Boxer cardiomyopathy
Encyclopedia
Boxer cardiomyopathy is a disease of the myocardium primarily affecting Boxer dogs. It is characterized by the development of ventricular tachyarrhythmias, resulting in syncope
and sudden cardiac death
. Myocardial failure and congestive heart failure are uncommon manifestations of the disease.
and ventricular fibrillation
. Affected dogs are at risk of syncope and sudden cardiac death.
The concealed form is characterized by an asymptomatic dog with ventricular premature contractions (VPCs).
The overt form is characterized by ventricular tachyarrhythmias and syncope
. Dogs with overt disease may also have episodic weakness and exercise intolerance, but syncope is the predominant manifestation.
The third form, which is recognized much less frequently, is characterized by myocardial systolic dysfunction. This may result in left-sided, right-sided, or bi-ventricular congestive heart failure
. It is not known if this form represents a separate clinical entity, or whether it is part of the continuum of disease.
. This includes asymptomatic dogs, meaning that sudden death may be the first sign of disease.
Sudden cardiac death is usually caused by the degeneration of ventricular tachycardia to ventricular fibrillation. Unless terminated promptly by defibrillation
, death usually occurs within minutes.
, and to help direct therapeutic interventions.
Physical examination
The physical examination is often unremarkable, although an arrhythmia characterized by premature beats may be detected.
Electrocardiogram:
An ECG often shows ventricular premature complexes (VPCs). These typically have an upright morphology on lead II (left bundle branch morphology). This occurs as the ectopic impulses usually arise in the right ventricle. In some case, the ECG may be normal. This is due to the intermittent nature of ventricular arrhythmias, and means that the diagnosis should not be excluded on the basis of a normal ECG.
Holter monitor:
A Holter monitor
allows for 24-hour ambulatory ECG monitoring. It facilitates quantification of the frequency and severity of ventricular ectopy, and is important in the management of affected dogs.
Echocardiogram:
Echocardiography
is recommended to determine if structural heart disease is present. A small percentage of dogs have evidence of myocardial systolic dysfunction, and this may affect the long-term prognosis.
is probably the most commonly used antiarrhythmic at this time. It has been demonstrated that sotalol alone, or a combination of mexiletine and atenolol, results in a reduction in the frequency and complexity of ventricular ectopy. It is likely that these medications also reduce syncopal episodes. However, the efficacy of any form of medical therapy in preventing sudden cardiac death is not known. Unfortunately, in some cases oral antiarrhythmic medications may be proarrhythmic, and can exacerbate the arrhythmia and syncopal episodes.
The ideal therapy for Boxer cardiomyopathy would be implantation of an implantable cardioverter-defibrillator
(ICD). This has been attempted in a limited number of dogs. Unfortunately, ICDs are programmed for humans and the algorithms used are not appropriate for dogs, increasing the risk of inappropriate shocks. In the future, reprogramming of ICDs may allow them to emerge as a viable option in the treatment for Boxer cardiomyopathy.
Syncope (medicine)
Syncope , the medical term for fainting, is precisely defined as a transient loss of consciousness and postural tone characterized by rapid onset, short duration, and spontaneous recovery due to global cerebral hypoperfusion that most often results from hypotension.Many forms of syncope are...
and sudden cardiac death
Sudden Cardiac Death
Sudden cardiac death is natural death from cardiac causes, heralded by abrupt loss of consciousness within one hour of the onset of acute symptoms. Other forms of sudden death may be noncardiac in origin...
. Myocardial failure and congestive heart failure are uncommon manifestations of the disease.
Overview
Boxer cardiomyopathy shares striking similarities to a human myocardial disease called arrhythmogenic right ventricular cardiomyopathy (ARVC). On histopathology, the disease is characterized by the progressive replacement of ventricular myocardium (primarily right ventricular myocardium) with fatty or fibro-fatty tissue. Clinically, the disease is characterized by the development of ventricular tachyarrhythmias, including ventricular tachycardiaVentricular tachycardia
Ventricular tachycardia is a tachycardia, or fast heart rhythm, that originates in one of the ventricles of the heart...
and ventricular fibrillation
Ventricular fibrillation
Ventricular fibrillation is a condition in which there is uncoordinated contraction of the cardiac muscle of the ventricles in the heart, making them quiver rather than contract properly. Ventricular fibrillation is a medical emergency and most commonly identified arrythmia in cardiac arrest...
. Affected dogs are at risk of syncope and sudden cardiac death.
Cause
Boxer cardiomyopathy is a genetic disease inherited in an autosomal dominant pattern. The presentation in affected offspring is quite variable, suggesting incomplete penetrance. Recently, a group led by Kate Meurs at Washington State University has announced that they have identified one genetic anomaly associated with Boxer cardiomyopathy but there is still debate over the discovery.Clinical presentation
Boxer cardiomyopathy is an adult-onset disease with three distinct clinical presentations:The concealed form is characterized by an asymptomatic dog with ventricular premature contractions (VPCs).
The overt form is characterized by ventricular tachyarrhythmias and syncope
Syncope (medicine)
Syncope , the medical term for fainting, is precisely defined as a transient loss of consciousness and postural tone characterized by rapid onset, short duration, and spontaneous recovery due to global cerebral hypoperfusion that most often results from hypotension.Many forms of syncope are...
. Dogs with overt disease may also have episodic weakness and exercise intolerance, but syncope is the predominant manifestation.
The third form, which is recognized much less frequently, is characterized by myocardial systolic dysfunction. This may result in left-sided, right-sided, or bi-ventricular congestive heart failure
Congestive heart failure
Heart failure often called congestive heart failure is generally defined as the inability of the heart to supply sufficient blood flow to meet the needs of the body. Heart failure can cause a number of symptoms including shortness of breath, leg swelling, and exercise intolerance. The condition...
. It is not known if this form represents a separate clinical entity, or whether it is part of the continuum of disease.
Sudden cardiac death
All dogs with Boxer cardiomyopathy are at risk of sudden cardiac deathSudden Cardiac Death
Sudden cardiac death is natural death from cardiac causes, heralded by abrupt loss of consciousness within one hour of the onset of acute symptoms. Other forms of sudden death may be noncardiac in origin...
. This includes asymptomatic dogs, meaning that sudden death may be the first sign of disease.
Sudden cardiac death is usually caused by the degeneration of ventricular tachycardia to ventricular fibrillation. Unless terminated promptly by defibrillation
Defibrillation
Defibrillation is a common treatment for life-threatening cardiac arrhythmias, ventricular fibrillation and pulseless ventricular tachycardia. Defibrillation consists of delivering a therapeutic dose of electrical energy to the affected heart with a device called a defibrillator...
, death usually occurs within minutes.
Diagnosis
A genetic test for Boxer cardiomyopathy is now commercially available. The genetic test is not yet accepted as a definitive test and additional diagnostic testing continues to be essential to characterize the phenotypePhenotype
A phenotype is an organism's observable characteristics or traits: such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior...
, and to help direct therapeutic interventions.
Physical examination
The physical examination is often unremarkable, although an arrhythmia characterized by premature beats may be detected.
Electrocardiogram:
An ECG often shows ventricular premature complexes (VPCs). These typically have an upright morphology on lead II (left bundle branch morphology). This occurs as the ectopic impulses usually arise in the right ventricle. In some case, the ECG may be normal. This is due to the intermittent nature of ventricular arrhythmias, and means that the diagnosis should not be excluded on the basis of a normal ECG.
Holter monitor:
A Holter monitor
Holter monitor
In medicine, a Holter monitor is a portable device for continuously monitoring various electrical activity of the cardiovascular system for at least 24 hours...
allows for 24-hour ambulatory ECG monitoring. It facilitates quantification of the frequency and severity of ventricular ectopy, and is important in the management of affected dogs.
Echocardiogram:
Echocardiography
Echocardiography
An echocardiogram, often referred to in the medical community as a cardiac ECHO or simply an ECHO, is a sonogram of the heart . Also known as a cardiac ultrasound, it uses standard ultrasound techniques to image two-dimensional slices of the heart...
is recommended to determine if structural heart disease is present. A small percentage of dogs have evidence of myocardial systolic dysfunction, and this may affect the long-term prognosis.
Treatment
Current treatment options for Boxer cardiomyopathy are largely restricted to the use of oral anti-arrhythmic medications. The aim of therapy is to minimize ventricular ectopy, eliminate syncopal episodes, and prevent sudden cardiac death. A number of medications have been used for this purpose, including atenolol, procainamide, sotalol, mexiletine, and amiodarone. Combinations can also be used. SotalolSotalol
Sotalol is a drug used in individuals with rhythm disturbances of the heart, and to treat hypertension in some individuals. It is a non-selective competitive β-adrenergic receptor blocker that also exhibits Class III antiarrhythmic properties by its inhibition of potassium channels...
is probably the most commonly used antiarrhythmic at this time. It has been demonstrated that sotalol alone, or a combination of mexiletine and atenolol, results in a reduction in the frequency and complexity of ventricular ectopy. It is likely that these medications also reduce syncopal episodes. However, the efficacy of any form of medical therapy in preventing sudden cardiac death is not known. Unfortunately, in some cases oral antiarrhythmic medications may be proarrhythmic, and can exacerbate the arrhythmia and syncopal episodes.
The ideal therapy for Boxer cardiomyopathy would be implantation of an implantable cardioverter-defibrillator
Implantable cardioverter-defibrillator
An implantable cardioverter-defibrillator is a small battery-powered electrical impulse generator which is implanted in patients who are at risk of sudden cardiac death due to ventricular fibrillation and ventricular tachycardia. The device is programmed to detect cardiac arrhythmia and correct it...
(ICD). This has been attempted in a limited number of dogs. Unfortunately, ICDs are programmed for humans and the algorithms used are not appropriate for dogs, increasing the risk of inappropriate shocks. In the future, reprogramming of ICDs may allow them to emerge as a viable option in the treatment for Boxer cardiomyopathy.