Definition of Sick Sinus Syndrome
Sick sinus syndrome (also called sinus node dysfunction) is a disorder characterized by the heart's inability to perform its pacemaker function properly, manifesting as symptomatic bradyarrhythmias (sinus bradycardia, sinus arrest, sinoatrial exit block) with or without alternating tachyarrhythmias (tachy-brady syndrome). 1
Core Diagnostic Criteria
The diagnosis requires direct correlation between symptoms and documented bradyarrhythmia on ECG monitoring 2. The syndrome is present when symptoms and sinus bradycardia or pauses occur simultaneously as proven by ECG monitoring, which is the gold standard 2.
Electrocardiographic Manifestations
Sick sinus syndrome encompasses several distinct ECG patterns 2:
- Sinus bradycardia: Sinus rate 40-50 bpm with normal P-wave axis and PR interval 2
- Severe sinus bradycardia: Sinus rate <40 bpm with normal P-wave axis and PR interval 2
- Sinus arrest: Sudden absence of sinus activity 2
- Sinoatrial exit block: Loss of sinus activity at an interval fixed to that of the basic P-P interval 2
- Tachycardia-bradycardia syndrome: Paroxysmal tachycardias followed by bradycardia upon termination 2
Pathophysiology
The underlying mechanism is age-dependent, progressive, degenerative fibrosis of the sinus nodal tissue and surrounding atrial myocardium 1. This same fibrotic process creates the substrate for both bradycardia and atrial tachyarrhythmias when they coexist 1, 3.
Clinical Presentation
Symptoms result from end-organ hypoperfusion 4:
- Cerebral hypoperfusion (most common): Syncope, presyncope, lightheadedness, or dizziness occurring in approximately 50% of patients 5, 4
- Cardiac symptoms: Palpitations, decreased exercise tolerance, angina 4
- Other manifestations: Muscular fatigue, oliguria 4
Critical caveat: Many patients are asymptomatic or have subtle, nonspecific symptoms early in the disease course, making diagnosis challenging 6, 5.
Distinguishing Intrinsic vs. Extrinsic Causes
Before confirming the diagnosis, exclude reversible extrinsic factors 1, 4:
Extrinsic Causes to Eliminate:
- Medications: Cardiac glycosides, beta-blockers, calcium channel blockers, membrane-active antiarrhythmics, Class IC agents 1, 3
- Metabolic abnormalities: Electrolyte disturbances 1
- Autonomic disturbances: Increased vagal tone 1
Intrinsic Causes (Irreversible):
- Degenerative fibrosis (most common) 1, 5
- Infiltrative diseases: Sarcoidosis, amyloidosis 1
- Cardiac remodeling: Particularly with frequent atrial arrhythmias 1
Diagnostic Thresholds
When electrophysiologic testing is performed, abnormal sinus node function is suggested by 2:
- Sinus node recovery time (SNRT) >2 seconds 2
- Corrected sinus node recovery time (CSNRT) >1 second 2
- Marked prolongation of SNRT >3 seconds increases likelihood that sinus node dysfunction is responsible for syncope 2
Important limitation: Invasive electrophysiology studies play a very limited role in assessment and are not routinely needed 2, 5.