Ischemic stroke

There are two main types of stroke: an ischemic stroke which is when there’s a blocked artery that reduces blood flow to the brain and a hemorrhagic stroke which is when an artery in the brain breaks, creating a pool of blood that damages the brain.

Of the two, ischemic strokes are much more common, and the amount of damage they cause is related to the parts of the brain that are affected and how long the brain suffers from reduced blood flow.

Now if symptoms self-resolve within 24 hours, it’s called a transient ischemic attack and there are usually minimal long-term problems.

OK - let’s start with some basic brain anatomy. The brain has a few regions - the most obvious is the cerebrum, which is divided into two cerebral hemispheres, each of which has a cortex - an outer region - divided into four lobes including the frontal lobe, parietal lobe, temporal lobe, and the occipital lobe.

There are also a number of additional structures - including the cerebellum, which is down below, as well as the brainstem which connects to the spinal cord.

The right cerebrum controls muscles on the left side of your body and vice versa.

The frontal lobe controls movement, and executive function, which is our ability to make decisions.

The parietal lobe processes sensory information, which lets us locate exactly where we are physically and guides movements in a three dimensional space.

The temporal lobe plays a role in hearing, smell, and memory, as well as visual recognition of faces and languages.

Finally there’s the occipital lobe which is primarily responsible for vision.

The cerebellum helps with muscle coordination and balance.

And finally there’s the brainstem plays a vital role in functions like heart rate, blood pressure, breathing, gastrointestinal function, and consciousness.

The brain receives blood from the left and right internal carotid arteries, as well as the left and right vertebral arteries, which come together to form the basilar artery.

The internal carotid arteries turn into the left and right middle cerebral arteries which serve the lateral portions of the frontal, parietal, and temporal lobes of the brain.

Each of the internal carotid arteries also give off branches called the anterior cerebral arteries which serve the medial portion of the frontal and parietal lobes and connect with one another with a short little connecting blood vessel called the anterior communicating artery.

Meanwhile, the vertebral arteries and basilar artery gives off branches to supply the cerebellum and the brainstem.

In addition, the basilar artery divides to become the right and left posterior cerebral artery which mainly serve the occipital lobe and some of the temporal lobe as well as the thalamus.

Finally, the internal carotid arteries each give off a branch called the posterior communicating artery which attaches to the posterior arteries on each side.

So together, the main arteries and the communicating arteries complete what is called the Circle of Willis - a ring where blood can circulate from one side to the other in case of a blockage.

The Circle of Willis offers alternative ways for blood to get around an obstructed vessel.

In general, the brain can get by on diminished blood flow - especially when it happens gradually because that allows enough time for collateral circulation to develop, which is where a nearby vessel starts sending out branches of blood vessels to serve an area that’s in need.

But once the supply of blood flow is reduced to below the needs of the tissue - it causes tissue damage, which we call an ischemic stroke.

There are two main ways that an ischemic stroke happens.

One mechanism is endothelial cell dysfunction, which is when something irritates or inflames the slippery inner lining of the artery—the tunica intima.

One classic irritant is the toxins found in tobacco which float around in the blood damaging the endothelium.

That damage becomes a site for atherosclerosis, which is where a plaque forms. This is when a buildup of fat, cholesterol, proteins, calcium, and immune cells forms and starts to obstruct arterial blood flow.

This plaque has two parts to it, the soft cheesy-textured interior and the hard outer shell which is called the fibrous cap.

Branch points in arteries and particularly the internal carotid and middle cerebral arteries are the most common spots for atherosclerosis.

Usually, though, it takes years for plaque to build up, and this slow blockage only partially blocks the arteries, and so even though less blood makes it to brain tissue, there’s still some blood.

So strokes happen when there’s a sudden and complete or near-complete blockage of an artery—so let’s see how that can happen.

Since plaques sit in the lumen of the blood vessel, they’re constantly being stressed by mechanical forces from blood flow, and interestingly it’s often the smaller plaques that are more dangerous.

Their fibrous caps are softer than the larger ones and are prone to getting ripped off. Once that happens, the inner cheesy filling is exposed to the blood and is thrombogenic, which means that it tends to form clots very quickly.

Platelets adhere to the exposed cheesy material, and they release chemicals that enhance the clotting process.

Within a minute that artery can be fully blocked.

Another mechanism for ischemic stroke formation is an embolism.

An embolic stroke typically happens when a blood clot breaks off from one location, travels through the blood, and gets lodged in an artery downstream, typically an artery, arteriole, or capillary with a smaller diameter. These blood clots typically emerge from atherosclerosis, but they can also form in the heart.

For example stagnant blood can form a clot, and blood can stagnate due to an atrial fibrillation or after a heart attack.

If a clot forms in the left atrium, it moves into the left ventricle and from there it has a direct route to the brain.