Causes

• When the intrinsic cause of this syndrome is unknown, the condition is called “idiopathic.” A degeneration or scarring of the node may be responsible for some cases, especially in elderly patients and after cardiac surgery. Some studies found a decrease in the number of nodal cells and detected antibodies against them. Coronary artery disease, connective tissue disorders and neuromuscular diseases can be associated with the syndrome. Several molecular defects have been linked to familial SSS.
  
  Extrinsic causes or aggravating factors include medications such as digitalis, calcium channel blockers and antiarrhythmics. Thyroid dysfunction and toxins (as in sepsis) have also been implicated in sinus node dysfunction. Despite recent advances, this syndrome remains incompletely understood and its clinical course unpredictable.
  
  

Symptoms

• With mild SSS, patients are usually asymptomatic. Their heart rate may be low. As the defect becomes more severe and organs receive insufficient blood, patients may develop various manifestations, such as fatigue, confusion, dizziness, fainting, palpitations, chest pain (angina) and shortness of breath.
  
  

Investigations and Differential Diagnosis

• Electrocardiography (EKG), the most important diagnostic method, reveals several potential patterns that are inappropriate for the individual’s physiological state. Instead of a steady pace of impulses, with a frequency of 60 to 100 beats per minute (the normal heart rate), the sick node produces a rhythm that is too slow (bradycardia) or too fast (tachycardia) and often alternates between these two extremes (brady-tachy syndrome). Abnormalities may include sinus arrest (when the node does not “fire” at all) or block (when the impulse fails to stimulate the cardiac muscle). Tachycardias can present as chaotic contractions of the upper heart chambers (e.g., atrial fibrillation, AF) and paroxystic supraventricular tachycardia, each with distinct electrocardiographic findings.
  
  Many patients have more than one EKG abnormality. Ambulatory EKG monitoring is sometimes used to correlate rhythm abnormalities and symptoms. Electrophysiologic testing, rarely employed, involves recording the electrical activity in the heart using electrode-tipped catheters driven through blood vessels.
  
  Routine laboratory studies are rarely of value, except to exclude extrinsic conditions such as hypothyroidism, electrolyte imbalances, hypoglycemia or inappropriate drug levels. A differential diagnosis will consider other potential causes of bradycardia (e.g., athletic status, atrioventricular block, heart attack) and alternative sources of palpitations (other “fast” arrhythmias, heart valve disease or anxiety). Additional factors that should be excluded are hypothermia and vascular or reflex causes for fainting.
  
  

Treatment

• Correcting extrinsic causes is the only effective medical approach to SSS treatment. Surgical care involves artificial pacemaker therapy, with the goal of relieving symptoms. Selection of pacing mode is important for the clinical outcome in patients with symptomatic bradycardia; it affects not only the quality of life, but also the development of complications such as AF, blood clots, stroke and congestive heart failure. The risk of AF, for example, is greatly reduced when a physiologic (atrial or atrial and ventricular) pacemaker is used. Antiarrhythmic drugs may be used prevent “fast” arrhythmias after pacemaker insertion.
  
  

Tips

• A cardiologist is usually responsible for the care of the patient with SSS.
  
  Patients with symptomatic SSS and not on an artificial pacemaker should monitor their activity level to minimize symptoms.