What causes Bell's palsy?The exact cause is not known. It is thought that inflammation develops around the facial nerve as it passes through the skull from the brain. The inflammation may compress (squash) the nerve as it passes through the skull. The nerve then
partly, or fully stops working until the inflammation goes. If the nerve stops working, the muscles that the nerve supplies also stop working.
The cause of the inflammation is not known, but it is probably due to a viral infection. There is some evidence that the cold sore virus (herpes simplex virus) causes most cases of Bell's palsy.
When the facial nerve is irritated for any reason, it swells up and presses against the inside of the bony tube. This puts pressure on the nerve, and it can't send the right signals to the muscles in the face, the salivary glands, or the tongue, leading to Bell's palsy. Without those signals, half of a kid's face may seem to freeze. One eye may stay open, even if she's trying to close them both, and one corner of her mouth may look droopy or unhappy, even if she's trying to smile. Bell's palsy then shows up pretty fast.