These three nerves are responsible for movements of the eyeball. In a routine clinical examination the movements are tested by asking the patient to keep his head fixed and to move his eyes in various directions i.e., upwards, downwards, inwards and outwards. An easy way is to ask the patient to keep his head fixed, and to follow the movements of your finger with his eyes. Such an examination can detect a gross abnormality in movement of the eyes.
Sometimes one of the ocular muscles may not be completely paralysed but may be weak. Two indications of such weakness are as follows.
This term means that objects are seen double. To understand this phenomenon remember that objects lying in different parts of the visual field produce images over different spots on the retina. The brain judges the position of an object by the position at which its image is formed on the retina. Normally the movements of the right and left eyes are in perfect alignment, and an object casts an image on corresponding spots on the two retinae so that only one image is perceived by the brain. When a muscle of the eyeball is weak, and a movement involving that muscle is performed, the movement of the defective eye is slightly less than that of the normal eye. As a result images of the object on the two retinae are not formed at corresponding points but over two points near each other. The brain therefore ‘sees’ two images, one from each retina.
To understand the causation of diplopia you can do a little experiment on yourself. Fix your gaze on any object. Place a finger below one eyeball and gently push it upwards. In addition to the normal bright image of the object you will see a second fainter image above the normal image. This illustrates that diplopia will be produced by any factor that distorts the normal alignment of the two eyes relative to each other.
This is a condition in which the two eyes do not look in the same direction. The squint becomes obvious when the eye movement involves a muscle that is paralysed or weak, because the weak muscle cannot keep up with the muscle of the normal side.
As explained above squint will be accompanied by diplopia. However, the patient compensates for lack of movement of the eyeball by turning the head in the direction of the object and on doing so the diplopia disappears.
If the normal eye is closed, the patient is unable to judge the position of objects in the field of vision correctly (because the image of the object does not fall on the part of the retina that corresponds to the true position of the object). All the features described above are those of paralytic squint.
There is another type of squint called concomitant squint. This condition is congenital, and manifests itself in early childhood. Squint is present in all positions of the eyeball. There is no muscular weakness and movements are normal in all directions. There is no diplopia.
All movements of the eyeball are lost in the affected eye. When the patient is asked to look directly forwards the affected eye is directed laterally (by the lateral rectus) and downwards (by the superior oblique). There is lateral quint (external strabismus) and diplopia.
As the levator palpebrae superioris is paralysed there is drooping of the upper eyelid (ptosis). As parasympathetic fibres to the sphincter pupillae pass through the oculomotor nerve, the sphincter pupillae is paralysed. Unopposed action of sympathetic nerves produces a fixed and dilated pupil.
Normally the pupil contracts when exposed to light (light reflex). It also contracts when the relaxed eye is made to concentrate on a near object (accommodation reflex). Both these reflexes are lost. The power of accommodation is lost because of paralysis of the ciliaris muscle.
The superior oblique muscle (supplied by the trochlear nerve) moves the eyeball downwards and laterally, and the inferior rectus (supplied by the oculomotor nerve) moves it downwards and medially. For direct downward movement synchronized action of both muscles is required. When the superior oblique muscle is paralysed the eyeball deviates medially on trying to look downwards.
This nerve supplies the lateral rectus muscle which moves the eyeball laterally. In looking forwards the lateral pull of the lateral rectus is counteracted by the medial pull of the medial rectus and so the eye is maintained in the centre. When the lateral rectus is paralysed the affected eye deviates medially (medial squint, or internal strabismus).