Sick sinus syndrome (SSS)
Sick sinus syndrome (sinus node dysfunction) is a clinical syndrome caused by impaired sino‑atrial (SA) node automaticity or exit block, producing a spectrum of bradyarrhythmias and chronotropic incompetence. It is a common cause of symptomatic bradycardia in older adults and an important ED diagnosis because it can cause syncope, unexplained falls and is a common indication for permanent pacing (NICE TA88/TA324).
Clinical manifestations and ECG features
- Sinus bradycardia: regular sinus rhythm < 60 bpm (symptomatic when inadequate for physiological demand).
- Sinus pauses / arrest: transient absence of P waves with missed QRS complexes; may cause abrupt syncope.
- SA exit block: dropped P waves (may show fixed pause multiples on rhythm strip).
- Tachy-brady syndrome: alternating atrial tachyarrhythmias (often AF/flutter) with marked bradycardia on termination.
- Variable P‑wave morphology or irregular atrial rate with stable PR interval may be seen.
Diagnosis requires correlation of symptoms (syncope, presyncope, falls, exertional intolerance, fatigue, palpitations) with documented rhythm disturbance; intermittent abnormalities often need ambulatory monitoring (Holter / event recorder / implantable loop recorder).
Pathophysiology
- Age‑related fibrosis and degeneration of the SA node (most common).
- Ischaemic injury to the sinus node artery, cardiomyopathy or infiltrative disease.
- Drug effects (β‑blockers, non‑DHP calcium‑channel blockers, digoxin, some antiarrhythmics) or metabolic causes (hypothyroidism, electrolyte disturbance) that unmask or worsen dysfunction.