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 two thirds of the tongue to detect tastes, and controls a muscle involved in hearing.
The cause of Bell palsy may be a viral infection or an immune disorder that causes the facial nerve to swell, or the cause may be unknown.
People may feel pain behind the ear, then one side of the face may become weak or completely paralyzed, and they may be unable to taste with the front of the tongue on the affected side.
Doctors usually base the diagnosis on symptoms.
Corticosteroids, which are used to reduce swelling of the nerve, help people recover and regain facial movement slightly more quickly.
With or without treatment, most people recover completely within several months.
(See also Overview of the Cranial Nerves.)
Bell palsy is a type of facial nerve palsy, which was originally thought to have no identifiable cause (idiopathic facial nerve palsy). However, current evidence suggests that in some cases, Bell palsy can be caused by a viral infection or another disorder.
Evidence suggests that common causes of Bell palsy include
Infection by herpes simplex virus type 1 (which causes mouth infections, such as cold sores)
Other viruses, such as coxsackievirus, cytomegalovirus, and the viruses that cause COVID-19, mumps, rubella, mononucleosis, or influenza or the vaccines for these viruses 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 other types of facial nerve palsy. Lyme disease can cause facial nerve palsy that, unlike Bell palsy, may affect both sides of the face. In African-Americans, sarcoidosis is a common cause of facial nerve palsy. Sometimes tumors and skull fractures cause facial palsy.
Symptoms of Bell Palsy
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, 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.
DR P. MARAZZI/SCIENCE PHOTO LIBRARY
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.
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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 of Bell Palsy
A doctor's evaluation
Sometimes various tests, depending on the suspected cause
Facial nerve palsy can usually be diagnosed and distinguished from other disorders 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 one side of the face rather than in the entire side of the 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. Thus, if doctors are not certain that Bell palsy is the cause or if symptoms developed gradually, tests are done. These tests may include
Blood tests
X-rays
Magnetic resonance imaging (MRI) or computed tomography (CT) of the brain
For example, blood tests may be done to check for other causes of facial nerve palsy such as Lyme disease and a blood test and chest x-ray may be done to check for sarcoidosis. Usually, doctors can exclude other causes based on the person’s symptoms and results of these tests.
Treatment of Bell Palsy
Eye drops or a patch to protect the cornea
Sometimes a corticosteroid
The eye must be protected from dryness to reduce the risk of eye damage. Eye drops consisting of artificial tears or a salt (saline) solution can be frequently applied to the eye. 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.
Prognosis for Bell Palsy
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) can be done to help predict the likelihood of recovery. Some people do not recover completely. The facial muscles may remain weak, causing the face to droop.
Recovery occurs when nerve fibers regrow. However, nerves may regrow in the wrong place—for example, to the eyes rather than lower face. As a result, unexpected muscles may contract when people make facial expressions or people may shed tears (called crocodile tears) when they eat.
