ABSTRACT
Transient loss of consciousness may result from any process that causes sudden reversible cerebral dysfunction. The commonest mechanism for this presentation is a transient drop in cerebral perfusion, known as 'syncope.' Cardiac causes of syncope include arrhythmias and structural heart diseases such as hypertrophic cardiomyopathy and aortic stenosis. Myocardial infarction may also cause a transient reduction in cardiac output and thus syncope. Postural hypotension may result from intravascular volume depletion or from loss of normal orthostatic blood pressure control, due to autonomic neuropathy or drugs. Vertebrobasilar ischaemia, subclavian steal syndrome and basilar migraine cause syncope through transient hypoperfusion of the brainstem and thus the reticular activating system. Transient loss of consciousness may also be caused by impaired delivery of oxygen or glucose to the brain by mechanisms other than reduced perfusion, such as hypoglycaemia, hypoxia and carbon monoxide poisoning.
