Electrophysiologic testing in the evaluation of patients with syncope of undetermined origin.

Thirty-two patients were prospectively evaluated for syncope (recurrent in 26 patients) with conventional neurologic and cardiovascular testing without elucidation of a cause. All patients underwent invasive electrophysiologic studies to assess sinus nodal function, atrioventricular conduction, and the inducibility of supraventricular and ventricular tachycardia (VT). Eleven patients (34%) were found to have laboratory-induced VT, 5 patients (15%) had evidence of sinus nodal dysfunction, 1 patient (3%) had infra-His atrioventricular block during atrial pacing, and 1 patient (3%) had probable quinidine-related VT. The remaining 14 patients had no detectable abnormalities during electrophysiologic study. After a mean follow-up period of 21.3 +/- 1.2 months, 10 of the 11 patients with inducible VT were asymptomatic while receiving laboratory-directed antiarrhythmic therapy, and 1 patient died from ventricular fibrillation after discontinuing the chosen antiarrhythmic regimen. Five of the remaining 7 patients with an electrophysiologic abnormality were asymptomatic after implantation of a permanent pacemaker or alteration of previous drug therapy, or both. The 14 patients with a normal electrophysiologic study were treated empirically, with recurrent syncope occurring in 4 of these patients during the follow-up period. Invasive electrophysiologic studies provided a presumptive diagnosis in 56% of patients with syncope of undetermined origin. Previously unsuspected VT accounted for 61% (11 of 18) of the detected abnormalities. Therapy specific for the electrophysiologic abnormality was usually successful in preventing recurrent syncope.