Bell's palsy is the most common facial nerve disorder, in which all divisions of the facial nerve are impaired within a few hours or days and then gradually recover. The cause is unknown, although viral or inflamatory mechanisms have been suggested. The most stiking feature is unilateral facial weakness of the lower neuron type, which can be mild but is often severe.

Upper Motor Neuron versus Lower Motor Neuron Facial Weakness: With an upper motor neuron lesion (Lesion A), the upper face is spared because both hemi-spheres contribute to movement of the upper face, and the unaffected hemisphere can compensate. With a lower motor neuron lesion (Lesion B), the entire face is affected on one side.