Astigmatism: How do you get astigmatism?
How do you get astigmatism? Well, really astigmatism is a function of the beautiful infinite variety that we see in biology in nature. We know that every parameter follows a normal distribution. So most people are average height, we just have some quite tall friends or quite short friends, and we just see that variation. And we see that variation in how peoples’ eyes are constructed, and this explains why we see patients with incredible optics.
I had the privilege of looking after an extraordinary, famous cricketer who lives locally and he had the most perfect optics of any patient I have ever seen, just incredible eyes, incredible vision. Literally biological perfection. Most of us fall short of biological perfection and we see astigmatism really as a function of that. So a lot of people are lucky enough to have a perfectly round spherical cornea giving them a really crisp sharp focus. But a lot of us have some tendency at least for that cornea to be a slightly uneven shape; a little steeper in one direction than another. And, the greater that difference between the two meridians, the greater the amount of astigmatism that we’re going to see in the way that lens performs. It will focus more powerfully in one direction than another. So that’s normal astigmatism that we see through a population.
But of course, we do see disease states where people can develop astigmatism in their prescription. So in the cornea, for example, trauma to the cornea may result in scarring and an uneven shape that distorts the light and creates astigmatism. There’s a condition called keratoconus where the cornea can move into an abnormal shape with steepening in the lower part of the cornea. And irregular astigmatism, where the light is refracted more strongly in a few directions, not just in in two meridians.
And then in older patients, who may not have had any astigmatism throughout our lives, as the lens inside their eye matures, ages and begins to turn into cataracts, that may cause uneven refraction of light and again may result in newly acquired astigmatism.