Chapter 22: Mixed Astigmatism

Mixed Astigmatism - occurs when one principal meridian is short-sighted and the other is long-sighted, Treatment of mixed astigmatism including coupling adjustments for myopic and hyperopic cylinder, Coupling, Mixed astigmatism treatment in refractive laser surgery There are more complicated forms of astigmatism such as mixed astigmatism. Can you just give us a definition of mixed astigmatism? Mixed astigmatism is when you have a greater amount of cylinder magnitude than sphere and the sphere and cylinder are of the opposite sign. Optically in cases of mixed astigmatism there are two focal planes – one in front of the retina and one behind the retina unlike hyperopic or myopic astigmatism where the focal planes lie either in front or behind the retina. How do you go about treating mixed astigmatism? So this is where it gets a little bit more complicated because you need to look at previous treatments so you look at all your plano cylindrical myopic treatments and you look at all your plano cylindrical hyperopic treatments to see how much coupling effect you have, you then combine both of them in to your paradigm for mixed astigmatism keeping in mind that you have a plano cylindrical myopic component of the mixed, you have a plano cylindrical component of the hyperopic but you have no spherical treatment. And the idea of having no spherical treatment, you don’t really need to have a spherical treatment. If you just want to move those two images, one in front of the retina and one behind the retina, then you want to move them on to the retina by differing amounts to get it to the retinal plane and so therefore you do not need to have a spherical treatment and this reduces the amount of ablation treatment that you need to have. Can you give us a simple explanation of why that separate coupling adjustment for myopic and hyperopic cylinder needs to be done. The adjustment is done for those two differing amounts to be able to determine how do you get this image on to the retina from one from in front of, being a myopic, and one from behind the retina being the hyperopic without the spherical treatment. So it goes back to high school type of calculations when you have two trains travelling in opposite directions from different places, at different speeds but you want them to arrive at the same station at the same time at the same place. It is kind of a good analogy of the complexity of these trains like the complexity of mixed astigmatism treatments, one being minus and one being plus.