There's convincing evidence that Bell's palsy, an acute peripheral facial weakness and paralysis, is related to a herpes simplex type I infection of the facial nerve. Current thinking is that fever blisters provide the initial route for the virus to gain access to the nerve and migrate to the ganglion, where it lies dormant until reactivated by unknown factors. The facial nerve runs through a bony canal in the skull between the brain and the face. In the meatal foramen, where the bony canal is the narrowest, the nerve takes up 99 percent of available space. When the reactivated virus causes inflammation and swelling, the nerve has no room to expand and shuts down.
Clinical diagnosis is one of exclusions, especially to rule out stroke or tumors.
An MRI scan is helpful. Electrical testing comparing conductivity of facial nerves on both sides of the face can help determine the potential for improvement. If denervation is 90 percent or greater, there's a 50 percent chance the patient will have a bad outcome. Treatment includes steroids, antivirals and analgesics. Eye symptoms related to the inability to move the eyelid must be managed.