Sick sinus syndrome

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Right lateral postoperative radiograph demonstrating the position of a dual-chambered pacemaker with leads in the right atrium (small arrow) and right ventricular apex (large arrow.) for a dog with sick sinus syndrome[1]

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[2].

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[3], 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[4] which presents as periods of superventricular tachycardia alternating with bradyarrhythmias and is therefore also known as 'brady-tachy syndrome'[5]. Bundle branch block may also be observed in some dogs.

Clinical signs may be lacking in some dogs, while others present with syncope, tachycardia, dyspnea, weakness, confusion, and incoordination[6].

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.

A differential diagnosis would include atrial fibrillation and myocarditis secondary to Bartonella spp infections[7].

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[8].

Treatment is usually unwarranted for clinical normal dogs, but in severe cases, particularly where risk of development of congestive heart failure may be present[9], a pacemaker implantation is the treatment of choice[10]. Pharmacological treatment has been attempted but is not always successful.

References

  1. Burrage H (2012) Sick sinus syndrome in a dog: treatment with dual-chambered pacemaker implantation. Can Vet J 53(5):565-568
  2. James R (2007) Use of pacemakers in dogs. In Practice 29:503–511
  3. Gladuli A et al (2011) Poincaré plots and tachograms reveal beat patterning in sick sinus syndrome with supraventricular tachycardia and varying AV nodal block. J Vet Cardiol 13(1):63-70
  4. Kavanagh K (2002) Sick sinus syndrome in a bull terrier. Can Vet J 43:46–48
  5. Bulmer BJ (2011) Sick sinus syndrome. In: Côté E, editor. Clinical Veterinary Advisor: Dogs and Cats. 2nd ed. St. Louis, Missouri: Elsevier. pp:1022–1024
  6. Vailati MC et al (2011) ECG of the month. J Am Vet Med Assoc 238:850–852
  7. Trafny DJ et al (2010) Cardiac troponin-I concentrations in dogs with bradyarrhythmias before and after artificial pacing. J Vet Cardiol 12(3):183-190
  8. Nakao S et al (2012) The anatomical basis of bradycardia-tachycardia syndrome in elderly dogs with chronic degenerative valvular disease. J Comp Pathol 146(2-3):175-182
  9. Ogawa M et al (2009) Cryoablation of stellate ganglia and atrial arrhythmia in ambulatory dogs with pacing-induced heart failure. Heart Rhythm 6(12):1772-1779
  10. Estrada AH et al (2012) Atrial-based pacing for sinus node dysfunction in dogs: initial results. J Vet Intern Med 26(3):558-564