Bell's palsy, also known as idiopathic facial palsy, is a form of temporary facial paralysis or weakness on one side of the face.  It results from dysfunction of cranial nerve VII (facial nerve) which directs the muscles on one side of the face, including those that control eye blinking and closing and facial expressions such as smiling.  (There are 12 pairs of cranial nerves, identified by Roman numerals.)  The facial nerve also carries nerve impulses to the tear glands, the saliva glands, and the muscles of a small bone in the middle of the ear.  The facial nerve also transmits taste sensations from the tongue.

Bell's palsy is the most common cause of facial paralysis, although its exact cause is unknown. Generally, Bell's palsy affects only one side of the face; however, in rare cases, it can affect both sides.  Symptoms appear suddenly over a 48 - 72-hour period and generally start to improve with or without treatment after a few weeks, with recovery of some or all facial function within six months.  In some cases, residual muscle weakness lasts longer or may be permanent. 



Because the facial nerve has so many functions and is so complex, damage to the nerve or a disruption in its function can lead to many problems. Symptoms of Bell's palsy can vary from person to person and range in severity from mild weakness to total paralysis. The most common symptom is sudden weakness of one side of the face. Other symptoms may include drooping of the mouth, drooling, inability to close eye (causing dryness of the eye), and excessive tearing in one eye. Individuals may also have facial pain or abnormal sensation, altered taste, and intolerance to loud noise. Most often these symptoms lead to significant facial distortion.



The cause of Bell's palsy is unknown.  Swelling and inflammation of the cranial nerve VII is seen in individuals with Bellís palsy.

Most scientists believe that reactivation of an existing (dormant) viral infection may cause the disorder. Impaired immunity from stress, sleep deprivation, physical trauma, minor illness or autoimmune syndromes are suggested as the most likely triggers.  As the facial nerve swells and becomes inflamed in reaction to the infection, it causes pressure within the Fallopian canal (a bony canal through which the nerve travels to the side of the face), leading to the restriction of blood and oxygen to the nerve cells.  In some mild cases where recovery is rapid, there is damage only to the myelin sheath (the fatty covering that acts as insulation of nerve fibers).

Several other conditions can also cause facial paralysis, for example, brain tumor, stroke, myasthenia gravis, and Lyme disease.  If no specific cause can be identified, the condition can be diagnosed as Bell's palsy.