Laughter-induced or gelastic (derived from the Greek word for laughter, 'gelos') syncope is extremely rare. It is a sub-type of the situational syncopes hypothesized to be the result of a neurally mediated reflex triggered by increased intrathoracic pressure. Intense laughter causes repetitive forced expirations in a staccato pattern with a Valsalva-type effect. The associated increase in intrathoracic pressure reduces venous return resulting in decreased cardiac output and a transient reduction in cerebral perfusion. It has also been proposed that strenuous laughter
might produce isometric muscle contraction resulting in