Pathophysiology of syncope

Abstract

Syncope or near-syncope is a not uncommon effect of gravitational or other stresses and it occurs when cerebral blood flow falls to below about half the normal value. It is not necessarily abnormal, and individuals who are usually asymptomatic show the same reaction if a stress is sufficiently great to result in hypotension. Blood pressure is regulated mainly by baroreceptor reflexes by their control of vascular resistance and heart rate. The ability to vasoconstrict powerfully is important in resisting syncope; heart rate responses are of much less physiological significance. The intriguing unanswered question is what suddenly changes vasoconstriction and tachycardia to vasodilatation and bradycardia. It is now known not to be due to stimulation of cardiac receptors and some cerebral signal is more probable. People are more likely to faint when upright, motionless, warm, following meals, dehydrated or emotionally stressed, and these factors may be involved in some reflex syncopes including micturition and defaecation syncopes. Plasma volume is of considerable importance and increasing this by interventions such as salt loading, exercise training, and even sleeping with the bed head raised can often be of clinical benefit.

Influence of age and gender on the occurrence and presentation of reflex syncope

“…The occurrence of reflex syncope is often preceded by precipitating events (triggers) such as fear, severe pain, or micturition [5]. Though the exact pathophysiological mechanisms by which triggers could lead to the occurrence of syncope are still unknown [14], reflex-mediated factors, physical factors, or a combination of these are thought to be involved [7].…”

Micturition syncope following intermittent catheterisation in a tetraplegic patient

“…The ability to vasoconstrict powerfully is important in resisting syncope; heart rate responses are of much less physiological importance. 7 Low arterial blood pressure and decreased vascular resistance can be found in case of low cardiac output due to inadequate venous return, or in case of low total peripheral vascular resistance, due in particular to autonomic neuropathies, reflex causes or vasodilator drugs. These factors are likely to summate.…”

“…These factors are likely to summate. 7 Autonomic impairment is seen in many SCI patients: the sympathetic outflow of the baroreceptors to the peripheral vessels is impaired. This results in a decreased capacity to regulate vascular tone and, consequently, a decreased capacity to redistribute blood by means of vasodilatation or vasoconstriction.…”

Identical pattern of cerebral hypoperfusion during different types of syncope

“…Although the exact pathophysiological mechanisms leading to the occurrence of syncope are still unknown, 31 reflex-mediated factors, physical factors or a combination of these are thought to be involved. 32 Traditionally, it has been thought that syncope is chiefly caused by hypotension produced by decreased systemic vascular resistance.…”