13 May Subvalvular Aortic Stenosis in Dogs: Symptoms, Signs, and Diagnosis

Subvalvular Aortic Stenosis in dogs is a congenital heart defect that predominantly affects medium to large-breed dogs, such as the Golden Retriever, Boxer, and Rottweiler. In this condition, the aortic valve starts narrowing below the level of the valve leaflets. As a result, the heart has to work harder to pump blood, leading to potential health complications such as heart murmurs, arrhythmias, and, in severe cases, heart failure. Management of SAS in dogs involves careful monitoring, medication, and, in some cases, surgical intervention to ensure the well-being and quality of life of the affected canine companions. Let’s evaluate SAS in dogs in more detail.

Causes and Risk Factors of Subvalvular Aortic Stenosis in Dogs

1. Breed Predisposition: SAS is more prevalent in certain breeds, suggesting a genetic component. So, breeds commonly affected include Golden Retrievers, Newfoundlands, Rottweilers, Boxers, German Shepherds, Bull Terriers, and Bouvier des Flandres.
2. Hereditary Transmission: Studies indicate that SAS can be inherited in an autosomal dominant pattern with incomplete penetrance, meaning that not all dogs with the genetic predisposition will develop the condition. The specific genes involved are not fully understood. Still, breeding studies have shown a higher incidence in offspring from affected parents.
3. In Utero Development: SAS is believed to originate during fetal development, where abnormal growth of heart structures leads to the formation of fibrous or muscular tissue below the aortic valve. The exact cause of this abnormal development is not entirely known. Still, it is likely influenced by genetic and possibly environmental factors.
4. Epigenetic Influences: Environmental factors that affect gene expression without altering the DNA sequence may play a role. These include maternal health, nutrition, and other intrauterine conditions during pregnancy, although specific environmental triggers have not been conclusively identified.
5. Inflammatory and Fibrotic Processes: In some cases, inflammation and fibrosis within the heart can contribute to developing subvalvular obstructions. However, this is less common compared to congenital causes.

Clinical Signs and Symptoms of Subvalvular Aortic Stenosis in Dogs

1. Heart Murmur: Often detected during a routine veterinary examination. The murmur is usually a systolic ejection murmur heard best over the left heart base.
2. Exercise Intolerance: Dogs with SAS may tire quickly and show reluctance to exercise or play.
3. Syncope (Fainting): Fainting or collapsing episodes, especially during or after exercise, due to inadequate blood flow to the brain.
4. Breathing Difficulties: Signs of respiratory distress, such as rapid breathing or shortness of breath.
5. Coughing: Some dogs may develop a cough due to congestive heart failure or fluid accumulation in the lungs.
6. Weakness or Lethargy: Generalised weakness and lethargy, especially after physical exertion.
7. Cyanosis: Bluish discolouration of the gums and mucous membranes indicates poor oxygenation.
8. Sudden Death: In severe cases, sudden death can occur, often related to arrhythmias or heart failure.

Diagnosis of SAS in Dogs

1. Physical Examination: A veterinarian may detect a heart murmur during a routine physical exam. The murmur is typically a systolic ejection murmur that is best heard over the left heart base.
2. Echocardiography (Ultrasound of the Heart): This is the gold standard for diagnosing SAS. It allows visualisation of the subvalvular area to identify the narrowing (stenosis), measure the pressure gradient across the stenosis, and assess the overall heart function and structure. Doppler echocardiography can measure blood flow velocities, helping to determine the severity of the obstruction.
3. Chest Radiographs (X-rays): These can show the size and shape of the heart and any signs of pulmonary congestion or oedema, which might suggest heart failure secondary to SAS.
4. Electrocardiogram (ECG): An ECG can detect arrhythmias common in dogs with SAS. It can also show signs of left ventricular hypertrophy, resulting from the heart working harder to pump blood through the narrowed outflow tract.
5. Cardiac Catheterisation: Although less commonly used due to its invasiveness, cardiac catheterisation can provide detailed pressure measurements within the heart chambers and across the aortic outflow tract, offering precise information about the severity of the stenosis.
6. Blood Pressure Measurement: Systemic hypertension may occur in dogs with severe SAS. Blood pressure measurements can help assess the dog’s cardiovascular status overall.
7. Breed-Specific Screening: Some breeds are more predisposed to SAS, such as Boxers, Golden Retrievers, Newfoundlands, and Rottweilers. In predisposed breeds, early screening and monitoring can be crucial.
8. Genetic Testing: While not commonly performed as part of the routine diagnosis, genetic testing can help breed programs to identify carriers and reduce the incidence of SAS in future generations.

Treatment Options

• Beta-blockers: Drugs like atenolol or propranolol are commonly used to reduce the heart rate and myocardial oxygen demand. Thus, they help alleviate symptoms such as syncope and exercise intolerance.
• Calcium Channel Blockers: Verapamil may help decrease myocardial contractility and reduce left ventricular outflow obstruction.
• Diuretics: These medications may be prescribed to manage fluid buildup in the lungs or other symptoms of congestive heart failure.
• Antiarrhythmic Medications: If arrhythmias are present, medications such as sotalol or amiodarone may be used to control them.
• Balloon Valvuloplasty: In some cases, balloon valvuloplasty may be considered, particularly for dogs with discrete subaortic stenosis (DSS), a subtype of SAS characterised by a discrete fibrous ring below the aortic valve. 
• Subaortic Resection: Surgical removal of the fibrous tissue causing the obstruction may be considered in severe cases. However, this is less common due to associated risks and variable outcomes.
• Activity Restriction: To manage the condition, limit strenuous exercise and avoid triggers to reduce complications.
• Weight Control: A healthy body weight is essential to reduce the workload on the heart.
• Regular Veterinary Check-Ups: Regular monitoring by a veterinarian, including periodic echocardiograms, is necessary to assess disease progression and adjust treatment as needed.
• Genetic Counseling and Breeding Practices: Genetic testing and responsible breeding can reduce SAS in predisposed breeds like boxers and golden retrievers.
• Palliative Care: In cases where the condition is severe and refractory to medical management, palliative care focuses on maintaining the affected dog’s comfort and quality of life.
• Prognosis: The outlook for dogs with SAS can vary based on the seriousness of the stenosis, the presence of symptoms, and the response to treatment. So, some dogs may have a relatively positive outlook on medical care. In contrast, others may experience worsening of the condition and a shorter lifespan, mainly if severe complications arise.

The Last Word

Subvalvular Aortic Stenosis in dogs is a complex condition influenced by genetic, environmental, and developmental factors. Understanding these causes and risk factors is essential for early detection and effective management. With responsible breeding practices, vigilant monitoring, and appropriate treatment, dogs with SAS can lead fulfilling lives despite the challenges posed by this condition.