Sick sinus syndrome refers to sinus node dysfunction causing slow, physiologically inappropriate, heart rates. Symptoms may be minimal or include weakness, effort intolerance, palpitations, and syncope. Diagnosis is by electrocardiography. Patients who have symptoms require a pacemaker.
(See also Overview of Arrhythmias.)
Sick sinus syndrome includes
Inappropriate sinus bradycardia
Alternating bradycardia and atrial tachyarrhythmias (bradycardia-tachycardia, or brady-tachy, syndrome)
Sinus pause or sinus arrest
Sinoatrial (SA) exit block
Chronotropic incompetence
Sick sinus syndrome affects mainly older patients, especially those with another cardiac disorder or diabetes.
In the brady-tachy variant of the sick sinus syndrome, sinus node dysfunction is associated with atrial tachyarrhythmias, most commonly atrial fibrillation.
Sinus pause is temporary cessation of sinus node activity, seen on electrocardiography (ECG) as disappearance of P waves for seconds to minutes. The pause usually triggers escape activity in lower pacemakers (eg, atrial or junctional), preserving heart rate and function, but long pauses cause dizziness and syncope.
In SA exit block, the SA node depolarizes, but conduction of impulses to atrial tissue (exit conduction) is impaired.
In first-degree SA block, the SA node impulse exit conduction is merely slowed, and, in the absence of another disorder, the ECG is normal and there are no symptoms.
In type I second-degree SA (SA Wenckebach) block, impulse exit conduction slows before blocking, seen on the ECG as a P-P interval that decreases progressively until the P wave drops altogether, creating a pause and the appearance of grouped beats; the duration of the pause is less than 2 P-P cycles.
In type II second-degree SA block, impulse exit conduction is blocked without slowing beforehand, producing a pause that is a multiple (usually twice) of the P-P interval and the appearance of grouped beats.
In third-degree SA block, impulse exit conduction is blocked; normal P waves are absent, giving the appearance of sinus arrest.
Chronotropic incompetence refers to the inability to increase sinus heart rate with exercise most commonly defined as achievement of less than 80% of the maximum age-predicted heart rate (220 beats/minute minus the person's age) on an exercise tolerance test (1).
General reference
1. Kusumoto FM, Schoenfeld MH, Barrett C, et al: 2018 ACC/AHA/HRS guideline on the evaluation and management of patients with bradycardia and cardiac conduction delay: A Report of the American College of Cardiology/American Heart Association Task Force on Clinical Practice Guidelines and the Heart Rhythm Society. Heart Rhythm 16(9):e128–e226, 2019. doi: 10.1016/j.hrthm.2018.10.037
Etiology of Sick Sinus Syndrome
The most common cause of intrinsic sinus node dysfunction in the sick sinus syndrome is
Idiopathic SA node fibrosis, which may be accompanied by degeneration of lower elements of the conducting system, manifested as concomitant atrioventricular (AV block and/or bundle branch block)
Other causes of sick sinus syndrome include sleep apnea, medications, excessive vagal tone, and many ischemic, inflammatory, and infiltrative disorders (1).
Etiology reference
1. Sathnur N, Ebin E, Benditt DG: Sinus Node Dysfunction. Cardiol Clin 41(3):349–367, 2023. doi: 10.1016/j.ccl.2023.03.013
Symptoms and Signs of Sick Sinus Syndrome
Many patients with sick sinus syndrome are asymptomatic. Depending on the heart rate, all the symptoms of bradycardias and, in the case of the brady-tachy variant of the sick sinus syndrome, all the symptoms of tachycardias can occur. Syncope may occur when the sinus node does not immediately resume function after spontaneous termination of an atrial tachyarrhythmia.
Diagnosis of Sick Sinus Syndrome
ECG
Measurement of thyroid stimulating hormone (TSH) to rule out hypothyroidism
Ambulatory ECG monitor
Exercise tolerance testing for chronotropic incompetence
Rarely, implantable cardiac monitoring or a catheter electrophysiologic study
A slow, irregular pulse suggests the diagnosis of sinus node dysfunction, which is confirmed by ECG, rhythm strip, or continuous 24-hour ECG recording. Some patients present with atrial fibrillation, and the underlying sinus node dysfunction manifests only after conversion to sinus rhythm with post-conversion sinus pause. The goal of ECG monitoring is to establish a correlation between symptoms and bradyarrhythmia.
If suggested by a history and physical examination, targeted evaluation is performed, including measurement of thyroid stimulating hormone (TSH) for suspected hypothyroidism, polysomnography for suspected sleep apnea, exercise tolerance testing for suspected chronotropic incompetence, and echocardiography for suspected structural heart disease (1). When symptoms compatible with the sick sinus syndrome are very infrequent, implantable cardiac monitoring may be required to enable correlation between symptoms and rhythm. Infrequently, the diagnosis of sick sinus syndrome is made during a catheter electrophysiologic study done for the evaluation of syncope of unknown etiology.
Diagnosis reference
1. Kusumoto FM, Schoenfeld MH, Barrett C, et al: 2018 ACC/AHA/HRS guideline on the evaluation and management of patients with bradycardia and cardiac conduction delay: A Report of the American College of Cardiology/American Heart Association Task Force on Clinical Practice Guidelines and the Heart Rhythm Society. Heart Rhythm 16(9):e128–e226, 2019. doi: 10.1016/j.hrthm.2018.10.037
Treatment of Sick Sinus Syndrome
Pacemaker
Treatment of symptomatic sick sinus syndrome in the absence of an extrinsic reversible cause is pacemaker implantation (1). Risk of atrial fibrillation is reduced when a physiologic (atrial or atrioventricular) pacemaker rather than a ventricular pacemaker is used (2). Dual-chamber pacemakers that minimize ventricular pacing may further reduce risk of atrial fibrillation (2). Dual-chamber pacemakers also further protect patients in the group who experience a 1 to 2% expected annual occurrence of AV block (3). Antiarrhythmic medications may prevent paroxysmal tachyarrhythmias after pacemaker insertion.
Treatment references
1. Kusumoto FM, Schoenfeld MH, Barrett C, et al: 2018 ACC/AHA/HRS guideline on the evaluation and management of patients with bradycardia and cardiac conduction delay: A Report of the American College of Cardiology/American Heart Association Task Force on Clinical Practice Guidelines and the Heart Rhythm Society. Heart Rhythm 16(9):e128–e226, 2019. doi: 10.1016/j.hrthm.2018.10.037
2. Liu Y, Zheng Y, Tse G, et al: Association between sick sinus syndrome and atrial fibrillation: A systematic review and meta-analysis. Int J Cardiol 381:20–36, 2023. doi: 10.1016/j.ijcard.2023.03.066
3. Nielsen JC, Thomsen PE, Højberg S, et al: A comparison of single-lead atrial pacing with dual-chamber pacing in sick sinus syndrome. Eur Heart J 32(6):686–696, 2011. doi: 10.1093/eurheartj/ehr022.
Prognosis for Sick Sinus Syndrome
Sinus node dysfunction in the sick sinus syndrome tends worsen over time. The prognosis is otherwise mixed; mortality is about 4%/year, primarily resulting from an underlying structural heart disorder (1). Each year, about 5% of patients develop atrial fibrillation with its risks of heart failure and stroke (2).
Prognosis references
1. Shaw DB, Holman RR, Gowers JI: Survival in sinoatrial disorder (sick-sinus syndrome). Br Med J 280(6208):139–141, 1980. doi: 10.1136/bmj.280.6208.139
2. Sutton R, Kenny RA: The natural history of sick sinus syndrome. Pacing Clin Electrophysiol 9(6):1110-1114, 1986. doi: 10.1111/j.1540-8159.1986.tb06678.x
Key Points
Sick sinus syndrome results in inappropriately slow heart rates.
Symptoms may be absent or result from bradycardia.
In the brady-tachy variant of the sick sinus syndrome, symptoms may also result from tachycardia or asystole when tachycardia terminates.
Diagnosis is mainly by electrocardiographic monitoring.
Symptomatic sick sinus syndrome is treated with a pacemaker.
