A single diagnostic test for boxer ARVC is unavailable, and the diagnosis is best based on a combination of findings. Genetic evaluation for the deletion mutation can be performed. A test result that is positive for the mutation means that the dog is at increased risk of the disease and supports a diagnosis in dogs with clinical signs. However, a positive genetic test result does not mean that the dog absolutely will develop the disease since about 30% of dogs with the mutation show no signs of the disease (due to the variable penetrance described earlier). Similarly, if the test result is negative, it does not mean that the dog does not have ARVC since there may be more than one cause for the disease. Therefore genetic testing is a useful tool for screening dogs for a breeding program but is less helpful as a single diagnostic test. A family history of disease, the presence of a ventricular tachyarrhythmia, a history of syncope or exercise intolerance, and the exclusion of other systemic and cardiovascular diseases that could be responsible for the clinical presentation all are factors that support the diagnosis. Generally a thorough physical examination, electrocardiography (ECG), blood pressure measurement, and echocardiography should be performed when a diagnosis is suspected. In addition, ambulatory ECG (Holter) monitoring provides important information for both the initial treatment and long-term management of the case and should be performed whenever possible.
Physical examination findings may include the auscultation of premature beats with post-extrasystolic pauses, a persistent or paroxysmal tachyarrhythmia, or no abnormalities at all. It should be emphasized that many affected dogs have very intermittent bouts of the arrhythmia, and the absence of a tachyarrhythmia during examination does not rule out the diagnosis. In addition, since this is primarily an electrical disease, the majority of affected boxers do not have a heart murmur, although a left apical systolic murmur of mitral regurgitation may be identified in dogs that also have myocardial dysfunction and many boxers have a left basilar ejection murmur of uncertain cause.
The classic ECG findings of ARVC include the presence of upright VPCs with positive QRS complexes with a morphology resembling that of left bundle branch block on lead II (Harpster, 1991). However, in some affected dogs the morphology of the VPCs is different or, when the arrhythmia is intermittent, the ECG may not demonstrate any VPCs. It is important to note that normal ECG findings do not exclude a diagnosis of ARVC; if suspicion persists because of clinical signs (syncope, exercise intolerance), auscultation of an arrhythmia, or a family history of disease, 24-hour Holter monitoring and genetic testing is strongly suggested. Even if occasional VPCs are identified on an in-house ECG, Holter monitoring is generally recommended to allow better assessment of the overall frequency and complexity of the arrhythmia. In addition, a pretreatment Holter ECG recording provides useful baseline information when one attempts to determine treatment efficacy as well as to assess for potential proarrhythmic effects of treatment once it has started.
The results of Holter monitoring can be very useful in establishing a diagnosis of ARVC, particularly if the ECG findings are within normal limits. It is unusual for mature adult dogs to have ventricular ectopy. The median number of VPCs detected on the Holter readings from 600 mature asymptomatic boxers was 10 VPCs in 24 hours. Therefore the identification of frequent ventricular ectopy (>100 VPCs in 24 hours) in an adult boxer dog is strongly suggestive of a diagnosis of ARVC, particularly if there is significant complexity (couplets, triplets, bigeminy, or ventricular tachycardia) within the arrhythmia. However, in some cases a diagnosis of ARVC in a boxer is strongly suspected, based on the breed and a history of syncope, but the Holter monitor recording clearly does not show abnormalities. This may be explained by the significant day-to-day variability in the number of VPCs (up to 83%) in affected dogs. Alternatively, reflex-mediated syncope (e.g., vasovagal syncope, characterized by vasodilation and bradycardia) may be occurring. Reflex-mediated syncope has been observed in boxers and should be considered in a fainting boxer with no evidence of ventricular arrhythmias. In cases of uncertain cause, a second round of Holter monitoring or ambulatory ECG event monitoring may be performed to better determine the relationship of heart rhythm and syncope in a boxer dog.
Blood pressure measurement, echocardiography, and in certain cases splenic ultrasonography are recommended in cases of suspected ARVC. Although in the majority of cases the blood pressure, cardiac chamber sizes, and systolic ventricular function are within normal limits, an echocardiogram can rule out other, less common causes of ventricular ectopy (e.g., neoplasia). Likewise, splenic masses are a known cause of ventricular arrhythmias in dogs and should be considered in the diagnostic workup of ventricular arrhythmias, particularly in older dogs. Blood pressure measurement may offer insight into additional systemic diseases capable of causing syncope. In addition, echocardiography is necessary to identify the small percentage of dogs with ARVC that develop right and left ventricular enlargement and myocardial dysfunction.
