The term leptomeningeal refers to the two deep layers of the meninges — the pia mater and the arachnoid mater. The leptomeningeal collateral circulation is a network of blood vessels supplying the brain that follows a diffuse course over the superficial surface of the brain.
Leptomeningeal Collateral Circulation and Cerebrovascular Accidents
If the middle cerebral artery is occluded during a CVA (or stroke), there is dramatic loss of blood flow to the area of the brain supplied by this artery (the lateral surfaces of the frontal, parietal and temporal lobes, including the primary motor and sensory cortexes). The leptomeningeal collateral circulation allows a very limited degree of compensation for this. However, this mild compensatory effect is nowhere near adequate - a CVA is a very serious medical emergency.