Subaortic stenosis

What is ... ?: 

Subaortic stenosis is a common, potentially devastating birth defect of the heart in dogs.  It consists of excessive tissue that partially blocks the path for outflow of blood from the heart to the circulation, creating a partial obstruction.  The result is an increase in strain on the main pumping chamber of the heart, the left ventricle, as the heart works harder to maintain a normal circulation.  In severe cases, subaortic stenosis can be fatal, but in mild cases it produces no problems (no symptoms, no need for treatment) in the affected dog.  Subaortic stenosis is almost always inherited, so all affected dogs, whether mild or severe, should be removed from the breeding pool.

How is ... inherited?: 

In virtually all cases in dogs, subaortic stenosis is inherited from the sire, the dam, or both.  The degree of severity is highly variable, and a mildly affected individual may produce offspring with severe subaortic stenosis, or no subaortic stenosis at all (silent carriers).  In Newfoundlands, subaortic stenosis has an autosomal dominant mode of inheritance, with variable expression. This means that if one parent has it, most of the offsrping will have it, but to a degree of severity that reaches across the spectrum, from undetectable to life-threatening. 
The two major challenges for breeders and veterinarians are: 1) to identify affected dogs and exclude them from the breeding pool, which can be difficult with very mild subaortic stenosis (overlap with normal, athletic hearts); and 2) to avoid overreacting and culling too many individuals (the "if there's a heart murmur, don't breed" mentality), which leads to excessive elimination of good breeding stock, a smaller gene pool, and inevitably, the emergence of other inbreeding-related problems later on.  Therefore, an accurate confirmation of subaortic stenosis, and judicious breeding decisions, are essential.

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

The degree of concern and of impact on an individual dog depends on how much of an obstruction the subaortic stenosis is causing; i.e., on the degree of severity of the obstruction.  Dogs with mild subaortic stenosis will generally experience no adverse health effects and have a normal life expectancy. With moderate to severe subaortic stenosis, symptoms and effects on health are variable. In general, some degree of exercise restriction in these cases is wise: at higher heart rates, the extra workload imposed on the heart is poorly tolerated and life-threatening complications can occur.  Therefore, dogs with moderate or severe subaortic stenosis should avoid "adrenaline rushes" (sympathetic surges) such as sprinting or jumping.  Rather, controlled, steady walks (on-leash) are much safer for the heart and still allow a dog to be outdoors and exercising.  Reduced exercise tolerance -when a dog just seems to run out of steam- or fainting, due to inadequate blood supply to the brain, are signs of excessive physical activity that is outstripping the heart's capacity; physical activity should never reach a point where it causes such symptoms because a dog may collapse and not survive this degree of strain on the heart.

In response to the obstruction to blood flow caused by subaortic stenosis, the heart muscle tissue becomes thicker over time (left ventricular hypertrophy), crowding out the space in the heart available for blood. Over time, dogs with severe subaortic stenosis can have serious limitations in heart function, and symptoms such as difficulty in breathing, fits of uncomfortable coughing, and/or poor growth (in puppies) are possible. Any of these symptoms warrants a visit to the veterinarian, as the problem may be life-threatening and medications may need to start to be given immediately.  At its worst, subaortic stenosis can cause changes in the heart muscle tissue that trigger an erratic, unstable heartbeat (cardiac arrythmias), which can be fatal.

How is ... diagnosed?: 

Subaortic stenosis is almost always first suspected based on a heart murmur detected by the veterinarian.  This presents a dilemma, because many other reasons exist for heart murmurs, many of which are harmless.  Subaortic stenosis is a common cause of heart murmurs, but many dogs with normal hearts and no subaortic stenosis have heart murmurs as well.  The dilemma cannot be resolved just by listening to the heart: even the most experienced veterinary cardiologists cannot confirm or eliminate subaortic stenosis just from listening to the heart.  Therefore, tests are necessary to identify subaortic stenosis.  The diagnostic test of choice is an echocardiogram, also called cardiac ultrasound or sonogram of the heart.  The severity of subaortic stenosis depends on the degree of narrowing it is causing; this is assessed by measuring the pressure gradient across the aortic valve (between the left ventricle and the aorta) using Doppler ultrasound.  Such testing is best performed by a veterinary cardiologist, and general practitioner veterianrians can refer their patients to one of these specialists for a confirmatory opinion (directories are available at www.acvim.org and www.ecvim-ca.org for veterinary cardiologists in North America and Europe, respectively).

It is important to realize that a grey zone exists between being clearly normal and being clearly affected with subaortic stenosis.  Occasionally, dogs can have features that are neither convincing for subaortic stenosis nor convincingly normal, even on cardiac ultrasound.  In these "indeterminate status" dogs, where subaortic stenosis is neither confirmed nor definitively eliminated, a moratorium may be imposed on breeding, but the dog should not be spayed or castrated.  Rather, a follow-up examination several months or a year later may reveal a result that is clear.

Finally, dogs that are affected to a degree that they show overt symptoms, such as laboured breathing or collapse/fainting, generally require an electrocardiogram (EKG/ECG), a blood test (complete blood count, serum biochemistry profile), and thoracic radiographs (chest X-rays) to stage the problem and make decisions regarding starting medications.

How is ... treated?: 

In dogs with mild subaortic stenosis, there is no treatment required. The dog should not be used for breeding, and littermates should be carefully screened.

With moderate to severe stenosis, the dog's exercise should be restricted: vigorous running and jumping are examples of activities that put a great deal of strain on the heart and could be permanently damaging; therefore, such activities shold be stopped or minimized, seeking the best balance between safety for the heart and enjoyment of controlled, mild to moderate physical activity. Beta-blocking medications such as atenolol, metoprolol, or carvedilol are often warranted with moderate or severe subaortic stenosis.  These medications are given every day and they have a protective effect: they minimize the strain on hypertrophied cardiac tissue.
In dogs showing overt symptoms due to fluid retention in the lungs, treatment may include diuretics (such as furosemide) and ACE inhibitors (such as enalapril, benazepril, ramipril, imidapril, or lisinopril).  These medications are given by mouth every day at home and help to alleviate the symptoms.
Various surgeries have been attempted to alleviate the obstruction of subaortic stenosis.  These are never required in cases of mild subaortic stenosis, but may be useful for moderate or severe cases.  The most promising at present is cutting-balloon valvuloplasty, a minimally-invasive (catheter-based) procedure that partially opens the narrowing caused by subaortic stenosis.  Surgeries of this type are available in limited numbers of secialized veterinary facilities, and general practitioner veterinarians can arrange a referral to a specialty institution for this purpose.

For the veterinarian: 
  1.    MURMUR: systolic, left hemithorax commonly radiating to the right (due to aortic root malposition) and possibly into thoracic inlet and up the neck; PMI left heart base (3rd to 4th intercostal space), may be equally loud or louder at right heart base.
  2.    ECG: often normal in mild or moderate cases; over time, ECG may show evidence of left ventricular enlargement (e.g., tall R waves in lead II), possibly evidence of myocardial hypoxia via ST segment depression or elevation, possibly ventricular arrhythmias.  Overall, these changes are highly variable from one case to the next, and ECG is a poor screening test for subaortic stenosis.
  3.    RADIOGRAPHS:  in severe cases, radiographs may show evidence of left ventricular enlargement, cranial aortic enlargement, left axis shift. Pulmonary vasculature is normal. Overall, these changes are highly variable, and radiographs are a poor screening test for subaortic stenosis.
  4.    ECHOCARDIOGRAPHY: diagnostic test of choice.  Results include an increased left ventricular outflow tract velocity and left ventricular : aortic pressure gradient.  Left ventricular hypertrophy, a subvalvular fibrous ring, and poststenotic dilation of the ascending aorta are possible in some cases.
  5.    OTHER: On physical exam in dogs with severe subaortic stenosis, the arterial pulse may be of reduced intensity and slow to rise ("pulsus parvus et tardus") and the systolic murmur may radiate such that it is audible on the dorsal surface of the skull. 
Breeding advice: 

Affected individuals should not be used for breeding, and littermates should be carefully screened.

FOR MORE INFORMATION ABOUT THIS DISORDER, PLEASE SEE YOUR VETERINARIAN.
 

Resources: 

Oyama MA, Sisson DD, Thomas WP, Bonagura JD.  Congenital heart disease.  In Ettinger SJ, Feldman EC, eds.  Textbook of Veterinary Internal Medicine, 7th ed (St. Louis, MO: Saunders Elsevier, 2010) pp. 1250-1298.
Belanger M-C.  Subaortic stenosis.  In Cote E, ed.  Clinical Veterinary Advisor: Dogs and Cats, 2nd ed (St. Louis, MO: Mosby Elsevier, 2011) pp. 1057-1059.


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