Sick Sinus Syndrome
Dysfunction of the sinoatrial node causing a combination of inappropriate sinus bradycardia, sinus pauses/arrest, and alternating tachycardia-bradycardia. Most common indication for pacemaker in elderly.
Key Facts
Also known as sinus node dysfunction; encompasses sinus bradycardia, sinus arrest/pauses, sinoatrial exit block, and tachy-brady syndrome Tachy-brady syndrome: alternating episodes of tachycardia (usually atrial fibrillation) and bradycardia Most common in elderly due to age-related fibrosis of the SA node Definitive treatment: permanent pacemaker (atrial-based pacing: AAI or DDD preferred over VVI to reduce AF risk) Drug challenge: antiarrhythmic drugs for tachycardia episodes may worsen bradycardia — pacemaker often needed first Prevalence: increases significantly with age; most common indication for pacemaker implantation
Overview
Key Facts
Sick sinus syndrome (SSS) or sinus node dysfunction encompasses a spectrum of SA node abnormalities including inappropriate sinus bradycardia, sinus pauses or arrest, sinoatrial exit block, chronotropic incompetence, and the tachy-brady syndrome.
Epidemiology
- Predominantly affects the elderly (mean age at diagnosis ~65-70 years)
- Prevalence: ~1 in 600 patients over 65 years
- Most common indication for permanent pacemaker implantation
- No significant sex difference
Aetiology
- Idiopathic degenerative fibrosis of the SA node (most common)
- Ischaemic heart disease (SA node artery occlusion)
- Drugs: beta-blockers, CCBs, digoxin, amiodarone, lithium
- Infiltrative: amyloidosis, sarcoidosis, haemochromatosis
- Inflammatory: myocarditis, pericarditis
- Post-surgical: cardiac surgery, especially atrial surgery
- Hypothyroidism, hypothermia
- Genetic: SCN5A mutations (familial SSS)
Pathophysiology
- SA node fibrosis and fatty infiltration reduce automaticity and conduction
- Diseased SA node fails to generate adequate impulses or exit to atrial myocardium
- Chronotropic incompetence: failure to increase heart rate appropriately with exercise
- Tachy-brady: atrial tachyarrhythmias (AF/flutter) terminate with prolonged pauses due to sinus node suppression
Clinical Presentation
Typical Presentation
- Episodic dizziness, presyncope, or syncope (Stokes-Adams attacks)
- Fatigue, exercise intolerance (chronotropic incompetence)
- Palpitations during tachycardic episodes
Subtypes
- Persistent sinus bradycardia: HR <50 bpm inappropriately
- Sinus pauses/arrest: pauses >3 seconds on Holter
- Sinoatrial exit block: grouped beating, similar appearance to Wenckebach
- Tachy-brady syndrome: paroxysmal AF/flutter alternating with prolonged bradycardic pauses
- Chronotropic incompetence: failure to achieve 80% of age-predicted maximum heart rate during exercise
Red Flags
- Syncope or near-syncope
- Pauses >3 seconds on monitoring
- Symptoms while on essential rate-limiting medication
- Heart failure symptoms
- Associated thromboembolic events (from AF episodes)
Differential Diagnosis
| Diagnosis | Key Features | Investigation |
|---|---|---|
| Drug-induced bradycardia | Beta-blockers, CCBs, digoxin | Medication review |
| Hypothyroidism | Fatigue, cold intolerance, weight gain, bradycardia | TFTs |
| Complete heart block | AV dissociation, cannon waves | ECG |
| Neurocardiogenic syncope | Vasovagal episodes, young patients | Tilt table test |
| Carotid sinus hypersensitivity | Syncope with head turning/tight collar, elderly | Carotid sinus massage |
| Obstructive sleep apnoea | Nocturnal bradycardia, snoring, daytime somnolence | Polysomnography |
Diagnosis / Investigation
Bedside
- 12-lead ECG: sinus bradycardia, sinus pauses, sinoatrial exit block
- Continuous cardiac monitoring: capture intermittent events
Bloods
- TFTs: exclude hypothyroidism
- U&Es: electrolyte abnormalities
- Digoxin level: if applicable
- FBC: anaemia may worsen symptoms
Imaging
- Echocardiography: structural heart disease
Special Tests
- 24-48 hour Holter monitor: correlate symptoms with rhythm; identify pauses >3s, tachy-brady
- 7-day or implantable loop recorder (ILR): if symptoms are infrequent
- Exercise test: demonstrate chronotropic incompetence (failure to reach 80% predicted max HR)
- Carotid sinus massage: exclude carotid sinus hypersensitivity (under monitored conditions)
- Electrophysiology study: measure sinus node recovery time (SNRT) — prolonged in SSS
Management
Non-pharmacological
- Review and withdraw causative/contributory drugs where possible
- Treat underlying causes (hypothyroidism, etc.)
Pharmacological
- No reliable drug therapy for SSS
- Tachy-brady management is challenging: antiarrhythmics (beta-blockers, flecainide, amiodarone) for AF may worsen bradycardia
- Usually requires pacemaker implantation BEFORE starting antiarrhythmic drugs
- Anticoagulation for AF component: follow standard AF guidelines (CHA₂DS₂-VASc score)
Surgical/Interventional
- Permanent pacemaker implantation: definitive treatment
- AAI (single-chamber atrial pacing): if isolated sinus node disease with intact AV conduction
- DDD (dual-chamber pacing): preferred if any concern about AV conduction
- Atrial-based pacing reduces risk of developing AF compared to VVI pacing
- Rate-responsive pacing (AAIR/DDDR) for chronotropic incompetence
- AF ablation: may be considered for the tachycardic component in tachy-brady if pacemaker in situ
Referral Criteria
- All symptomatic SSS: cardiology referral for pacemaker assessment
- Syncope with documented sinus pauses: urgent referral
- Tachy-brady syndrome requiring antiarrhythmic initiation: pacemaker first
Prognosis
- SSS itself rarely causes mortality, but is associated with significant morbidity (syncope, falls, AF)
- With permanent pacemaker: symptoms resolve in >90% of patients
- ~50% of patients with SSS develop atrial fibrillation over 5-10 years
- Atrial-based pacing reduces AF risk compared to ventricular pacing
- Thromboembolic risk from associated AF must be managed with anticoagulation
- Overall survival after pacemaker implantation is similar to age-matched controls
Other Relevant Information
Sinus Node Dysfunction Subtypes
| Subtype | ECG Feature |
|---|---|
| Sinus bradycardia | Persistent HR <50 bpm |
| Sinus pause/arrest | Absent P waves for >2 seconds |
| Sinoatrial exit block | Grouped beating, pauses multiples of PP interval |
| Tachy-brady syndrome | AF/flutter alternating with sinus pauses |
| Chronotropic incompetence | Inadequate HR response to exercise |
Pacing Mode Selection
| Mode | Chamber(s) Paced | Best For |
|---|---|---|
| AAI | Atrium | Isolated sinus node disease |
| DDD | Atrium + ventricle | SSS with possible AV disease |
| VVI | Ventricle only | Avoid if possible (pacemaker syndrome, higher AF risk) |