High cholesterol, specifically high LDL and low HDL, is clearly a risk factor for the development of heart disease, heart attack, and death. High cholesterol promotes the development of atherosclerosis or fatty plaques in the heart arteries. The blood flow to the heart muscle can then be slowly interrupted by this gradual narrowing or suddenly cut off if the plaque ruptures and forms a blood clot. The neck and brain arteries can also develop fatty plaques as well. One would then think that a clear association exists between high cholesterol and stroke.
A recent analysis looked at 61 studies involving almost 900,000 people and tried to make a clear connection between cholesterol and death from stroke. (The fact that there have been 61 studies looking at this question should give you some clue as to what the final conclusion was) In the end, there was no clear independent relationship between high cholesterol and death from stroke. However, the relationship between high cholesterol and the risk of death from heart disease was again confirmed.
Contrast to the above findings, statins are a class of medications that specifically lower cholesterol mainly by reducing bad LDL. Numerous studies have shown that statins lower the risk of heart attack in those with high cholesterol. A recent study from 2006 showed that those who had suffered a stroke can reduce their risk of another stroke by taking this cholesterol lowering medication. In this particular study, LDL was reduced by more than a third, and the relative risk of stroked was reduced by ~15%.
So how do we reconcile the two paradoxical findings? On the one hand, high cholesterol does not seem to be a strong independent risk factor for stroke and yet a potent cholesterol lowering medication can protect against recurrent stroke. The answer seems to be that statins are more than just cholesterol lowering medicines. They are anti-inflammatory agents that may stabilize fatty plaques and reduce the risk of rupture. They also seem to help blood vessel walls relax and may prevent blood from clotting. Other cholesterol lowering treatments such as diet/exercise have not been shown to reduce the risk of stroke thereby confirming that statins prevent stroke independent of cholesterol.
One other thing to consider is that not all strokes are caused by narrowing of the neck and brain arteries by fatty plaque. A blood clot or embolus from the heart or another part of the body can interrupt blood flow to the brain and smaller brain arteries may narrow as a consequence of high blood pressure. Also, bleeding into the brain can cause a stroke as well. Scientists were once concerned that lowering cholesterol too low with statins may increase the risk of a bleeding stroke, but this does not seem to the case.
In summary, high cholesterol is weakly associated with an increased risk of stroke and the use of cholesterol lowering medicines called statins can protect you from a recurrent stroke. But, lowering cholesterol is not the main reason for this effect. These medicines are safe to take even when cholesterol is already low and do not appear to increase the risk of a bleeding stroke.