Introduction
Background
The term sick sinus syndrome first appeared in the literature in 1967 to describe a rapid atrial rhythm interspersed with varying periods of bradycardia that followed cardioversion. Today, the term is applied to a broad range of electrophysiological abnormalities, including inappropriate sinus bradycardia, sinus arrest, sinus node exit block, chronic atrial fibrillation, and bradycardia-tachycardia syndrome. Although the term remains popular, the more comprehensive title sinus nodal dysfunction (SND) is gaining favor.
The disease commonly is observed in older patients with a history of concomitant heart disease. Its natural history is largely unknown, but it frequently runs an erratic and progressively malignant course. For this reason, cardiac pacing has become the cornerstone of therapy for symptomatic patients.
Pathophysiology
SND is characterized by delayed or failed conduction between the sinus node and the atria, either due to inadequate sinus node pacemaking or because of intrinsic or extrinsic conduction disturbance. If the degree of dysfunction is minimal, the patient usually is asymptomatic. Advanced SND is unpredictably malignant, however, and it often is characterized by significant cardiovascular impairment. Clinical manifestations are due to hypoperfusion of the vital organs, specifically the brain and heart, usually as a result of an inadequate ventricular rate. Patients commonly present with complaints related to cerebral or cardiac dysfunction.
Frequency
United States
The exact incidence rate of SND is unknown because it never has been evaluated adequately in asymptomatic patients. One study estimates the incidence to be about 3 cases in 5000 patients older than 50 years.
International
SND is more prevalent in countries where citizens have a longer life expectancy, indicating that the disease is more common in elderly people.
Mortality/Morbidity
Duration from the onset of symptoms to death is not well defined. Untreated patients may live a couple of weeks to more than 10 years. SND almost always is progressive, and patients usually become increasingly symptomatic if they remain untreated. Survival appears to depend primarily on the severity of the underlying cardiac disease. Thromboembolic complications are a frequent cause of morbidity and mortality. Chronic atrial fibrillation and tachycardia-bradycardia syndrome are the variants with the most significant stroke risk. Sudden cardiac death is possible at any point during the disease
Race
No racial preponderance exists.
Sex
Men and women are affected in equal numbers.
Age
- SND tends to be a disease of elderly people, with a peak incidence in the sixth and seventh decades of life.
- The disease may be observed in any age group, including adolescents and children.
- Pediatric patients are most susceptible during the postoperative period of major cardiac surgery, especially transposition of the great vessels.
Clinical
History
Because SND is a heterogenous condition, its clinical manifestations may vary widely. In the early stages of SND, most patients are asymptomatic. As the disease advances, however, patients often seek medical attention for bradycardia-related symptoms. Syncope, near-syncope, and dizziness are the most frequently reported complaints, followed by palpitations, angina, or shortness of breath. Because these symptoms are relatively nonspecific, a high index of suspicion is necessary to make the correct diagnosis.
- Cerebral symptoms
- Patients with mild symptoms present with vague complaints of fatigue, irritability, labile mood swings, or forgetfulness.
- As the disease progresses and blood flow is further compromised, the cerebral manifestations become more profound. They can include lightheadedness, slurred speech, near-syncope, and, finally, syncope. Syncope almost always implies marked bradycardia. It is believed to occur in most patients with SND.
- Cardiac symptoms
- Early in the course of the disease, patients may note an abnormally slow or irregular pulse.
- As the disease progresses, the 3 most common cardiac manifestations are palpitations, angina, and shortness of breath, which is due to congestive heart failure (CHF).
- Palpitations may be attributed to runs of supraventricular tachycardia (SVT) or to a mixture of tachycardia and bradycardia.
- Angina and CHF symptoms usually are due to cardiac hypoperfusion. Cases of flash pulmonary edema have been reported.
- In late stages, the incidence of ventricular tachycardia or fibrillation increases, thereby augmenting the risk for sudden cardiac death.
- Other symptoms
- Other symptoms include hypoperfusion of the kidney with resultant oliguria.
- Some patients have a history of vague gastrointestinal complaints, probably also related to inadequate oxygenation.
Physical
Certain physical findings are suggestive, but the results of the physical examination alone never are diagnostic. The most persistent finding is long periods of bradycardia, appearing in as many as 75% of patients. Any person with unexplained, marked bradycardia probably requires further workup. Occasionally, arrhythmia may be detected with careful assessment of the pulse. Several basic maneuvers can help establish the diagnosis.
- Valsalva maneuver
- In a healthy individual, the Valsalva maneuver will increase heart rate.
- A patient with SND, however, usually will not respond appropriately (heart rate will not change).
- Carotid sinus massage: Carotid sinus massage may be helpful. If carotid massage provokes a sinus pause of longer than 3 seconds, SND must be considered carefully.
Causes
Although the exact etiology usually is not identified, most cases are believed to be attributable to a combination of intrinsic and extrinsic influences. The most common intrinsic causes are idiopathic degenerative disease and coronary artery disease. The most common extrinsic causes are medications and autonomic hyperactivity. Remember that SND is always an acquired condition.
- Intrinsic sinus nodal dysfunction
- Idiopathic degenerative disease is the most common intrinsic cause. With aging, the myocardium surrounding the sinus node gradually becomes replaced by fibrous stroma. As this fibrosis progresses, automaticity and sinoatrial conduction can become impaired. In a patient with SND, fibrosis of the sinus node is virtually complete. Why certain patients with diffuse fibrosis develop SND while others do not remains unclear.
- Coronary artery disease may cause SND either by chronic hypoperfusion or as a complication of an acute ischemic event. An acute inferior or lateral myocardial infarction sometimes is complicated by significant bradycardia or sinus arrest; these arrhythmias are due to local neural effects and usually are transient. Temporary SND has been reported in 5-10% of acute myocardial infarctions.
- The following also may cause intrinsic SND:
- Infiltrative diseases (amyloid, hemochromatosis, neoplasms)
- Cardiomyopathy
- Hypertension
- Collagen vascular diseases (systemic lupus erythematosus [SLE], scleroderma)
- Congenital heart disease
- Surgical trauma/heart transplant
- Musculoskeletal disorders (myotonic dystrophy, Friedreich ataxia)
- Myocarditis/pericarditis
- Extrinsic sinus nodal dysfunction
- Medications that depress sinus nodal function often are implicated as the cause of SND. Common offending agents include the following:
- Beta-blockers
- Nondihydropyridine calcium channel blockers (diltiazem, verapamil)
- Cardiac glycosides (digoxin)
- Sympatholytic antihypertensives (clonidine, methyldopa, and reserpine)
- Membrane-active antiarrhythmics (amiodarone, sotalol, bretylium)
- Less commonly, phenytoin, amitriptyline, lithium, and phenothiazine
- Autonomic dysfunction may be caused by vagal stimulation slowing the sinus rate and lengthening the refractory period of the sinus node. Conditions associated with marked hypervagotonia can result in SND. Symptomatic bradycardias secondary to excess vagal tone have been observed in well-trained athletes. Vasovagal syncope, specifically in elderly people, can present with SND. Finally, carotid sinus syndrome has been associated with increased vagal tone that infrequently leads to symptomatic bradyarrhythmias.
- The following also can cause extrinsic SND: electrolyte imbalance (eg, hypokalemia or hypocarbia), hypothyroidism or hyperthyroidism, hypothermia, and sepsis.
- Medications that depress sinus nodal function often are implicated as the cause of SND. Common offending agents include the following:
More on Sinus Node Dysfunction |
 Overview: Sinus Node Dysfunction |
| Differential Diagnoses & Workup: Sinus Node Dysfunction |
| Treatment & Medication: Sinus Node Dysfunction |
| Follow-up: Sinus Node Dysfunction |
| References |
| Next Page » |
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Further Reading
Keywords
SND, sinoatrial disease, sinoatrial dysfunction, sick sinus syndrome, sluggish sinus syndrome, sinus nodal dysfunction, Valsalva maneuver, carotid sinus massage, idiopathic degenerative disease, coronary artery disease, autonomic hyperactivity, intrinsic sinus nodal dysfunction, infiltrative diseases, cardiomyopathy, hypertension, collagen vascular diseases, congenital heart disease, heart transplant, musculoskeletal disorders, myocarditis, pericarditis, beta-blockers, nondihydropyridine calcium channel blockers, cardiac glycosides, sympatholytic antihypertensives, membrane-active antiarrhythmics, autonomic dysfunction, vasovagal syncope, carotid sinus syndrome, extrinsic sinus nodal dysfunction, electrolyte imbalance, hypothyroidism, hyperthyroidism, hypothermia, sepsis
