Watershed infarcts in the brain caused by microemboli.

Multiple vascular occlusions are frequently found in the leptomeningeal arteries over watershed infarcts in the brain. These occlusions have largely been interpreted as thrombi secondary to slowing of the blood flow. This report suggests that most of the occlusions are microemboli, which may lodge preferentially in these areas, and that they are the cause of the infarcts rather than secondary events. These suggestions are based upon the analysis of three groups of patients. The first group consists of four cases, two of which had atheromatous masses and the other two, tumor emboli in the overlying leptomeningeal arteries. These cases prove beyond doubt that microemboli can lodge preferentially in the watershed areas and cause infarcts in the brain. The second group consists of the cases of watershed infarcts that were precipitated by hypotensive episodes. Only one of these showed occlusion of the overlying arteries, although all of them obviously had slowing of the blood flow during the acute phase. These cases thus discredit the concept that stagnation thrombosis is a frequent event. Finally, three cases with watershed infarcts and vascular occlusions interpreted as platelet microemboli are presented to demonstrate different pathogenetic mechanisms effective in the process of embolization.