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Am Fam Physician. 2021;104(2):179-185

Patient information: A handout on this topic is available at https://familydoctor.org/condition/sick-sinus-syndrome.

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Sinus node dysfunction, previously known as sick sinus syndrome, describes disorders related to abnormal conduction and propagation of electrical impulses at the sinoatrial node. An abnormal atrial rate may result in the inability to meet physiologic demands, especially during periods of stress or physical activity. Sinus node dysfunction may occur at any age, but is usually more common in older persons. The causes of sinus node dysfunction are intrinsic (e.g., degenerative idiopathic fibrosis, cardiac remodeling) or extrinsic (e.g., medications, metabolic abnormalities) to the sinoatrial node. Many extrinsic causes are reversible. Electrocardiography findings include sinus bradycardia, sinus pauses or arrest, sinoatrial exit block, chronotropic incompetence, or alternating bradycardia and tachycardia (i.e., bradycardia-tachycardia syndrome). Clinical symptoms result from the hypoperfusion of end organs. About 50% of patients present with cerebral hypoperfusion (e.g., syncope, presyncope, lightheadedness, cerebrovascular accident). Other symptoms include palpitations, decreased physical activity tolerance, angina, muscular fatigue, or oliguria. A diagnosis is made by directly correlating symptoms with a bradyarrhythmia and eliminating potentially reversible extrinsic causes. Heart rate monitoring using electrocardiography or ambulatory cardiac event monitoring is performed based on the frequency of symptoms. An exercise stress test should be performed when symptoms are associated with exertion. The patient's inability to reach a heart rate of at least 80% of their predicted maximum (220 beats per minute – age) may indicate chronotropic incompetence, which is present in 50% of patients with sinus node dysfunction. First-line treatment for patients with confirmed sinus node dysfunction is permanent pacemaker placement with atrial-based pacing and limited ventricular pacing when necessary.

Sinus node dysfunction, previously known as sick sinus syndrome, is characterized by abnormal initiation and propagation of electrical impulses from the sinoatrial node (SAN). The resulting abnormalities include bradycardia (less than 50 beats per minute [bpm]), sinus pause (more than three seconds), sinus arrest, and sinoatrial exit blocks, which are sometimes associated with supraventricular tachyarrhythmias in bradycardia-tachycardia syndrome14 (Table 1511 ). Bradycardia-tachycardia syndrome occurs in approximately 50% of patients with sinus node dysfunction and increases the risk of stroke and death.5,12 Symptoms manifest as end-organ hypoperfusion, including palpitations, decreased physical activity tolerance, easy fatigability, dizziness, and syncope.2,5,6,13 To diagnose sinus node dysfunction, a combination of symptoms and documented electrical abnormalities must be present.5,7

FindingDescription
BradycardiaSinus bradycardia: sinus rhythm with HR < 50 bpm
Atrial fibrillation with slow ventricular rate: ventricular rate with HR < 50 bpm
Wandering pacemaker: at least three distinct P waves with HR < 50 bpm
Bradycardia-tachycardia syndromePresent in 50% of patients at time of sinus node dysfunction diagnosis; atrial tachyarrhythmias alternating with bradycardia
Sinus node arrestComplete loss of automaticity of the sinus node; prolonged asystole with or without escape rhythm (junctional or idioventricular)
Sinoatrial exit blockFirst-degree: conduction delay between the sinoatrial node and atrial tissue; bradycardia as above
Second-degree type 1: single impulse blocked; pause following progressive decrease in the P–P interval*
Second-degree type 2: single impulse blocked; distinct pause preceded by generally constant P–P interval*
Third-degree: multiple impulses blocked within sinus node; pause followed by a beat originating with atrial depolarization (i.e., preceded by a P wave)

Epidemiology

Sinus node dysfunction may occur at any age7,14; however, increasing age is the most significant risk factor with the highest disease prevalence in patients 70 to 89 years of age.2,7,8,14 The incidence of sinus node dysfunction is 0.8 per 1,000 person-years and is expected to double by 2060 due to the aging population.15 Conditions associated with advanced age such as hypertension, chronic kidney disease, diabetes mellitus, and coronary heart disease are overlapping risk factors and potential causes of sinus node dysfunction.2,15 Brugada syndrome, a rare inherited ion channel disorder that results in ventricular tachyarrhythmias and sudden cardiac death, is also associated with sinus node dysfunction.5,16,17

Etiology

Causes of sinus node dysfunction are generally categorized as intrinsic or extrinsic based on their effect on the SAN (Table 22,58,18 ). It is important to note that sinus node dysfunction is usually a progressive condition and most causes are chronic and irreversible.5

Intrinsic causes
Acute myocardial infarction
Cardiomyopathies
Congenital ion channel dysfunction (HCN4, SCN5A mutations)
Connective tissue diseases
Embolization of the sinoatrial node artery
Idiopathic degenerative fibrosis
Infection (Chagas disease [cardiomyopathy], diphtheria [myocarditis], rheumatic heart disease, typhoid)
Infiltrative disease (amyloidosis, hemochromatosis, sarcoidosis)
Muscular dystrophy
Postsurgical changes or surgical injury to the sinoatrial node
Extrinsic causes
Diabetes mellitus (risk factor for structural or physiologic changes)
Excessive vagal tone (athletic training, during sleep, postmyocardial infarction)
Hypertension (risk factor for structural or physiologic changes)
Medications: antiarrhythmic drugs (Class I and II); amiodarone, amitriptyline, beta blockers, cimetidine (Tagamet), digoxin, lithium, marijuana, nondihydropyridine calcium channel blockers; sympatholytic drugs
Metabolic abnormalities (hyperkalemia, hypokalemia, hypocalcemia, hypothermia, hypothyroidism, hypoxia)
Toxins

INTRINSIC CAUSES

Intrinsic causes originate from structural or functional changes within the SAN. These changes can occur because of fibrosis, ischemia, cardiac remodeling, infiltrative disease, or ion channel dysfunction. 8,18,19 Degenerative idiopathic fibrosis of the SAN is the most common cause of sinus node dysfunction.4,5,7,8,15 Elastic fiber and fatty and fibrous tissue buildup at the SAN and surrounding myocardial tissue increases with age and may lead to prolonged SAN refractory time and, therefore, a decreased intrinsic heart rate.2,4,8 Ischemic heart disease and embolization of the sinus node artery may cause ischemic necrosis of the node, resulting in sinus node dysfunction.8 Acute myocardial infarction may induce a transient sinus node dysfunction caused by autonomic disturbance and increased vagal tone.2,12 Cardiac remodeling following myocardial infarction, congestive heart failure, or advanced age can result in structural changes that decrease cardiac tissue voltage transmission and ultimately delay or block the SAN and result in sinus node dysfunction.3,8,14

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