Sick Sinus Syndrome
Sick Sinus Syndrome (SSS) is not a specific disease but rather a group of clinical signs which point to an issue with the sinus node, the Sinoatrial (SA) node specifically. The SA node is a small body of specialized muscle tissue in the wall of the right atrium of the heart that acts as a pacemaker by producing a contractile signal at regular intervals. Normally, the sinus node produces a regular, steady pattern of signals.
Sick sinus syndrome is characterized by a heart rhythm whereby the sinus node does not discharge an impulse to trigger the heart to contract. As a result the heart literally stops beating. If the heart stops for more than 8 seconds then the dog will collapse/faint. Sometimes the heart will have another part of the heart initiate a beat to rescue the heart from complete arrest. Most of the time the sinus node will eventually start up again to do its job but the dog has a rhythm with many long pauses. Some dogs with sick sinus syndrome have a more constant sinus bradycardia (too slow) because the sinus node has a low firing rate. Other dogs with sick sinus syndrome will have periods of excessive tachycardia (rapid rate) in addition to the pauses or bradycardia.
Symptoms
Some dogs will not show any symptoms of sick sinus syndrome, especially if they tend to be fairly inactive under normal circumstances. Generally, the symptoms that will present are:
Weakness
Fainting
Fatigue
Collapse
Seizure
Abnormally fast, or abnormally slow heart rate
Pauses in the heart rate
Rarely, sudden death
Breeds predisposed:
West Highland White Terriers, Dachshunds, Miniature Schnauzers, Boxers, Cocker spaniels. The cause is unknown but there is a likely genetic component.
Diagnosis
A provocative atropine response test may be done to assess sinus node function. This test uses the drug atropine to stimulate the firing action (sending electrical impulses out) of the SA Node. Dogs with SSS generally will have no response, or will have an incomplete response to the atropine.
An ECG may be indicated in certain breeds which are predisposed to SSS, as these same breeds are often predisposed to other diseases of the heart valves (the valves that separate the four chambers of the heart). Hence, if there is a heart murmur, disease of any of the heart’s valves should first be ruled out.
Treatment: When a dog has clinical signs of sick sinus syndrome it is almost always required that a pacemaker be implanted. The implantation of a pacemaker is today a common procedure in dogs. Veterinary cardiologists who are experienced in the implantation of pacemakers and the programming of these pacemakers can best insure the best treatment for afflicted dogs. The response to treatment is usually very good. In dogs that also have the tachycardia this is treated with medication(s) if it does not subside after pacing.
References:
One, Two, Three
