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If you have Bell's palsy, the muscles of your face suddenly become weak. This usually happens on just one side of the face. It means you can't move your face properly. You may find only half of your face moves when you smile, and you can't move one eyelid to open or close it.

Bell's palsy happens because one of the nerves sending messages from your brain to your face muscles has stopped working properly.

Some people who have a stroke also get weakness in the muscles of the face. But this is different from Bell's palsy. If you have a stroke, other parts of your body are affected, too. And after a stroke you can usually move your forehead and eyelids.

Bell's palsy happens when a nerve in your face is damaged.¹ It's called the facial nerve. There's one on each side of your face.

Each facial nerve comes out from your brain through a small tunnel under your ear, and then divides into many branches. These branches are connected to the muscles that you use to blink, smile and frown.

The nerves also go to the glands that make your tears and saliva, and they carry the sense of taste from your tongue.

We don't know for certain what causes Bell's palsy.

Doctors think there may be swelling (inflammation) around the facial nerve, which squashes the nerve as it passes through your skull. The nerve can't work properly until the inflammation goes. The messages the brain sends to your face muscles get interrupted. So the muscles of your face become weak or paralyzed (they can't move).

Doctors think the swelling around the nerve is probably caused the by the same viruses that cause cold sores and chicken pox:²

• The virus that causes cold sores is called herpes simplex virus
• The virus that causes chicken pox is called herpes zoster virus. Bell's palsy caused by herpes zoster virus is less common but can be more serious.³

You might have had cold sores or chicken pox in the past. But some people have these viruses for a long time without getting any symptoms. Getting Bell's palsy does not mean you will go on to get cold sores or chicken pox.

Not everyone who has weak face muscles has Bell's palsy. About a third of people have another reason for their muscle weakness.⁴ These include:

• A stroke
• An injury
• An ear disease
• Lyme disease (caused by a tick bite)
• A type of cancer (but this is rare).

Your doctor will examine you and ask you questions to rule out other possible reasons for your face muscles becoming weak. You won't be diagnosed with Bell's palsy until other problems have been ruled out. There isn't a test that can tell you for certain you have Bell's palsy. But you can have investigations to look for other things that might make your face muscles weak. You might need to go to the hospital for an MRI scan or a CT scan. And you might also need a test called electromyography (EMG) to see how badly your nerve is damaged.

To read more, see Tests and scans if you have Bell's palsy.

This information is for educational use only, and is not a substitute for prompt professional medical advice. Readers should always consult a physician or other professional for advice and treatment. ©BMJ Publishing Group Limited 2008. All rights reserved.