, frequently called syncope
in a medical context, is a sudden, and generally momentary, loss of consciousness
, or blacking out caused by the Central Ischaemic Response, because of a lack of sufficient blood and oxygen in the brain. The first symptoms a person feels before fainting are dizziness
; a dimming of vision, or brownout
; and feeling hot. Moments later, the person's vision turns black, and he or she drops to the floor (or slumps if seated in a chair).
Central Ischaemic Response
The Central Ischaemic Response is the brain's response to a chronic lack of blood available to the brain. The brain attempts to prioritise its own needs above any others by commandeering all available blood flow at the expense of all other bodily functions and increasing the rate of blood oxygenation.
It diverts blood to itself by reducing the supply to most of the rest of the body through vasoconstriction and increasing the pulse rate, tachycardia. It attempts to increase oxygenation of the blood by accelerating the breathing rate hyperventilation. This gives rise to the typical symptoms of fainting: pale skin, notably a white face, rapid breathing and weakness of the limbs, particularly the legs, progressing to collapse and is subjectively felt as weakness and nausea.
Weakness of the legs tends to cause victims to lay themselves down or fall down involuntarily thereby countering low blood pressure, hypotension, in the brain. It is unclear whether this is an evolved response or merely a serendipitous result of collapsing.
Factors that influence fainting are taking in too little food and fluids, low blood pressure
, growth spurts
, physical exercise in excess of the energy reserve of the body, emotional distress, and lack of sleep. Orthostatic hypotension
caused by standing up too quickly or being in a very hot room can also cause fainting.
More serious causes of fainting include cardiac (heart-related) causes such as an abnormal heart rhythm (an arrhythmia), where the heart beats too slowly, too rapidly or too irregularly to pump enough blood to the brain. Some arrhythmias can be life-threatening. Other important cardio-vascular conditions that can be manifested by syncope include subclavian steal syndrome and aortic stenosis.
Fainting can also occur following hyperventilation prior to a breath-hold dive in shallow water or on ascent from a breath-hold dive in deep water.
Vasovagal (situational) syncope, one of the most common types, may occur in scary, embarrassing, or uneasy situations or during blood drawing, coughing, or urinating. Other types include postural syncope (caused by a changing in body posture), cardiac syncope (due to heart-related conditions), and neurological syncope (due to neurological conditions). There are many other causes of syncope including low blood sugar levels and lung disease such as emphysema and a pulmonary embolus. The cause of the fainting can be determined by a doctor using a complete history, physical, and various diagnostic tests.
The vasovagal type can be considered in two forms:
- Isolated episodes of loss of consciousness, unheralded by any warning symptoms for more than a few moments. These tend to occur in the adolescent age group, and may be associated with fasting, exercise, abdominal straining or circumstances promoting vaso-dilatation (eg heat, alcohol). The subject is invariably upright. The tilt-table test, if performed, is generally negative.
- Recurrent syncope with complex associated symptoms. This is so-called Neurally Mediated Syncope (NMS). It is associated with any of the following: preceding or succeeding sleepiness, preceding visual disturbance ("spots before the eyes"), sweating, light-headedness. The subject is usually but not always upright. The tilt-table test, if performed, is generally positive.
A pattern of background factors contributes to the attacks. There is typically an unsuspected relatively low blood volume, for instance, from taking a low salt diet in the absence of any salt-retaining tendency. Heat causes vaso-dilatation and worsens the effect of the relatively insufficient blood volume. That sets the scene, but the next stage is the adrenergic response. If there is underlying fear or anxiety (e.g. social circumstances), or acute fear (e.g. acute threat, needle phobia), the vaso-motor centre demands an increased pumping action by the heart (flight or fight response). This is set in motion via the adrenergic (sympathetic) outflow from the brain but the heart is unable to meet requirement because of the low blood volume, or decreased return. The high (ineffective) sympathetic activity is always modulated by vagal outflow, in these cases leading to excessive slowing of heart rate. The abnormality lies in this excessive vagal response. The tilt-table test typically evokes the attack.
Much of this pathway was discovered in animal experiments by Bezold (Vienna) in the 1860s. In animals, it may represent a defence mechanism when confronted by danger ("playing possum"). This reflex occurs in only some people and may be similar to that described in animals.
The mechanism described here suggests that a practical way to prevent attacks would be, counter-intuitively, to block the adrenergic signal with a beta-blocker. A simpler plan might be to explain the mechanism, discuss causes of fear, and optimise salt as well as water intake.
Pure cardiac syncope
Fainting can also occur if pressure on the carotid artery in the neck triggers a vagal signal to the Vaso-Motor Centre, reflexly causing a vagal response to slow the heart.
A pure cardiac arrhythmia is a serious matter that can appear as syncope but this is unusual. Severe narrowing of the Aortic Valve leading to syncope is included for completeness.
Syncope from vertebro-basilar arterial disease
Arterial disease in the upper spinal cord, or lower brain, causes syncope if there is a reduction in blood supply, which may occur with extending the neck or after drugs to lower blood pressure.
Signs and symptoms
A pre- or near-syncope is diagnosed if the individual can remember events during the loss of consciousness (i.e. reports remembering dizziness, blurred vision
and muscle weakness
and the fall previous to hitting their head and losing consciousness). If the individual remembers feeling dizzy and loss of vision, but not the fall, then it is considered a syncoptic episode.
As loss of consciousness is a symptom for a variety of conditions and syncope is difficult to rule out outside of a hospital, a thorough examination is required to determine the cause, including interviews with witnesses as well as evaluation with an electrocardiogram.
If one is suffering from syncope, there are many underlying causes that may be contributing to the episodes. It is important to understand that there is no master list
of tests that are currently being used to diagnose the underlying cause(s). That being said, there are some common diagnostic tests for fainting. Blood Tests
:A hemoglobin count may indicate anemia or blood loss. However, this has been shown to be useful in only about 5% of patients being evaluated for fainting Electrocardiograms
:An electrocardiogram (ECG) records the electrical activity of your heart. It is estimated that from 20%-50% of patients will have an abnormal ECG. However, while an ECG may identify conditions such as atrial fibrillation, heart block, or a new or old heart attack, it typically does not provide a definite diagnosis fo the underlying cause for fainting.Holter monitor testing
:Sometimes, one may be asked to wear a holter monitor
. This is a portable ECG device that can record your heart rhythms during daily activities over an extended period of time. Since fainting usually does not occur upon command, a holter monitor can provide a better understanding of your heart's activity during fainting episodes.Tilt table test
:This is perhaps the most common test performed for syncope. Though it can be helpful, the purpose is to induce a fainting episode, and thus is not necessarily conclusive in why
Recommended treatment involves returning blood to the brain by laying the person on the ground with legs slightly elevated or they should lean forwards with their head between their knees. As the dizziness and the momentary blindness passes, the person may experience a brief period of visual disturbances
in the form of phosphenes
, sudden sore throat, nausea and general shakiness. After the symptoms have passed, sleep
Fainting in women was a commonplace trope or stereotype in Victorian England
and in contemporary and modern depictions of the period. Partly this may have been due to genuine ill-health (the respiratory effects of corsets
are frequently cited), but it was encouraged by the myth of female invalidity, in which it was fashionable for women to affect an aristocratic frailty and create a scene by fainting at a dramatic moment.
Children sometimes will play the 'fainting game', deliberately restricting blood flow to the brain in order to induce syncope.
- Grubb, Blair P. The Fainting Phenomenon; Understanding Why People Faint and What to Do About It. 2001. 2nd ed. New York: Blackwell Publishing, 2007