The pathogenesis of watershed infarcts in the brain.

WATERSHED INFARCTS are ischemic lesions which are situated along the border zones between the territories of two major arteries, for example the anterior and middle or the middle and posterior cerebral arteries (fig. 1). They may also be located between the territories of the major cerebellar arteries and they have even been described between the territories of the small arteries in the basal ganglia. However, the exact nature of the latter lesions is more uncertain. The infarcts may be pale or hemorrhagic or mixed but generally the hemorrhagic component is not prominent. Altogether, approximately 10% of all brain infarcts are watershed lesions. Similar infarcts are also found in other organs, such as the heart and the kidneys, but they are more easily recognized in the brain because of the well defined course and extent of the cerebral arteries. The mechanisms whereby watershed infarcts develop have been debated for many years and they have been variously ascribed to cerebral thromboangiitis obliterans, 12 episodes of systemic hypotension, •'• "• carotid occlusions" and to microembolism.'It has now been convincingly shown that both hypotension and microembolism may cause such lesions. However, in some cases, particularly in those with a progressive or stepwise clinical course, the mechanism still remains uncertain.