Elsevier

Autonomic Neuroscience

Volume 184, September 2014, Pages 40-45
Autonomic Neuroscience

The role of autonomic testing in syncope

https://doi.org/10.1016/j.autneu.2014.05.011Get rights and content

Highlights

  • Syncope frequently results in expensive medical evaluation and hospitalization.

  • Autonomic testing can determine the cause of syncope in ~28% of cases.

  • Neurally mediated syncope, POTS and orthostatic hypotension are common diagnoses.

  • Therapeutic interventions can target the underlying disorders, improving outcomes.

  • Autonomic testing is a necessary part of the evaluation for syncope.

Abstract

Syncope is a common presenting complaint in both the inpatient and outpatient settings. The main goals in the clinical evaluation of syncope are to identify an underlying etiology, to stratify risk and to guide plans for therapeutic intervention. Testing begins with an initial electrocardiogram to screen for any cardiac rhythm abnormalities. Heart rate variability to paced breathing provides a standard measure of cardiac parasympathetic function and offers clues towards an autonomic cause of syncope. A Valsalva maneuver is used to evaluate for parasympathetic dysfunction through the Valsalva ratio. In addition, sympathetic adrenergic function is assessed through evaluation of blood pressure response during the Valsalva maneuver. Abnormalities to the Valsalva maneuver can suggest clues towards an autonomic cause of syncope. Head-up tilt table testing is an important part of the autonomic evaluation of patients with syncope, and can be diagnostic for many disorders that result in syncope including orthostatic hypotension, neurally mediated syncope, postural tachycardia syndrome or delayed orthostatic hypotension. 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. While the initial clinical evaluation of syncope involves a detailed history and physical examination; in situations where the diagnosis is unknown, the addition of autonomic testing is complementary and can lead to identification of autonomic causes of 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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