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Long‐Term Insertable Loop Monitors: How Much Should We Watch to Get an Answer (Diagnosis)?

Syncope is one of the most commonly encountered medical conditions and a frequent reason for seeking medical care. Despite extensive medical and diagnostic evaluation, the cause of syncope is unexplained in about one-third of patients. This lack of a clear etiology for unexplained syncope leaves both patients and clinicians uncertain as to future prognosis in regard to potential recurrent events. This uncertainty often results in further testing, in part, so as to not miss diagnosing a potentially preventable condition such as a transient arrhythmia, for example a bradycardia, or to make a diagnosis and thus possibly answer the question “why do I keep passing out?” Unfortunately, the answer is often “I don’t know.” A crucial piece of information is whether the etiology of syncope is arrhythmic or nonarrhythmic in origin. In this setting when there is no clear diagnosis, the insertable loop recorder (ILR) has been utilized as a diagnostic tool, typically for 12–18 months. Such technology has clearly been an advance in monitoring in terms of yielding a recording of the electrocardiogram (ECG) at the time of recurrent syncope in comparison with standard holter or event monitoring, especially for infrequent episodes. Even more long-term or repeat noninvasive monitors are still less likely to capture an event simply by the obvious—more time for “watching” with the ILR. In this issue of the journal, Furukawa et al.1 present the data on very prolonged ILR monitoring up to 4 years and the impact on yielding a diagnosis in a highly selected and appropriate ILR population with unexplained syncope. Nearly half (n = 70) of the reported patients (n = 157) were monitored ≥18 months. Diagnoses were made in 68 patients with ILR event analysis with an increased diagnostic yield over time: 30% (year 1); 43% (year 2); 52% (year 3); 80% (year 4). Of the diagnoses, 26% were made after 18 months and overall diagnoses were not dependent on clinical factors at presentation. Consistent with prior studies,2-4 the di-