Epidemiology and Etiology
- More common in the elderly
- Most common reason for placement of an artificial pacemaker
- 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:
- 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).
- Calcium channel blockers
- Underlying medical conditions:
- 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:
- SA node initiates impulse/P wave:
- Atrial depolarization
- Atrial contraction
- PR segment:
- Impulse reaches AV node.
- Impulse passes to the His bundle.
- QRS complex:
- Atrial repolarization
- Depolarization of ventricles
- Ventricular contraction
- ST segment: completion of ventricular depolarization
- T wave: repolarization of ventricles
- Completion of repolarization
- SA node initiates impulse/P wave:
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
Symptoms of bradycardia or tachycardia–bradycardia syndrome include:
- Chest pain
- Manifestations of low cardiac output:
- 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 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.
- 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:
- 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.
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.
- 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
- 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