Physiological Mechanisms Before and During Fainting (Syncope)
Fainting (syncope) is caused by a temporary reduction in cerebral blood flow that leads to global cerebral hypoperfusion, resulting in transient loss of consciousness due to insufficient oxygen delivery to the brain's reticular activating system. 1
Pre-Syncope Phase
During the pre-syncope phase, several physiological changes occur:
Autonomic Nervous System Changes
- Reflex syncope (vasovagal): The most common type involves:
- Increased parasympathetic activity causing bradycardia
- Reduced sympathetic vasoconstrictor outflow causing vasodilation 1
- Blood vessels in the body open too widely, causing blood to pool in lower extremities
- Blood pressure drops significantly
Prodromal Symptoms
Before loss of consciousness, patients may experience:
- Lightheadedness/dizziness
- Nausea and sweating
- Visual disturbances (blurring, constriction of visual field)
- Pallor
- Feeling of warmth
- Abdominal discomfort 1
These prodromal symptoms are directly related to autonomic activation and cerebral hypoperfusion. They typically occur seconds to minutes before actual syncope 1.
During Syncope
When cerebral blood flow becomes critically reduced:
Cerebral Effects
- Cerebral blood flow falls below the threshold needed to maintain consciousness
- Unconsciousness occurs when blood supply to the reticular activating system in the brainstem becomes insufficient
- Complete loss of consciousness typically lasts no longer than 20 seconds in reflex syncope 1
Cardiovascular Changes
- Blood pressure drops significantly
- In vasovagal syncope: heart rate may slow dramatically or even stop temporarily 1
- Systemic vascular resistance decreases
Postural Changes
- Loss of postural tone occurs
- Patient falls to a horizontal position (which is actually protective as it helps restore cerebral blood flow)
Recovery Phase
- Unconsciousness abolishes voluntary influence over respiration, restoring autonomic control
- Horizontal position facilitates return of blood flow to the brain
- Recovery is usually accompanied by almost immediate restoration of appropriate behavior and orientation
- Post-recovery period may be marked by fatigue 1
Different Types of Syncope
Reflex (Neurally-Mediated) Syncope
- Most common type in healthy individuals
- Triggered by emotional stress, pain, standing for prolonged periods
- Involves a reflex pathway with afferent, central, and efferent components 1
Orthostatic Syncope
- Occurs when standing up due to impaired peripheral vasoconstriction or reduced intravascular volume
- Blood pressure decreases without appropriate increase in heart rate 1
Cardiac Syncope
- Due to arrhythmias or structural heart problems limiting cardiac output
- Associated with higher morbidity and mortality than other types 1
Important Clinical Considerations
Syncope must be differentiated from other causes of transient loss of consciousness like epilepsy, metabolic disorders, or psychogenic pseudosyncope 1
Pre-syncope symptoms provide an opportunity for intervention with counter-pressure maneuvers:
- Leg crossing with muscle tensing
- Hand gripping
- Arm tensing 1
These maneuvers can increase blood pressure rapidly and abort an impending syncopal episode by increasing venous return and cardiac output 1
Common Pitfalls and Caveats
Misdiagnosis: Syncope can be confused with seizures, especially when brief myoclonic movements occur during profound cerebral hypoperfusion.
Duration assessment: Complete loss of consciousness in typical syncope lasts less than 20 seconds, though it may seem longer to observers.
Hyperventilation: While hyperventilation can reduce cerebral blood flow through vasoconstriction, it remains uncertain whether hyperventilation alone can cause complete loss of consciousness 1.
Post-syncope confusion: Unlike seizures, syncope typically does not cause prolonged confusion after the event. Patients usually recover orientation quickly, though they may feel fatigued 1.