Causes

History

Orthostatic hypotension is a drop in blood pressure on standing. This causes a transient decrease in cerebral perfusion and therefore loss of consciousness. Prolonged bedrest can result in the deconditioning of the baroreceptors in the body, resulting in a postural drop of blood pressure. A drug history may exclude offending medication such as antihypertensives and opiates. Hypovolaemia is also a cause of postural hypotension (see Shock, p. 421); it is associated with a pallor, tachycardia and a low urine output. Disease states such as diabetes mellitus and Guillain–Barré syndrome can result in autonomic failure and inability of the body to maintain an appropriate blood pressure.

Cardiac outflow obstruction, which occurs with aortic stenosis and hypertrophic obstructive cardiomyopathy, will result in syncope on effort as the cardiac output cannot be increased on demand.

In carotid sinus syndrome, the receptors of the carotid sinus are more sensitive than normal, thus minor stimulation, such as turning the head or pressure from a tight collar, may elicit the carotid sinus reflex and precipitate syncope.

Seizures are paroxysmal discharges in the cortex, which are sufficient to produce clinically detectable events, e.g. convulsions, loss of consciousness or behavioural symptoms. Although it is not strictly syncope, atonic seizures may present in a similar fashion, with a sudden loss of muscle tone and collapse. Patients may be incontinent during a seizure and drowsy or confused during the post-ictal phase.

Hysterical syncope tends to be very dramatic with normal examination findings during the attack.

Hypoglycaemia causes faintness and even results in loss of consciousness. It tends to be more common with insulin-treated diabetics but may also occur in normal individuals after an alcohol binge. Symptoms usually occur when the glucose is below 2.5 mmol/L.