Page 176 - Small Animal Internal Medicine, 6th Edition

148 PART I Cardiovascular System Disorders

systolic dysfunction, presence of ventricular tachycardia or likely to develop the DCM phenotype, although some het-
atrial fibrillation, and higher values of cardiac biomarkers. erozygous dogs also manifest this more severe form of
VetBooks.ir In individual cases, however, it is reasonable to assess the ARVC.
animal’s response to initial treatment before pronouncing an
Clinical Findings
unequivocally dismal prognosis. Overall prognosis for DCM,
in both preclinical stages and in CHF, is improved with use of Signs may appear at any age, but the median age at diagnosis
pimobendan. is 6 years. Syncope is the most common clinical complaint.
Ventricular tachyarrhythmias underlie most instances of
syncope in Boxers with ARVC. However, another potential
ARRHYTHMOGENIC RIGHT cause for syncope in young adult Boxers is neurocardiogenic
VENTRICULAR CARDIOMYOPATHY (reflex-mediated) syncope, where a sudden surge in sympa-
thetic activity triggers reflex vagal stimulation and inappro-
ARVC is the most common acquired heart disease of Boxer priate bradycardia and hypotension. Neurocardiogenic
dogs. This inherited primary myocardial disease shares syncope can occur in normal Boxers and in Boxers with
many similar features to those of ARVC in people. Histologic ARVC, and can potentially be exacerbated by use of sotalol
changes in the myocardium are more extensive than other or (other) β-blocker therapy.
canine cardiomyopathies and are characterized by fatty or The physical examination could be normal, although a
fibrofatty infiltration, usually most severe in the right ven- soft left basilar systolic murmur is common in Boxers,
tricle (RV) free wall. Atrophy of myofibers and myocardial whether ARVC is present or not. In many Boxers, this is a
fibrosis are also common. Focal areas of myocytolysis, necro- breed-related physiologic murmur related to aortic annular
sis, hemorrhage, and mononuclear cell infiltration may be hypoplasia relative to body size, or it may be associated with
seen. Ultrastructural abnormalities, including reduced underlying subaortic stenosis. In some dogs, a cardiac
numbers of myocardial gap junctions and desmosomes, are arrhythmia with pulse deficits is found on physical examina-
apparent throughout the myocardium (including the atria), tion; in others, the resting heart rhythm is normal. When
suggesting that the disease process is not confined to the RV. CHF occurs in dogs with DCM phenotype, left-sided signs
ARVC in Boxers is familial with an autosomal dominant are more common than ascites or other signs of right-sided
inheritance pattern. A mutation in the striatin gene on chro- heart failure; a mitral insufficiency murmur can be present
mosome 17, which encodes for a protein involved in cell-to- in these cases as well.
cell adhesion, has been associated with Boxer ARVC. Boxers
with at least one copy of the striatin mutation are 40 times Diagnosis
more likely to develop ARVC than homozygous negative Radiographic findings are variable. Boxers with ARVC and
dogs. Overall genetic penetrance of this mutation is approxi- normal myocardial function have no visible abnormalities.
mately 80%, with nearly 100% of homozygous positive dogs Those with DCM phenotype and CHF generally show evi-
affected. Yet the fact that this mutation is not present in all dence of cardiomegaly and pulmonary edema. Echocardio-
Boxers with ARVC and is present in some without ARVC graphic findings also vary between disease manifestations.
suggests that it may collocate with, rather than being, the Most Boxers with ARVC have normal cardiac size and func-
causative mutation. However, as in people, there may be a tion; dogs with DCM phenotype show reduced fractional
number of gene mutations associated with ARVC in different shortening and chamber dilation, similar to other dogs with
bloodlines. Genetic testing for the striatin gene mutation is DCM.
available (North Carolina State University Veterinary Cardiac The characteristic ECG finding is ventricular ectopy.
Genetics Laboratory; https://cvm.ncsu.edu/genetics/). VPCs occur singly, in pairs, in short runs, or as sustained
Clinical manifestations of ARVC can appear in three ventricular tachycardia. Most ectopic ventricular complexes
forms, although these are thought to represent the same originate in the RV and thus appear upright in leads II and
underlying disease. Dogs with the occult form have ventricu- aVF (Fig. 7.4). However, some Boxers have multiform VPCs.
lar arrhythmias without clinical signs. Dogs with overt Usually an underlying sinus rhythm exists; AF is less
ARVC have syncope or weakness associated with paroxys- common. Supraventricular tachycardia, conduction abnor-
mal or sustained ventricular tachycardia, usually despite malities, and evidence of chamber enlargement also are
normal heart size and LV function. Approximately 10% of sometimes seen on ECG, particularly in patients with the
affected Boxers have a form of ARVC where ventricular DCM phenotype.
tachyarrhythmias are accompanied by a DCM phenotype, Twenty-four-hour Holter monitoring is used to quantify
with poor myocardial function that progresses to CHF, the frequency and complexity of ventricular tachyarrhyth-
unless sudden death occurs first. Myocardial changes in the mias and as a screening tool for Boxer ARVC. It also is
DCM phenotype typically involve both the LV and RV, and recommended to evaluate the efficacy (and any proarrhyth-
left-sided CHF is most common. Some dogs with normal LV mic adverse effects) of antiarrhythmic drug therapy. Fre-
systolic function at the time of ARVC diagnosis progress to quent VPCs and/or complex ventricular arrhythmias are
develop the DCM phenotype later in life. Dogs that are characteristic findings in affected dogs. Absolute criteria for
homozygous-positive for the striatin mutation appear more separating normal from abnormal Boxers are not entirely