Causes of Bell's Palsy
Originating from the base of the brain are twelve pairs of nerves called the cranial nerves. They are named and numbered and are responsible for a variety of specialised functions such as smell, equilibrium (balance), pupil contraction and hearing. The seventh cranial nerves are the facial nerves that are involved in eyelid closure, facial expression, saliva and tear production, and the sensation of taste at the front of the tongue.
 
Bell’s palsy is thought to occur when facial nerves become compressed or inflamed. Part of the nerve is encased within a bony canal and is therefore susceptible to compression when swollen. Although not fully understood, the swelling is thought to occur as the result of a viral infection of the facial nerve. The herpes simplex virus, which causes cold sores, is one virus thought to be responsible for the condition.
 
Other factors linked to the development of facial nerve paralysis include ear infections, tumours, brain injury and several viruses including herpes zoster (the virus that causes chickenpox and shingles) and Epstein-Barr virus (the virus that causes glandular fever). These conditions do not cause Bell’s palsy, but they can produce very similar symptoms. Bell's palsy is differentiated from these conditions by the fact that its cause remains unknown.