Arterial occlusion by an embolus or a thrombus is usually followed by infarction of a portion of the region supplied. Anastomotic channels are present between branches of the major arteries on the surface of the brain. There are also communications at the arteriolar level, and the capillary bed is continuous throughout the brain.
These anastomoses, however, are usually insufficient to sustain the circulation in the region normally supplied by a major artery. The size of an infarction depends on the caliber of the occluded artery, existing anastomoses, and the time elapsing before complete obstruction.
In addition to intracranial occlusions, impairment of the cerebral circulation is often caused by stenosis of a carotid or vertebral artery in the neck. The slender, thin-walled arteries that penetrate the ventral surface of the brain to supply the internal capsule and adjacent gray masses are especially prone to rupture
Hypertension and degenerative changes in these arteries are major factors that lead to cerebral hemorrhage. An aneurysm usually occurs at the site of branching of one of the larger arteries at the base of the brain.
An aneurysm may leak or rupture, and there is bleeding into the subarachnoid space. In some cases, adhesion of the aneurysmal sac to adjacent structures can give rise to hemorrhage that is intracerebral or into a cranial nerve.
Aneurysms often develop at sites of branching of arteries in and near the arterial circle, and they can rupture or leak, causing subarachnoid hemorrhage. The most common sites for such aneurysms are the terminal part of the internal carotid artery, the anterior communicating artery, the proximal part of the middle cerebral artery, and the posterior communicating artery.
A subarachnoid hemorrhage causes a severe headache of sudden onset, with a stiff neck and other signs of meningeal irritation