“Boxer Cardiomyopathy” is a well known condition, but how much do you know about it? In this blog, we’ll be exploring this relatively common condition!
Surely that’s not its real name?
Strictly speaking, the correct name is Arrhythmogenic Right Ventricular Cardiomyopathy or ARVC; however, it is so commonly referred to as Boxer Cardiomyopathy that the name even appears in textbooks! It is also useful to distinguish the disease in dogs from that in humans.
What dogs are affected?
Take a guess! It’s almost exclusive to Boxers, although a very similar (and possibly identical) condition has been recognised in closely related breeds like Bulldogs. The disease can occur at any age (cases range from 6 months to over 10 years of age at diagnosis) but is usually detected in mid-adult life (5 to 8 years). It is thought that it tends to gradually worsen as the affected dog ages.
What is it?
ARVC is a disease of the muscle of the heart wall, like Dilated Cardiomyopathy in other breeds (e.g. Dobermans or Wolfhounds). However, in ARVC there is often no dilation (swelling) or deformation of the heart, and it looks perfectly normal on an X-ray or scan. The trouble is that the condition causes an arrhythmia (an abnormal heartbeat) which intermittently prevents the heart from pumping effectively. Instead of the four chambers of the heart pumping together in sync, the ventricles (the large bottom chambers) sometimes contract on their own, disrupting the cardiac rhythm and preventing effective circulation of oxygenated blood. This is known as a ventricular premature complex, or VPC, and occurs periodically in affected dogs. The more frequently they occur, the more severe the symptoms. In some cases, there may be so many VPCs that the heart stops, resulting in sudden death.
What causes it?
Boxer Cardiomyopathy is a genetic disease, with a mutation in the Striatin gene definitively identified as the major cause (although there is at least one other gene as well). Dogs with one copy of this mutant gene are at risk of the disease; dogs with two copies (i.e. one from their mother and one from their father) are usually more severely affected. It is reported that up to 50% of Boxers carry at least one mutated copy.
What are the symptoms?
There are four different ways that the disease can manifest itself:
Apparently Normal – these dogs appear completely fine; however, occasionally abnormal heartbeats are detected when a vet is checking for something else – for example, listening to the heart at vaccination, or using an ECG monitor during surgery – or with a screening test such as a Holter Test (see below).
Overt Disease – the abnormal heartbeats are frequent enough to cause the “classical” symptoms of the disease, as the dog periodically faints as their brain is temporarily starved of oxygen. Other possible signs include loss of exercise tolerance, weakness and collapse.
Congestive Heart Failure – the failure of the heart as a pump may result in coughing, blue gums, swollen abdomen and difficulty breathing. This is, however, rare with less than 5% of affected dogs presenting like this.
Sudden Death – sadly, in some cases this is the first symptom seen.
How is it diagnosed?
Unlike most other heart diseases, a heart scan (echocardiogram) is not useful in diagnosis (although it can rule in or out other causes of heart disease). Instead, it is necessary to carry out an ECG to measure the heart’s electrical activity. However, because the numbers of VPCs per minute is very variable, this needs to be conducted over a 24 hour period. For that reason, we use a portable ECG called a Holter Monitor which the dog wears on a harness for a day or two, and the device then goes back to the heart vets for analysis. The device is about the size of a pack of cards and most dogs don’t even notice it’s there!
How can it be treated?
There is no cure for ARVC at the moment (although modern research in genetic modification may result in a genetic therapy in the future). Instead, treatment is aimed at reducing the numbers of VPCs and the severity of the symptoms, and usually involves the use of antiarrhythmic drugs such as sotalol. Most of these are human drugs and must be prescribed by a vet on the Cascade.
The decision on when to start treatment is complex, partly because any of the heart drugs may make it better, or worsen the condition. In general, the more severely affected and/or the more frequent the VPCs, the more urgent it is to start medication.
Most affected dogs have episodes that are triggered by stress or excitement, so changes to management to minimise this may help – although it’s easier said than done with an enthusiastic and excitable Boxer!
What is the expected prognosis?
Affected dogs are always at risk of sudden death; however, with early diagnosis and effective treatment, they may live happily for several years with a good quality of life. The more severely affected the dog is at diagnosis, the worse the prognosis is likely to be.