Most of my vision-centric posts on this blog so far have focused on the more dramatic of my conditions, myopic macular degeneration. But after seeing my ophthalmologist last week, I had the harsh reminder that the underlying cause of my MMD still comes with it’s own frustrations. Most notably, the relationship between pathological myopia and visual acuity.
As discussed in an earlier post, pathological (or degenerative) myopia is the progressive elongation of the eyeball. While it is common for people to have myopia (or near-sightedness), it becomes severe in a small percentage of individuals.
Typically when people reach their 20s to 30s, their eyes settle and don’t experience continual major growth or changes. For people with pathological myopia, the eye grows longer than it should, altering its shape from spherical to oblong. It is responsible for the strain to the retina that causes lacquer cracks, tears and abnormal blood vessels which can develop into myopic macular degeneration.
Visual acuity measures the clarity of your best corrected vision (that is, while wearing prescribed contacts or glasses). A person with 20/20 vision can see clearly at 20 feet what should normally be seen at that distance. Someone with a visual acuity of 20/100 must be as close as 20 feet to see what a person with normal vision can see at 100 feet. The interactive chart below from All About Vision allows you to see how images and letters appear at different levels of visual acuity.
The Impact of Pathological Myopia on Visual Acuity
So at the very beginning of this post I referenced my recent eye appointment. The prescription I received when I left the office was -25.00 in my left eye, -21.00 in my right eye. However, while adjusting my prescription, they determined that to best improve my vision in my left eye, I should have a lens with a strength of -26.50. So why the difference?
Two reasons. One involves how my eyes work together. When the machine was set with lenses to best correct the vision in each of my eyes, it caused me to have double vision. My right eye has been over compensating for the left one for so long that when both attempt to work an equal amount, my eyes don’t know how to handle it. The second reason is it is incredibly difficult to cut lenses stronger than a -25.00 without adding additional distortion to the vision.
If I were to have a -26.50 lens for my left eye, my best corrected visual acuity would be 20/100; with the -25.00, it’s 20/150. I’ve always anticipated the distortion caused by scarring on my macula caused by the myopic macular degeneration would be the primary factor towards reaching the threshold for legal blindness of 20/200. But my appointment reminded me that even when my MMD experiences periods of stability, the pathological myopia continues progressing. There is not much research being performed at the moment on how to stall or improve pathological myopia, but I am grateful that there are many ways to treat the more damaging symptoms that result from it.