Page 296 - Clinical Small Animal Internal Medicine

264 Section 3 Cardiovascular Disease

The mode of inheritance in the boxer is autosomal may have pulse deficits. Soft left base murmurs are
VetBooks.ir dominant with incomplete and age‐related penetrance common in boxers and not specifically indicative of
myocardial disease.
and variable expression. Dogs homozygous for the stri-
atin mutation exhibit a more severe form of disease and
there may be association with myocardial failure. Diagnosis
The natural history of the disease in the boxer can be
quite variable. Frequently, dogs may present with There is no gold standard for antemortem diagnosis of
incidental VA and remain asymptomatic for years. The ARVC in dogs. However, Holter recording is the most
percentage of boxers that develop syncope has varied valuable diagnostic tool, and should be performed in any
from 33% to 68% in different studies. The percentage boxer with an auscultable arrhythmia, with a history of
that experienced SD in one prospective longitudinal exercise intolerance or syncope, for active screening in
study was 31%. Less than 10% of boxers develop myocar- any asymptomatic adult boxer especially with a family
dial failure and CHF. In contrast, in a case series of 31 history of ARVC or positive genotype (starting at >3
English bulldogs with ARVC, 52% had CHF (38% left‐ years of age), or for those in breeding programs.
sided, 33% right‐sided, and 19% biventricular failure). Diagnostic criteria have yet to be clearly defined. Greater
Risk factors for the development of ARVC apart from than 100 VPCs per 24 hours of RV origin (LBBB configu-
breed are unknown. Risk of SD in boxers appears to be ration, “upright” orientation in lead II) is suspicious for
higher with a previous history of syncope, more frequent ARVC in the absence of another identifiable cause,
VPCs, more complex VA (couplets, triplets, runs of VT), particularly if there is increased complexity like couplets,
and more runs of VT. In humans, being a competitive triplets, or VT, and >300 VPCs/24h is a more suggestive
athlete has been linked with increased risk of SD, threshold.
suggesting the cardiac strain associated with intense One study proposed three diagnostic criteria (>100
exercise can exacerbate VA. VPCs/24h, presence of couplets, and presence of R‐
on‐T phenomenon) with the presence of two out of
three possibly being a more robust way to identify the
Signalment
disease. However, these or any other cut‐offs have yet to
Arrhythmogenic right ventricular cardiomyopathy is be validated prospectively, and to rely solely on the total
most common in the boxer breed. Males tend to be over- VPC count may be erroneous. The complexity of the
represented. The average age at diagnosis is 5–7 years arrhythmia and the complete clinical presentation
but younger and older dogs may be affected. Multiple should be considered when making the diagnosis.
cases are now also reported in the English bulldog. Holter recordings can have a high degree of day‐to‐day
Isolated cases of a similar RV cardiomyopathy have been variability (as much as 80%) and therefore may need to
reported in a dachshund, bullmastiff, Labrador retriever, be repeated in dogs with high suspicion yet equivocal
Siberian husky, Shetland sheepdog, Dalmatian, and results.
Weimaraner. Myocardial biopsy is not typically used for diagnosis
owing to anesthetic requirements and invasiveness, as
lesions may be missed given the epicardial and patchy
History and Clinical Signs
distribution of pathology, and the risk of adverse effects
Many boxers present without any clinical signs and are including provocation of arrhythmias or cardiac
detected incidentally through identification of an puncture.
arrhythmia on physical exam or through active screen- Unlike in humans, imaging modalities including echo-
ing programs. Roughly one‐third to two‐thirds may have cardiography have low diagnostic yield as they are often
a history of syncope, and roughly one‐quarter experi- normal. In few boxers will RV or LV dilation and systolic
ence exercise intolerance. Few present with respiratory dysfunction be appreciable whereas in a case series of
signs typical of CHF, whereas this appears to be more English bulldogs, 59% had subjective RV dilation. Cardiac
common in English bulldogs. It is important to note that MRI may have utility in detecting the fatty infiltration
there are other causes of syncope in the boxer apart from that characterizes the disease, but this remains to be
ARVC. investigated thoroughly in dogs.
Physical examination may be normal since the VA are Circulating biomarkers including cTnI and NT‐
intermittent. Alternatively, intermittent premature beats proBNP are not useful for detection of ARVC, based on
may be heard on auscultation, or rather the “pause” after studies in boxers. While cTnI levels did correlate with
a premature beat may be more apparent than the prema- degree of arrhythmia, the degree of overlap with pre-
ture beat itself. Bursts of tachycardia may also be noted sumably normal boxers renders it an unsatisfactory
in more severely affected dogs. Dogs with arrhythmias screening tool.