I’m a medical student and I want to share with everyone the knowledge I have acquired about astigmatism by reading pretty much every post from endmyopia about it and also by learning the mainstream medical point of view and asking doctors questions in my 2 weeks long ophthalmology rotation. One reason I’m doing this is because lately I’ve seen a lot of posts here and on the facebook group about astigmatism and a lot of people are asking questions about reducing it, spherical equivalent and so on. Hope you’ll find the information useful, I acknowledge my humane limitations and I’m open to discussions.
1. What exactly is astigmatism? - Basically, astigmatism happens when the cornea is not perfectly shaped (like a sphere), but resembles a rugby ball. That is, the length of the horizontal and vertical axes are different, thus the refractive power of the lens (cornea) varies. I don’t want to get into a lot of details, but what this means, is that there are 2 images formed on the retina because of the difference in refractive power along the 2 axes of the cornea. 2 images mean what we on endmyopia call astigmatic blur.
2. What does the axis number mean? - It is the angle (that’s why it’s written in degrees) between the two principal axes of the cornea that are different. This can be anywhere between 0 and 180 and the 2 different images formed on the retina are misaligned on this particular direction.
3. Why is astigmatism corrected with cylindrical lenses? - To correct myopia we use spherical lenses that diverge the all the rays of light, thus moving the image further back in your eye. To correct astigmatism, we use cylindrical lenses that diverge the rays of light only on a certain axis. Not gonna go further into high school physics about lenses.
4. Are there different types of astigmatism? - Yes, I’m not gonna go into all the types, but you should (understand at least) know what’s the basis of defining different types. You have 2 images formed and one retina in one eye. One image can be formed either in front of the retina, on the retina, or behind it. The other image - the same. By playing with different possibilities, you can have different combinations, so different types of astigmatism. I will exemplify just 2 types:
a) Simple myopic astigmatism is when one image is on the retina and the other is in front of the retina. You need a cylindrical correction that moves that image back on the retina.
b) Compound myopic astigmatism is when both images are formed in front of the retina, but not in the same place (that would basically mean myopia). You need both spherical correction (that moves both images back, one being on the retina now) and cylindrical correction (that moves the other image further back, aligning them both on the retina taDA! the sharp image is formed)
Most of endmyopia users that also suffer from astigmatism probably have this type
*Astigmatism can also be classified based on the axes number, but I don’t feel like going into these details now. Try wiki. *
5. What is spherical equivalent? - Giving you some numbers because it’s easier to understand. Let’s say you have (talking only one eye for the sake of simplicity) -2 spherical and -1 cylindrical (with axis y). You have one image that is formed in front of the retina and you need 2 diopters to move it back. Second image - you need 3 diopters to move it back, 2 are from the spherical lens and 1 additional is from the cylindrical on the y axis. Spherical equivalent means something in between that, let’s assume is -2.5 spherical that moves both images back at the same time, but not putting any of them on the retina. Think of this like a compromise that cannot work perfectly because neither image is where it should be.
6. Why does Jake says you don’t need correction for astigmatism when it’s under a certain value (0.5-1.0) ? Because it’s not that much and if you can see fine without it, there’s no need to use it.
7. What can cause astigmatism? - Medically speaking, the exact cause of astigmatism is unknown. The eye is a VERY VERY VERY sensitive and delicate organ (a third of a millimeter of misalignment between the image that is formed on the retina and the retina equals to 1 dioptre of myopia) and just small inaccuracies in the lens of the eye that can arise from unknown factors (genetics for example) can cause problems. Mainly, the cornea is affected in some way that makes it irregular (picture the rugby ball again).
8. Then is there such a thing like lens induced astigmatism from progressive myopia? - Yes and No. I don’t have a definite answer to this. Intuitively, when the eye is very big (high myopia) there is the possibility that it didn’t enlarge uniformly, thus making it astigmatic.
9. Why does Jake say that genetic astigmatism is rare? - It’s not that rare. According to wiki, 30-60% of adults in Europe and Asia have astigmatism. There is a rare disease called Keratoconus and it’s caused by a very nasty shaped cornea that gives progressive and very high astigmatism. This is pretty rare, but low genetic astigmatism can occur with higher frequency than Keratoconus.
10. How do I know that my astigmatism is from my cornea? - You go to a ophthalmologist that will look at your eyes with a refractometer AFTER he put cycloplegic drops in your eyes, eliminating cilliary spasm and thus any accomodation reflex that might occur during the measurement. On the piece of paper that comes out from the machine, it should be written (if you have astigmatism) how much is from the cornea, also giving you other numbers for nerds. The measurement of the the topography (how it looks) of the cornea is called Keratometry (it measures your cornea).
11. Why is astigmatism so frequent? I’m so sad to hear so many people have astigmatism - Actually, there is something called Physiologic astigmatism. This means that is normal. Wait, what?! Yes, a lot of people have slight astigmatism and they usually don’t need correction (because it’s very low). It is caused by the weight of the lids that press unequally on the cornea every time you blink, thus changing the shape of it just a little (VERY little actually). This would be another argument for point 6.
12. Is astigmatism caused by uneven cilliary spasm or imbalance between the muscles that move the eye? - I HIGHLY doubt it, but think for yourself. This is my opinion based on my anatomy knowledge.
13. Enough with the talking, can I reverse my astigmatism? - If we assume that you can reverse myopia by changing back the eye to it’s normal size (through AF, positive stimulus, outdoor time, bifocal planes etc), we could also assume that you can reverse back the astigmatism that was created during the elongation of the eye. If you have low myopia, and higher astigmatism (that you can pinpoint through keratometry to an asymmetric cornea), my feeling is that you might not be able to see clearly without changing the shape of your cornea somehow.
TOO LONG DID NOT READ
To sum things up (because this post has gotten way bigger than I expected), astigmatism is usually caused by a different shape of the cornea (we don’t know why this happens) that forms 2 different images on the retina. There are a lot of types of astigmatism. If you have less than 0.75 it’s probably physiologic and you should wear cylindrical correction only if it bothers you that much.
P.S. What I’ve written above is my personal knowledge about astigmatism and I thought that by sharing it with you, you’ll gain something from it. This is by no means medical advice! I don’t want to hurt anybody’s feelings with my opinions that might be different than what others think. I’m aware that what I think I know might be wrong. I’m open to discussions and I’l respond the best I can and when I have the time. Feel free to leave a comment and tell me what you think! Have a lovely weekend