Discussion

Wolff-Parkinson-White pattern is defined by the manifestation of preexcitation on an electrocardiogram caused by an atrioventricular accessory pathway bypassing the atrioventricular node. The prevalence of Wolff-Parkinson-White pattern is estimated at 0.2% of the general population with 65% of adolescents and 40% of young adults remaining asymptomatic. ^{Reference Averill and Lamb1–Reference Khairy, Van Hare and Balaji3} Young patients with Wolff-Parkinson-White pattern are at a greater risk of developing life-threatening sentinel symptoms compared with adults. ^{Reference Etheridge, Escudero and Blaufox4} Accessory pathways are associated with sporadic development, a familial form due to a mutation in the PRAKAG2 gene, Ebstein’s anomaly, genetic hypertrophic cardiomyopathy, and certain metabolic cardiomyopathies such as Pompe disease and Danon disease. ^{Reference Gollob, Green and Gollob5} Complete atrioventricular block is associated with congenital heart disease (heterotaxy with accompanied atrioventricular canal defects and L-transposition of the great arteries), myocardial infarction, idiopathic fibrosis (Lenegre disease) or degradation of the conducting system due to neuromuscular (muscular dystrophy, Kearns-Sayre syndrome) or neoplastic (primary cardiac lymphoma) disorders, infiltrative (sarcoidosis, amyloidosis) and inflammatory myocardial disease, and iatrogenesis due to ablation or surgical complication. Symptoms of an accessory pathway are typically due to tachyarrhythmias but can rarely be secondary to bradyarrythmias as in this case. In the setting of complete atrioventricular node blockade, an accessory pathway has the potential to offer an alternative method of atrioventricular conduction if it is durable, conducts antegrade, and has a suitable refractory period. Symptoms can still develop in one-to-one accessory pathway conduction due to rate-dependent conduction abnormalities or the development of conduction delays. Atrioventricular pathways may spontaneously degrade and disappear. ^{Reference Chen, Chiang and Tai6} We describe a case of a child with an atrioventricular accessory pathway that bypassed a non-functional atrioventricular node and began to degrade leading to symptomatic intermittent complete heart block.

This case demonstrates the coexistence of presumed iatrogenic complete atrioventricular nodal block with failing antegrade accessory pathway conduction. This is a rare occurrence, and case report observations have described constant or transient atrioventricular block in patients with Wolff-Parkinson-White syndrome in the setting of congenital and iatrogenic causes of atrioventricular nodal block. ^{Reference Seipel, Both and Breithardt7} The presence of an accessory pathway can be advantageous in the setting of complete atrioventricular nodal block by providing physiologic one-to-one atrioventricular conduction. ^{Reference Rubio, Saez and Jimenez8} Occasionally, an accessory pathway can manifest long after a complete atrioventricular node block is diagnosed and lead to the development of one-to-one atrioventricular conduction without retrograde conduction. ^{Reference Rubio, Saez and Jimenez8} Alternatively, accessory pathway conduction can appear at different developmental stages in childhood and adolescence and can exist into adulthood without complication. ^{Reference Erickson, Yetman and Jones9}

An early clue that there had been atrioventricular node damage during the patient’s surgery was the postoperative electrocardiogram change. At the time, the patient was described as having a new left bundle branch block, which can sometimes mimic the Wolff-Parkinson-White pattern as seen in Figure 1, which shows a short PR interval, delta wave, and QRS morphology inconsistent with the left bundle branch block. While atrioventricular conduction block can be associated with ostium primum atrial septal defects, our patient had new evidence of conduction delay on postoperative and follow-up electrocardiograms eight years later leading us to suspect an iatrogenic etiology. We hypothesise that the surgery unmasked the Wolff-Parkinson-White pattern. The pathway was most likely concealed preoperatively due to competitive conduction through the atrioventricular node, which led to the pathway being revealed when the atrioventricular node was injured.

Our patient had a non-functional atrioventricular node and an intermittently conducting accessory pathway, the durability of which was unpredictable and required permanent pacemaker placement. Intermittent failure of accessory pathway conduction may manifest as episodic light-headedness, fatigue, exertional intolerance, or syncope in those with atrioventricular nodal disease, and this may be the first clue to the presence of the underlying atrioventricular nodal disease. ^{Reference Dinckal, Davutoglu and Bayata10} Others have described presentations of syncope that was initially thought to be due to a tachyarrhythmia associated with an accessory pathway which was revealed to be secondary to complete infrahissian atrioventricular block. ^{Reference Palanca, Quesada and Roda11} Such cases along with ours highlight the importance of perioperative electrocardiogram monitoring and a thorough understanding of a patient’s conduction system prior to performing an accessory pathway ablation, which would have left this patient without atrioventricular conduction if performed prior to pacemaker implantation. Regardless of the electrophysiological properties of the atrioventricular accessory pathway, a permanent pacemaker is indicated whenever a high degree atrioventricular node disease is identified because of the propensity of accessory pathways to degrade leading to complete atrioventricular conduction blockade. ^{Reference Mackintosh, Chamberlain and Curry12}

This is a rare instance of Wolff-Parkinson-White syndrome presenting with symptoms of bradyarrhythmia rather than tachyarrhythmia. When a previously concealed accessory pathway is unmasked, it is important to consider the reason for the change. This case demonstrates the propensity of accessory pathways to spontaneously degrade and reveal underlying atrioventricular node disease, and for this reason, permanent pacemaker placement is indicated for all high-degree atrioventricular node blocks, regardless of the presence of one-to-one atrioventricular conduction through an accessory pathway. Finally, this case demonstrates the importance of a thorough electrophysiology study prior to accessory pathway ablation.