The role of autonomic testing in syncope
Introduction
Syncope is defined as a transient loss of consciousness, associated with cerebral hypo-perfusion, no lasting neurologic deficits and complete recovery (Moya et al., 2009, Freeman et al., 2011a, Freeman et al., 2011b). It is a common presenting complaint among patients in both outpatient clinics and emergency rooms. Patients presenting with syncope account for approximately 1–3% of emergency room visits and are the admitting diagnosis for up to 6% of all hospitalizations (Grossman et al., 2014). In one large study (Brignole et al., 2006) the average inpatient hospital days for syncope work up was 5.5 days. It is estimated that in the United States, the annual costs for syncope related admissions were $2.4 billion, with a mean hospitalization cost of $5400 (Sun et al., 2005).
Section snippets
Pathophysiology and differential diagnosis
Syncope is caused by a decrease in cerebral blood flow. A cessation of blood flow for even 4–10 s has been shown to cause loss of consciousness (Moya et al., 2009). In addition, data acquired from tilt table studies reveal that a drop in systolic blood pressure to 60 mm Hg or lower has been associated with syncope (Moya et al., 2009, Smith et al., 2011). Cardiac output (CO) and total peripheral resistance (TPR) are the determinants of blood pressure. A fall in either of these variables can lead to
Preparation for autonomic testing
All patients referred for autonomic testing should adhere to a standard set of guidelines prior to testing. Patients should avoid nicotine and caffeine the day of testing. Ideally, all medications should be held for 5 half-lives before autonomic testing. However this may not be feasible or safe. Holding medications for 48 h may be a practical compromise (Low, 2003). In some cases it may be preferable to test patients on medications to determine the clinical response in their typical outpatient
Reflex syncope
Reflex syncope is caused by an intermittent, inappropriate vasodilation and/or bradycardia, which occurs in response to a particular trigger. It is reported to be the etiology in 21–60% of patients presenting after a syncopal episode (Moya et al., 2009). In vasovagal syncope, the event is most often triggered by pain, fear, or prolonged standing. The pathophysiology of neurally mediated syncope is not completely elucidated. However, it is clear that after standing, there is pooling of blood in
Conclusion
Syncope is a common presenting complaint in both the inpatient and outpatient settings. The main goals in clinical evaluation of syncope are for identification of an underlying etiology and risk stratification. Given the multiple etiologies that may result in syncope, a focused approach is essential. Autonomic function testing provides a safe and controlled environment for evaluation of patients, and plays a pivotal role in the diagnosis of syncope, particularly in challenging cases. The
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(2010) Chapter 79—tilt table studies