The trigeminal nerve has a wide sensory distribution. It also supplies the muscles of mastication.
The sensation of touch in the area of distribution of the nerve can be tested by touching different areas of skin with a wisp of cotton wool. The sensation of pain can be tested by gentle pressure with a pin.
Motor function is tested by asking the patient to clench his teeth firmly. Contraction of the masseter can be felt by palpation when the teeth are clenched.
Injury to the trigeminal nerve causes paralysis of the muscles supplied and loss of sensations in the area of supply. Some features of special importance are as follows:
1. Apart from their role in opening and closing the mouth, the muscles of mastication are responsible for side to side movements of the mandible. Contraction of these muscles on one side moves the chin to the opposite side. Normally the chin is maintained in the midline by the balanced tone of the muscles of the right and left sides. In paralysis of the pterygoid muscles of one side the chin is pushed to the paralysed side by muscles of the opposite side.
2. Loss of sensation in the ophthalmic division (specially the nasociliary nerve) is of great importance. Normally the eyelids close as soon as the cornea is touched (corneal reflex). Loss of sensation in the cornea abolishes this reflex leaving the cornea unprotected. This can lead to the formation of ulcers on the cornea which can in turn lead to blindness.
3. Pain arising in a structure supplied by one branch of the nerve may be felt in an area of skin supplied by another branch: this is called referred pain. Some examples are as follows:
a. Caries of a tooth in the lower jaw (supplied by the inferior alveolar nerve) may cause pain in the ear (auriculotemporal).
b. If there is an ulcer or cancer on the tongue (lingual nerve) the pain may again be felt over the ear and temple (auriculotemporal).
c. In frontal sinusitis (sinus supplied by a branch from the supraorbital nerve) the pain is referred to the forehead (skin supplied by supraorbital nerve). In fact headache is a common symptom when any structure supplied by the trigeminal nerve is involved (e.g., eyes, ears, teeth).
4. A source of irritation in the distribution of the nerve may cause severe persistent pain (trigeminalneuralgia). Removal of the cause can cure the pain. However, in some cases no cause can be found.In such cases pain can be relieved by injection of alcohol into the trigeminal ganglion, into one of the divisions of the nerve, or into its sensory root. In some cases it may be necessary to cut fibres of the sensory root. In this connection it is important to know that the fibres for the maxillary and mandibular divisions can be cut without destroying those for the ophthalmic division. This is possible as the fibres for the ophthalmic division lie separately in the upper medial part of the sensory root. Finally, it may be noted that trigeminal pain can also be relieved by cutting the spinal tract of the trigeminal nerve: this procedure is useful specially for relieving pain in the distribution of the ophthalmic division as pain can be abolished without loss of the sense of touch and, therefore, without the abolition of the corneal reflex.
5. Mandibular nerve block: This is used for anaesthesia of the lower jaw (for extraction of teeth).Palpate the anterior margin of the ramus of the mandible. Just medial to it you will feel the pterygomandibular raphe (ligament). The needle is inserted in the interval between the ramus and the raphe. The tip of the needle is now very near the inferior alveolar nerve, just before it enters the mandibular canal. Anaesthetic injected here blocks the nerve.
6. The lingual nerve lies very close to the medial side of the third molar tooth, just deep to the mucosa. The nerve can be injured in careless extraction of a third molar. In cases of cancer of the tongue, having intractable pain, the lingual can be cut at this site to relieve pain.