Hemorrhagic stroke occurs when a blood vessel bursts inside the brain. The brain is very sensitive to bleeding and damage can occur very rapidly, either because of the presence of the blood itself, or because the fluid increases pressure on the brain and harms it by pressing it against the skull.
Bleeding irritates the brain tissue, causing swelling (cerebral edema). The surrounding tissues of the brain resist the expansion of the bleeding, which is finally contained by forming a mass (hematoma). Both swelling and hematoma will compress and displace normal brain tissue.
Most often, hemorrhagic stroke is associated with high blood pressure, which stresses the artery walls until they break.
Another cause of hemorrhagic stroke is an aneurysm. This is a weak spot in an artery wall, which balloons out because of the pressure of the blood circulating inside the affected artery. Eventually, it can burst and cause serious harm. The larger the aneurysm is, the more likely it is to burst. It is unclear why people develop aneurysms, but genes may play a role, since aneurysms run in certain families.
Stroke can also be caused by the accumulation of a protein called amyloid within the artery walls, particularly in the elderly. This makes the arteries more prone to bleeding.
Amyloid protein is also implicated in the brain damage related to Alzheimer's disease, but the difference is that people with Alzheimers disease have amyloid accumulatation in the brain tissue instead of in the arteries. Therefore people with Alzheimers usually do not develop brain bleeding.
Some people with brain hemorrhage have abnormal connections between arteries and veins. Under normal circumstances, circulating blood travels through the arteries into the capillaries, where it provides nutrients and oxygen to the tissues. Once the blood has deposited the nutrients and oxygen, it is carried back to the heart from the capillaries via the veins.
However, in some people, a brain artery may connect directly to a vein, instead of going through the capillaries first. This is called an arterial-venous malformation (AVM). Since blood pressure inside the arteries is much greater that inside the veins, the latter may rupture, causing bleeding into the brain.
Another important brain disease that can cause bleeding is cancer. This is especially true for cancers that spread to the brain from distant organs, such as breast, skin, and thyroid.
About 20% of strokes are hemorrhagic -- but the other 80% are caused by the opposite problem: too little blood reaching an area of the brain, which is usually due to a clot that has blocked a blood vessel. This is called "ischemic stroke." This type of stroke can sometimes lead to a brain hemorrhage because the affected brain tissue softens and this can lead to breaking down of small blood vessels.
In addition, brain hemorrhage can be caused when people have problems forming blood clots, which is the natural way that the body stops any bleeding. Clots are formed by proteins called coagulation factors and by sticky blood cells called platelets. Whenever the coagulation factors or platelets do not work well or are insufficient in quantity, people may develop a tendency to bleed excessively.
Some medications (often used, ironically, to prevent ischemic stroke) prevent clot formation. These work by blocking the production of clotting factors (such as the blood thinner warfarin) or interfering with the function of platelets (such as aspirin). The most common side effects of such medications is bleeding, which may occasionally affect the brain. Controlling bleeding to avoid stroke is a very fine balancing act.
Other drugs that can be associated with brain hemorrhage are cocaine and certain cough remedies, although the mechanism by which they do so is unclear.
