The facial nerve supplies the muscles of the face including the muscles that close the eyelids, and the mouth. The nerve is tested as follows.
1. Ask the patient to close his eyes firmly. In complete paralysis of the facial nerve the patient will not be able to close the eye on the affected side. In partial paralysis the closure is weak and the examiner can easily open the closed eye with his fingers (which is very difficult in a normal person).
2. Ask the person to smile. In smiling the normal mouth is more or less symmetrical, the two angles moving upwards and outwards. In facial paralysis the angle fails to move on the paralysed side.
3. Ask the patient to fill his mouth with air. Press the cheek with your finger and compare the resistance (by the buccinator muscle) on the two sides. The resistance is less on the paralysed side. On pressing the cheek air may leak out of the mouth because the muscles closing the mouth are weak.
4. The sensation of taste should be tested on the anterior two thirds of the tongue (as described under glossopharyngeal nerve).
Paralysis of the facial nerve is fairly common. It can occur due to injury or disease of the facial nucleus (nuclear paralysis) or of the nerve any where along its course (infranuclear paralysis). In the most common type of infranuclear paralysis called Bell’s palsy the nerve is affected near the stylomastoid foramen. Facial muscles can also be paralysed by interruption of corticonuclear fibres running from the motor cortex to the facial nucleus: this is referred to as supranuclear paralysis.
The effects of paralysis are due to the failure of the muscles concerned to perform their normal actions. Some effects are as follows:
1. The normal face is more or less symmetrical. When the facial nerve is paralysed on one side the most noticeable feature is the loss of symmetry. (Also see para 4 in this regard).
2. Normal furrows on the forehead are lost because of paralysis of the occipitofrontalis.
3. There is drooping of the eyelid and the palpebral fissure is wider on the paralysed side because of paralysis of the orbicularis oculi. The conjunctival reflex is lost for the same reason.
4. There is marked asymmetry of the mouth because of paralysis of the orbicularis oris and of muscles inserted into the angle of the mouth. This is most obvious when a smile is attempted. As a result of asymmetry the protruded tongue appears to deviate to one side, but is in fact in the midline.
5. During mastication food tends to accumulate between the cheek and the teeth. (This is normally prevented by the buccinator).Additional effects are observed in injuries to the facial nerve at levels higher than the stylomastoid foramen, as follows:
a. If the injury is proximal to the origin of the chorda tympani there is loss of the sensation of taste on the anterior two thirds of the tongue.
b. The transmission of loud sounds to the internal ear is normally dampened by the stapedius muscle. When the lesion is proximal to the origin of the branch to the stapedius this muscle is paralysed. As a result even normal sounds appear too loud (hyperacusis).
c. In fractures of the temporal bone, or in lesions near the exit of the nerve from the brain the vestibulocochlear nerve may also be affected (leading to deafness).
d. In nuclear lesions (within the brainstem) other neighbouring nuclei may be affected leading to lesions of the abducent or trigeminal nerves.
e. Supranuclear lesions can be distinguished from nuclear or infranuclear lesions because these are usually accompanied by hemiplegia. Movements of the lower part of the face are more affected than those of the upper part: the explanation for this is that the corticonuclear fibres concerned with movements of the upper part of the face are bilateral, whereas those for movements of the lower part of the face are unilateral. Another difference is that while voluntary movements are affected, emotional expressions appear to be normal. It has been suggested that there are separate pathways from the cerebral cortex to the facial nucleus for voluntary and emotional movements; and that usually only the former are involved.