Bell's palsy (also known as peripheral facial paralysis or acute benign cranial polyneuritis) is a disorder characterized by a disruption of the motor branches of the facial nerve (cranial nerve VII) on one side of the face in the absence of any other disease such as a stroke. It is an acute, peripheral facial paresis of unknown cause.

Although the exact cause of Bell's Palsy is unknown, there is evidence that reactivated herpes simplex virus (HSV) may be involved in some cases, causing inflammation, edema, ischemia, and eventual demyelination of the nerve, creating pain and alterations in motor and sensory function.

Bell's Palsy is considered benign with full recovery after 6 months for most patients, while others continue to be bothered by asymmetric movement of the facial muscles.

Clinical manifestations of Bell's palsy may include some or all of the below:

- fever
- tinnitus (ringing in the ears)
- hearing deficit
- flaccidity of the affected side of the face
- drooping of the mouth accompanies by drooling
- inability to close the eye, with an upward movement of the eyeball when closure is attempted
- widened palpebral fissure (the opening between the eyelids)
- flattening of the nasolabial fold
- inability to smile, frown, or whistle
- unilateral loss of taste
- loss of tearing/excessive tearing
- pain behind the ear on the affected side, especially before the onset of paralysis.

Corticosteroids, especially prednisone, are started immediately, preferably before paralysis is complete, for best results.