Bell's palsy is a condition that affects the facial nerve, and therefore the function of the muscles, on one side of the face. The facial nerve is one of the 12 cranial nerves and is given the designation of the 7th cranial nerve (CN VII). There is a facial nerve on each side of the face. This nerve can be considered to be the nerve that controls facial expression, so when its function is disrupted, the control of facial expression is impaired.
While a number of different conditions can cause damage to the facial nerve, including tumors, trauma, toxins, and neurologic diseases, true Bell's palsy is an idiopathic (meaning that the cause is unknown) paralysis of the facial nerve. It typically occurs on one side of the face and comes on suddenly, sometimes overnight. The diagnosis of Bell's palsy is a diagnosis of exclusion, meaning that this diagnosis is made when all the potential other causes of nerve damage have been ruled out. Most investigators believe that Bell's palsy results from a viral infection of some type.
The degree to which facial muscle control varies among affected individuals. Symptoms are paralysis (to a varying degree) of all muscles on one side of the face, including the forehead muscles, which results in an asymmetric or distorted appearance to the face and an abnormal facial expression. The face may appear to droop, and it can be hard to close the affected eye. It can be difficult to eat or drink without spilling food out of the side of the mouth. There can be associated numbness or pain in the ear, face, neck, or tongue. Some patients report an increased hearing sensitivity (hyperacusis) as well. The paralysis may be accompanied by twitching of the face, dryness of the mouth or eyes, drooling, or changes in taste perception.
Bell's palsy tends to occur more frequently with age, and is over three times more common in pregnant women than in the general population. Fortunately, the condition generally resolves on its own within about six weeks. Specific treatment is not generally necessary, but lubricating eye drops may be prescribed to relieve dryness of the eyes.
Some studies suggest that early treatment (within three days of symptoms) with either corticosteroids, antivirals or both may reduce the duration and severity of symptoms; others suggest there is little or no benefit to the patient with these treatments.
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WebMD Medical Reference. "Bell's Palsy - Topic Overview." Healthwise. 18 Feb. 2011. <https://www.webmd.com/brain/tc/bells-palsy-topic-overview>.