Sick Sinus Syndrome

Etiology

The etiologic factors leading to sinus node dysfunction can be classified into two categories: intrinsic pathology to the sinus node itself, typically results from fibrosis of the nodal tissue and external causes that affect the SA node function.

While the most common intrinsic factor leading to sinus node dysfunction is age-related degeneration of the SA node [1], sinus node dysfunction can be a result of congenital disorders, arrhythmias, infiltrative disorders and surgery. Recent studies have identified several mutations in the ion channels explaining familial and congenital forms of sick sinus syndrome. Heart failure and atrial tachyarrhythmias have been shown to induce cellular remodeling of the sinus node in animal models [2]. Infiltration of the SA node by sarcoidosis, amyloidosis, hemochromatosis, collagen vascular disease or metastatic cancer results in SA node dysfunction. Damage to SA node or the sinus nodal artery may occur after cardiothoracic surgery from valve replacement, correction of congenital heart disease or heart transplant. Infectious agents such as bacterial endocarditis and Chagas disease commonly result in atrioventricular conduction problems rather than sinus node dysfunction. Because the SA node is located within the atrial wall, ischemic injury by atherosclerosis of the arteries feeding the SA node is unusual [3]. 

Several external causes that can affect the pacing function of the SA node. These can occur in conditions where there is abnormally increased vagal tone such as carotid sinus hypersensitivity, vasovagal syncope, and autonomic dysfunction. Furthermore, metabolic derangements such as hypothyroidism, hyperkalemia, hypokalemia, hypocalcemia, hypoxia, and hypothermia can lead to depression of the pacing function of the SA node. Obstructive sleep apnea may cause bradycardia by profound hypoxia during episodes of apnea. Increased intracranial pressure (Cushing’s reflex) also causes bradycardia. Several pharmacologic and toxic substances can produce a similar effect on the SA node; these include class I to IV antiarrhythmic medications, digoxin, lithium and sympatholytic medications.

Epidemiology

Sinus node dysfunction is a disease of older adults, although it can occur at any age. The mean age of a patient with sinus node dysfunction is 68 years. Sinus node dysfunction develops in one of every 600 cardiac patients of 65 years of age or older. Males and females are equally affected. Sinus node dysfunction accounted for more than 50% of the pacemaker implantations in the United States in the 1990s [4].

Pathophysiology

The sinoatrial (SA) node is composed of a collection of atrial myocytes that are specialized into two different functional cells. Pacemaker “P cells” with intrinsic pacemaker function and transitional “T cells” which are responsible for propagating the impulse into the right atrium. Dysfunction of these cells leads to the several arrhythmias:

1. Abrupt, inappropriate, severe sinus bradycardia.
2. Sinus pause or arrest where there are pauses of 3 seconds or more without atrial activity. This is often rescued by an escape rhythm which can be atrial, junctional and ventricular in origin. It usually reflects a failure of P cells to generate the action potential.
3. Sinoatrial (SA) exit block which is caused by the failure of the T cells to transmit the impulse. Three degrees of sinoatrial (SA) block parallel atrioventricular (AV) block: (1) First-degree SA block is usually subclinical and undetectable on the surface electrocardiogram (ECG); (2) Second-degree-type 1 SA block is characterized by progressive shortening of PP interval prior to the dropped P wave, and Second-degree-type 2 SA block refers to a dropped P wave without any preceding change in the PP interval; (3) Third-degree SA block refers to complete failure of the SA node to conduct impulses to the atrial tissue and is reflected by the absence of P waves on surface ECG. Third-degree SA exit block is clinically identical to sinus arrest.
4. Tachy-brady syndrome is identified by bradycardia alternating with paroxysmal supraventricular arrhythmias, most frequently atrial fibrillation (AF). This results from abnormal automaticity and conduction within the atrial tissue. It affects at least 50% of patients with SND.
5. Failure to resume sinus rhythm after cardioversion, manifesting as a prolonged sinus pause.
6. Atrial fibrillation with slow ventricular response in the absence of AV node blocking agents. This is likely due to the simultaneous degeneration in the AV node. In these patients, spontaneous or therapeutic termination of AF (cardioversion) results in a sinus pause due to concomitant SND. The annual incidence of complete AV block ranges from (0% to 4.5% with a median of 0.6% [5][6].
7. Chronotropic incompetence is defined as inappropriate bradycardia resulting in an inability to meet the metabolic demands. It is estimated to occur in 20% to 60% of patients.

History and Physical

The natural history of sinus node dysfunction is usually a slow progression over decades. In the early stages, most patients are asymptomatic. When symptoms do develop, they are usually attributed to hypo-perfusion to vital organs with high oxygen demand. Sinus pauses or severe bradycardia results in central nervous system (CNS) underperfusion which manifests in paroxysmal presyncope or syncope. The sudden termination of AF in tachy-brady syndrome typically results in a prolonged sinus pause and failure to return to sinus rhythm which can manifest in syncope. Renal and gastrointestinal hypoperfusion may result in oliguria and abdominal pain, respectively. Rhythms with AF or atrial flutter pose a significant increased risk of thromboembolism, manifesting as transient ischemic attack or cardioembolic stroke, thus carrying the worst prognosis. Chronotropic incompetence results in fatigue and exercise intolerance. Palpitations are usually felt during episodes of tachycardia, after termination of tachyarrhythmias or due to prolonged pauses. Worsening of angina pectoris or congestive heart failure are also reported.

Evaluation

The first step in evaluating sinus node dysfunction is to exclude reversible causes such as electrolyte abnormalities, metabolic abnormalities, or uncontrolled sleep apnea, and removal of possible offending medications (e.g., beta-blockers, calcium channel blockers). There should be clear documentation of bradycardia that correlates with patient’s symptoms to make a diagnosis of sinus node dysfunction. Given the episodic nature of the disease, diagnosis on 12 lead ECG is unusual and prolonged monitoring to detect and document bradycardic episodes is often indicated. This is achieved by 24-48 Holter monitor or telemonitoring, depending on the severity of the symptoms. If telemetry or Holter monitoring fails to demonstrate bradycardia, then an external or implantable long-term event monitor or a loop recorder is warranted. If the diagnosis of sinus node dysfunction is strongly suspected, but no arrhythmias were detected, then the electrophysiologic study is suggested. Autonomic modulation testing such as prolonged sinus pause more than 3 seconds after carotid sinus massage is suggestive of sinus node dysfunction but not by itself diagnostic. Testing for chronotropic incompetence is usually done by treadmill exercise test especially in those who experience fatigue and exertional symptoms. Chronotropic incompetence is diagnosed when the patient achieves less than 80% of maximal predicted heart rate; however, this has not been clinically validated [6].

Treatment / Management

The management of sinus node dysfunction includes identifying and correcting reversible factors, as an initial step. After treating reversible causes, the treatment of sinus node dysfunction can include the placement of a permanent pacemaker. A permanent pacemaker is indicated in symptomatic patients who have documented bradycardia responsible for their symptoms or in those who have demonstrated chronotropic incompetence. A permanent pacemaker is also indicated in sinus node dysfunction secondary to medications necessitated by another medical condition. This is commonly encountered in tachy-brady syndrome where beta-adrenergic blockers are required for treatment of AF, but they suppress the SA node causing prolonged sinus pauses and syncope upon the cessation of tachyarrhythmia episodes. Moreover, it is reasonable to consider a permanent pacemaker in symptomatic patients who have bradycardia less than 40 beats per minute that were not documented to be associated with their symptoms. A permanent pacemaker is also reasonable in patients who have had a diagnostic electrophysiologic study.

When considering the type of pacemaker, the dual-chamber pacing is usually preferred due to the increased risk of AV block in sinus node dysfunction. Anticoagulation should be addressed in patients with paroxysmal atrial tachyarrhythmias since these patients are at increased risk of stroke. Dual-chamber pacemaker is recommended in patients with sinus node dysfunction due to increased risk of AV block. Moreover, atrial pacing, attained with a dual-chamber pacemaker, may reduce the incidence of AF and subsequent risk of systemic thromboembolism and stroke compared to patients who receive ventricular pacing only [7]. Importantly, sinus node dysfunction follows a benign course and the risk of sudden cardiac death is low, as sinus node dysfunction has not been shown to impact survival whether treated with a pacemaker or not.[8][9][10][11](B3)

Differential Diagnosis

• Atrial Fibrilation

• Atrial Flutter

• AV Block

• Bradycardia

• Carotid Sinus Hypersensitivity

• Neurocardiogenic Syncope