Cranial nerves III, IV and VI contain motor fibres. CN III also contains sympathetic and parasympathetic fibres which also have a motor function, namely dilation and constriction of the pupil, lens accommodation and raising the upper eyelid.
CN III innervates:
- the levator muscle of the upper eyelid
- the superior rectus muscle
- the medial rectus muscle
- the inferior rectus muscle
- the inferior oblique muscle
The parasympathetic fibres of the CN III innervate:
- the pupillary sphincter muscle
- the ciliary muscle
The sympathetic fibres which come from the upper thoracic segments of the spinal cord, and not from the brainstem, run from the ciliary ganglion together with the CN III and innervate:
- the pupillary dilatator muscle
- the superior tarsal muscle
The CN IV innervates:
- the oblique superior muscle
The CN VI innervates:
The lateral rectus muscle [Figure 9].
Figure 9
A Top view of the eye muscles of the right eye
B Side view of the eye muscles of the right eye
C Function of the eye muscles.
1 superior rectus muscle (III)
2 inferior rectus muscle (III)
3 medial rectus muscle (III)
4 lateral rectus muscle (VI)
5 superior oblique muscle (IV)
6 inferior oblique muscle (III)
7 levator muscle of the upper eyelid (III)
The eyelid slit
Procedure
- Compare the left and right eyelid slits.
- Note whether the patient is holding their head tilted backwards or is pulling the eyebrows up to keep the eyes open.
- If you suspect left, right or bilateral ptosis ask the patient to close their eyes for a few minutes and then open them again. Note whether the ptosis is less pronounced now.
Interpretation
Ptosis can be caused by loss of function of the sympathetic fibres of CN III (superior tarsal muscle) as part of Horner’s syndrome (myosis, ptosis and anhydrosis). Ptosis can also be caused by loss of function of the CN III motor fibres (levator muscle of upper eyelid). In this case there will also be mydriasis caused by loss of function of accompanying parasympathetic fibres, and other eye muscles will also be affected. Ptosis is also seen with muscular diseases (having the patient close the eyes briefly will show an improvement in the case of myasthenia gravis). In these cases ptosis is often bilateral. It can also be congenital, in which case it will have been present for a longer period of time and will be non-progressive.
Ocular position, ocular movement and reaction to convergence
Procedure – ocular position
- Inspect the position of both eyes.
- Note whether the patient has a squint [Figure 10].
- Inquire whether the patient suffers from double vision. If you are unsure whether they have strabismus, shine a lamp onto the patient’s face from a distance of 30 cm and ask them to look at it.
- Look at the reflection of the lamp in the patient’s eyes (the Hirschberg test).Normally the reflection will be approximately in the middle of both pupils.
Figure 10
To interpret an abnormal ocular position or abnormal ocular movements resulting from loss of function of the oculomotor, trochlear or abducent nerves, it is essential to understand the function of the different eye muscles [Figure 9].
The movements which the six eye muscles allow the eye to make are dependent on the eye’s starting position. For example, if the eye is turned inwards as a result of fixation on a closely located point immediately in front of the patient, a downwards movement of that eye will be brought about through contraction of the superior oblique muscle. On the other hand, if the eye is in a neutral position, contraction of this muscle would cause the eye to roll inwards and to turn outwards and downwards.
Procedure – ocular movements
- Hold up a finger and have the patient follow it with their eyes [Figures 11, 12].
- Ask them to keep their head still.
- Move your finger left and right horizontally, upwards to the left and right, down to the left and the right, and up and down along the midline.
- Never allow the angle of vision to be greater than 45°.
- Ask the patient whether he or she experiences double vision at any point when following your finger.
- If so, ask them to indicate where this was most pronounced.
- Have the patient cover one eye and tell you which image disappears. If, upon covering the right eye, the right-hand image disappears, the term uncrossed double vision is used. If the left-hand image disappears, it is known as crossed double vision.
- While the patient follows your finger, look at both eyes and assess whether you observe conjugate ocular movement.
NB. Refer to the examination of CN VIII as well.
Figure 11
Figure 12
Procedure – convergence reaction
- Ask the patient to look at the finger you are holding up about 1 meter from their face [Figures 11, 13].
- While looking carefully at the patient’s pupils, swiftly move your finger towards a point between the patient’s eyebrows.
- Ask the patient to follow your finger.
- By monitoring the patient’s ocular position, check that they are actually following your finger.
- Note the pupillary reaction during this convergence test.
Figure 13
Interpretation
To interpret the ocular position, pupillary light reaction and convergence [Figure 14] in a comatose patient, please refer to the section concerning ‘The unconcious patient’.
Figure 14
If there is clear strabismus in the absence of double vision and with unimpaired bilateral vision, the strabismus has probably been present for a long time and one of the two images is being suppressed.
If double vision (diplopia) is caused by paralysis of an eye muscle, the double vision will become worse when looking in the direction in which the paralysed muscle would normally move the eye. If there is loss of function of the right trochlear nerve, the right eye will be looking inwards (nasally) and slightly upwards into the distance. The diplopia worsens if the patient looks downwards to the left. The image in the right eye is not projected onto the macula, but outwards (temporally) and above the macula. The right eye therefore perceives the image as being more to the left and lower than the left eye. This is known as crossed double vision [Figure 15].
Figure 15
In the case of loss of right abducent nerve function, the right eye will turn inwards when looking into the distance. The patient’s double vision will worsen when looking to the right. The image in the right eye is not projected onto the macula, but nasally to the macula. The right eye therefore perceives the image as being more to the right than the left eye does. This is known as uncrossed double vision [Figure 16].
Figure 16
If right oculomotor nerve function is lost, right-sided ptosis will develop and the right eye will turn downwards and outwards due to the dominant action of the lateral rectus muscle (CN VI) and the superior oblique muscle (CN IV). The pupil will also be dilated due to paralysis of the pupillary sphincter muscle. The reflex to light (direct and consensual) will be lost. The patient’s diplopia will deteriorate when looking upwards and to the left. The image in the right eye is not projected onto the macula, but outwards and below the macula. The patient therefore perceives the image through the right eye as being more to the left and higher than through the left eye. This is crossed double vision [Figure 17].
Figure 17
Double vision that does not disappear when one eye is covered does not suggest a lesion in the nervous system, but rather a refractive error, a detached retina or a psychogenic cause. If there is no double vision, but the patient is unable to move both eyes in a particular direction, it is known as gaze paresis or paralysis. This always indicates a central condition (refer to the section concerning ‘The unconcious patient’).