Why can't my 20-20 MS patient see? Every so often, I get a call of one my patients with MS because he "can't see anything". He comes in, reads the entire eye chart without error, but still insists he can't read 20/20. So what's wrong?
To address this question, it is important to be familiar with the eye and how Multiple Sclerosis affects it. The eye has a fundus in which the retina layers are distributed. This is where the foveal area is and, therefore, where visual acuity is most acute. For purposes of multiple sclerosis, it is the optic nerves and damage to the optic nerve with which we are often concerned. The thing to remember about the eye (and I am always amazed at how many researchers forget this), is that it is actually a direct extension of the brain. The retina is literally brain tissue. When it comes to vision problems associated with multiple sclerosis, first we're concerned with the optic nerve and then with the rest of the visual pathway, including the visual cortex. There are primarily three suspects in vision loss: visual acuity, visual fields, and contrast sensitivity.
The first suspect is visual acuity. There are three types of visual acuity. The first is detection visual acuity. That is, can you simply see something, whether or not it is there? The second is resolution visual acuity. If you have two objects, how close can they get together before they seem as if they are only one object? The third is identification visual acuity, which we typically use in eye clinic. This is the measurement and identification of whether or not the patient can see the eye chart.
The second suspect is visual fields. These are composed or made of an island of vision, surrounded by a sea of blindness. When we have problems associated with multiple sclerosis or other disorders, what we end up with are scotomata, or blind spots. Each person has a blind spot in each eye as part of the normal anatomical and physiological structure. We fill it in physiologically, but they are there.
In multiple sclerosis, it is possible to have a lot of different types of blind spots. Some of the most common involve notching of the peak of a hilar vision, a central scotoma. The notch can also be to on the side of the hilar vision, a paracentral scotoma.
Visual acuity and visual fields are routinely measured in many eye practices. Unfortunately, contrast sensitivity, the third suspect, is not. There are a lot of eye doctors who talk about it, but few who do anything about it or measure it.
Contrast sensitivity is basically the ability to see different size objects or different size contrasts. It is very important in many activities of daily living, such as driving, reading, and walking around. Even patients with good vision, apparently can have changes in their contrast sensitivity.
Contrast sensitivity is measured in the laboratory using a sine-wave generator. The sine-wave generators have different frequencies, phases, orientations, and amplitudes and by putting the different sinusoidal gradings up on a screen for a patient to look at, you develop a contrast sensitivity curve.
The interesting thing about using this method is that you can build almost any scene if you have the right number of sinusoidal curves. The process is called Fourier analysis. Just about any visual scene, if you have an appropriate computer to break it down into a sinusoidal component, can be generated as an image by using different types of sine waves.
The thing about our eye is that it is a fantastic mechanism: we see some things better than other things. When
it comes to contrast sensitivity, we see things at about four to five cycles per degree on the sine wave,
much better than we see things at about thirty cycles per degree. The other interesting thing is where four to
five cycles per degree falls on the vision curve is not where we have 20/20 vision. It's really where we have
about 20/50, 20/60 vision. When we measure visual acuity with the standard Snellen chart, we are actually
measuring the 20/20 end of the curve. So visual acuity is indeed built into this test.
ALIGN="JUSTIFY">Contrast sensitivity is an area that has not been explored in vision disorders related to Multiple Sclerosis.
And it may hold the answer to why patients with 20/20 vision still can't see.
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