More properly termed idiopathic facial nerve palsy, this refers to isolated paralysis of the facial muscles on one or both sides. The cause is often unclear, though damage to the seventh cranial or FACIAL NERVE, possibly of viral origin, is thought likely in most cases. Rare causes include mastoiditis (see EAR, DISEASES OF), LYME DISEASE and HYPERTENSION. It tends to start with pain on the affected side of the face, followed by an inability to close the eye or smile. The mouth appears to be drawn over to the opposite side, and fluids may escape from the angle of the mouth. Lines of expression are flattened and the patient is unable to wrinkle the brow.
Many patients recover without treatment, but oral steroids, if started early, increase the rate of recovery. This usually starts after two or three weeks and is complete within three months. Permanent loss of function with facial contractures occurs in about 5 per cent of patients. Recurrence of Bell's palsy is unusual.