Sick Sinus Syndrome

Sick sinus syndrome (SSS), also known as sinus node dysfunction, is characterized by degeneration of the sinoatrial (SA) node, the heart’s primary pacemaker. Patients with SSS may be asymptomatic or may present with tachycardia or bradycardia. In cases of bradycardia, patients can experience fatigue, light-headedness, and syncope. Diagnosis is made by physical exam and ECG. Management can include a pacemaker.

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Epidemiology and Etiology

Epidemiology

  • More common in the elderly
  • Most common reason for placement of an artificial pacemaker

Etiology

  • Degenerative changes and fibrosis of the sinoatrial (SA) node and surrounding myocardium:
    • Age-related tissue change
    • Tissue damage from acquired heart disease (e.g., myocardial infarction)
    • Cardiac deposition disease:
      • Sarcoidosis
      • Amyloidosis
      • Hemochromatosis
  • Hereditary sick sinus syndrome (SSS) can be due to mutations of the gene responsible for the formation of the alpha subunit of the sodium channel (SCN5A).
  • Medications:
    • Beta-blockers
    • Calcium channel blockers
  • Underlying medical conditions:
    • Hypothyroidism
    • Hyperkalemia

Pathophysiology

Normal physiology

  • Sinus or SA node:
    • A cluster of cells near the junction of the right atrium and superior vena cava
    • “Natural pacemaker” that initiates the electrical impulses to stimulate contraction
  • Innervation of the heart: 
    • Parasympathetic: reduces sinus node automaticity (↓ heart rate)
    • Sympathetic: increases sinus node automaticity (↑ heart rate)
  • Sequential events of a cardiac cycle:
    1. SA node initiates impulse/P wave: 
      • Atrial depolarization  
      • Atrial contraction
    2. PR segment:
      • Impulse reaches AV node.
      • Impulse passes to the His bundle.
    3. QRS complex: 
      • Atrial repolarization 
      • Depolarization of ventricles
      • Ventricular contraction
    4. ST segment: completion of ventricular depolarization
    5. T wave: repolarization of ventricles
    6. Completion of repolarization

Conduction system of the heart

Image by Lecturio.

Pathophysiology

Degeneration of the SA node:

  • Affects the ability to generate or transmit impulses to the atria → the conduction system as a whole fails to function effectively
  • Beats fail to originate in the SA node → may result in tachycardia–bradycardia syndrome

Clinical Presentation

Symptoms of bradycardia or tachycardia–bradycardia syndrome include:

  • Chest pain
  • Palpitations
  • Manifestations of low cardiac output:
    • Syncope
    • Hypotension 
    • Pallor
    • Dyspnea
    • Confusion
    • Dizziness
  • Adams–Stokes attacks: sudden loss of consciousness due to a disorder of the heart rhythm in which there is a slow or absent pulse

The following heart rhythm abnormalities may be seen as a part of SSS:

  • Sinus pause: 
    • Temporary failure of the SA node to depolarize
    • Can be seen in a 12-lead ECG as absent P waves
    • Usually, the AV node will take over as a pacemaker (escape rhythm).
    • A sinus pause of < 3 seconds is occasionally found in healthy adults.
    • A pause > 3 seconds may require intervention.
  • SA block: 
    • Condition in which the SA node depolarizes but the signal does not progress through the rest of the atria
    • May be viewed as “temporary” SSS; shares many common etiologic factors with SSS
  • Atrial fibrillation
    • Most common supraventricular arrhythmia
    • Characterized by disorganized atrial contraction and an increased risk of thromboembolic disease
  • Atrial flutter: supraventricular tachyarrhythmia characterized by “sawtooth” flutter waves on ECG

Diagnosis

  • Diagnosis is made by ECG.
    • Ambulatory (Holter) monitoring might be needed if the arrhythmia is transient.
    • ECG findings can vary widely:
      • Sinus arrhythmia
      • Sinus arrest
      • Sinus block
      • Asystolic pauses
      • Reentrant tachycardia
  • Cardiac stress test: Exercise or pharmacologic stress test shows an inadequate increase in heart rate in relation to stress/physical activity.
  • Electrophysiology studies can show prolonged sinus recovery time but are not frequently used for diagnostic purposes owing to low sensitivity and specificity.
  • Tilt-table testing can be used to reveal some forms of SSS.

24-Hour Holter monitor recording demonstrating both bradycardia (panel a) and tachycardia (panel b); bradycardia–tachycardia syndrome

Image: “24-Hour Holter” by Department of Pediatrics, Seth G.S. Medical College and KEM Hospital, Mumbai, India. License: CC BY 2.0

Management

  • Treat reversible causes that may be affecting SA function:
    • Electrolyte disturbances
    • Withhold offending medications.
  • Stable, asymptomatic patients may not require medical intervention.
  • In hemodynamically unstable (systolic BP < 90 mm Hg) or symptomatic (altered mental status, chest pain) patients:
    • Best initial step is atropine and transcutaneous pacing.
    • Sympathomimetics can be used if transcutaneous pacing fails:
      • Epinephrine
      • Dopamine
      • Dobutamine
    • A temporary external or transvenous pacemaker may be required.
  • The most definitive long-term therapy for refractory disease is pacemaker placement.
  • Concurrent tachycardia syndromes should be treated accordingly.

Clinical Relevance

The following conditions may predispose to SSS.

  • Amyloidosis: a disorder caused by extracellular deposition of insoluble abnormal amyloid that alters the function of tissues, especially the cardiac conduction system
  • Cardiomyopathies: a varied group of myocardial diseases (dilated, restrictive, hypertrophic) associated with impaired systolic and diastolic function
  • Hypothyroidism: deficiency of triiodothyronine (T3) and thyroxine (T4): Hashimoto’s disease (autoimmune thyroiditis) is the leading cause of hypothyroidism in non–iodine-deficient regions. Hypothyroidism may decrease the overall metabolic rate.
  • Hemochromatosis: a genetic autosomal recessive disorder caused by mutations in the HFE gene that results in increased intestinal iron absorption 
  • Sarcoidosis: a multisystem disease for which the genesis is not yet completely known and which is characterized by noncaseating granulomas in the affected organs: The main locations for this disease to manifest itself are the lungs and the hilar lymph nodes.
  • Hyperkalemia: serum potassium level > 5.5 mEq/L: Severe hyperkalemia is rare but can cause respiratory paralysis, generalized muscle paralysis, and cardiac arrest.

The following conditions are included in the differential diagnosis for SSS.

  • Syncope: temporary loss of consciousness caused by a fall in blood pressure: Syncope can be due to many causes, ranging from benign to life-threatening conditions.
  • Myocardial infarction: ischemia of myocardial tissue due to obstruction of a coronary artery: This is usually accompanied by an increase in cardiac enzymes, ECG abnormalities, and chest pain.

References

  1. Kusumoto F. M., et al. (2019). 2018 ACC/AHA/HRS guideline on the evaluation and management of patients with bradycardia and cardiac conduction delay. Journal of the American College of Cardiology, 74(7). doi:10.1016/j.jacc.2018.10.044
  2. Keller, K. B., Lemberg, L. (2006). The sick sinus syndrome. American Journal of Critical Care, 15(2), 226-229. doi:10.4037/ajcc2006.15.2.226

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