ARVC is a recendy reported and rare form of feline cardiomyopathy. It has been identified in humans, dogs (boxer dogs), and cats. It is characterized by fibrofatty or fatty infiltration of primarily the right ventricular free wall. The right ventricular wall is commonly thinned in humans and cats with the disease. Ventricular tachyarrhythmias are common in all three species, and sudden death is a common feature of the disease in humans and boxer dogs. The disease is also commonly known as arrhythmogenic right ventricular dysplasia (ARVD).
Cause of Arrhythmocenic Right Ventricular Cardiomyopathy
The cause of arrhythmogenic right ventricular cardiomyopathy is unknown in cats. In humans at least six forms of the disease (ARVD 1 to 6) are inherited as an auto-somal dominant trait, and one (Naxos syndrome) is inherited as an autosomal recessive trait.Iu The cause of ARVD 2 has been recently identified as several mutations in the gene that encodes for the calcium release channel (also known as the ryanodine receptor) on the myocardial sarcoplasmic reticulum. Ryanodine receptor dysfunction has been identified in boxer dogs with ARVC.
Pathophysiology of Arrhythmocenic Right Ventricular Cardiomyopathy
In cats, arrhythmogenic right ventricular cardiomyopathy most commonly (8 of 12 cats in the one study reported to date) produces right heart failure, presumably through the destruction of right ventricular myocardium resulting in right ventricular systolic and, possibly, diastolic dysfunction along with secondary tricuspid regurgitation. The changes in the right ventricular free wall also commonly produce ventricular tachyarrhythmias (9 of 12 cats) and supraven-tricular tachyarrhythmias (5 of 12 cats). Tumor necrosis factor (TNF) is commonly increased in cats with right heart failure, which may contribute to the systemic effects of the disease.
Pathology of Arrhythmocenic Right Ventricular Cardiomyopathy
The right ventricular and atrial chambers are markedly enlarged in cats that die of the disease.IH Thinning of the right ventricular free wall is a consistent feature of the disease and may be focal or diffuse. Aneurysms of the wall may occur, especially at the apex of the right ventricle (RV). The wall is often so thin that light can be seen through it. Histopathologically either fibre-fatty or fatty replacement of myocardium exists. Inflammatory cells are commonly present, especially in regions of fibrofatty replacement. Although most prominent in the right ventricular free wall, these changes are also commonly present in the left ventricular and occasionally identified in the left atrium. Apoptosis is common.
Evidence of right heart failure, including ascites, pleural and pericardia! effusions, and jugular vein distension, is common. The pleural effusion may be severe enough to cause tachypnea and dyspnea. A heart murmur secondary to tricuspid regurgi-tation is common. Arrhythmias are also commonly heard on auscultation, and ECG evidence of a ventricular arrhythmia may be one clue that one is dealing with arrhythmogenic right ventricular cardiomyopathy and not tricuspid valve dysplasia. Supraventricular tachyarrhythmias, including atrial fibrillation, may also occur. In the severe stage, echocardiography reveals marked enlargement of the right ventricular and right atrial chambers. The right ventricular chamber enlargement may be segmental. Tricuspid regurgitation is usually present on color flow Doppler. Careful echocardiographic interrogation may reveal localized regions of right ventricular wall thinning or regions of aneurysmal dilation. The right ventricular trabeculae may appear abnormal, especially at the apex. The disease may not be confined to the right heart, which means the left atrial and ventricular chambers may also be enlarged. Syncope due to ventricular tachycardia may be present.
Cats with severe arrhythmogenic right ventricular cardiomyopathy are commonly misdiagnosed as having tricuspid valve dysplasia because tricuspid regurgitation is a common sequel to the disease. Cats from 1 to 20 years of age have been diagnosed with the disease.
Therapy of Arrhythmocenic Right Ventricular Cardiomyopathy
Right heart failure is treated with furosemide and an angiotensin-converting enzyme inhibitor. Digoxin combined with either diltiazem or atenolol may be used to control supraventricular tachycardia or the ventricular rate in cats with atrial fibrillation. Malignant ventricular tachycardia or ventricular tachycardia that causes clinical signs (eg., syncope) can be managed acutely with lidocaine (5 to 20 µg/lb/min) and chronically with sotalol (1 to 2 mg/lb every 12 hours orally).