Boxer Cardiomyopathy

1. Some General Statements:

Boxer cardiomyopathy is a disorder characterized primarily by ventricular arrhythmias, syncope, and sudden death. LV systolic dysfunction and CHF, the typical characteristics of canine dilated cardiomyopathy, are relatively rare in Boxer cardiomyopathy. Boxer cardiomyopathy closely resembles an arrhythmic disorder in humans called Arrhythmogenic Right Ventricular Cardiomyopathy (ARVC), and as such, the name ARVC is now used to refer to the arrhythmic cardiomyopathy of Boxers. (Boxers in the UK appear to be much more commonly affected with the DCM form of the disease).

2. Etiology of Boxer cardiomyopathy (ARVC):

ARVC is a familial disease in Boxers that is inherited as an autosomal dominant trait with variable penetrance.
An 8 base pair deletion on chromosome 17 located within the striatin gene was found to be associated with affected Boxers and absent in normal Boxers and normal other dogs (Meurs et al. JVIM 2009;23:687-688).
A disorder of carnitine metabolism is involved in a few cases of the relatively uncommon form of Boxer cardiomyopathy with LV failure and CHF.

3. Signalment:

• Age range = 1 – 15 years
• Most cases (60%) are 6 to 10 years of age

4. History:

Based on clinical signs, three categories can be defined:

1. Dogs with no clinical signs of heart disease.
2. Dogs with syncope or weakness.
3. Dogs with signs of left-sided congestive heart failure.

5. Clinical Signs:

• May be free of clinical signs
• May have syncope or episodic weakness
• aA small percentage (<10%) have cough and/or dyspnea

6. Physical Examination:

• Many have a normal physical exam
• Cardiac arrhythmias may be ausculted
• Pulse deficits may be present with dysrhythmias
• Very few have a murmur of mitral valve insufficiency

7. ECG:

The ECG provides the most consistent findings.

1. Ventricular premature beats are common, and may occur

• as single beats
• as pairs or triplets
• in runs of ventricular tachycardia

Most ventricular premature beats are of right ventricular origin (QRS is upright in lead II).
Sometimes the ventricular premature beats are multiform.
While the exact number of VPCs above which a dog should be considered affected is unknown, the observation of >100 VPC’s/24 hours is highly suspicious, and periods of couplets, triplets, or runs of ventricular tachycardia are generally considered abnormal.

Note: Since the arrhythmias are intermittent, a normal routine (i.e. 2-minute) ECG does NOT rule out the disease, thus it can be a very poor screening test. A 24-hour Holter monitor is much more useful for screening, and is also recommended to evaluate the quantity and complexity of arrhythmia in a Boxer that does exhibit ventricular arrhythmias on routine ECG.

2. Supraventricular arrhythmias are much less common, and are typically associated with the uncommon myocardial dysfunction form of the disease:

• Supraventricular premature beats
• Aupraventricular tachycardia
• Atrial fibrillation

8. Radiography:

• Normal findings are most common
• General cardiomegaly with mild to moderate left atrial enlargement, pulmonary venous congestion, and pulmonary edema are seen in the uncommon form with LV systolic dysfunction

9. Echocardiography:

• The majority of affected Boxers have a normal echocardiogram
• A small percentage (~7%) have evidence of left ventricular dilation and hypokinesis (systolic dysfunction)

10. Therapy:
11. Cases free of clinical signs:

The criteria for initiating therapy in asymptomatic dogs remain ill-defined, and depend not only on the absolute number of VPCs observed but also on the complexity of the arrhythmia. Antiarrhythmic therapy is generally indicated if:Frequent premature beats are present (certainly if >1000 VPC’s/24 hours on a Holter monitor, and potentially if >500/24 hours)There are runs of ventricular tachycardiaThe VPCs exhibit a very short coupling interval (R-on-T phenomenon)

Based on work by Dr. K. Meurs et al, the most efficacious antiarrhythmics for Boxers with ARVC are:

1. Sotalol: 1.5-3.5 mg/kg BID (PO)
2. Combination of mexiletine and atenolol
   mexiletine: 5-8 mg/kg TID (PO)
   atenolol: 0.3-0.6 mg/kg or 12.5 mg/DOG BID (PO)

2. Syncopal/weakness cases: secondary to ventricular dysrhythmias but unassociated with signs of heart failure.

• Antidysrhythmic therapy can be instituted as described per asymptomatic cases
• In acute situations, procainamide or lidocaine may be used:
• Procainamide 6-8 mg/kg (IV) over 5 min, then 25 to 30 ug/kg/min CRI for 3 hours
• Lidocaine 2 mg/kg IV bolus, then 25 to 75 ug/kg/min

3. Cases with congestive heart failure and cardiac dysrhythmias:

• Management of heart failure as discussed in General Therapeutic Concepts
  • Should include supplementation with carnitine 50 mg/kg BID-TID (PO)
• Management of cardiac dysrhythmias.
  1. Supraventricular dysrhythmias including atrial fibrillation are managed as per Electrocardiology section
  2. Ventricular dysrhythmias are managed as per above
• Prognosis:
• Asymptomatic dogs may live for several years without clinical signs
• Cardiac dysrhythmias with syncope but without heart failure:
• With antiarrhythmic therapy syncope can usually be dramatically reduced and most dogs survive for several to many years
• If they develop CHF their prognosis falls dramatically
• Congestive heart failure with cardiac dysrhythmias
• Usually fail to survive for more than 6 months