Sick sinus syndrome

Overview

Sick sinus syndrome (SSS), also known as sinus node dysfunction, is a group of heart rhythm disorders caused by malfunction of the sinoatrial (SA) node—the heart’s natural pacemaker. The SA node, located in the right atrium, is responsible for initiating electrical impulses that regulate the heartbeat. In SSS, this node fails to function properly, resulting in abnormal heart rhythms (arrhythmias) such as bradycardia (slow heart rate), tachycardia (fast heart rate), or alternating episodes of both (tachy-brady syndrome). The condition is more common in older adults and may lead to dizziness, fainting, fatigue, or even heart failure if left untreated.

Causes

Sick sinus syndrome arises from damage or degeneration of the SA node or its surrounding tissue, leading to impaired impulse generation or conduction. Causes can be broadly categorized as intrinsic (originating within the heart) or extrinsic (external factors affecting heart rhythm):

Intrinsic Causes:

• Age-related degeneration: The most common cause; fibrosis or scarring of the SA node occurs with aging

• Ischemic heart disease: Reduced blood flow to the heart may impair SA node function

• Cardiomyopathies: Conditions that damage heart muscle tissue

• Congenital heart defects: Rare but possible in children and young adults

• Infiltrative diseases: Sarcoidosis, amyloidosis, or hemochromatosis affecting heart tissue

Extrinsic Causes (Reversible):

• Medications: Beta-blockers, calcium channel blockers, digoxin, and antiarrhythmic drugs

• Electrolyte imbalances: Especially hyperkalemia or hypokalemia

• Hypothyroidism: Reduced thyroid hormone levels can slow heart rate

• Sleep apnea: Intermittent hypoxia may trigger arrhythmias

In many cases, especially among the elderly, the condition is progressive and irreversible.

Symptoms

Symptoms of sick sinus syndrome vary depending on the type and severity of arrhythmia. Some individuals may be asymptomatic, while others experience disabling symptoms. Common manifestations include:

• Bradycardia: Slow heart rate leading to fatigue, dizziness, lightheadedness, or syncope (fainting)

• Tachy-brady syndrome: Alternating episodes of slow and fast heart rates, which may cause palpitations, chest discomfort, or fainting

• Exercise intolerance: Inability to perform physical activity due to inadequate cardiac output

• Confusion or memory problems: Especially in older adults, due to poor brain perfusion

• Shortness of breath or angina: Particularly when SSS is associated with underlying heart disease

In some cases, the first sign may be sudden syncope or cardiac arrest.

Diagnosis

Diagnosing sick sinus syndrome requires a careful evaluation of symptoms, physical examination, and confirmation of abnormal heart rhythms using various diagnostic tools:

• Electrocardiogram (ECG): Initial test to detect bradycardia, pauses, or atrial arrhythmias

• Holter monitor: A 24- to 48-hour portable ECG monitor to record intermittent arrhythmias

• Event recorder or implantable loop recorder: For capturing infrequent symptoms over weeks or months

• Exercise stress test: To assess the heart’s response to exertion and uncover chronotropic incompetence (inability to increase heart rate with activity)

• Electrophysiological study (EPS): An invasive test to map electrical pathways and confirm SA node dysfunction, used in selected cases

• Blood tests: To rule out thyroid dysfunction, electrolyte imbalances, or drug effects

Treatment

Treatment of sick sinus syndrome focuses on relieving symptoms and preventing complications. The approach depends on the severity and type of arrhythmia, and whether symptoms are present.

Lifestyle and Medication Adjustments:

• Review and discontinue medications that may exacerbate bradycardia

• Treat underlying conditions (e.g., hypothyroidism, electrolyte imbalances)

• Manage contributing factors such as sleep apnea

Pacemaker Implantation:

• Permanent pacemaker: The mainstay treatment for symptomatic bradycardia or chronotropic incompetence

• Dual-chamber pacemaker: Preferred for patients with tachy-brady syndrome to allow atrial pacing and AV synchrony

Pharmacologic Therapy:

• Antiarrhythmic medications: May be used in patients with tachyarrhythmias, but only after pacemaker placement to prevent exacerbation of bradycardia

• Anticoagulation: In patients with atrial fibrillation or flutter to reduce stroke risk

Patients with asymptomatic or mild SSS may be monitored without immediate intervention.

Prognosis

The prognosis for sick sinus syndrome depends on the underlying cause and the patient’s overall health. With appropriate treatment, especially pacemaker therapy, most individuals can manage the condition effectively and maintain a good quality of life. However:

• The condition is progressive in many cases and may lead to complete SA node failure

• Patients with SSS have an increased risk of developing other arrhythmias, particularly atrial fibrillation

• Untreated SSS can result in recurrent syncope, injury from falls, or sudden cardiac arrest

With timely diagnosis and proper management, including lifestyle changes, medication adjustments, and pacemaker implantation when necessary, the outlook for patients with sick sinus syndrome is generally favorable.