

Does a normal heart ultrasound rule out ARVC?

A normal ultrasound does not exclude ARVC, as the hearts of most affected Boxers may look normal on it. Intermittent arrhythmias are therefore investigated with prolonged ECG recording, while ultrasound assesses structure and pumping function alongside it.

Is one normal Holter recording enough?

A normal result describes that recording period, but the disease can develop later. Premature-beat counts can also vary between recordings, so dogs with uncertain findings or at increased risk are monitored with repeat tests rather than tested just once when young. Repeat Holter monitoring is agreed according to your dog's signs and previous findings.

Does a positive genetic test mean disease is certain?

Not all dogs carrying a variant develop severe heart disease. Some may remain free of signs or only become affected when older, so genetic results are combined with monitoring of heart rhythm and structure. Genetic testing is not used as the sole measure of the severity of a current arrhythmia.

How long can a Boxer live with the disease?

Without DCM, many dogs live for several years with arrhythmia treatment. Heart failure often reduces the outlook to months, and survival with severe DCM may be shorter still. Sudden death remains possible even in a dog that has been well for a long time.

Is a faint that passes quickly harmless?

Even a brief loss of consciousness can result from an arrhythmia reducing blood flow to the brain. The same ventricular arrhythmia can, at worst, become life-threatening ventricular fibrillation, so recovery does not remove the need for investigation. A dog with fainting episodes and a ventricular arrhythmia needs treatment appropriate to its situation.