7th Cranial Nerve – Tests for the facial nerve

Definition:

The use and interpretation of medical examinations to determine the integrity and adequate function of the facial nerve (seventh cranial nerve). This nerve provides the innervation for one of the special senses: taste.

The facial nerve’s motor component innervates the muscles of facial expression as well as the digastric, styloid and stapedius muscles. Its sensory component relays afferent information relating to taste sensation from the anterior two-thirds of the tongue, and cutaneous sensation from the skin in and around the auricle. The facial nerve also has a parasympathetic component which sends fibres to the submandibular, sublingual and lacrimal glands.

Test procedure:

Facing the patient, the practitioner instructs the patient to go through a series of facial movements (grimaces). Instruct the patient to raise their eyebrows, frown, close their eyes lightly and have the practitioner attempt to open them, move the their lips and show their teeth, smile, and puff out their cheeks.

The sensory component of the facial nerve can be assessed by placing a small amount of salt or sugar on the anterior two-thirds of the patient’s tongue and asking them to identify the taste.

The efferent limb of the corneal reflex is under the control of the facial nerve and can be assessed by lightly touching the patient’s cornea with a clean piece of cotton wool.

Test findings (inc Positive & Negative results):

The two limbs forming the corneal reflex are distinct nerves and therefor an absent or weakened reflex may be indicative of a lesion or lesions either at the trigeminal nerve, facial nerve, higher centres or a combination of the aforementioned.

A lower motor neuron lesion affecting the facial nerve can result in what is clinically referred to as facial nerve palsy (referred to as Bell’s palsy when idiopathic). The clinical signs include weakness in both the upper and lower muscles of facial expression on the same side of the lesion.

In an upper motor neuron lesion, clinically referred to as central facial palsy, or central seven, the clinical manifestations depict a weakness in only the muscles of the lower portion of the face on the contralateral side of the lesion. This is due to the bilateral innervation of the upper muscles of facial expression.

Taste sensation can be reduced and/or absent on the anterior two-thirds of the tongue.

Considerations:

Often iatrogenic, acute facial nerve paralysis has numerous hypothesised causes which include: post-viral infection (typically herpes), Lyme disease, poorly administered inferior alveolar nerve block, tongue piercings, and stress.