Bell's palsy (or facial palsy) is characterized by facial drooping on the affected half, due to malfunction of the facial nerve (VII cranial nerve), which controls the muscles of the face. Facial palsy is typified by inability to control movement in the facial muscles. The paralysis is of the infranuclear/lower motor neuron type.
The facial nerves control a number of functions, such as blinking and closing the eyes, smiling, frowning, lacrimation, and salivation. They also innervate the stapedial (stapes) muscles of the middle ear and carry taste sensations from the anterior two thirds of the tongue.
Clinicians should determine whether the forehead muscles are spared. Due to an anatomical peculiarity, forehead muscles receive innervation from both sides of the brain. The forehead can therefore still be wrinkled by a patient whose facial palsy is caused by a problem in one of the hemispheres of the brain (central facial palsy). If the problem resides in the facial nerve itself (peripheral palsy) all nerve signals are lost, including to the forehead.
One disease that may be difficult to exclude in the differential diagnosis is involvement of the facial nerve in infections with the herpes zoster virus. The major differences in this condition are the presence of small blisters, or ''vesicles'', of the external ear and hearing disturbances, but these findings may occasionally be lacking (zoster sine herpete).
Lyme disease may produce the typical palsy, and may be easily diagnosed by looking for Lyme-specific antibodies in the blood. In endemic areas Lyme disease may be the most common cause of facial palsy.
The degree of nerve damage can be assessed using the House-Brackmann score.
Although defined as a mononeuritis (involving only one nerve), patients diagnosed with Bell’s palsy may have "myriad neurological symptoms" including "facial tingling, moderate or severe headache/neck pain, memory problems, balance problems, ipsilateral limb paresthesias, ipsilateral limb weakness, and a sense of clumsiness" that are "unexplained by facial nerve dysfunction".
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