In Bell's palsy the forehead usually freezes too, a clue that separates it from stroke
Within a day or two, one side of the face can suddenly stop obeying: the eyebrow droops, the smile goes lopsided, taste changes and ordinary sounds may seem painfully loud. Bell's palsy looks alarming, and its cause is still unknown, yet most people regain normal or nearly normal movement within months.
The trouble lies in the seventh cranial nerve, the facial nerve, which works the muscles for blinking, smiling, frowning and raising the eyebrows, and also helps drive tears and saliva. It carries taste from the front two-thirds of the tongue, which explains the altered taste, and it controls a tiny muscle in the middle ear that damps loud noise, so its failure can make everyday sounds seem too loud. The leading theory is that the nerve swells, perhaps after a dormant herpes-family virus reactivates, and gets squeezed inside the narrow bony channel where it leaves the skull. Studies disagree on how often herpes viruses are actually found.
It is the commonest cause of one-sided facial paralysis, behind about 70% of cases, striking 1 to 4 people per 10,000 each year and around 1.5% of people over a lifetime. Men and women are equally affected, most often between 15 and 60, and diabetes, pregnancy and a recent cold raise the risk. Charles Bell, a Scottish surgeon who lived from 1774 to 1842, first tied the condition to the facial nerve.
Diagnosis works by elimination; there is no routine test. Stroke is the big worry, but a stroke usually spares the upper face, so the forehead still wrinkles, and it tends to bring arm or leg weakness. Lyme disease accounts for about a quarter of facial palsies where it is common and is more likely to affect both sides. Shingles of the facial nerve, called Ramsay Hunt syndrome, usually adds blisters on the ear and severe pain, and carries a worse outlook.
Research findings on treatment are uneven. Corticosteroids started within about 3 days improved the chance of recovery by 14% in studies, while antivirals show little or no added benefit and the evidence is weak. Because the eyelid may not close, keeping the eye moist and covered matters, and surgery is generally not advised. Improvement often begins within 14 days and full recovery within six months, though some people are left with lasting weakness or later relapse.
Source: Bell's palsy
