Cranial nerves 1, 3–6

Olfactory nerve (1st)

This nerve is usually only tested formally in patients with specific complaints regarding the sense of smell. The examination consists of the presentation of a range of smells such as peppermint, camphor and rose water to each nostril while closing off the other and asking the patient to identify them. Ammonia is also tested: this is not perceived by the olfactory nerve but is a direct irritant to the nasal mucosa, stimulating the trigeminal nerve. Failure to respond suggests non-organic anosmia.

Olfactory nerve lesions are usually post-traumatic, but can occur rarely in patients with frontal lesions, and subtle abnormalities are found in degenerative conditions such as Parkinson’s disease. Anosmia occurs commonly with nasal blockage (e.g. colds).

Eye movements (3rd, 4th, 6th)

Eye movements are controlled in several ways. They can be moved as follows:

Voluntarily (e.g. look right) – movement is initiated from the frontal eye fields. These are also called saccadic movements.

On pursuit – following objects (e.g. follow my finger); this is controlled by the occipital lobe.

Under vestibular control, reflex movements maintain eye posture with the head and other movements. This is tested as the vestibulo-ocular reflex, which subserves the doll’s head phenomenon.

With convergence, to look at close objects (rarely of clinical importance).

These movements are integrated in the brain stem with the 3rd and 4th nuclei in the midbrain, and the parapontine reticular formation and 6th nerve nucleus in the pons. These are connected by the median longitudinal fasciculus (MLF). Eye movements result from contraction of the extraocular muscles. All but two of the extraocular muscles are supplied by the 3rd nerve, the superior oblique and lateral rectus being supplied by the 4th and 6th nerves, respectively (Fig. 1). Weakness of the extraocular muscles results in double vision in the direction of movement of that muscle. The image that is further out arises from the affected eye. Get the patient to look in the direction where diplopia is maximal and then cover each eye in turn to find which eye sees the outer image. The relative position of the two images is helpful: if the images are horizontally displaced but parallel, this indicates a weakness in the lateral, or occasionally the medial, rectus. If the images are at an angle then other muscles are affected.

Fig. 1 Muscles involved in eye movement of the left eye.

In simple terms, examination of eye movements involves answering the following questions:

What is the position of the eyes at rest looking straight ahead (in primary gaze)?

Does each eye move to all positions of gaze (Fig. 1)?

Is there double vision in any direction of gaze?

Do the eyes appear to move together in each direction of gaze?

Can the eyes converge when looking at a close object?

Infranuclear lesions

Double vision usually results from a 3rd, 4th or 6th nerve palsy and these are illustrated in Figure 2.

Fig. 2 Single nerve palsies.

If double vision does not conform to a single cranial nerve then consider neuromuscular junction failure, as in myasthenia gravis, or muscle disturbance, such as ocular myopathies, or mechanical distortion, as in thyroid eye disease. Associated ptosis is seen in myasthenia and ocular myopathies.

Internuclear and nuclear lesions

Lesions of the MLF (Fig. 3) usually do not produce double vision.

Lesions of the nucleus for lateral gaze lead to lateral gaze palsies; this is where neither eye can look in one direction.

Inability to look up (upgaze paresis) or look down (downgaze paresis) results from upper and lower brain stem lesions, respectively.

Lesions to the MLF lead to an internuclear ophthalmoplegia (INO; Fig. 3). The lateral gaze nucleus in the pons stimulates the ipsilateral 6th nerve nucleus, causing the ipsilateral eye to abduct; however, the message fails to reach the opposite 3rd nerve nucleus and the contralateral eye does not adduct. This is associated with nystagmus in the abducting eye, so called ataxic nystagmus.

Fig. 3 Internuclear connections. (a) Normal; (b) internuclear ophthalmoplegia (INO). PPRF, Parapontine reticular formation.

Supranuclear abnormalities of eye movements arise when there is an abnormality of voluntary or pursuit movements yet the eye movement can be produced using the doll’s head manoeuvre. These are rare.

Nystagmus

Nystagmus is rhythmic to-and-fro movements of the eyes. This should be watched for during testing of eye movements, paying attention to the direction of gaze in which the nystagmus occurred and the direction of the fast phase. Be careful not to consider the normal nystagmoid jerks found on extreme lateral gaze as nystagmus. Nystagmus is normal in certain situations, such as looking out of the window in a moving train.

The following patterns can be seen:

Pendular nystagmus. This is symmetrical nystagmus in primary gaze, and indicates impaired fixation. It occurs in albinism and, classically, but now rarely, coal miners.

Unidirectional jerk nystagmus. This can be classified as first degree if it occurs only on looking in the direction of the fast phase, second degree if occurring on primary gaze, and third degree if looking away from the direction of the fast phase. This can occur in central or peripheral vestibular syndromes. Peripheral lesions are associated with vertigo while central ones may not be.

Multidirectional jerk nystagmus. This indicates a central vestibular syndrome, for example phenytoin toxicity.

Vertical nystagmus. Up- or down-beat nystagmus indicates a central brain stem lesion but has poor localizing value.

Ataxic nystagmus. This occurs with internuclear ophthalmoplegia – see above.

Vestibular syndromes are discussed further on pages 46–47. Other abnormal eye movements are rare and include opsoclonus or dancing eyes and chaotic movements of the eyes.

Cranial nerves 1, 3–6

Anosmia is usually due to blocked nostrils.

Neurogenic anosmia is usually due to prior head trauma.

Diplopia is most commonly due to lesions of the oculomotor (3rd), abducens (6th) and rarely trochlear nerves (4th).

Myasthenia gravis is a cause of complicated and variable eye movement abnormalities with ptosis, fatiguability and normal pupils.

Nuclear and supranuclear lesions produce abnormalities that can be recognized on clinical examination.