Bell's Palsy

Bell's palsy is the term used to describe the sudden onset of paralysis or weakness of the muscles on one side of the face, caused by impaired function of the facial nerve.
The facial nerve is the seventh (VII) of twelve paired cranial nerves. It emerges from the brainstem between the pons and the medulla, and travels through the short bony auditory canal, then past the middle ear in the facial canal and exits the skull through the stylomastoid foramen. The facial nerve then travels through the parotid gland and terminates in the zygomatic, buccal, mandibular, and cervical branches. It controls the muscles of facial expression, and taste to the anterior two-thirds of the tongue.

Facial nerve

The facial nerve shares the narrow auditory canal with the vestibulocochlear nerves and the labyrinthine artery and the narrow facial canal has two tight turns. Damage to the facial nerve occurs in the narrow auditory and facial canals and is thought to be due to compression, (possibly as a result of inflammation) in a tightly confined space, or infection with herpes virus.

Facial nerve passing through the inner auditory canal (1,2,3)

Herpes virus normally resides in cells of the nervous system. After infection has subsided, the virus lies dormant, usually in a ganglion (node or junction of the nervous system comprising a cluster of nerve cells). If the virus is reactivated by stress or depressed immune system it emerges from the ganglion and reinfects the host.
It has been postulated that herpes virus from the geniculate ganglion in the facial canal is reactivated and damages the nerve causing a facial paralysis.
The paralysis extends from the forehead down to the jaw and usually includes the eyelid.
Bell‘s palsy affects around one in every 2000 people every year, with young adults of either sex more susceptible for unknown reasons. Bell's palsy usually resolves by itself within a few months in most cases.
Bell's palsy (and other types of facial nerve paralysis) 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 a leg.   
  

One sided facial paralysis of bell's palsy

Bells palsy may occasionally present as a bilateral (both sides) facial paralysis and may recur in some individuals, particularly if there is a family history.