Diplopia comprises one of the most sweeping differential diagnoses in all of ophthalmology. The patient who complains of double vision can have something as benign as dry eye or as life-threatening as an intracranial tumor.
Diplopia occurs when the eyes are not moving together so that the brain is getting two slightly different pictures simultaneously. This typically occurs when MS affects the brainstem, where the coordination of eye movements is controlled. One common cause of double vision in MS is an internuclear ophthalmoplegia (also known as an INO). Rarely, MS patients may develop double vision from a sixth nerve palsy or other neuro-ophthalmologic disorder.
In Multiple Sclerosis patients this condition is caused by scarring in the brainstem where thecranial nerves serving the eye are situated.
Double vision occurs when the nerves that control the eye muscles develop plaques or demyelination. This leads to weakness of the eye muscle(s) involved and loss of particular eye movements or loss of coordination of eye movements. These lackings of the muscles that control the eye movements in turn causes double vision.
Sometimes the patient does not see two completely separate images. MS patients may report a “shadow” or a “blur” instead of frank double vision. An important question to ask is whether the visual problem goes away if either eye is closed. Because diplopia is caused by the brain receiving two different images, one from each eye, as soon as eithereye is closed, this type of visual problem will go away. On examination, there may be an obvious problem with the movement of the eyes, but sometimes the misalignment is not easy to see without special equipment.
Binocular double vision:
This occurs when the images produced by the two eyes do not absolutely match, so that the images produced are misaligned relative to one another. The diplopia disappears when one eye is covered.
Monocular double vision:
This is much less common. It affects one eye only and continues when the unaffected eye is covered. It can be caused by abnormalities of the lens, cornea or retina, which result in splitting of the image.
Diplopia Causes In MS:
Abducens (also called sixth) nerve palsy is the most common in MS and affects the nerve which operates lateral rectus muscle which pulls the eye outwards. Sixth nerve palsy causes double vision when looking from side to side. It is usually unilateral which means that the double vision occurs only when looking to one side and not to the other.
Oculomotor (also called third) nerve palsy is much rarer than abducnes in MS and it’s not really understood why. The third cranial nerve supplies four muscles to each eye; the medial rectus (pulls the eye inwards), the superior and inferior rectus (which pull the eye up and down and tend to turn the eye inwards), and the inferior oblique.
Trochlear (also called fourth) nerve palsy is also somewhat rare. The fourth cranial nerve controls the one muscle, the superior oblique, which moves the eye down and out. Forth nerve palsy is often associated with vertical diplopia.
All three of these cranial nerves arise in the brainstem – the third and forth in the midbrain and the sixth in the pons (a favoured site for MS). Lesions to these areas will often show up in MRI scans and your neuro can often pinpoint the actual lesion that is causing the problem on the image.
A related paralysis, called internuclear ophthalmoplegia, sometimes, but not always, leads to double vision. What usually happens is that the outward-looking eye fails to move outwards. This can occur in the absence of paralysis of the sixth nerve. It is due to a lesion of the medial longitudinal bundle, which is also commonly affected by MS.
Diplopia often resolves on its own in six to 8 weeks time. As with optic neuritis, intravenous corticosteroids are often prescribed, in the hopes of speeding up the recovery. Patients may need to wear an eye patch temporarily. The eye patch is guaranteed to “cure” the diplopia, since only one eye will be sending an image to the brain, but some patients may feel self-conscious while wearing the patch. Sometimes, if recovery is incomplete, eyeglasses with prisms can be used to bring the eyes back into alignment. Prism eyeglasses are similar to prescription eyeglasses for reading. The prism prescription and can be added to an already existing eyeglass prescription. In rare cases, strabismus surgery (surgery to correct crossed eye) may be recommended to realign the eyes.
For some people, wearing a patch over one eye or frosted tape over one lens of their glasses helps block visual input from one eye, thereby achieving single vision until the exacerbation is over. There is no danger in developing a “lazy eye” from patching. Dr. Thomas Hedges, director of neuro-ophthalmology at the New England Eye Center at Tufts University in Boston, says, “The muscles in both eyes are always stimulated together so they work in tandem whether one eye is covered or not.” People can wear the patch for as long as their doctor prescribes. Disagreement exists regarding how often or how long an eye patch should be used.
If patching is not effective, prism lenses may be incorporated into eyeglasses to redirect the way light enters the eye. An ophthalmologist adjusts the prisms for each individual to bring the images closer together. Over time, the eye muscles learn to fuse the images into one.
Prisms were effective for Ned, who after three months of wearing them, has not had any recurrences of double vision. However, prisms do have some drawbacks: They can take considerable time to get the fit just right, and the extra weight of the lens can feel heavy.
When patching and prisms don’t work, doctors may prescribe steroids. Their anti-inflammatory action sometimes helps resolve double vision brought on by an exacerbation, which really is an inflammatory episode of demyelination.
Steroids may be prescribed alone, with other medications or used in combination with more conservative methods, such as patching and prisms.