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Bell's palsy is a form of facial paralysis resulting from a dysfunction of the cranial nerve VII (the facial nerve) that results in the inability to control facial muscles on the affected side. Several conditions can cause facial paralysis, e.g., brain tumor, stroke, and Lyme disease. However, if no specific cause can be identified, the condition is known as Bell's palsy. Bell's palsy is defined as an idiopathic unilateral facial nerve paralysis, usually self-limiting. The hallmark of this condition is a rapid onset of partial or complete palsy that often occurs overnight. In rare cases (1%), it can occur bilaterally resulting in total facial paralysis. It is thought that an inflammatory condition leads to swelling of the facial nerve. The nerve travels through the skull in a narrow bone canal beneath the ear. Nerve swelling and compression in the narrow bone canal are thought to lead to nerve inhibition, damage, or death.
Bell's palsy is characterized by facial drooping on the affected half, due to malfunction of the facial nerve (VII cranial nerve), which controls the muscles of the face. Facial palsy is typified by inability to control movement in the facial muscles. The paralysis is of the infranuclear/lower motor neuron type. The facial nerves control a number of functions such as blinking and closing the eyes, smiling, frowning, lacrimation, salivation, flaring nostrils, and raising eyebrows. Due to an anatomical peculiarity, forehead muscles receive innervation from both sides of the brain. The forehead can therefore still be wrinkled by a patient whose facial palsy is caused by a problem in one of the hemispheres of the brain (central facial palsy). If the problem resides in the facial nerve itself (peripheral palsy) all nerve signals are lost on the ipsilateral (same side of the lesion) half side of the face, including to the forehead (contralateral forehead still wrinkles).
Some viruses are thought to establish a persistent (or latent) infection without symptoms, e.g., the varicella-zoster virus, HSV-1 and Epstein-Barr viruses, all of the herpes family. In a few cases, bilateral facial palsy has been associated with acute HIV infection. Reactivation of an existing (dormant) viral infection has been suggested as cause behind the acute Bell's palsy. Studies suggest that this new activation could be preceded by trauma, environmental factors, and metabolic or emotional disorders, thus suggesting that stress - emotional stress, environmental stress (e.g., cold), or physical stress (e.g., trauma) - in short, a host of different conditions, may trigger reactivation.
It is thought that as a result of inflammation of the facial nerve, pressure is produced on the nerve where it exits the skull within its bony canal, blocking the transmission of neural signals or damaging the nerve. Patients with facial palsy for which an underlying cause can be found are not considered to have Bell's palsy per se. Possible causes include tumor, meningitis, stroke, diabetes mellitus, head trauma, and inflammatory diseases of the cranial nerves (sarcoidosis, brucellosis, etc.). In these conditions, the neurologic findings are rarely restricted to the facial nerve. Babies can be born with facial palsy. This has given hope for anti-inflammatory and anti-viral drug therapy (prednisone and acyclovir). Bell's palsy is therefore a diagnosis of exclusion; by elimination of other reasonable possibilities. Bell's palsy is commonly referred to as idiopathic or cryptogenic, meaning that it is due to unknown causes.
Bell's palsy affects each individual differently. In patients presenting with incomplete facial palsy, where the prognosis for recovery is very good, treatment may be unnecessary. However, the more severe cases may require treatment. Patients presenting with complete paralysis, marked by an inability to close the eyes and mouth on the involved side, are usually treated, some of them with smile surgery. Early treatment (within three days after the onset) is necessary for therapy to be effective. Steroids have been shown to be effective at improving recovery while antivirals have not. Corticosteroid such as prednisone significantly improves recovery at six months and is thus recommended.
Even without any treatment, Bell's palsy tends to carry a good prognosis. In a 1982 study, when no treatment was available, of 1,011 patients, 85% showed first signs of recovery within three weeks after onset. For the other 15%, recovery occurred three to six months later. After a follow-up of at least one year or until restoration, complete recovery had occurred in more than two-thirds (71%) of all patients. Recovery was judged moderate in 12% and poor in only 4% of patients. Another study found that incomplete palsies disappear entirely, nearly always in the course of one month. The patients who regain movement within the first two weeks nearly always remit entirely. When remission does not occur until the third week or later, a significantly greater part of the patients develop sequel.