Q: What are the common causes of subarachnoid bleeding secondary to vascular malformation?

A:There can be two kinds of malformations in the blood vessels of the brain that can bleed into the space between the inner coverings of the brain: 1. Aneurysms: These are balloon like outpouchings of the walls of arteries (blood vessels that carry blood from the heart to the brain). These balloons have very thin walls that can burst. 2. Arteriovenous Malformations (AVM): These are abnormal connections between arteries and veins (blood vessels that carry blood from the brain back to the heart). The thin capillary network that normally exists between arteries and veins is missing here. The result is that blood under high pressure in the arteries (80 - 120 mm mercury) rushes into the thin walled veins that have a tolerance for pressures less than 1 - 2 mm mercury. The commonest cause for rupture in either case is a sudden rise in blood pressure, as when straining.