Sick sinus syndrome
Sick sinus syndrome (sinus node dysfunction) is an idiopathic heart disease of dogs characterized by sinoatrial node and/or atrioventricular node dysfunction, usually accompanied by depressed subsidiary pacemaker automaticity.
Microscopically, there appears to be dysfunction of both the calcium clock and voltage clock within the sinoatrial node which leads to conduction disturbances associated with this condition.
A number of breeds appear predisposed including the Boxer and Bull Terrier and a genetic component has been suspected in middle-aged to older West Highland White Terriers, Miniature Schnauzers and Cocker Spaniels.
As a result of this conduction disorder of the heart, dogs often develop asystole due to sinus arrest with failure to generate appropriate escape rhythms which presents as periods of superventricular tachycardia alternating with bradyarrhythmias and is therefore also known as 'brady-tachy syndrome'. Bundle branch block may also be observed in some dogs.
Diagnosis can occasionally be ascertained from an ECG, but arrhythmias may be intermittent and a Holter monitor or cardiac event monitor may be necessary for a diagnosis. Thoracic radiographs, blood test, specifically measurement of cardiac troponin-I measurements) are necessary to eliminate other causes of arrhythmias. An atropine response test may be used to determine if the bradyarrhythmia is vagally mediated.
Histological examination of postmortem cases has shown that sick sinus syndrome is associated with depletion of sinoatrial nodal cells with fibrous or fibro-fatty tissue, resulting in interrupted contiguity between the sinoatrial node and the surrounding atrial myocardium.
Treatment is usually unwarranted for clinical normal dogs, but in severe cases, particularly where risk of development of congestive heart failure may be present, a pacemaker implantation is the treatment of choice. Pharmacological treatment has been attempted but is not always successful.
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