Sick sinus syndrome

Definition. Sick sinus syndrome (sinus node dysfunction) is a spectrum of inappropriate sinus bradycardia, sinus arrest or pause, sinoatrial exit block, chronotropic incompetence, and tachycardia–bradycardia syndrome. Dysfunction may progress until no sinus beats occur and a junctional escape rhythm takes over. It is primarily a disease of older people, resulting from degenerative or ischaemic change that may also involve the AV node.

Causes. Idiopathic age-related fibrosis of the sinus node is commonest. Others: ischaemic heart disease, drugs (beta-blockers, verapamil, diltiazem, digoxin, amiodarone, ivabradine, lithium, donepezil), hypothyroidism, hyperkalaemia, infiltrative disease (amyloid, sarcoid), obstructive sleep apnoea, and cardiac surgery. Always exclude a drug cause before implanting a pacemaker.

Clinical features. Recurrent syncope from sinus pauses with an inadequate escape rhythm, presyncope, dizziness, unexplained falls, fatigue, confusion, and exercise intolerance from chronotropic incompetence. In tachy–brady syndrome, palpitations from atrial fibrillation or flutter are followed by a long post-tachycardia pause and syncope on termination. Many older patients are asymptomatic. Both paroxysmal and chronic atrial fibrillation are commonly associated with sinus node disease.

Diagnosis. Resting ECG, then ambulatory Holter monitoring; if symptoms are infrequent use prolonged external monitoring or an implantable loop recorder. Symptom–rhythm correlation is essential — an abnormal ECG without symptoms does not justify pacing. Exercise testing demonstrates chronotropic incompetence. Electrophysiological study (sinus node recovery time) is rarely required. Check thyroid function, electrolytes and the drug chart.

Prognosis. Mortality appears to be unaffected by sinus node dysfunction itself; survival is determined by associated pathology such as ischaemic heart disease. The main morbidity is syncope with injury and thromboembolic stroke from associated atrial fibrillation — assess stroke risk and anticoagulate accordingly, independently of the pacing decision.

Management and pacing indications. First withdraw culprit drugs and correct reversible causes. Pacing is Class I when (i) symptoms are directly attributable to sinus node dysfunction, and (ii) sinus bradycardia develops secondary to guideline-directed treatment of another condition which must be continued. Pacing should be considered (Class IIa) in tachy–brady syndrome where symptoms are attributable to the bradycardia, and rate-responsive pacing should be considered for symptomatic chronotropic incompetence. AAI pacing is used only in pure sinus node disease with no existing or expected disease elsewhere in the conduction system; otherwise DDD(R) is preferred, since VVI pacing causes pacemaker syndrome and more atrial fibrillation.

The 2023 HRS/APHRS/LAHRS Cardiac Physiologic Pacing Guideline reinforces that in sinus node dysfunction with intact AV conduction the aim is to preserve intrinsic ventricular activation — use atrial-based (AAI/DDD) pacing and enable algorithms that minimise unnecessary right-ventricular pacing, since a high right-ventricular pacing burden increases atrial fibrillation and heart failure. Conduction system pacing is reserved for those who also need ventricular pacing.