Definitions of Syncope and Presyncope
Syncope is a transient, complete loss of consciousness caused by transient global cerebral hypoperfusion, characterized by rapid onset, short duration (typically ≤20 seconds), inability to maintain postural tone, and spontaneous complete recovery. 1, 2
Syncope: Key Defining Features
The most recent and authoritative definition from the 2017 ACC/AHA/HRS guidelines specifies that syncope must include 2, 3:
- Abrupt, transient, complete loss of consciousness
- Inability to maintain postural tone (usually leading to falling)
- Rapid and spontaneous recovery
- Presumed mechanism: cerebral hypoperfusion
Critical Distinguishing Elements
The definition explicitly excludes other causes of loss of consciousness such as 1:
- Epileptic seizures
- Head trauma/concussion
- Metabolic disorders (hypoglycemia, hypoxia)
- Pseudosyncope (psychogenic)
- Stroke or TIA
Duration and Clinical Course
- Typical syncope episodes are brief: complete loss of consciousness in reflex syncope usually lasts no longer than 20 seconds 1, 4
- Recovery is accompanied by almost immediate restoration of appropriate behavior and orientation 4, 1
- Post-recovery may include fatigue, though confusion is typically absent 4
- Retrograde amnesia may occur more frequently than previously recognized, particularly in older individuals 4
Prodromal Symptoms (When Present)
Some forms include a premonitory period with 4, 1:
- Lightheadedness
- Nausea
- Sweating
- Weakness
- Visual disturbances (tunnel vision, graying out)
However, loss of consciousness often occurs without warning 1, 4.
Presyncope (Near-Syncope): Definition and Clinical Significance
Presyncope describes a state resembling the prodrome of syncope but without complete loss of consciousness—patients feel as though syncope is imminent but consciousness is not lost. 1, 2
Key Characteristics
The 2017 ACC/AHA/HRS guidelines define presyncope as 2, 3:
- Symptoms before syncope including extreme lightheadedness
- Visual sensations (tunnel vision, graying out)
- Variable degrees of altered consciousness WITHOUT complete loss of consciousness
- May progress to syncope or abort without syncope
Important Clinical Caveat
The mechanisms underlying presyncope remain uncertain—doubts exist as to whether they are identical to syncope mechanisms. 1 However, recent evidence demonstrates that presyncope carries similar short-term serious outcomes and mortality risk as syncope (5.1% serious outcomes at 30 days), with arrhythmia being the most common serious outcome 5, 6.
Practical Implication
Management and risk stratification of patients with presyncope should mirror that of syncope—this approach is endorsed by multiple medical societies 7, 8. Emergency physicians should exercise the same level of caution for presyncope as for syncope, as the risk of serious adverse outcomes (including death, arrhythmia, myocardial infarction) is comparable 5, 6.
Terminology Clarification
- "Pre-syncopal" (adjective): Describes symptoms/signs occurring before unconsciousness in syncope—synonymous with "warning" or "prodromal" 1
- "Presyncope" or "near-syncope" (noun): The clinical state where prodromal symptoms occur without subsequent loss of consciousness 1, 2
Underlying Pathophysiology
The fundamental mechanism is transient global cerebral hypoperfusion 4, 1. A sudden cessation of cerebral blood flow for 6-8 seconds is sufficient to cause complete loss of consciousness 4. This distinguishes true syncope from other causes of transient loss of consciousness where the mechanism is not cerebral hypoperfusion 1.