CN V consists of sensory and motor fibres. This nerve transmits sensory stimuli from the face. Another important function that the CN V fulfils is innervation of the mastication muscles.
The CN V has three branches: the ophthalmic branch, the maxillary branch and the mandibular branch. These come together in the trigeminal (Gasser’s) ganglion, which is located near the apex of the petrous part of the temporal bone [Figure 18].
Figure 18
Facial sensory function
The section concerning ‘The sensory function’ describes how to carry out this examination.
Corneal reflex
The trigeminal nerve is the afferent pathway for the corneal reflex. The efferent pathways are the facial nerve and the oculomotor nerve.
Procedure
- Stand diagonally behind the sitting patient.
- Pull the upper eyelid up slightly with one finger on the patient’s eyebrow
[Figure 19]. - Ask the patient to look away from you.
- Wipe a clean cotton bud across the sclera from the side to over the cornea.
- Observe whether the patient closes their eyes in a reflex reaction or whether they turn their eyes away in an upwards direction.
- Compare left and right and ask the patient whether they felt left and right to the same extent.
Figure 19
Mastication muscles
Procedure
The strength of the mastication muscles can be examined in two ways.
Method 1
- Have the patient clamp their jaw tightly together [Figure 20].
- Palpate the masseter muscle on both sides and assess the muscle tone.
Figure 20
Method 2
- Place a spatula between the patient’s molars [Figure 21].
- Ask the patient to bite down.
- Try to pull the spatula out the patient’s mouth (be aware for dentures).
- Compare left and right.
Figure 21
Masseter reflex
Procedure
- Ask the patient to open their mouth slight and to keep the lower jaw as relaxed as possible.
- Place one finger on the patient’s chin [Figure 22].
- Strike your own finger with the reflex hammer.
- Look for a reflex closing movement of the lower jaw.
Figure 22
Interpretation
Facial sensory function can be reduced by a lesion in the peripheral nerve, the brainstem or a supranuclear lesion. If the lesion is located in the peripheral nerve (for example due to compression caused by a cerebellopontine angle tumour), there will be loss of sensory function. There will also be loss of motor function. If the patient does not feel the left or right cornea being touched, the trigeminal nerve is affected. If the patient does feel the cornea being touched on both sides, but the left / right blink reaction differs, the facial nerve is involved.
The masseter reflex will be stronger than usual if there is a lesion above the pons (corticobulbar pathway). It is practically impossible to compare left and right with this reflex.
The most common conditions involving the CN V is trigeminal neuralgia, which can be diagnosed based on typical findings during specific history-taking. There are no physical diagnostic findings relevant to establishing the diagnosis.