Manifestations of sick sinus syndrome

Manifestations of sick sinus syndrome

Sick sinus syndrome is often considered to be a disease of the sinus node alone. But it is often associated with disease of the atrioventricular node (AV node) and sometimes with pan conduction tissue disease. Hence the alternate name structural nodal disease. Other terms which have been used earlier are lazy sinus syndrome and sluggish sinus syndrome [1]. When both sinus and AV node are diseased, it has been called binodal disease or double nodal disease.

Usual manifestation of sick sinus syndrome is a prolonged pause, usually due to sinus arrest. Pauses can also be due to sinoatrial block. In case of sinoatrial blocks, there may be a numerical relationship between the length of the pause and basic sinus cycle length. Pause in sinus arrest is at least one and a half times the basic sinus cycle length.

There may be difficulty in ascertaining these timings in case of exaggerated sinus arrhythmia, which is another manifestation of sick sinus syndrome. Still, symptomatic sinus pauses are the clinical hallmarks of sick sinus syndrome. It is often mentioned that though these pauses can cause syncope, they seldom cause death, unlike the asystole in complete heart block.  In type I second degree sinoatrial block, there is progressive shortening of PP interval, like the progressive shortening of RR interval in type I second degree AV block.

Another important clinical manifestation of sick sinus syndrome is the so called tachy-brady syndrome with episodes of sinus pauses and atrial fibrillation with fast ventricular rate. Ventricular rate need not always be fast in atrial fibrillation with sick sinus syndrome. Presence of associated AV nodal disease causes atrial fibrillation with controlled ventricular rate and even slow ventricular rate.

Inter atrial and intraatrial blocks may also be associated with sick sinus syndrome. Both can cause widening of the P wave without left atrial enlargement documented by imaging studies like echocardiography. Advanced interatrial block can manifest as wide biphasic P waves in inferior leads, known as Bayes syndrome [2]. Terminal negative component of the P wave in inferior leads is due to delayed activation of left atrium from below upwards secondary to the block in Bachmann’s interatrial bundle in the upper part.

Delay in the onset of sinus rhythm after cardioversion for atrial flutter or atrial fibrillation is another manifestation of sick sinus syndrome. Prolonged junctional rhythm may occur prior to resumption of sinus rhythm in some cases. Failure of the sinus node to accelerate adequately with exercise and fever, known as chronotropic incompetence, is another manifestation of sick sinus syndrome. Chronotropic incompetence has been dealt with in detail earlier.

Apart from a good clinical history, routine ECG and ambulatory ECG or Holter monitoring are useful in documenting sinus node dysfunction in sick sinus syndrome. It is essential to document symptoms in a diary during Holter monitoring for correlation. Futility of treating asymptomatic pauses has to be borne in mind. Length of the pause has poor correlation with symptoms and prognosis. If symptoms are infrequent a loop recorder, a home recording device or an implantable loop recorder is needed to pick out the pauses.

Autonomic testing in sinus node dysfunction is another aspect. Pharmacologic interventions/maneuvers test reflex responses of the heart rate. Carotid sinus massage causing pauses of more than 3 seconds is considered significant. Occasionally such pauses may occur in asymptomatic elderly individuals. Intrinsic heart rate is the heart rate during complete autonomic blockade with atropine and beta blockade. In a case of clinical bradycardia, a low intrinsic heart rate suggests intrinsic sinus nodal dysfunction while a normal intrinsic heart rate indicates abnormal autonomic regulation.

Invasive testing is reserved for symptomatic patients whose bradycardia cannot be documented by noninvasive means. Sinus node recovery time, corrected sinus node recovery time and sinoatrial conduction time are measured. But the ability to provide a definitive diagnosis by these parameters is limited. Concomitant AV nodal disease is seen in about 17% of patients with sinus nodal dysfunction. New AV conduction abnormalities develop at the rate of 2.7%/year.

References

1. Ginks WR. Lazy sinus syndrome. Proc R Soc Med. 1970 Dec;63(12):1307-8. PMID: 5490793; PMCID: PMC1812355.
2. Baranchuk A, Torner P, de Luna AB. Bayés Syndrome: What Is It? Circulation. 2018 Jan 9;137(2):200-202. doi: 10.1161/CIRCULATIONAHA.117.032333. PMID: 29311351.