Sick sinus syndrome
Sick sinus syndrome (also called sinus node dysfunction or bradycardia-tachycardia syndrome) is a disturbance of the normal rhythm of the heart in adult/older dogs. Normally, a dog's heart will beat at a rate as low as 40 beats/minute during sleep and as high as 280 beats/minute during intense exercise. In sick sinus syndrome, the heart's natural ability to beat is compromised. Some dogs suddenly have a heart rate as slow as 10 beats/minute, even during physical exercise; this causes a drastic drop in circulation, and then they recover for hours or days before having another sudden drop in heart rate. Other dogs have the opposite: a heart rate that suddenly is very fast, causing a pounding heartbeat and also compromising the circulation. Overall, then, sick sinus syndrome is a disorder that causes an erratic change in the heartbeat, and that begins mildly and then develops over time into a potentially life-threatening problem. The hallmark symptom of sick sinus syndrome is sudden loss of consciousness (fainting), also called syncope.
Sick sinus syndrome occurs much more often in specific breeds: miniature Schnauzers (females more than males), cocker spaniels, West Highland white terriers, and dachshunds. However, it can occur in any breed of dog. The exact genetic mechanism/mode of transmission is not known.
Sick sinus syndrome is a serious, potentially life-threatening disorder. That said, it does not affect the strength of the heart muscle tissue, and it does not cause pain. In other words, if the rhythm of the heartbeat can be stabilized, the outlook for dogs with sick sinus syndrome is very good. In dogs, as in people, the heart rhythm is stabilized through surgical implantation of a pacemaker. The vast majority of dogs who have sick sinus syndrome and receive a pacemaker live normal lives; the vast majority of dogs who have sick sinus syndrome but do not receive a pacemaker develop increasingly frequent and severe episodes of collapse and loss of consciousness, eventually proving fatal.
The first indication of this disorder may be that a veterinarian finds that an affected dog has an unusually slow heart rate, which is not increased by exercise. These asymptomatic dogs tend to have earlier, milder forms of sick sinus syndrome and may not develop life-threatening problems as quickly.
In other cases, the overt symptom of collapse and loss of consciousness may be the first clue that leads to finding sick sinus syndrome. These symptomatic dogs tend to require treatment quickly to avoid fatal collapse in the subsequent days to months.
Either way, an electrocardiogram (ECG, EKG) is necessary for observing the rhythm of the heartbeat and confirming that sick sinus syndrome is or is not present. Occasionally, an in-office ECG at the veterinary hospital is normal, and yet the intermittent loss of consicousness is still highly suggestive of sick sinus syndrome. If this is the case, the veterinarian may recommend a portable ECG monitor (Holter monitor or event monitor) that a dog can wear, and which can record the ECG under natural circumstances.
Confirming (or eliminating) sick sinus syndrome through definitive ECG recording is important in order to follow the correct treatment and give an accurate assessment of future outlook.
When there are no overt symptoms (and sick sinus syndrome is only identified at a check-up, for example), treatment generally is not necessary. Two exceptions are if general anesthesia is planned (because sick sinus syndrome decreases cardiac reserves that are essential for safe anesthesia) or if the veterinarian identifies very, very slow heart rates. Treatment should be considered in such cases, and in all cases where there are marked ECG changes and/or significant symptoms such as frequent fainting. Certain medications may temporarily improve the heart's rhythm, but virtually all dogs with sick sinus syndrome ultimately require surgical implantation of a permanent pacemaker. The prognosis (outlook for a good life) with a pacemaker is very good.
- ELECTROCARDIOGRAM (ECG): common findings include any combination of severe and persistant sinus bradycardia, asystole, supraventricular premature complexes or tachycardia, bradycardia-tachycardia syndrome (periods of severe sinus bradycardia alternating with ectopic supraventricular tachycardias). Often, syncopal patients 1) have syncopal episodes that may not fit the description of classic syncope (e.g., the signs may overlap somewhat with seizures) and 2) may have perfectly normal ECG findings in the hospital (and no syncope in the hospital). The finding of a normal in-hospital ECG is inconclusive in such cases. For these reasons, portable ECG monitoring, such as Holter monitors or owner-activated event monitors, is extremely helpful for ruling in or ruling out sick sinus syndrome. Portable ECG monitoring is much safer than proceeding with a neurologic workup (cerebrospinal fluid tap, brain imaging) in ambiguous cases because general anesthesia is required for these neurologic procedures, and it may be catastrophic for an undiagnosed sick sinus syndrome patient.
- Classically, neither exercise nor intravenous atropine were said to elevate the heart rate significantly. We now know that some dogs with sick sinus syndrome do respond to exercise or atropine, although such a response is not reliably predictive of successful response to oral (anticholinergic) medications.
Even though the specific mode of inheritance is not known, the occurrence of sick sinus syndrome in a breeding dog should be a discouraging factor with regards to future breeding.
FOR MORE INFORMATION ABOUT THIS DISORDER, PLEASE SEE YOUR VETERINARIAN.
Cote E. Electrocardiography and cardiac arrhythmias. In Ettinger SJ, Feldman EC, eds. Textbook of Veterinary Internal Medicine, 7th ed (St. Louis, MO: Saunders Elsevier, 2010) pp. 1159-1187.
Bulmer BJ. Sick sinus syndrome. In Cote E, ed. Clinical Veterinary Advisor: Dogs and Cats, 2nd ed (St. Louis, MO: Mosby Elsevier, 2011) pp. 1022-1024.
- Disorder Type: