Why Do Strokes Cause Hemiplegia?

Photo © A.D.A.M.

 Hemiplegia after stroke is common. But what is hemiplegia anyway? Hemiplegia is the term we use to describe paralysis of one side of the body. The term can be broken down into “hemi” which means “half,” and “plegia,” which means paralysis.

To understand how strokes cause hemiplegia, let's try to understand the anatomy of the brain, brainstem, and spinal cord. Imagine looking at a highway from a helicopter.

For simplicity, this highway only travels south and is made up of 16 lanes. Several exits serve the 8 right-sided lanes (the right side of the body), and an equal number of exits serve the 8 left-sided lanes (the left side of the body).

This highway represents the long nerve fibers which originate in the brain and travel south towards the brainstem and spinal cord where they exit in order to control the nerves that move our muscles on each side. But where is the brain in this mental picture? Let's just say that the towns and neighborhoods located on each side of the highway represent different parts of the brain.

One last thing. Each neighborhood performs a specific task such as movement, hearing or feeling. And in the “movement neighborhood,” specific areas are dedicated to moving the face, arm, and leg.

Phew! That's a lot of imagination-stretching. But bear with me. To understand hemiplegia, let's now follow a nerve signal (a car) leaving from the “movement neighborhood” (your motor cortex) on the left side of the brain.

Because the left side of the brain controls the right side of the body, this car’s final goal will be to reach the exit to the right hand, where it will cause it to move.

First, the car leaves its driveway, takes a short ride through a few local 2-lane roads, which then bring it to a larger, 4-lane road.

On this road, it meets other cars which initiated their trip in adjacent parts of the movement neighborhood, destined to reach different muscles on the right side of the face. Not long after, it enters a 6-lane road, where it meets with most cars traveling south, to face, leg and other parts of the right side of the body. Then, along with the rest of the cars, it crosses over and enters the right side of the highway where it now can travel to the right-hand exit. The other cars mentioned will exit at the right face, and right leg exits.

Now imagine that a stroke is a meteorite. Yes, the type that shows up in your driveway on a very unlucky day. If the meteorite falls on the driveway where the signal (car) to the right hand normally starts its trip, then the signal will never be able to leave the driveway and the right hand will become “plegic,” or paralyzed. However, signals to other places such as the face and the leg won’t become affected by this relatively small catastrophe.

But if the meteorite hits the 4-lane road described above, it will permanently interrupt the road that allows cars (signals) to both the arm and the face from reaching the highway. This causes a partial hemiplegia affecting the face and arm.

Finally, if the meteorite falls on the entrance to the highway (internal capsule or brainstem), or on the entire right side of the highway (spinal cord), none of the signals traveling to the right face, arm or leg will be able to get past it, causing a full right-sided hemiplegia.

Unfortunately, besides the obvious difficulties that come along with not being able to move one whole side of the body, hemiplegia often comes accompanied by side effects such as:


Bradley G Walter, Daroff B Robert, Fenichel M Gerald, Jancovic, Joseph; Neurology in clinical practice, principles of diagnosis and management.

Philadelphia Elsevier, 2004.

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