Walsh & Hoyt: Collateral Circulation Between Cerebral Vessels

Superficial intracranial arteries are not true end arteries. Diffuse leptomeningeal anastomoses over the surface of the brain were first described by Heubner. He found that ligation of the major artery distal to the Circle of Willis did not prevent filling of its supply area by colored fluid injected into adjacent vascular beds. Penetrating intracranial arteries, however, have no effective collateral connections. Leptomeningeal anastomoses thus can protect superficial areas of the brain from infarction, but they rarely are able to support the deep regions from the effects of vascular occlusion. Sparing of the striate cortex at the occipital pole following an occlusion of the ipsilateral PCA is a classic example of the behavior of leptomeningeal collaterals from the middle cerebral supply area. The clinical expression of this phenomenon is sparing of the macula in a patient with homonymous hemianopia. Venous drainage from the head is also quite variable and there are a number of venous collaterals present that may help maintain blood flow in the event of a venous occlusion.