Other cranial nerves

Published on 10/04/2015 by admin

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Last modified 22/04/2025

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Other cranial nerves

Face, motor and sensory

Trigeminal nerve (5th)

Facial sensation is supplied by the trigeminal nerve (Fig. 1), which has three branches:

1. The ophthalmic, supplying forehead to vertex and tip of the nose, and including the cornea.
2. The maxillary, supplying the cheek to the angle of the jaw and the inner aspect of the mouth and upper palate.
3. The mandibular, supplying the lower jaw but not the angle of the jaw.
image

Fig. 1 Facial sensation is supplied by the trigeminal (V) nerve.

The sensation to pinprick, light touch and temperature can be tested in each division. The afferent component of the corneal reflex is the ophthalmic branch of the trigeminal nerve.

Trigeminal motor function is rarely abnormal. This can be tested by getting the patient to clench the teeth and feeling the masseter and temporalis.

Facial nerve (7th)

Look at the patient’s face for asymmetries. Compare the blink rate.

To test facial nerve function, ask the patient to look up at the ceiling (look at the frontalis), screw up the eyes (look at the orbicularis oculi), to whistle and show the teeth. Three patterns of abnormality are seen (Fig. 2):

1. Lower motor neurone (LMN) 7th: all facial muscles are affected. If severe, the patient is unable to close the eye and the eye is seen to roll up on attempted closure (Bell’s phenomenon) (common cause is Bell’s palsy; see Box 1). Reduced blink rate.
2. Upper motor neurone 7th: relative sparing of the frontalis.
3. Extrapyramidal weakness: limited spontaneous movement, full voluntary movement.
image

Fig. 2 Facial nerve lesion. (a) Right-sided lower motor neurone weakness; note droopy mouth, loss of nasolabial fold and eyebrow lines; (b) right-sided upper motor neurone lesion; note weakness in lower face with sparing of forehead muscles – especially when asked to look up.

Box 1 Bell’s palsy

This is a common condition (25 per 100 000 per year) of unknown aetiology. The patient develops a unilateral facial weakness, often with pain behind the ear, altered taste on one side of the tongue and hyperacusis. It usually improves spontaneously, but patients may be left with some facial weakness or aberrant renervation. Early treatment with oral steroids hastens and improves recovery. The evidence for the use of the antivirals, aciclovir or valaciclovir, is less clear cut. It is important to protect the cornea from trauma and abrasions (artificial tears, patching eye closed).

Bilateral facial weakness is difficult to spot. The facial nerve also supplies (i) the stapedius, so LMN 7th can lead to hyperacusis (noises sound particularly loud); (ii) taste to the anterior two-thirds of the tongue, so LMN 7th can lead to altered taste; and (iii) the lacrimal and parotid glands, so aberrant recovery can lead to crocodile tears (crying when hungry).

The corneal reflex is carried out by touching the cornea (not the conjunctiva) with cotton wool; the normal response is a brisk contraction of both orbicularis oculi. A lesion to the afferent (ophthalmic branch of the 5th) produces loss of the reflex. Lesions to the efferent (7th) can impair the reflex on that side but not on the other side.

Hearing and vestibular function (8th)

Hearing can be tested at the bed side in several ways. The volume of a sound can be compared between ears, for example using a whisper, a tuning fork or a ticking watch. If the hearing is impaired unilaterally, the Weber and Rinne tests will determine whether this is conductive or sensineural (Box 2).

Box 2 Weber and Rinne tests

Rinne test. Hold a tuning fork (512 Hz) in front of the patient’s ear, then transfer it to the mastoid process behind the ear. Ask which is louder. Conduction in air should be louder than that in bone.

Weber test. Place the tuning fork on the vertex. Ask where the sound is heard. Normally it is heard in the centre of the head.

Patterns of abnormality

image Bone conduction is better than air conduction:

– Weber test lateralizes to the affected ear = conductive hearing loss
– Weber test lateralizes to the other ear = a complete sensineural deaf ear, with bone conduction appreciated from the good ear

image Hearing diminished but air conduction better than bone conduction

– Weber test lateralizes to the other ear = sensineural hearing loss

Vestibular function is assessed by looking at eye movements, gait and coordination.

Mouth, tongue and palate (9th, 10th, 12th)

The glossopharyngeal nerve (9th) supplies sensation to the posterior pharynx. This is tested as part of the gag reflex. Touching this area is normally appreciated and provokes pharyngeal movement (vagus nerve, 10th). This is rarely affected in isolation. There is no clinically relevant motor output.

The vagus nerve has many functions: the parasympathetic supply to the stomach, upper gastrointestinal tract and heart, the supply to the larynx, muscles of deglutition and the palate. Palatal movement can be assessed by asking the patient to say ‘Ahh’. The uvula normally lifts centrally; unilateral weakness causes the uvula to be pulled to the good side, and bilateral lesions mean the uvula does not lift (Fig. 3).

image

Fig. 3 Palatal weakness. The palate is pulled to the normal side.

Laryngeal function can be tested by asking the patient to speak and to cough. In dysphonia, speech is a whisper and the cough is weak. This is usually due to laryngeal disease. Neurological causes include recurrent laryngeal or vagal nerve lesions and myasthenia gravis. Neurological dysphonia is usually associated with a bovine cough as a result of failure of laryngeal closure. Dysphonia may be non-organic and then the cough is usually normal.

The tongue is supplied by the hypoglossal nerve (12th). Look at the tongue for wasting; ask the patient to put out the tongue and move it quickly from side to side watching the speed of tongue movement. Lower motor neurone lesions lead to wasting and weakness of the tongue; upper motor neurone weakness tends to make the tongue slow moving. If the tongue is bilaterally wasted with fasciculations, this is usually a sign of progressive bulbar palsy, a form of motor neurone disease.

Fasciculation of the tongue is an important sign. The tongue should only be assessed when relaxed in the mouth; there is often the appearance of fasciculation when a normal patient puts out the tongue. It is important to be absolutely sure before saying there is tongue fasciculation.

Dysarthria

Dysarthria is a distortion of the articulation of speech. Listen to speech for a variation in tone, melody and rhythm. Phrases such as ‘yellow lorry’ test the lingual sounds, while ‘baby hippopotamus’ tests the labial sounds. The abnormalities are classified in Table 1.

Table 1 Type of dysarthria

  Clinical features
Spastic Laboured speech, with slow tongue and lip movements
Cerebellar Slurred as if drunk; staccato with loss of normal variation of emphasis, as if scanning a poem
Extrapyramidal Quiet and monotonous, running out of steam before the end of the sentence
Lower motor neurone, neuromuscular junction and muscle Normal rhythm, difficulty with sounds depends on affected muscle groups: tongue – sound t; face – sound b (think ventriloquist); palate – sound c or k (as if patient has a bad cold)

Accessory nerve (11th)

This nerve supplies the sternocleidomastoid (SCM) and trapezius muscles, which can be tested as follows. SCM: (i) ask the patient to flex the neck and resist the movement with a hand against the forehead; (ii) place a hand on the patient’s right temple and ask the patient to turn the head to the right – watch the left SCM. Trapezius: ask the patient to shrug the shoulders.

Other cranial nerves

image Trigeminal nerve abnormalities are usually sensory and rarely motor.
image Bell’s palsy is the commonest, but not the only, cause of lower motor neurone facial weakness.
image Hearing loss can be classified as conductive or sensineural on bed-side clinical testing.
image Dysarthria can be clinically categorized into spastic, cerebellar, extrapyramidal or lower motor neurone in type.

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