Sick Sinus Syndrome

Sick sinus syndrome (SSS, sinus node dysfunction) is a group of disorders of sinus node impulse formation or exit, producing bradyarrhythmias, pauses and often tachyarrhythmias (tachy brady syndrome).

Mostly age related fibrosis of the sinus node (peak incidence 65 to 74 years, 1 in 600 over age 65).

Causes: idiopathic degeneration, ischaemia, cardiomyopathies, infiltrative disease (amyloid, sarcoid, haemochromatosis), myocarditis, Chagas disease, cardiac surgery, genetic (SCN5A, HCN4, MYH6), drugs (beta blockers, calcium channel blockers, digoxin, amiodarone, antiarrhythmics, clonidine, donepezil), hypothyroidism, hypothermia, raised intracranial pressure, obstructive sleep apnoea, athletes' high vagal tone.

Forms: sinus bradycardia, sinus pause or arrest, sinoatrial exit block, chronotropic incompetence, bradycardia tachycardia syndrome (atrial fibrillation or flutter alternating with bradycardia).

Atrial fibrillation present in 50% and thromboembolism common.

• Fatigue, dizziness, presyncope, syncope (Stokes Adams), reduced exercise tolerance, palpitations (tachy phase), dyspnoea, confusion, heart failure.
• Bradycardia, irregular pulse with pauses.
• Asymptomatic bradycardia in athletes or at night is physiological and needs no treatment.

• ECG: sinus bradycardia under 50 beats per minute, pauses over 3 seconds, sinoatrial block, junctional escape rhythm, atrial fibrillation with slow ventricular response after conversion.
• Ambulatory monitoring: Holter, patch monitor, implantable loop recorder for infrequent symptoms; symptom rhythm correlation is essential.
• Exercise testing: chronotropic incompetence (failure to reach 80% of predicted maximum heart rate).
• Reversible causes: medication review, thyroid tests, electrolytes, sleep apnoea screen.
• Electrophysiology study (sinus node recovery time over 1.5 seconds) rarely needed.
• Echocardiography for structural disease.

• Remove reversible causes: stop or reduce nodal blocking drugs, treat hypothyroidism, sleep apnoea, hypothermia.
• Acute symptomatic bradycardia: atropine 0.5 to 1 mg IV (repeat every 3 to 5 minutes, maximum 3 mg), dopamine 5 to 20 micrograms/kg/min or epinephrine 2 to 10 micrograms/min, transcutaneous or transvenous pacing.
• Permanent pacing is the only effective therapy for symptomatic irreversible SSS. Dual chamber pacemaker (DDDR, or AAIR/DDD with minimised ventricular pacing) reduces atrial fibrillation and stroke compared with ventricular only pacing. Leadless or conduction system pacing in selected patients.
• Asymptomatic patients with pauses or bradycardia do not need pacing.
• Tachy brady syndrome: pacemaker first, then antiarrhythmic drugs (flecainide, sotalol, amiodarone) or catheter ablation of atrial fibrillation (may eliminate the need for pacing in some).
• Anticoagulation by CHA2DS2 VASc (apixaban 5 mg twice daily).
• Cardiac rehabilitation, follow-up of pacemaker every 6 to 12 months with remote monitoring.
• Avoid theophylline or aminophylline for chronic bradycardia in elderly due to arrhythmia risk.