Wolff-Parkinson-White Syndrome

Introduction
Background

Preexcitation was defined by Durrer et al in 1970 when they wrote, "Preexcitation exists, if in relation to atrial events, the whole or some part of the ventricular muscle is activated earlier by the impulse originating from the atrium than would be expected if the impulse reached the ventricles by way of the normal specific conduction system only." Wolff-Parkinson-White (WPW) syndrome is a congenital abnormality involving the presence of abnormal conductive tissue between the atria and the ventricles that is often in association with supraventricular tachycardia (SVT). It involves activation of the ventricles that occurs earlier than anticipated (preexcitation), which occurs because of conduction of an atrial impulse not by means of the normal conduction system, but via an extra atrioventricular (AV) muscular connection, termed an accessory pathway, that bypasses the AV node.1

Classic electrocardiographic (ECG) findings that are associated with WPW syndrome include the following:
Presence of a short PR interval (<120 ms)
A wide QRS complex of longer than 120 milliseconds with a slurred onset of the QRS waveform producing a delta wave in the early part of QRS
Secondary ST-T wave changes

Patients with WPW syndrome are potentially at an increased risk of dangerous ventricular arrhythmias due to extremely fast conduction across the bypass tract if they develop fast supraventricular arrhythmias, such as atrial flutter or fibrillation.

Patients with a concealed bypass tract are also discussed in this article. While they have an accessory AV connection, it lacks antegrade conduction, and accordingly they do not have the classical abnormalities of the surface ECG.

Only a small number of patients with WPW syndrome are at risk for sudden death. In these patients, cardiac electrophysiologic (EP) studies and radiofrequency (RF) catheter ablation may be curative. Other presentations include symptomatic SVT, which can also be cured by catheter ablation. Asymptomatic patients, who form the majority, may merely need periodic observation.

This review discusses the pathogenesis, clinical presentation, evaluation, and treatment of patients with WPW syndrome.

Pathophysiology

Patients with preexcitation are at risk of developing supraventricular tachycardia (SVT) generally due to a reentrant mechanism, of which the accessory connections often play an essential role. The genesis of reentrant SVT involves the presence of dual conducting pathways between the atria and the ventricles.2 These pathways are as follows:
The natural AV nodal His-Purkinje tract
An AV accessory tract (ie, AV connection or bypass tract, Kent fibers, Mahaim fibers)

These 2 pathways usually exhibit 2 different conduction properties and refractory periods that facilitate reentry. The effective refractory period (ERP) of the accessory tract is usually longer than that of the normal AV nodal His-Purkinje tract. Several types of SVT have been described, including orthodromic tachycardias, orthodromic tachycardia with a concealed accessory pathway, and antidromic tachycardia.

Orthodromic tachycardia

When a premature ectopic atrial impulse begins to traverse down towards the ventricle, it may block at the accessory tract but conduct in antegrade fashion down the normal pathway, which includes the AV node. The impulse then reenters the accessory tract in retrograde fashion to perpetuate a circus movement of the impulse. Such reentrant tachycardia is described as orthodromic. Premature ventricular contractions (PVCs) can also initiate orthodromic tachycardia.

In orthodromic tachycardia, the normal pathway is used for ventricular depolarization and the accessory tract is used for the retrograde conduction essential for reentry. On ECG findings, the delta wave is absent, QRS complex is normal, and P waves are inverted in the inferior and lateral leads.

Orthodromic tachycardia with a concealed accessory pathway

Some accessory (bypass) tracts are unable to conduct in the antegrade fashion. These are called concealed accessory pathways (ie, concealed WPW syndrome). Although no evidence of the pathway is present during sinus rhythm (ie, no preexcitation), orthodromic tachycardias can occur. Orthodromic tachycardia may also occur when there are 2 or more accessory connections, and, in that case, the retrograde conduction may occur either through the AV node and/or one of the accessory connections.

Differentiation between this type of SVT and usual AV nodal reentrant tachycardia (AVNRT) may be difficult on the standard surface ECG. Nonetheless, if the heart rate is higher than 200 beats per minute and a retrograde P wave is visible following the QRS complex, a concealed accessory pathway may be the diagnosis.

Antidromic tachycardia

Less commonly, a shorter refractory period in the accessory tract may cause block of an ectopic atrial impulse in the normal pathway, with antegrade conduction down the accessory tract and then retrograde reentry of the normal pathway. This type of tachycardia produced is called antidromic tachycardia.

On ECG findings, the QRS is wide, which is an exaggeration of the delta wave during sinus rhythm (ie, wide-QRS tachycardia). Such tachycardias are difficult to differentiate from ventricular tachycardia.

Thus, the mechanism underlying the majority of the tachycardias in patients with WPW syndrome is macroreentry caused by antegrade conduction over the AV node His bundle pathway and retrograde conduction over an accessory pathway (orthodromic). Less common in patients with WPW syndrome is antidromic tachycardia. Even when the accessory pathway conducts only in retrograde fashion, it can still participate in the reentrant circuit and produce an orthodromic AV reciprocating tachycardia.

Frequency
United States

The prevalence of ventricular preexcitation is thought to be 0.1-0.3% in the general population. Estimates of arrhythmia incidence in patients with preexcitation vary widely, ranging from 12-80% in several surveys.

Incidence of preexcitation and WPW syndrome varies from 0.1-3 cases per thousand population (average of 1.5 cases per thousand population) in otherwise healthy persons.
In a review of ECG findings from 22,500 healthy aviation personnel, 0.25% exhibited findings consistent with the WPW pattern, with 1.8% incidence of tachycardia.
The location of the accessory pathways, in descending order of frequency, is (1) the left free wall, (2) posteroseptal, (3) right free wall, and (4) anteroseptal.
The presence of concealed accessory pathways accounts for approximately 30% of patients with apparent SVT referred for EP evaluation.
Approximately 80% of patients with WPW syndrome have a reciprocating tachycardia, 15-30% have atrial fibrillation, and 5% have atrial flutter. Ventricular tachycardia is uncommon.
International

Incidence and prevalence of WPW syndrome worldwide parallels that in the United States.
Mortality/Morbidity

Patients with WPW syndrome have a very small risk of sudden arrhythmic death. Medical therapy with agents such as digoxin may increase this risk. The risk in asymptomatic patients is extremely low.

Overall, sudden death occurs rarely, with an estimated frequency rate of 0.1%.
Other factors that appear to influence risk are the presence of multiple bypass tracts and a family history of premature sudden death. Sudden cardiac death is unusual without preceding symptoms.
Morbidity may be related to rapid near syncopal or syncopal arrhythmias. Even when syncope is absent, the arrhythmia episodes may be highly symptomatic. The potential for syncope, hemodynamically compromising rhythms, or sudden death may prevent patients with WPW from participating in competitive sports or hazardous occupations until the substrate is definitively addressed in an ablative procedure.

Race

No clear racial predilection appears to exist.
Sex

Prevalence may be higher in males.
Age

Certain factors exist regarding age and the prevalence of preexcitation and WPW syndrome.

WPW syndrome is found in persons of all ages, from those in fetal and neonatal age groups to elderly individuals.
Prevalence decreases with age because of loss of preexcitation. Cases have been described in which electrocardiographic evidence of preexcitation disappears.
In patients with abnormal ECG findings indicative of WPW syndrome, the frequency of SVT paroxysms increases from 10% in people aged 20-39 years to 36% in people older than 60 years.3
Clinical
History

WPW syndrome can result in SVT that uses an AV accessory (bypass) tract. The accessory pathway may also act as an innocent bystander and allow conduction during other supraventricular arrhythmias, such as atrial fibrillation or flutter. The possibility of a concealed bypass tract as a mechanism underlying certain types of SVT should be considered because treatment options may vary. Paradoxically, the use of digoxin and perhaps other AV nodal blocking agents may accelerate conduction through the bypass tract, causing potentially lethal ventricular arrhythmias or hemodynamic instability during atrial fibrillation.

SVT in WPW syndrome may begin in childhood or may not appear clinically until the patient reaches middle age. In some patients in whom it first presents during childhood, it may then cease for some time, only to recur. In fact, the probability is 75% that the tachycardia will persist if it is still present in patients older than 5 years.
In asymptomatic patients, the probability of losing the capacity for antegrade conduction across the accessory pathway increases with advancing age. This probably results from fibrotic changes at the site of insertion of the accessory bypass tract.
In patients with WPW syndrome, the tachycardia that produces symptoms may be an SVT, atrial fibrillation, or atrial flutter. In a series of 212 patients with tachyarrhythmias and WPW syndrome, SVT alone occurred in 64%, atrial fibrillation alone in 20%, and both occurred in 16% of patients.
Light-headedness and near syncope appear to occur more commonly in persons with WPW syndrome who have paroxysmal SVT (PSVT) or atrial fibrillation than in those with AV nodal reentry.
Syncope can occur because of inadequate cerebral circulation due to a rapid ventricular rate or because the tachyarrhythmia is depressing the sinus pacemaker, causing a period of asystole at the point of tachycardia termination.
PSVT can be followed after termination by polyuria, which is due to atrial dilatation and release of atrial natriuretic factor.

Physical

During SVT, the rhythm is unvarying and regular, with constant intensity of the first heart sound.

The jugular venous pressure can be elevated, but the waveform generally remains constant.
Clinical features of associated cardiac defects may be present, such as the following:
Cardiomyopathy
Ebstein anomaly: Patients with right-sided accessory pathways should be screened for the Ebstein anomaly.
Hypertrophic cardiomyopathy (AMPK mutation)
The abnormal QRS complexes of WPW syndrome, when present, may appear similar to those observed in acute myocardial infarction (MI). Repolarization abnormalities are common in patients with WPW syndrome.

Causes

In patients with WPW syndrome, the underlying cardiac structural abnormality consists of accessory conduction tissue that bypasses the normal AV node His-Purkinje system pathway. Such pathways are generally believed to be congenital in nature.

The causes of WPW syndrome can be summarized as follows:
Congenital or hereditary
An accessory pathway is quite likely to be congenital, although its manifestations can be detected in later years and it may appear to be acquired.
Relatives of patients with preexcitation, particularly those with multiple pathways, have an increased prevalence of preexcitation, suggesting a hereditary mode of acquisition.
Associated with congenital cardiac defects
Patients with the Ebstein anomaly may develop WPW syndrome. Patients with the Ebstein anomaly frequently have multiple accessory bypass tracts, mostly right-sided, in the posterior part of the septum or the posterolateral wall. Preexcitation generally occupies the atrialized ventricle. The orthodromic reciprocating tachycardia in such patients exhibits right bundle-branch block (RBBB) and a long ventriculoatrial (VA) interval.
Hypertrophic cardiomyopathy may include idiopathic hypertrophic subaortic stenosis or asymmetric septal hypertrophy.
Associated with other acquired cardiac defects - Cardiomyopathies

http://emedicine.medscape.com/article/159222-overview