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less blood pressure is required to achieve adequate blood flow. It is unclear whether this is a mechanism evolved in response to the circulatory di culties of human bipedalism or merely a serendipitous result of a pre-existing circulatory response .

2.1.1 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.

2.2Vasovagal

Main article: Vasovagal syncope

Vasovagal (situational) syncope is one of the most common types which may occur in response to any of a variety of triggers, such as scary, embarrassing or uneasy situations, during blood drawing, or moments of sudden unusually high stress. There are many di erent syncope syndromes which all fall under the umbrella of vasovagal syncope related by the same central mechanism, such as urination ("micturition syncope"), defecation (“defecation syncope”), and others related to trauma and stress.

Vasovagal syncope 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-dilation (e.g., 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-dilation and worsens the e ect of the relatively insu cient 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

2 DIFFERENTIAL DIAGNOSIS

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 (ine ective) 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 tilttable 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 other animals.

The mechanism described here suggests that a practical way to prevent attacks might seem counter-intuitive— specifically to block the adrenergic signal with a betablocker. A simpler plan might be to explain the mechanism, discuss causes of fear, and optimise salt as well as water intake.

Psychological factors also have been found to mediate syncope. It is important for general practitioners and the psychologist in their primary care team to work closely together, and to help patients identify how they might be avoiding activities of daily living due to anticipatory anxiety in relation to a possible faint and the feared physical damage it may cause. Fainting in response to a blood stimulus, needle or a dead body are common and patients can quickly develop safety behaviours to avoid any recurrences of a fainting response. See link for a good description of psychological interventions and theories.[4]

An evolutionary psychology view is that some forms of fainting are non-verbal signals that developed in response to increased inter-group aggression during the paleolithic. A non-combatant who has fainted signals that she or he is not a threat. This would explain the association between fainting and stimuli such as bloodletting and injuries seen in blood-injection-injury type phobias such as trypanophobia as well as the gender di erences.[5]

2.2.1 Deglutition (Swallowing) syncope

Syncope may occur during deglutition. Manisty et al. note: “Deglutition syncope is characterised by loss of consciousness on swallowing; it has been associated not only with ingestion of solid food, but also with carbonated and ice-cold beverages, and even belching.”[6]

2.3 Cardiac

2.3.1 Cardiac arrhythmias

The most common cause of cardiac syncope is cardiac arrhythmia (abnormal heart rhythm) wherein the heart beats too slowly, too rapidly, or too irregularly to pump