Sick sinus syndrome (SSS) also known as sinus node dysfunction is a term used to describe a number of different abnormalities in the heart rhythm (known as arrhythmias). Typically, the arrhythmias associated with sick sinus syndrome involve persistent bradycardia with intermittent tachyarrhythmias.
Sick sinus syndrome is generally caused by a dysfunction of the sinus node, which is the heart’s natural pacemaker responsible for the regular, rhythmic heartbeat. The sinus node, also known as the sinoatrial node (SA), consists of a collection of specialized cells located at the top of the right atrium, which is the right upper chamber of the heart. Normally, an electrical wave comes out of the sinus node and propagates through the rest of the heart, keeping the heart rate between 60 and 100 beats per minute (bpm). In sick sinus syndrome, the sinus node is not able to properly propagate its electrical wave.
Types of Sick Sinus Syndrome
Sick sinus syndrome can include different types of heart rhythm problems including arrhythmias.
Sinus bradycardia: The average heart beats 60 to 100 times per minute. Someone with sinus bradycardia’s heart beats less than 60 times per minute. People like endurance athletes can have a heart rate lower than 60 that don’t cause them any problems. But for most people that heart rate may be too low to pump oxygen-rich blood to the rest of the body.
Risk factors for bradycardia include coronary heart disease, diabetes, high blood pressure, heart valve disease, or cardiomyopathy.
Tachycardia: Tachycardia is when your heart beats faster than 100 beats per minute. A fast heart rate can happen normally during exercise or when your body is under stress or has a fever. But it may lead to serious cardiac events such as heart failure stroke, cardiac arrest, or even death if it happens at other times and is not treated.
Bradycardia-tachycardia: This is when you have alternating fast and slow heartbeats.
Overall incidence of sinus node dysfunction increases with age with 1 in 1000 in adults over 45 years old and 1 in 600 cardiac patients over 65 years old. Sinus node dysfunction is the primary indication for approximately 30%-50% of all pacemaker implantation in the United States. Sinus node dysfunction is a relatively uncommon syndrome in the young and middle-aged population.
Epidemiology of Sick Sinus Syndrome
Sick Sinus Syndrome 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:
- Abrupt, inappropriate, severe sinus bradycardia.
- 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.
- 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.
- 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.
- Failure to resume sinus rhythm after cardioversion, manifesting as a prolonged sinus pause.
- 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%.
- 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.
ECG showing sick sinus syndrome and atrial fibrillation
Normal rhythm strip
Most of the time, sick sinus syndrome doesn’t cause any symptoms. When it does cause symptoms, they are very vague and can be attributed to many other disorders. Therefore, it is very hard to diagnose sick sinus syndrome just on the basis of symptoms.
Symptoms may include:
- Decreased exercise tolerance
- Dizziness or lightheadedness
- Fainting or near-fainting
- The sensation of feeling the heart beat (palpitations)
- Shortness of breath
Over the long term, sick sinus syndrome can increase your risk for stroke and heart failure. Injuries can occur if you fall during a fainting episode.
Causes of Sick Sinus Syndrome
Sick sinus syndrome has several causes. Sometimes, providers don’t know the cause.
Known causes of sick sinus syndrome include:
- Injury or breakdown of your heart’s electrical route or SA node due to aging.
- Injury to your SA node (from surgery for a heart transplant).
- Some medicines for high blood pressure or arrhythmia.
- Your genetics (a rare problem with your genes).
- Metabolic problems.
Other heart conditions can cause sick sinus syndrome, including heart failure and atrial tachyarrhythmias.
Other health problems can cause sick sinus syndrome, such as sarcoidosis, collagen vascular disease, inflammation, muscular dystrophy or cancer that has spread.
The risk of developing SSS increases with age. Having SSS at birth is called congenital sick sinus syndrome. Congenital heart conditions are the main cause of SSS in young children and adults.
Risk factors for SSS include:
- History of congenital heart disease (even with repairs, the heart is still weak)
- History of thyroid disease
- Sleep apnea disorder
- The coronary artery becomes clogged, and blood flow to the heart is restricted
- Older age
Complications of sick sinus syndrome include:
- Syncope and pre-syncope (sometimes with injury)
- During periods of profound tachycardia (e.g. fast AF) patients may experience rate-related myocardial ischaemia, which can cause a troponin rise.
- Tachyarrhythmias may also precipitate acute heart failure
- Heart block due to AVN fibrosis
- Patients with tachy-brady syndrome (paroxysmal atrial fibrillation) are at risk of thromboembolic events, such as stroke.
- Sudden cardiac death (rare, more common in AVN disease/heart block)
Your healthcare provider may think you have SSS based on your symptoms, but they are common in many other diseases. To diagnose your condition, your healthcare provider will do an electrocardiogram (ECG). This is a machine that records your heart’s rate and rhythm. If you don’t have symptoms at the time of your ECG, it may look normal.
Other possible tests include:
- An ECG while you walk on a treadmill (stress test).
- A Holter monitor. This is a recorder you wear for over 24 hours that takes an ECG. Some holters can be worn for several weeks.
- An event recorder. This is a recorder you wear over several days to weeks that samples your heart rate.
- An implantable loop recorder. This is a small heart recorder put underneath the skin over the heart. It records the heart rhythm for up to 3 years.
- Electrophysiologic testing. This is a hospital test that involves threading catheters into your heart through a vein in your groin.
- Echocardiogram or ultrasound of your heart. This is done to check for structural heart problems.
How to treat Sick Sinus Syndrome?
The goals of sick sinus treatment are to reduce or eliminate symptoms and to manage any other contributing health conditions.
Treatment of sick sinus syndrome may include:
- Regular checkups
- Catheter procedures
- Surgery to implant a device to maintain a regular heartbeat (pacemaker)
If you don’t have symptoms, your health care provider may simply recommend regular health checkups to monitor your condition. Most people with symptoms need to have a procedure to implant a device to maintain a regular heartbeat (pacemaker).
Some medications, including those used to treat high blood pressure or heart disease, may interfere with sinus node function. Your health care provider will likely review the medications you take and may adjust them or prescribe different ones.
Medications may be needed to prevent or to slow down fast heartbeats.
Blood-thinners (anticoagulants), such as warfarin (Jantoven), dabigatran (Pradaxa) or others, may be prescribed if sick sinus syndrome is associated with atrial fibrillation or other irregular heart rhythms linked to stroke.
Surgeries or other procedures
Most people with sick sinus syndrome eventually need a permanent device to control the heart rhythm (pacemaker). A pacemaker is a small, battery-powered device that’s implanted under the skin near the collarbone during a minor surgical procedure. The pacemaker stimulates (paces) the heart as needed to keep it beating regularly.
If sick sinus syndrome symptoms are mild or infrequent, the decision to use a pacemaker will depend on the results of electrocardiograms (ECGs), your overall health and the risk of more-serious problems.
The type of pacemaker you need depends on the type of irregular heart rhythm you have. Types of pacemakers include:
- Single chamber pacemaker. This type usually carries electrical signals to the right lower heart chamber (ventricle) of the heart.
- Dual chamber pacemaker. This type paces the right lower heart chamber (ventricle) and the right upper heart chamber (atrium) separately. Most people with sick sinus syndrome benefit from dual-chamber pacemakers.
- Biventricular pacemaker. Biventricular pacing, also called cardiac resynchronization therapy is for people who have heart failure and heartbeat problems. This type of pacemaker stimulates both lower heart chambers (the right and left ventricles) to make the heart beat more efficiently.
If your heart rate is still irregular after getting a pacemaker, you may need medications or a catheter-based procedure called cardiac ablation to correct or control it. Cardiac ablation uses heat or cold energy to create tiny scars in the heart to block faulty signals and restore a regular heartbeat. It’s most often done using thin, flexible tubes called catheters inserted through the veins or arteries. Less commonly, ablation is performed during cardiac surgery. A type of cardiac ablation called AV node ablation is often used to control fast heart rhythms in people with pacemakers.
Prevention of Sick Sinus Syndrome
While most risk factors for sick sinus syndrome cannot be controlled, there are ways you can help prevent high blood pressure, heart disease and other conditions that can damage the heart muscle and its electrical system.
Practice good heart health: Watch what you eat, exercise and avoid smoking.
Take your medications as prescribed: If you have high blood pressure, high cholesterol or diabetes, be certain to take your prescribed medications as directed.
Get regular check-ups: Abnormal heart rhythms can be detected at a regular appointment. If you have sick sinus syndrome, the earlier you’re diagnosed and treated, the better.