Current literature reveals three types of paroxysmal atrioventricular block (AVB) that can cause syncope:
Intrinsic paroxysmal atrioventricular block is due to an intrinsic disease of the AV conduction system; this type of “cardiac syncope” is also called Stokes-Adams attack;
Extrinsic vagal paroxysmal atrioventricular block is linked to the effect of the parasympathetic nervous system on cardiac conduction and is one of the mechanisms involved in “reflex syncope.”
Extrinsic idiopathic paroxysmal atrioventricular block is associated with low levels of endogenous adenosine and is supposed to be one of the mechanisms involved in “low-adenosine syncope.”
These three types of paroxysmal AVB present different clinical and electrocardiographic features. Additionally, the efficacy of cardiac pacing and theophylline therapy to prevent syncopal recurrences is also different for these three types of AVB.
Intrinsic AVB, they type we are most familiar with, can take the form of either phase 3 or, less commonly, phase 4 block. Vagally mediated AVB is characterized by sinus slowing leading up to and during the AVB, and evidence that pacing is helpful is lacking. Extrinsic idiopathic paroxysmal AVB due to low baseline adenosine levels is a more recently described entity.