Q1: Why is the middle cerebral artery excluded from the Circle of Willis?
A1: The middle cerebral artery branches directly off the internal carotid and extends laterally away from the base of the brain into the Sylvian fissure. It does not loop back or participate in forming the communicating ring that links the anterior and posterior circulations.
Q2: How does cranial nerve III palsy distinguish an aneurysm from diabetic microvascular disease?
A2: Compressive lesions (such as a posterior communicating artery aneurysm) push against the external surface of the third nerve, damaging superficial parasympathetic pupillomotor fibers early and producing a dilated, non-reactive pupil. Diabetic microvascular ischemia strikes deep central motor fibers while sparing superficial pupillomotor axons, typically leaving pupillary reflexes intact.
Q3: What causes macular sparing during a PCA cortical stroke?
A3: The occipital pole, which processes central visual information from the macula, benefits from dual arterial supply. Terminal branches of the middle cerebral artery anastomose with distal posterior cerebral artery twigs at watershed zones, preserving perfusion to the macular cortex even if the primary PCA stem occludes.
