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Bell palsy
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Bell palsy is the most common paralysis of the seventh cranial nerve, the facial nerve, the one that moves the muscles of facial expression. Among people who present with facial weakness, it is the cause behind 38% to 83% of cases.
What happens is that this nerve becomes inflamed, almost always along a stretch called the labyrinthine segment, and half the face stops moving. It comes on fast and affects one side only.
To understand why it happens right there, you have to look at where the nerve runs. That stretch travels inside a canal in the temporal bone, the hard bone of the temple, and it is the narrowest part of that canal. The nerve runs through a bony tunnel with no room to spare: if it becomes inflamed, it has nowhere to swell. And if edema appears, that is, fluid building up, the nerve can end up pressed against the wall of the canal.
In brief
Bell palsy is the most common paralysis of the seventh cranial nerve and accounts for 38% to 83% of facial weakness.
It is a rapid-onset paralysis of one side of the face that results from inflammation of the facial nerve, usually in its labyrinthine segment.
That segment is the narrowest part of its canal in the temporal bone, and that narrowness predisposes the nerve to failure, possibly from edema.
The nerve runs through a bony tunnel with no room to spare: if it becomes inflamed, it has nowhere to swell.
Sources (1)
- National Center for Biotechnology Information, StatPearls PublishingBell Palsy, StatPearlsPublished
This page explains the topic in general terms; it does not tell anyone what treatment to follow.
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