Well, here's why.
Keratoconus is a disease of the cornea, which is the front clear part of the eye.
Keratoconus causes the cornea to thin and bulge outward resulting in distorted vision.
There really isn't a great awareness in the general public about the disease and that means that people don't really understand how it affects those of us that have it.
I have had Keratoconus for over twenty five years and have relied on contact lenses for most of that time.
But, I have had bad days, days when the contact lenses irritate, or even become too painful to wear.
On those days it's always helpful when someone asks why I don't just wear glasses.
Now it's not true to say that glasses don't work with Keratoconus.
For early stages or very mild Keratoconus they work just fine.
Once the disease progresses however they are pretty much useless and here's why.
To understand the limitations of glasses let's first take a quick look at how your eye works.
At the back of your eye is a layer of specialised cells called the Retina.
These cells convert light into nerve impulses that your brain then interprets as vision.
Because your eye is a sphere the Retina is curved.
For all this to work three things need to happen * The image must be reduced in size to fit onto the retina.
* The scattered light must come together -- that is, it must focus -- at the surface of the retina.
* The image must be curved to match the curve of the retina.
Glasses are used to correct vision conditions such as: Myopia (nearsightedness) which occurs when a distant object looks blurred because the image comes into focus before it reaches the retina.
Hyperopia (farsightedness) which occurs when a close object looks blurred because the image doesn't come into focus before it gets to the retina..
Astigmatism which is caused by a distortion that results in a second focal point.
Keratoconus often results in a very extreme and irregular Astigmatism Now, how do glasses work? The lenses of the glasses are shaped to refract or bend light so that the focal point changes.
In other words making sure that the image comes into focus just as it gets to the Retina.
It is possible to correct regular astigmatism with glasses This is because there is a limited amount of distortion and it is still just a matter of shifting the second focal point to the correct position so that it aligns with the first focal point.
Early stages or mild Keratoconus will often result in a regular Astigmatism so glasses can be quite effective.
However, once the Keratoconus progresses the Astigmatism becomes very irregular and the cornea can?often have multiple curves (giving multiple focuses).
Once this happens it becomes impossible for glasses to address all of the focal points that this creates.
The net effect is that the light cannot be focused correctly on the retina, which results in blurring, and the scattering and incorrect curvature of the image results in ghosting (or multiple overlapping images).