The elucidation of double vision depends on knowledge of the anatomy of the extra-ocular muscles and their actions.
The extra-ocular muscles are divided into two groups. The first group arises from the apex of the orbit to attach to the sclera anterior to the equator of the eye. Included in this group are the rectus muscles: medial, lateral, superior and inferior.
The second group consists of the oblique muscles. The superior oblique arises from the apex of the orbit and is deviated through a pulley in the anterior orbit so that it passes backwards and laterally above the globe to attach to the postero-lateral area of the upper surface of the eye. The inferior oblique arises in the anterior orbit and passes backwards and laterally under the globe to attach to the postero-lateral quadrant of the eye inferiorly.
The extra-ocular muscles are supplied by three cranial nerves. The third cranial nerve passes forward in the lateral wall of the cavernous sinus and divides anteriorly into the superior and inferior divisions, which enter the orbit through the superior orbital fissure. The superior division supplies the levator and superior rectus, while the inferior division supplies the medial and the inferior rectus and the inferior oblique.
The fourth cranial nerve supplies the superior oblique muscle, and the sixth cranial nerve supplies the lateral rectus.
The medial and lateral rectus pass forward from the apex of the orbit to the globe, and their actions are not influenced by the position of the globe. The medial rectus only adducts the eye, that is turns it towards the nose and the lateral rectus only abducts the eye, turning it outwards away from the nose.
The remaining rectus muscles have three actions depending on the direction of gaze. Clinically one need only consider the action of a muscle when the eye is directed along the axis of that muscle.
Thus, when the eye is abducted, the superior or inferior rectus each has only one action - the superior elevates the eye and the inferior depresses the eye.
When the eye is adducted to look along the line of action of the superior oblique, it has one action, to depress the eye. Its antagonist, the inferior oblique, when the eye is adducted, elevates the eye. The actions can be simply represented diagramatically (see Actions of ocular muscles.).
The superior and the inferior oblique muscles also rotate the eye around an antero-posterior axis. When the 12 o'clock meridian is rotated towards the nose it is called intorsion. Rotation of the 12 o'clock meridian away from the nose is extorsion. The oblique muscles have no torsional effect when the eye is adducted, but when the eye is abducted the torsional effect is maximal. The superior oblique intorts the eye, the inferior oblique extorts the eye.
Diplopia results when the visual axes are not parallel. Very occasionally, it is due to abnormalities in the refracting surfaces of the eye. The commonest cause of diplopia is palsy of the third, fourth or sixth cranial nerves resulting from vascular accidents, cerebral aneurysm, giant cell arteritis, multiple sclerosis, diabetes or trauma. Muscular causes are myositis, myasthenia gravis, thyroid eye disease and trauma, particularly a blowout fracture of the orbital floor (see Eye injuries and infections).
When a patient complains of sudden onset of painful diplopia, one should always think of cerebral aneurysm or diabetes. Diabetes causes occlusion of the small vessels supplying the fourth or sixth nerve causing pain due to ischaemia.
The key to the diagnosis of diplopia is the fact that the eye muscles work in pairs (see Actions of ocular muscles.). When the eye turns to the left, the left lateral rectus and the right medial rectus combine to produce the movement. When the eyes look down to the left the movement is produced by the left inferior rectus and the right superior oblique.
When the patient complains of diplopia the first question to ask is, “In which direction is the double vision maximal?” This then isolates the cause to the two muscles involved in turning the eye in that direction.
Because the image in the eye is inverted, the next step is to cover one eye to determine which eye gives rise to the image that is furthest away (image inversion dictates that the eye that moves least has the maximum displacement of the image). This indicates the muscle responsible for the diplopia.
Lateral rectus palsy
If the patient complains of double vision looking to the left, the left lateral rectus or the right medial rectus is responsible for the diplopia. Covering one eye elicits the information that the furthest image comes from the left eye and thus the cause of diplopia is the left lateral rectus - a lesion of the sixth cranial nerve. Furthermore, the medial rectus is one of four muscles supplied by the third nerve, and if the other muscles are acting normally it is unlikely that one of the four would be defective in the presence of three healthy muscles.
Fourth cranial nerve palsy
It is easy to diagnose a fourth nerve palsy because this is the muscle which takes the eye down in adduction. That is it is the muscle used in reading. Thus a person with a fourth nerve palsy will complain of double vision, a line of print will be double, with the second line below the line being read, and that line will be tilted, while the other remains level. Again, covering one eye will enable the diagnosis to be made, as the eye with the tilted image (due to the unopposed action of the inferior oblique) will be the eye at fault.
In unilateral fourth nerve palsy, one line is tilted; in a bilateral fourth nerve palsy both lines of print are tilted in opposite directions.
The sequelae of muscle palsy
The action of the eye muscles is not as simple as detailed here because in every eye movement all the extraocular muscles are involved, some contracting and others relaxing. When one extra-ocular muscle is paretic, changes occur in all the muscles.
The direct antagonist of the paretic muscle undergoes contracture because it is opposed by a weaker muscle. In a right lateral rectus palsy the right medial rectus undergoes contracture. This tends to increase the separation of the two images.
The contralateral synergist of the paretic muscle over-acts. In the case of a right lateral rectus palsy, the left medial rectus will overact, increasing the separation of the images. This is due to the paretic muscle receiving an increased innervation in an attempt to increase its range of movement: because the contralateral synergist, in this case the medial rectus, receives the same innervation it over-acts. Finally, to allow this muscle to overact, there must be an inhibitional palsy of its antagonist (in the example this will be the left lateral rectus).
Another example - a patient with a right superior oblique palsy suffers a contracture of the right inferior oblique, over-action of the left inferior rectus and a secondary palsy of the left superior rectus.
This increases the deviation - this is of value to the sufferer, as the further apart the images, the easier it is for one to be suppressed.
Sometimes the secondary effects of the paretic muscle do not recover when the paresis recovers. Surgery to the overacting, but healthy, muscle may then be necessary to weaken it.
Aberrant regeneration is commonly seen after a third nerve palsy resulting usually from head trauma. When the nerve recovers, some fibers will be misdirected. Typically, when the patient converges the eyes, the upper eyelid elevates because fibers to the medial rectus have reinnervated the levator of the upper lid. (See This patient has a right third cranial nerve palsy. In the upper picture he is attempting to look up. The right eye fails to elevate. In the lower picture when he attempts to look down the upper lid elevates owing to misdirection of nerve fibers. In this case fibers intended for the inferior rectus are innervating the levator muscle of the upper lid.).
Aberrant regeneration is untreatable.
Surgery of diplopia
The principles of the surgery of double vision are:
- to strengthen weak muscles by lengthening them,
- to weaken overacting muscles by shortening them.
A muscle is strengthened by excising some of the tendon and then resuturing it to its original insertion. A muscle is weakened by removing it from the globe and re-attaching it closer to its origin.
A paretic muscle may be strengthened by excising a small portion (usually 3–6 mm) and its direct antagonist weakened by recessing its attachment to the globe. If this is insufficient to correct the double vision one next proceeds to the contralateral synergist. The patient who has a weak lateral rectus would have an excision of a small length of the tendon of that muscle, a recession of the medial rectus on the same side initially, followed later by recession of the opposite medial rectus if required.
Surgery for diplopia never completely cures the patient because five muscles cannot do the work of six. Rather, the intention is to move the double vision away from the straight-ahead position to the extremes of ocular movement where it is less troublesome.A patient with a completely paralysed extra-ocular muscle will always have intractable double vision in some direction of gaze. In such cases, surgery is disappointing. Often single vision can be secured in straight ahead gaze, but not elsewhere.