by Rebecca Quinn, DVM, DACVIM (Internal Medicine)
Arrhythmogenic right ventricular cardiomyopathy (ARVC) is an inherited disease in Boxer Dogs and results in unique right ventricular fibrofatty infiltration and ventricular arrhythmias. The condition was initially described in 1983 as a disease associated with ventricular ectopy, syncope, and sudden death. There are three classifications of ARVC, which are now thought of as a spectrum of illness:
- Concealed ARVC: a patient with asymptomatic ventricular ectopy
- Overt ARVC: a patient with ventricular ectopy and symptoms of weakness or collapse
- Myocardial dysfunction: a patient with echocardiographically and radiographically apparent myocardial abnormalities (cardiac dilation, poor function), with or without congestive heart failure (CHF)
An association has been found between ARVC in Boxers and a mutation in the striatin gene. Striatin is a desmosomal gene, and contributes to normal cardiomyocyte cell-to-cell adhesions and mechanical stability. Mutations in striatin lead to loss of normal cell coupling and arrhythmias. The mutation is inherited, and most studies support an autosomal dominant inheritance pattern with incomplete penetrance. Boxer dogs with homozygous striatin mutations have nearly 100% disease penetrance, higher numbers of ventricular premature complexes (VPCs), and are more likely to have myocardial dysfunction. Boxer dogs with heterozygous striatin mutations have lower disease penetrance and fewer VPCs. While the striatin mutation is clearly associated with ARVC in Boxers, studies indicate that other unknown mutations also play a role in the disease.
Most Boxers are diagnosed with ARVC around six years of age, and there is a slight predisposition in male dogs. The patients present with arrhythmias, intermittent weakness or lethargy, collapse, seizure-like episodes, and rarely CHF. When assessing Boxers with symptoms of ARVC, it is important to avoid tunnel vision and rule-out other conditions. Baseline diagnostics such as CBC, chemistry panel, and urinalysis should be evaluated; infectious and endocrine disease testing is recommended if indicated. Additional imaging, such as thoracic radiographs, abdominal ultrasound, and echocardiogram should be considered, but are often normal.
The diagnosis of ARVC is often made via ECG, or by ECG in combination with a 24-hour Holter monitor. Ventricular premature complexes usually originate from the right ventricle, but left ventricular origin and polymorphic VPCs have been reported. The VPCs may occur as isolated beats, patterns (such as bigeminy or trigeminy), runs of ventricular tachycardia, or as R-on-T phenomenom. Normal Boxers should have fewer than 50 VPCs per 24 hours; Boxers demonstrating more than 100 complex VPCs per 24 hours or 300 – 1000 or more VPCs per 24 hours likely have the condition. A recently released striatin test can also be used to aid in diagnosing Boxers with ARVC.
Once diagnosed, anti-arrhythmic medications are often indicated. Commonly used emergent therapies include lidocaine, procainamide, or magnesium sulfate. Once stabilized, arrhythmias can be well controlled with sotalol, mexilitine, or a combination of the two. Some Boxers are refractory to what are thought of as “standard therapies,” and require less frequently used medications such as amiodarone, flecainide, or propafenone. Repeat Holter monitoring is recommended ten days after initiating therapy to ensure the arrhythmia is well controlled. Holter monitors should be repeated every 3 ‑ 6 months or as indicated based on clinical signs to ensure appropriate longterm management.
Despite therapy, Boxers with ARVC remain at risk for sudden death, and client education is an important aspect of ARVC. Prognosis in ARVC Boxers varies, and other than sudden death, some ARVC Boxers decompensate and experience myocardial failure. The prevalence of ARVC in Boxer Dogs is not known, but one study estimates that 25% of the Boxer population may be affected by the condition. Sound breeding programs and thorough veterinary care are essential in order to ensure the healthy future of the Boxer breed.
Please do not hesitate to contact the Angell Cardiology Service to discuss individual considerations for your patient and any other questions you may have. You may contact either Dr. Rebecca Quinn at email@example.com, send a message to the general Cardiology inbox at Cardiology@angell.org, or call the Cardiology Service at 617-541-5038.