What is sick sinus syndrome?

Sick Sinus Syndrome: Definition and Clinical Implications

Sick sinus syndrome (SSS), now more commonly referred to as sinus node dysfunction (SND), is a spectrum of cardiac disorders characterized by the heart's inability to perform its pacemaker function properly due to age-dependent, progressive, degenerative fibrosis of the sinus nodal tissue and surrounding atrial myocardium. 1, 2

Pathophysiology

• SND results primarily from intrinsic causes including degenerative fibrosis, ion channel dysfunction, and remodeling of the sinoatrial node 3
• The condition predominantly affects older adults, typically in their 70s or 80s 2
• Extrinsic factors can exacerbate or mimic SND, including:
  • Pharmacologic agents (cardiac glycosides, beta-blockers, calcium channel blockers, antiarrhythmic drugs) 1, 2
  • Metabolic disturbances 3
  • Autonomic dysfunction 3

Clinical Manifestations

• SND encompasses a variety of cardiac arrhythmias including:

  • Sinus bradycardia (inappropriately slow heart rate) 1
  • Sinoatrial exit block 1
  • Sinus pauses or arrest 1
  • Chronotropic incompetence (inadequate heart rate response to exercise) 1, 4
  • Alternating bradycardia and tachycardia (tachy-brady syndrome) 1, 2

• At least 50% of patients with SND develop tachy-brady syndrome, where the same degenerative fibrosis responsible for bradycardia also causes the development of atrial arrhythmias 2, 3

• Symptoms are often related to end-organ hypoperfusion and may include:

  • Syncope or near-syncope (occurring in about 50% of patients) 3
  • Dizziness or lightheadedness 3
  • Fatigue 3
  • Palpitations (especially with tachy-brady syndrome) 3
  • Exertional intolerance 4

• Many patients may be asymptomatic or have subtle, nonspecific symptoms, especially early in the disease course 3, 5

Diagnosis

• Diagnosis is made by correlating symptoms with electrocardiographic evidence of arrhythmias 1, 3

• Diagnostic findings may include:

  • Sinus bradycardia (heart rate <60 beats/min) 1
  • Sinus pauses or arrest 1
  • Sinoatrial block 1
  • Alternating bradycardia and tachycardia 1
  • Inadequate heart rate response to exercise (chronotropic incompetence) 1, 4

• If initial ECG is non-diagnostic, extended monitoring may be required:

  • Ambulatory ECG monitoring (Holter) 1
  • Event recorders 1
  • Implantable loop recorders 3
  • Exercise testing to assess chronotropic response 4

• Electrophysiologic studies may be used in select cases but have limited sensitivity and specificity 1

• It's important to differentiate pathological SND from physiologic sinus bradycardia in trained athletes, who may have resting heart rates of 40-50 beats/min and sleeping rates as low as 30 beats/min with sinus pauses up to 2.8 seconds 1

Treatment

• Permanent pacemaker implantation is the definitive treatment for symptomatic sick sinus syndrome 1, 2

• Treatment considerations include:

  • Removal of extrinsic factors when possible (medication adjustment) 1, 2
  • Physiological pacing (atrial or dual-chamber) is superior to ventricular pacing for SND 1, 2
  • Atrial-based rate-responsive pacing is preferred to minimize exertion-related symptoms 1, 2
  • For tachy-brady syndrome, catheter ablation may be considered for atrial tachyarrhythmia control 2, 6

• Clinical outcomes:

  • Pacemaker therapy effectively relieves symptoms and improves quality of life 3
  • Physiological pacing lowers the risk of developing atrial fibrillation compared to VVI pacing 1
  • Pacemaker therapy may not improve survival, as mortality is primarily related to underlying cardiac disease 1, 3
  • Despite adequate pacing, syncope recurs in approximately 20% of patients during long-term follow-up due to associated vasodepressor reflex mechanisms 1, 2

Special Considerations

• The presence of structural heart disease or left ventricular dysfunction may impact prognosis and treatment decisions 2
• Biventricular pacing may be considered in patients with depressed left ventricular ejection fraction, heart failure, and prolonged QRS duration 2
• Anticoagulation therapy should be considered in patients with tachy-brady syndrome due to the risk of atrial tachyarrhythmias and thromboembolism 1