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Bell palsy (a type of facial nerve palsy) is sudden weakness or paralysis of muscles on one side of the face due to malfunction of the 7th cranial nerve (facial nerve). This nerve moves the facial muscles, stimulates the salivary and tear glands, enables the front part of the tongue to detect tastes, and controls a muscle involved in hearing.
When the facial nerve is paralyzed (called facial nerve palsy), doctors can find a cause (such as Lyme disease or sarcoidosis) in about half the cases. When no cause is obvious, facial nerve palsy is said to be idiopathic. Idiopathic facial nerve palsy is called Bell palsy. Although the cause of Bell palsy has been thought to be unknown, recent evidence suggests that some cases are caused by a viral infection or an immune disorder that causes the facial nerve to swell. Thus, what causes Bell palsy is no longer always unknown.
Common causes of Bell palsy include infection by herpes simplex virus type 1 (which causes mouth infections, such as cold sores) and shingles. Other viruses, such as coxsackievirus, cytomegalovirus, and the viruses that cause mumps, rubella, mononucleosis, or influenza, may also cause Bell palsy. Infection causes the nerve to swell. When the nerve is swollen, it is squeezed (compressed) by the narrow passageways in the skull that it passes through.
Other disorders can cause facial nerve palsy. Lyme disease can cause facial nerve palsy, but unlike Bell palsy, it may affect both sides of the face. In blacks, sarcoidosis (see Interstitial Lung Diseases: Sarcoidosis) is a common cause of facial nerve palsy. Sometimes tumors and skull fractures cause facial palsy.
Symptoms
In Bell palsy, pain behind the ear may be the first symptom. Facial muscles weaken suddenly, usually within hours. The effect ranges from mild weakness to complete paralysis. By 48 to 72 hours, the weakness is as severe as it will be. Only one side of the face is affected.
In facial nerve palsies (including Bell palsy), the face becomes flat and expressionless. However, when only one side is affected, people often feel as though the face is twisted because the muscles on the unaffected side tend to pull the face to that side every time they make a facial expression. Wrinkling the forehead, blinking, and grimacing may be difficult or impossible on the affected side. For most people, the face feels numb or heavy, even though sensation remains normal.
Closing the eye on the affected side may be difficult. People may be unable to close the eye completely, and they blink less frequently. The eye also tends to turn upward when it is closed.
The production of saliva and tears may be impaired. People may have dry eyes and mouth, or they may drool. Because fewer tears are produced and the eye blinks less often (blinking helps moisten the eye's surface), the eye becomes dry, resulting in pain and eye damage. Eye damage is usually minor but can be serious if the eye is not moistened and protected another way.
People may be unable to taste with the front part of the tongue on the affected side. The ear on the affected side may perceive sounds as abnormally loud (a condition called hyperacusis) because the muscle that stretches the eardrum is paralyzed. This muscle is located in the middle ear.
Occasionally, as the facial nerve heals, it forms abnormal connections, resulting in unexpected movements of some facial muscles or in watering of the eyes (“crocodile tears”) during salivation. Because the facial muscles are not used for a long time, permanent tightening of the muscles (contractures) occasionally occurs.
Diagnosis
No specific test can determine whether people have facial nerve palsy. However, facial nerve palsy can usually be diagnosed based on symptoms. For example, facial nerve palsy can be distinguished from a stroke because a stroke usually causes weakness only in the lower part of the face rather than in the entire face. People who have had a stroke can close the eyes tightly and wrinkle the brow. Also, a stroke typically causes weakness of an arm and/or a leg.
Doctors can usually distinguish Bell palsy from other, less common disorders that cause facial nerve palsy (such as tumors, Lyme disease, other infections, sarcoidosis, diabetes, and skull fractures). These other disorders typically cause different symptoms, and in many of them, symptoms develop more slowly. Usually, doctors can exclude these disorders based on the person's history and results of x-rays, magnetic resonance imaging (MRI), or computed tomography (CT). A blood test may be done to check for Lyme disease, and a blood test and a chest x-ray may be done to check for sarcoidosis.
Prognosis
When facial paralysis is partial, most people recover completely within several months whether they are treated or not.
When the paralysis is total, the outcome varies. Tests (nerve conduction studies and electromyography—see Symptoms and Diagnosis of Brain, Spinal Cord, and Nerve Disorders: Electromyography and Nerve Conduction Studies) can be done to help predict the likelihood of recovery. Many people do not recover completely. The facial muscles may remain weak, causing the face to droop.
Treatment
If symptoms have been present less than 48 hours, a corticosteroid, such as prednisone, is given by mouth to reduce swelling of the nerve. Taking a corticosteroid slightly speeds and improves recovery of movement.
Antiviral drugs, even those that are effective against the herpes simplex virus and the virus that causes shingles (such as acyclovir, famciclovir, or valacyclovir), do not seem to help.
If the eye cannot close completely, it must be protected from dryness to reduce the risk of eye damage. Eye drops consisting of artificial tears or a salt (saline) solution are applied to the eye until it can close completely. People may need to wear an eye patch some of the time, particularly during sleep. Rarely, in severe cases, the upper and lower eyelids are sewn together.
Last full review/revision September 2012 by Michael Rubin, MDCM
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