[Sudden loss of consciousness: clinical presentation and pathophysiologic mechanisms].

Most cases of sudden and temporary loss of consciousness [syncope] are caused by hypoperfusion of the formatio reticularis. more rarely by primary neurologic or metabolic disorders. The most common etiology is vasodepressor (vasovagal) syncope, which is caused by peripheral vasodilation due to acute withdrawal of the efferent sympathetic tone, while the parasympathetic outpour is increased at the same time. Although the efferent limb of the baroreflex manifests in a rather uniform way, the afferent parasympathetic limb is very variable, leading to a variety of clinical presentations and triggers (orthostatic hypotension, pain, fear, cough, micturition, emotions). While vasodepressor syncope mainly occurs in young people with healthy hearts, cardiac syncope caused by arrhythmias or obstructive lesions are more frequently found in elderly patients with organic heart disease. Neurogenic syncope comprises either primary neurologic disorders, such as epilepsia, or hypoperfusion of the vertebrobasilar system (TIA). Rarely, an acute increase of intracerebral pressure may cause syncope. Similarly, metabolic disorders or side effects of drugs are rare causes of syncope; however, drugs may act as important cofactors in the pathogenesis of syncope.