Recurrent unexplained syncope and presyncope comprise approximately 1–6% of emergency room and approximately 1% of hospital admissions.^1,2 The estimated incidence of self-reported syncope is 6.2 per 1,000 person years in the Framingham study.³ The overall incidence is approximately 3–6% over 10 years, and in a select group of patients, the prevalence of syncope could reach almost 50%.⁴ A symptom-rhythm correlation is often difficult to obtain during spontaneous episodes because of the sporadic, infrequent and unpredictable nature of syncope.¹ A significant percentage of patients referred for Holter monitoring, tilt-table test and electrophysiology study have inconclusive results. In addition, many of these patients also have a negative neurological workup. Recent advances in loop recorder technology have permitted implantation of a subcutaneous nonvascular device to provide for a longer duration of electrocardiographic monitoring, thereby improving the overall diagnostic yield in patients with recurrent infrequent episodes of syncope. The purpose of this study was to review our 5-year experience with implantable loop recorders (ILRs) in identifying the etiology of recurrent unexplained syncope.

Methods
Patients with syncope or presyncope of unknown etiology (who had a negative tilt-table test, electrophysiologic study and neurologic workup) underwent implantation of an ILR. Two types of implantable loop recorders (Reveal™ and Reveal Plus™; Medtronic, Inc., Minneapolis, Minnesota) were used in our study. The main difference between the two being that the Reveal Plus™ has an autoactivation feature, which is helpful when patients are unable to activate the device or are noncompliant. All devices were implanted subcutaneously on the left side of the chest inferior to the clavicle. Once the device was implanted, the gain and the sensitivity were adjusted according to the manufacturer’s recommendations. The device records a bipolar electrocardiographic signal. It contains enough memory to record 42 minutes of compressed data. The episodes can be downloaded after interrogation with a standard Medtronic pacemaker programmer. Patients were chosen for implantation based on a nondiagnostic neurocardiogenic syncope workup. Regular follow up was performed every 2 months. If patients had recurrence of symptoms, they were seen prior to their scheduled follow-up visit. During follow up, loop recorders were interrogated and the data evaluated. These patients who did not follow up in the clinic were called and queried. Likewise, patients in whom a significant arrhythmia was detected were appropriately treated.

Results
One hundred patients (70 women, 30 men) were enrolled in this study. They initially presented with a chief complaint of either syncope or near-syncope. All patients prior to ILR placement had a negative conventional work-up. The presence of significant underlying structural heart disease is listed in Table 1. The mean age of patients enrolled was 68 ± 18 years. Of the 100 patients, 10 received a first-generation (manual activation) ILR, and the remainder had a second-generation device implanted. After 9 ± 8 months of follow up, ILR interrogation identified an arrhythmogenic etiology to the syncope/presyncope in 45 of the 93 patients (7 patients who were lost to follow up were not included). There were a total of 55 different events recorded in the ILR, which included a broad range of arrhythmias. Of the 45 patients identified with an arrhythmia, 8 had a diagnosis by ILR less than 2 months from the date of implantation. Of the 45 patients identified with an arrhythmia, 12 were men. These findings are summarized in Table 2. Twenty six of the patients had documented symptomatic bradycardia (asystole, sinus pauses, atrial fibrillation with long pauses). Eleven patients had episodes of sinus tachycardia with heart rates of 130–140 beats/minute. Four of these 11 patients were asymptomatic at the time the events were recorded. Two patients had atrial tachycardia. Five patients had multiple episodes of nonsustained ventricular tachycardia. One patient had sustained ventricular tachycardia. Four patients had paroxysmal supraventricular tachycardia. One patient had sick sinus syndrome recorded in the ILR. One patient had a seizure, which was recorded as high-frequency noise by the ILR. Two of the 10 patients who had a first-generation device failed to activate the ILR during an event. Only 1 patient developed an infection of the device that required removal. All arrhythmias were treated successfully by pacemaker/ICD implantation, radiofrequency ablation and/or medication. Table 3 shows the modes of treatment of documented problems in these patients. Twenty eight patients were treated with a permanent pacemaker. Six patients received an implantable defibrillator. AV junction ablation was performed on 4 patients. Medical management (amiodarone, beta-blockers) was utilized in the remaining 7 patients. Seven patients were lost to follow up, 3 who died during the course of the study (etiology unknown).

Discussion
The etiology of recurrent syncope is difficult to determine if the electrophysiologic and neurologic workup is negative. The cost of investigating syncope is also high. In 38–47% of patients, no etiology may be identified despite Holter monitoring, tilt-table testing and invasive electrophysiology studies.^1,2 It is often difficult to connect the infrequent symptoms with arrhythmias due to the inability to monitor and record these sporadic events. Mortality and morbidity associated with syncope is generally low overall, but 1-year mortality may be as high as 33% in certain subgroups of patients with a cardiac etiology.^1,2 Syncope may be benign or may be the only warning before an episode causing sudden death. Even if the cause is benign, recurrent syncope can result in injury and provoke substantial anxiety among patients and their families.⁵ Even though there are no data on mortality benefits of ILR, it can be inferred that these devices may play a role in reducing mortality and morbidity in syncope. In this study, the utility of the implantable loop recorder in diagnosing unexplained syncope was demonstrated. The automatic arrhythmia detection feature helped to record additional events beyond those recorded by manual activation. Also in this study, arrhythmia detection was observed in 48% of patients, leading to a specific treatment. The results are comparable to other studies in which ILR diagnosed between 17–42% of patients with unexplained syncope.^6,7 One of the largest studies combined data from 3 different centers, and a total of 206 patients had negative neurologic and electrophysiologic studies prior to ILR implantation. Symptoms recurred in 69% of patients 93 ± 107 days after implantation. The above-cited study demonstrated that bradycardia was more frequently identified (as opposed to tachyarrhythmias) and led to pacemaker implantation.⁸ Our study confirmed the results of prior studies, namely that ILRs were more likely to make a diagnosis when compared to conventional testing.⁸
In summary, Winthrop University Hospital’s 5-year experience with the ILR in 100 consecutive patients confirms the utility of this device in the diagnosis of recurrent, infrequent, unexplained syncope or presyncope. Despite a negative cardiac and neurologic workup, long-term monitoring with ILR helped diagnose almost 50% of patients with unexplained syncope with negative electrophysiologic and neurologic workup. Medical device and/or catheter ablation helped successfully treat all patients with an arrhythmogenic etiology detected by ILR.