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.
In most cases, the exact cause of nerve inflammation in people with Bell's palsy is unclear. Researchers suspect the inflammation is most often triggered by a viral infection, such as 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.