Dilated cardiomyopathy

What is ... ?: 

Cardiomyopathy refers to disease of the heart muscle tissue (the myocardium). In veterinary medicine, there are 3 main types of cardiomyopathy:

  1.      Dilated cardiomyopathy. This is by far the most common type in the dog, and is the subject of all the sections, below. With dilated cardiomyopathy, there is dilation of the chambers of the ventricles of the heart, caused by an inherent weakness in the muscle structure of the heart.  The heart itself becomes enlarged, bloated, and unable to contract adequately, severely hampering the circulation.
  2.      Hypertrophic cardiomyopathy. In this form of cardiomyopathy, there is a tremendous increase in the mass of the heart muscle tissue, with a resultant decrease in internal heart chamber size. While it is very common in the cat, hypertrophic cardiomyopathy is very rare in the dog.
  3.      Restrictive cardiomyopathy. In this form of cardiomyopathy, the heart appears normal but infiltration of stiff, inelastic tissue within the walls of the heart severely restricts the heart's ability to fill adequately.  Restrictive cardiomyopathy occurs with some frequency in cats but has never been identified in dogs.

With any type of cardiomyopathy, the heart and circulation are forced to work harder to maintain blood perfusion to the organs of the body.  If it is severe enough, cardiomyopathy may disrupt the circulation, causing fluid accumulation in the lungs and body cavities (congestive heart failure), or may give rise to erratic, unstable rhythms to the heartbeat (arrhythmias) that are potentially fatal.

How is ... inherited?: 

Dilated cardiomyopathy can be transmitted genetically from parent to offspring, as evidenced by the high prevalence in certain specific giant breeds of dogs (see below).  The exact mode of transmission is unknown, but a sex-linked mode may exist given the higher occurrence in males than females.

What does ... mean to your dog & you?: 

Dilated cardiomyopathy is a potentially severe, debilitating, but painless, disease of adult dogs (typically between 5 and 10 years of age).  Overall, it almost always worsens over time, beginning with an asymptomatic period where dogs (and their families/breeders) have no way of suspecting that it exists: there are no symptoms or detectable abnormalities.  This asymptomatic period, where the condition is referred to as "occult dilated cardiomyopathy" because it remains hidden, can last for years.  Indeed, one of the reasons dilated cardiomyopathy spreads so widely within breeds is that it may only be detected long after a dog's breeding career is underway and the condition has been passed on to subsequent generations.
Dilated cardiomyopathy usually is first suspected when symptoms emerge, including respiratory difficulty (even severe breathlessness/unrelenting gasping for air), loss of appetite, sudden decrease in willingness or ability to exercise, weakness, episodes of collapse, and/or a bloated, pot-bellied abdomen. Prior to the appearance of such symptoms, no medication has been convincingly shown to help prolong quality of life or lifespan.  Once these kinds of symptoms occur, however, medications (given at home, daily, as oral tablets or capsules) are indispensable for survival.  When Doberman dogs develop these symptoms, the outlook may be very serious: even with medication, approximately one-third may not survive for more than a few days; about one-third improve and feel well for several weeks before worsening despite medication and either beign euthanized or dying; and the remaining one-third do well for months to a year or more.

In addition to the symptoms outlined above, a distinctive feature of this disorder in Doberman pinschers and boxers is that abnormal heart rhythms may originate in the heart and can be quite serious.  These arrhythmias (atrial fibrillation, ventricular tachycardia) can worsen over time and may cause the heartbeat to be unstable.  This is perhaps the most suddenly devastating aspect of dilated cardiomyopathy: that it can cause immediate, collapse (like a person who has a heart attack) and that a dog may not survive.
When a dog is suspected of having dilated cardiomyopathy, the most important first step is confirmation, because this heart disorder can be so serious.  An echocardiogram (also called ultrasound of the heart or cardiac sonogram) should be performed by a board-certified veterinary cardiologist.  This painless, noninvasive procedure lets the cardiologist examine the heart in real time, and assess its structure and function.  The abnormality of dilated cardiomyopathy may be subtle, and there is much overlap between normal and early dilated cardiomyopathy.  For this reason, a skilled specialist should be consulted.
If dilated cardiomyopathy is confirmed, treatment may or may not be necessary depending on the degree/stage of the problem.  The outlook depends on findings obtained in diagnostic tests (see below).

How is ... diagnosed?: 

As stated above, the test of choice for confirming dilated cardiomyopathy is an echocardiogram: an ultrasound of the heart.  The echocardiogram cannot rely only on left ventricular fractional shortening for providing the diagnosis; it must include absolute left ventricular dimensions and, if there is any uncertainty (given the large area of overlap between normal athletic hearts and early dilated cardiomyopathy), it needs to include left ventricular volume calculations using Simpson's method of disks.  This is usually best accomplished by a veterinary cardiologist, and general practitioner veterinarians can refer their patients to these specialists (directories are available at www.acvim.org and www.ecvim-ca.org for North America and Europe, respectively).
Thoracic radiographs (X-rays of the chest) are appropriate in all suspected cases of dilated cardiomyopathy.  In affected dogs, an enlarged heart silhouette is a common abnormal finding, and there may be evidence of fluid retention within the lung tissue (pulmonary edema) or surrounding and collapsing the lungs (pleural effusion).
An electrocardiogram (ECG, EKG) may be performed if an irregular heartbeat, or arrhythmia, is detected by the veterinarian when listening with the stethoscope.

If dilated cardiomyopathy is suspected and results from all routine diagnostic tests are normal, a 24 hour ambulatory electrocardiogram (Holter monitor) is often recommended. The unobtrusive monitor is worn by the dog during normal daily activities, and it records irregular heart rhythms.
When dilated cardiomyopathy is confirmed in a dog, routine blood and urine tests (complete blood count, serum biochemistry profile, urinalysis) are run prior to beginning medications, in order to identify any other problems that could interfere with treatment, and to establish a baseline for future comparison when monitoring the effects of medications.

How is ... treated?: 

Treatment for dilated cardiomyopathy consists of medications given one or more times a day.  Initially, if severe symptoms are observed, the medications may need to be given in teh hospital for the first 1-3 days, where some of the medications can be given by injection.  Ultimately, home treatment consists of giving the medications orally, avoiding strenuous exercise, avoiding salty foods or treats, and monitoring for any return of symptoms that woud prompt a recheck visit and adjustment of medications.  Commonly-used medications include a diuretic such as furosemide (Lasix), spironolactone, or torsemide (Demadex); a type of medication that vasodilates, or decreases the resistance to outflow of blood from the heart, thereby easing the heart's workload (ACE inhibitors such as enalapril [Enacard], benazepril [Fortekor], ramipril [Vasotop], or imidapril [Prilium]); digitalis (digoxin, Lanoxin); and inodilators such as pimobendan (Vetmedin).  It is common for dogs to need to be on all 4 of these classes of medications, which demonstrates that close attention and medication administration at home is essential, and that treatment may become costly, especially in the largest breeds (since the amount of medication given is based on body weight).  Dogs that respond well to medications usually do so within days or 1-2 weeks of starting treatment, and show an often dramatic improvement in their energy level, appetite and overall demeanour.
Decisions about initiating (and later, adjusting) treatment are based on several factors: whether the dog is showing symptoms such as weakness or collapse, whether arrhythmias are seen on the electrocardiogram, and whether there is fluid retention in the tissues due to disruption of the circulation. There is no surgery to correct dilated cardiomyopathy in dogs.

For the veterinarian: 

 

  1.      RADIOGRAPHS (CHEST X-RAYS): may show generalized cardiomegaly with evidence of left atrial and ventricular enlargement predominating.  In Dobermans, only left atrial enlargement may be evident.
  2.      ELECTROCARDIOGRAM: atrial fibrillation is seen in 75 to 80 per cent of giant-breed dogs with dilated cardiomyopathy. Premature ventricular complexes and/or ventricular tachycardia also are very common.  There may be subtle changes such as tall R waves or wide QRS complexes (both consistent with left ventricular enlargement), or wide P waves (suggesting left atrial enlargement).
  3.      ECHOCARDIOGRAM: diagnostic test of choice.  Left ventricular dilation is the earliest change (increase in left ventricular volume as measured by Simpson's method of discs; increase in absolute left ventricular diastolic diameter and E-point-to-septal separation as measured on M-mode).  Reduced left ventricular contractility is observed last (=least reliable) and there is a great deal of overlap between LV fractional shortening in normal athletic dogs and early dilated cardiomyopathy.  Left atrial enlargement and right-sided chamber enlargement also are common.
  4.      PHYSICAL EXAM: occasional to frequent premature beats, pulse deficits, paroxysmal tachyarrhythmias or a totally irregular ventricular rhythm, variability in femoral pulse strength.
Breeding advice: 

Affected individuals and their parents should not be used for breeding. Siblings should only be used after careful screening.
 

How can cardiomyopathy be controlled?
There are promising ways to approach the control of this disease. Although signs of heart failure are often not evident until middle or older age, abnormalities on the electrocardiogram are often apparent earlier. In affected breeds with a family history of cardiomyopathy and in ALL Doberman pinschers, breeding animals should be evaluated yearly for evidence of cardiac arryhthmias, using echocardiography with left ventricular volume measurement whenever possible, or an ambulatory (Holter) monitor otherwise.  Asymptomatic dogs in which dilated cardiomyopathy has been identified through this type of screening (i.e., "occult dilated cardiomyopathy") should not be used for breeding.
 
FOR MORE INFORMATION ABOUT THIS DISORDER, PLEASE SEE YOUR VETERINARIAN.
 

Resources: 

Prosek R.  Dilated cardiomyopathy.  In Cote E, ed.  Clinical Veterinary Advisor: Dogs and Cats, 2nd ed (St. Louis, MO: Mosby Elsevier, 2011) pp. 309-312.
Meurs KM. Myocardial disease: canine.  In Ettinger SJ, Feldman EC, eds.  Textbook of Veterinary Internal Medicine, 7th ed (St. Louis, MO: Saunders Elsevier, 2010) pp. 1320-1328.


What breeds are affected by ... ?
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