Syncope is a transient loss of consciousness due to transient global cerebral hypoperfusion, characterised by rapid onset, short duration, and spontaneous complete recovery.  It is associated with the inability to maintain postural tone.  It has many causes, and is distinct from vertigo, seizures, coma, falls, psychogenic pseudosyncope, drop attacks, transient ischaemic attacks, and states of altered consciousness. 
Incidence rates vary depending on features of the population being studied. There are also some difficulties interpreting epidemiological estimates because of inconsistencies in methodology and definitions used between studies.
In a cross-sectional study of 305,932 patients, the adjusted incidence rate of syncope varied between 0.80 and 0.93 per 1000 person-years and was unchanged from 2000 to 2005. Overall mortality has also remained the same at around 0.28%.  A cause for syncope is determined in 55% to 82% of cases.    In older patients admitted to hospital with syncope, non-cardiovascular causes (such as vasovagal or orthostatic episodes) are twice as common as cardiovascular causes (such as arrhythmias or ischaemia). 
Risk factor identification is the most important step in evaluating a patient with syncope. It is also critical to identify those with high risk of death (e.g., people with structural heart disease or abnormal ECG). 
Kearney Regional Medical Center
SW declares that he has no competing interests.
Dr Sanjeev Wasson would like to gratefully acknowledge Dr Sarabjeet Singh and Dr Nishant Kalra, the previous contributors to this topic. SS and NK declare that they have no competing interests.
Assistant Professor of Medicine
Harvard Medical School
The Clinical Decision Unit and Cardiac Emergency Center
Beth Israel Deaconess Medical Center
SG declares that he has no competing interests.
Professor of Medicine
VA Greater Los Angeles Healthcare System
UCLA School of Medicine
DL declares that he has no competing interests.
Heart and Lung Centre
New Cross Hospital
SP has received funds for attending conferences, and fees for speaking and consultation, from Medtronic Inc. SP was also in receipt of funding for research, between 2007 and 2009, from Medtronic Inc. to the University of Manchester. SP was a Guideline Development Group member of the National Institute or Health and Care Excellence (formerly National Institute for Clinical Excellence) guidelines on transient loss of consciousness ('blackouts') management in adults and young people, published in August 2010.
Department of Cardiology
King's College Hospital
NG declares that he has no competing interests.
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