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August 1, 2017 Circulation. 2017;136:e60–e122. DOI: 10.1161/CIR.0000000000000499 e96 Several investigators have estimated the costs per clinically meaningful test result. Physician reviewers determined whether the results of a diagnostic test affected clinical management at a US tertiary referral hospital after an episode of syncope.77 The cost per informative diagnosis (as ordered in routine practice) affecting clinical management varied widely by specific diagnostic test, from postural blood pressure ($50) through telemetry ($1100) to EEG ($32 973).77 Similar high costs per actionable diagnosis occur in children admitted for new-onset syncope. Finally, mean costs per diagnostic result were also high in an outpatient ($19 900) specialty clinic for unexplained recurrent syncope.163 12. EMERGING TECHNOLOGY, EVIDENCE GAPS, AND FUTURE DIRECTIONS The writing committee created a list of key areas in which knowledge gaps are present in the evaluation and management of patients presenting with syncope. These knowledge gaps present opportunities for future research to ultimately improve clinical outcomes and effectiveness of healthcare delivery. 12.1 Definition, Classification, and Epidemiology Reported incidence and prevalence of syncope vary significantly because of several confounders: variable definitions for syncope versus transient loss of consciousness, different populations, different clinical settings, and different study methodologies. Definition and classification of syncope provided in this document will set the standard for future research. Standardized national registries and large sample databases are needed to gather data on a continuous basis to understand the true incidence and prevalence of syncope, understand patient risk, inform driving policies, improve patient outcomes, and improve and streamline health service delivery. Aortic dissection580–582 Aortic dissection may manifest with neurological symptoms, myocardial infarction, and HF. Syncope can occur in as many as 13% of aortic dissections. The risk of in-hospital death, tamponade, and neurological deficits is higher in patients with syncope. Otherwise, syncope alone does not appear to increase the risk of death. Subclavian steal583–587 The phenomenon of flow reversal in a vertebral artery ipsilateral to a hemodynamically significant stenosis of the subclavian artery. Severe cases resulting in vertebrobasilar ischemia may rarely result in syncope. Syncope is generally associated with upper-extremity activity. Coarctation of the aorta588 If severe, it can result in HF or aortic dissection. Associated bicuspid aortic valve stenosis may be considered with syncope. Rheumatoid arthritis589 Chronic, autoimmune inflammatory disorder with systemic manifestations. Rarely associated with complete heart block and syncope. Syringomyelia590–597 Chiari malformation598 Arnold Chiari malformations are the most common form of syringomyelia. Syringomyelia-induced disruption of sympathetic fibers in the thoracic spinal cord is a rare mechanism of syncope.599 Neck/vagal tumor600,601 Recurrent syncope is an uncommon complication of neck malignancy. The mechanism may be invasion of the carotid sinus or the afferent nerve fibers of the glossopharyngeal nerve. Endocrine Carcinoid syndrome602 Pheochromocytoma602,603 Mastocytosis602–609 Vasoactive intestinal peptide tumor These tumors can release vasoactive peptides and cause vasodilation, flushing, pruritus, and gastrointestinal symptoms. Syncope is usually due to transient hypotension. Hematologic Beta thalassemia major610 Severe anemia, multiple organ failure, and dilated cardiomyopathy due to iron overload. Syncope may be arrhythmic. Neurological disorders Seizure-induced bradycardia/ hypotension611–614 Generally due to temporal lobe epilepsy. Postictal bradyarrhythmia is uncommon and likely originates from the temporal lobe or limbic system. Migraine615,616 Migraine headaches are statistically associated with syncope. Syncope may be vasovagal or due to orthostatic intolerance. ACC indicates American College of Cardiology; AHA, American Heart Association; AV, atrioventricular; ECG, electrocardiogram; HCM, hypertrophic cardiomyopathy; HF, heart failure; HRS, Heart Rhythm Society; LV, left ventricular; LVH, left ventricular hypertrophy; SCD, sudden cardiac death; VA, ventricular arrhythmias; VT, ventricular tachycardia; and V VS, vasovagal syncope. Table 9. Continued Condition Clinical Characteristics Notes Downloaded from http://ahajournals.org by on October 7, 2026