Syncope in patients with WPW syndrome is usually related to episodes of paroxysmal reentrant supraventricular tachycardia, or even preexcitated atrial fibrillation. Although intermittent preexcitation suggested long refractory periods in accessory pathway, making tachyarrhythmia unlikely, the observation in our case of extreme bradycardia as the cause of syncope was unexpected. Holter tracing shows conducted sinus rhythm with delta wave, followed by abrupt complete AV block after one beat showing AV node conduction (figure).
Unfortunately, electrophysiological study (EPS) was not performed in our patient, so we cannot define mechanism underlying sudden ventricular asystole. However, certain mechanisms can be reasonably excluded.
First of all, as AV block starts following AV node conduction it could have been related to a previous ablation attempt that injured normal conduction system. This was not our case, since the patient had no underwent any previous EPS.
An alternative explanation is a neuromediated reaction. However, it is surprising that conduction block was also not preceded by sinus rate bradycardia (PP intervals remain at the same rate) in the context of increased parasympathetic tone.
Fatigue phenomenon is described as the failure of conduction through the accessory pathway after a period of repetitive excitation (or intermittent preexcitation) 1. This is a common finding, and its presence suggest a low likelihood of clinical tachycardia. Another possible mechanism of intermittent preexcitation is linking, defined as concealed conduction into the accessory pathway from the normal conduction system. The absence of regular antegrade conduction through the normal conduction system in this patient makes these explanations highly unlikely.
We made the diagnoses of both WPW syndrome and AV block, whether congenital or acquired. Disorders of conduction in both normal AV node and AV by-pass tracts, although rarely observed, may exist simultaneously in patients with WPW syndrome, intermittent, or fixed. There are just a few cases reported in the literature since the first description by Coumel et al in the early 70’s 2. However, patients rarely experience syncope or bradycardia-related symptoms in adulthood.
There are some entities in which WPW and AV block may be associated, such as congenitally corrected transposition of the great arteries, or familiar cardiomyopathies such as PRKAG2 mutations 3. Our patient had no structural heart disease (ruled out through an extensive cardiac imaging study) nor family history of cardiovascular diseases, and was not receiving any antiarrhythmic drug.
A permanent ventricular VDD pacemaker was implanted. The patient remained asymptomatic, with no new syncopal episodes for the next years. Follow up was lost as patient finally left town.
Although no definitive conclusions can be made without an EPS, this case illustrates two interesting points: (1) Preexcitation might occur in association with AV conduction disorders. This should be kept in mind before ablating a manifest accessory pathway, especially in children. (2) Symptoms such as syncope or dizziness in a patient with intermittent WPW pattern should not always be attributed to tachyarrhythmia. However, intermittent preexcitation does not imply impaired conduction in the normal AV conduction system.
