Sick sinus syndrome is a cardiac condition of unknown cause that affects the heart rate and rhythm of both humans and dogs. In this disease, the heart's electrical impulse-generating sites (called sinuses) fail to function normally. As a result, dogs with this problem will be unable to maintain normal heart rates and many also suffer other changes in heart rhythm.
This disease will vary in severity among dogs, with some suffering catastrophic failure to pump blood (when the heart rhythm is severely disturbed) while others can live perfectly normal lives.
For more severely affected dogs the syndrome can be divided into two different classes of patients based on their abnormal heart rhythms: those for whom the rhythm varies from slow to fast ("bradycardia-tachycardia") and those in which the rhythm is characterized by slowness and occasional stopping ("bradycardia and sinus arrest").
Symptoms and Identification
A low heart rate (or another abnormal rhythm) is the most common finding, but some dogs will be diagnosed only after they've fainted (known as a "syncopal episode") or had a seizure. Episodic weakness is also commonly associated with sick sinus syndrome. Sudden death is always a risk with these patients.
Diagnosing the disease is typically achieved via EKG (electrocardiogram) and/or a Holter monitor (an EKG the dog wears for continuous monitoring of the heart rhythm), though an echocardiogram (heart ultrasound) is a necessary step to rule out underlying or concurrent cardiac disease. A trial with a drug that speeds up the heart (atropine) may also be employed by way of testing the abnormal heart's rhythm.
For all these cases, a veterinary cardiologist is the go-to veterinarian when it comes to diagnosing and treating this disease.
Sick sinus syndrome has been identified in older female Miniature Schnauzers, Dachshunds, Cocker Spaniels, West Highland White Terriers and Pugs, though other breeds have occasionally been diagnosed with it as well.
Dogs who suffer symptoms of sick sinus syndrome are candidates for treatment. Others may not necessitate any treatment whatsoever. Drug therapy with "anticholinergics" (like atropine) may be employed for mildly affected patients, but implanting a pacemaker is considered best for all symptomatic dogs. A pacemaker ensures that dogs suffering significant heart rhythm abnormalities (in which little or no blood can be pumped) will have their hearts electrically "reset" by the device.
The cost of diagnosis and pacemaker placement for sick sinus syndrome patients varies according to the facility and geographic location, but $5,000 to $15,000 is a fairly typical expenditure for affected dogs.
Those for whom this great expense is not assumable should know that some medical therapies are available, though they are by no means as reliable as the pacemaker approach and dogs may suffer significant side effects from their use. Typical costs run from $30-$100 a month for these drugs when used in small breed dogs. Pacemaker patients may also be best served by instituting these drug therapies, thereby increasing the long-term cost of the condition.
The mode of inheritance is not known, nor are any genetic screening tests available. Affected dogs and their parents and siblings should not be bred. This is the only known means of prevention at the current time. Veterinarians are advised to be on the lookout for slow heart rates among affected breeds to forestall fainting episodes and initiate therapy early in most severely affected patients.
Zipes DP. Specific arrhythmias: Diagnosis and treatment. In: Braunwald E, ed. Heart Disease: A Textbook of Cardiovascular Medicine. 4th ed. Philadelphia: WB Saunders, 1992:677-679.
Rishniw M, Thomas WP. Bradyarrhythmias. In: Kirk RW, ed. Current Veterinary Therapy XIII. Philadelphia: WB Saunders, 2000:719-725.
Bolton GR, Bradycardia. In: Kirk RW, ed. Current Veterinary Therapy VII. Philadelphia: WB Saunders, 1980:376-381.
Tilley LP, ed. Essentials of Canine and Feline Electrocardiography: Interpretation and Treatment. 3rd ed. Malvern: Lea and Febiger, 1992:184-187.
Edwards NJ, ed. Bolton's Handbook of Canine and Feline Electrocardiography. 2nd ed. Philadelphia: WB Saunders, 1987: 66-151.