Stroke vs Non-Stroke Facial Paralysis

Pic from Wikipedia

A recent blog post about a ER doctor who was found negligent for not pursuing a stroke workup for an isolated facial paralysis did bring up an interesting medical phenomenon about what differentiates a facial paralysis due to stroke vs facial paralysis due to facial nerve inflammation.

To begin, the facial nerve innervates the entire half of the face. The nerve itself exits from the skull behind the ear and sends finger-like branches to all the muscles of the face (nerve is yellow).

So if something happens to the nerve, the entire face including forehead, around the eye, cheek, and lip would become paralyzed.

HOWEVER, if a stroke occurs that knocks out the part of the brain that governs facial movement on just one side, the patient will have facial paralysis only on the lower half of the face. The forehead on both sides will still move just fine!

Why is that?

At the brain level, the facial nerve branch that governs forehead movement is innervated by BOTH sides of the brain. However, the facial nerve branches for the remainder (lower part) of the face is innervated by only ONE side of the brain.

As such, if complete facial paralysis including forehead is present, it is NOT due to a stroke or any other brain problem. This condition is called Bell's Palsy.

If the forehead moves fine, but the rest of the face is paralyzed on one side, than it may be due to a stroke or some other brain-level problem. Of course, with a stroke, one would see not only facial paralysis with forehead sparing, but also paralysis involving other parts of the body (which makes it obvious).

Pic from Wikipedia

Take a look at the picture to the left illustrating the facial nerve branches. Nerve branches in solid yellow or orange come from either the right or left side of the brain. Nerve branches in yellow/orange hashes come from both sides of the brain.