Bell’s palsy is caused by compression or damage to the facial nerve that controls the facial muscles. The 7th cranial nerve (facial muscle nerve) may be damaged by compression, swollen areas, or inflammation and this results in facial weakness or paralysis. Exactly what causes this damage, however, is still unknown.
It has been hypothesized that a viral infection such as viral meningitis or the common cold sore virus (herpes simplex) is responsible for the disorder. That the facial nerve swells and becomes inflamed in reaction to the infection, causing pressure within the Fallopian canal and leading to ischemia. In some mild cases there is damage only to the myelin sheath of the nerve and recovery is usually rapid. The myelin sheath is the fatty covering-which acts as an insulator-on nerve fibers in the brain.
The disorder has also been associated with influenza or a flu-like illness, headaches, chronic middle ear infection, high blood pressure, pregnancy, diabetes, sarcoidosis, tumors, Lyme disease, and trauma such as skull fracture or facial injury.
Sources: National Institute of Health 2011. Bell’s Palsy Fact Sheet.