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Syncopal Episode Mechanisms and Causes

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A syncopal episode is an alternate term for syncope. There are several reasons it might occur. But there are three main mechanisms by which it occurs. They are:

  • Neurological
  • Postural
  • Cardiac

What Is Syncope  

The definition of syncope partially addresses the question – why do people faint?  Syncope or a syncopal episode is the professional term for a faint, fainting spell or swoon.  Other terms for it are syncopal attack, blackout and passing out. It is the sudden loss of consciousness and posture due to temporary insufficient blood flow to a large portion of the brain.   

Reduced blood flow to a sizable portion of the brain is also the cause of near syncope – a feeling of almost fainting without actual loss of consciousness. Presyncope is the preferred alternate medical term for near syncope. 

A syncopal episode differs from other forms of loss of consciousness in that recovery is spontaneous and complete.  It is not associated with brain damage and is of short duration. It usually lasts for just seconds to minutes and rarely more than five minutes.

The risk of injury notwithstanding, the falling which occurs during a faint is teleological in that it serves an adaptive purpose.  That purpose is to restore adequate blood flow to the brain.  The reason is an upright posture after passing out would cause blood to pool in parts of the body below the heart due to the effects of gravity. This would cause a decrease in venous return and cardiac output. The resulting insufficient blood flow to the brain if present for a long enough period of time would cause brain damage.

Syncope Causes

A syncopal episode usually belongs to one of three main categories.  It depends upon the cause of the reduced blood flow to the brain.  Those up-to-date categories are the following:    syncopal episode

  • Neurally-mediated syncope
  • Orthostatic hypotension syncope
  • Cardiac syncope

Neurally-mediated Syncope 

Neurally mediated syncope is fainting resulting from an imbalance in the activity of the autonomic nervous system as it regulates blood pressure and heart rate.  This in turn has an adverse impact on blood flow to the brain. The most common types of fainting in this category are:

  • vasovagal syncope
  • situational syncope
  • carotid sinus syncope

VASOVAGAL SYNCOPE   

Vasovagal syncope is fainting caused by a vasovagal reaction. Some refer to it as neurocardiogenic syncope. It is the most common type in this category.  It is also the most common type in young people, especially females.  It can occur in the elderly though. According to current scientific insight it is the result of a specific sequence of physiologic events. Common factors that can trigger it are emotional stress, a warm environment, dehydration and prolonged sitting or standing.  It can also occur when a reason is not apparent.

Based on findings from research using the tilt table test, vasovagal syncope appears to be the result of a specific order of events. The first event is more forceful than usual contractions of the heart resulting from excessive stimulation of the sympathetic nervous system. In the case of emotional stress the stimulation originates in the brain. If the trigger is dehydration, prolonged standing or heat exposure though, the effect is part of a baroreflex in response to decreased venous return. With greater than normal force of contractions of the heart the cavity size diminishes as the amount of blood pumped out exceeds venous return. As the partially filled heart pumps harder than usual mechanical receptors within it undergo activation. The result is a vasovagal reaction with typical hypotension and relative bradycardia. If the trigger is one that involves decreased venous return the ensuing vasovagal reaction ends up being a vicious cycle in that the vasodilation which accompanies it causes a further drop in the blood pressure.

Fainting that occurs after exertion is oftentimes a form of vasovagal syncope which occurs via the same mechanism is that discussed above. Fainting in that circumstance though does raise the possibility of a cardiac cause.  

SITUATIONAL SYNCOPE

neurogenic syncope Other situations can serve as triggers for fainting. Micturition, deglutition, defecation, weight lifting, laughing and coughing are some of the more common ones. Researchers have not studied these forms with tilt table testing to the same extent as they have studied vasovagal syncope. But they believe they involve similar autonomic reflexes.  

CAROTID SINUS SYNCOPE

Fainting from carotid sinus activity occurs primarily in people over the age of 50. It can be spontaneous or induced. Spontaneous fainting occurs when a hypersensitive carotid sinus is inadvertently stimulated. The exaggerated carotid sinus response causes a greater than usual drop in blood pressure and decrease in heart rate.

Factors that might trigger the response are things such as turning the head, extending the head, wearing a tight collar or shaving. Induced fainting on the other hand is that which is provoked with carotid massage. It is more an unmasking of a diseased SA node or AV node in the heart than it is a problem with the carotid sinus itself. Thus, syncope which occurs with carotid massage is a marker for heart disease affecting the electrical conduction system of the heart.  

Orthostatic Hypotension Syncope (Postural Syncope)

Orthostatic hypotension syncope is fainting caused by orthostatic hypotension. It is the most common form of fainting in elderly people. It makes up roughly 10% of all cases of fainting. The blood pressure drop usually occurs between 30 seconds and five minutes after standing. The drop can be immediate or delayed. Therefore, in testing for it the blood pressure should be checked immediately upon standing and three and five minutes later. The cause of postural syncope can be hypovolemia or dysfunction of the autonomic nervous system.

Some common causes of fainting due to hypovolemia are the following:

  • Diuretics taken for high blood pressure or heart disease
  • Excessive perspiration with inadequate fluid replacement
  • Excessive alcohol consumption
  • Various types of anemia (low blood count)
  • End-stage liver disease causing it to not produce enough protein
  • Kidney disease causing the spillage of protein

Autonomic dysfunction can be the result of certain medical conditions or medications. Common medical conditions that can cause it are diabetes, Parkinson’s disease and kidney failure. The medicines that can cause it are those that block some parts of the autonomic reflex which regulate blood pressure. They are mainly those used for treating high blood pressure and certain psychiatric disorders.

Checking the heart rate along with the blood pressure helps in determining if a postural drop in BP is due to low blood volume or autonomic dysfunction. If blood volume is low the heart rate should speed up. In the case of autonomic dysfunction the expected increase in heart rate to maintain cardiac output does not occur. Hence, with the latter, not only does the blood pressure drop. The heart rate does not increase at all, or to the degree that it should. Doctors can test for orthostatic syncope at the bedside or via a tilt table test.  

Cardiac syncope is fainting due to reduced cardiac output caused by heart disease. It is the most serious of the three categories of syncope in terms of future mortality. It consists of three subcategories. They are:

  • Tachyarrythmias – excessively fast heartbeats which can be regular or irregular
  • Bradyarrythmias  – excessively slow heartbeats which can be regular or irregular
  • Structural heart disease – with or without obstruction to blood flow from the heart

A heart rhythm that is too fast and irregular causes fainting because of insufficient time between heartbeats for the heart to adequately fill with blood.  As a result cardiac output and blood flow to the brain decrease. The cause of this problem can be structural heart disease or disease of the electrical conduction system of the heart.

A heart rhythm that is too slow causes fainting because the rate at which the heart pumps blood out syncopal episodeis too slow to maintain adequate cardiac output and blood flow to the brain. This problem can also be due to structural heart disease or isolated disease of the electrical system involving the AV node or SA node such as that accompanying previously mentioned induced carotid sinus syncope.

In addition to causing electrical disturbances structural disease of the heart can cause fainting due to a reduction in stroke volume if the ejection fraction is low. When structural heart disease is a type that causes partial obstruction to the outflow of blood from the heart it is even more problematic. The reason is during exertion peripheral vasodilation occurs in order to increase blood flow to the muscles. At the same time though, the heart cannot increase its output because of the obstruction. The net result of the two processes is a drop in blood pressure and fainting. Fainting which occurs during exertion is a red flag that the cause might be cardiac.

Cardiac syncope accounts for 10% to 20% of cases of faints.  Unlike the other causes though, it is associated with an increased risk of sudden death. Therefore, identifying it is of high priority in the absence of evidence of other causes of fainting spells. Studies have shown that if a person suffers a syncopal episode three times or more the cause is probably not heart- related.  The reason is most people don’t survive three such events.

 

 

 

 

 

 

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