Health A-Z

Medical Content Created by the Faculty of the
Harvard Medical School

Bells Palsy

What Is It?

Bell's palsy is a weakness of the muscles on one side of the face caused by problems with a facial nerve. The nerve becomes inflamed and swollen and stops functioning properly.

There are two facial nerves, one for the right side of the face and one for the left. Each has several branches. The main branch controls most of the muscles on one side of the face, including the muscles that control facial expression and the muscles that close and open the eyes and the lips. Other smaller branches go to the tongue and ear.

The nerve inflammation of Bell's palsy usually is caused by a viral infection. The most common viral infection that causes Bell's palsy is herpes simplex, the same virus that causes cold sores (fever blisters). A variant of Bell's palsy, called Ramsay-Hunt syndrome, is caused by the herpes zoster virus, the virus that causes chickenpox and shingles. A less common cause of Bell's palsy is Lyme disease. People with diabetes are more likely to develop Bell's palsy.