So far we discussed astigmatism reduction for low myopia degrees. The higher you go in correction, the more likely it is that there is some axial growth in the eyeball (the eye grew longer to permanently move focus, in response to close-up strain). The more axial growth happens, the more the eyeball is likely to be deformed (cylinder), causing vision to be sharp in some parts of the eye more than others.
The higher the myopia, the more likely some degree of astigmatism correction is warranted. Reducing the astigmatism correction becomes a question of priority – do we first want to have optimum expediency in reducing myopia, or do we want to reduce astigmatism as much as we can?
We must generalize a bit here, since the Web program can not properly account for each individual case. I start with these generalizations when evaluating clients, then adjusting to their case – so while general, this premise is relevant for you as well:
The Relative Correlation of Astigmatism
If you have a -2.5D myopia prescription and your astigmatism correction is -1D, we can assume with some degree of certainty that the prescription does not address physiological realities (ie. your eye probably not actually deformed to warrant a -1D astigmatism prescription).
Your astigmatism correction should be less than 20% of your myopia degree.
There are exceptions, for a whole number of reasons. Still, in most normal cases, 20% is on the high side of reasonable. 10% to 15%, for myopia degrees past -3D might be explicable. More, is less likely.
Consider too, why we want to reduce myopia – to reduce prescription complexity. We want a less complex prescription, to closer align your natural eyesight (uncorrected) to your current, corrected eyesight. When you take off your glasses, we are doing much better if it is just a simple focal plane correction change that your brain has to process. If the change also involves cylinder, axis (degree and location of astigmatism correction), and variance (difference between left and right eye), there is a huge amount of change between glasses and no-glasses. The less this change is, the easier it is, psychologically, to adjust to the various focal-plane states (normalized prescription, differential prescription, no prescription).
On the other hand, if your eye is deformed and requires some astigmatism correction, and we forego it, your vision is always compromised. This of course is also not beneficial, since the focusing error makes it more difficult for you to work on focus pulling (at a distance), and getting the healthy baseline of ‘this is what your vision should be’.
So there is the compromise. We don’t want to make your every-day vision unnecessarily handicapped, and at the same time we don’t want an overly complex prescription to deal with.
Generally, in a case below -5D of correction, we can critically look at everything above 20% of astigmatism correction.
If you have access to a test lens kit, this is an ideal time to look at changes. You will want to wear test lenses of your normalized prescription, and reduce the astigmatism value – 0.50D at a time, until it becomes noticeable. Then wear the test lens kit setup for at least 10-15 minutes, getting some distance vision, to assess the change. After 10-15 minutes, do the Snellen test (again), and see what has changed. This is what we did in the office, to help work on some astigmatism correction, while reducing the guesswork.
You shouldn’t, of course, just cut out a big part of your astigmatism correction, unless ….
You have been making good progress, you have been aggressive (and successful) with normalized prescription reductions, you have found that you can push the boundaries of pushing focus. In a case like that, you could take everything above 20% (of the astigmatism correction relative to your myopia correction), and ‘trade’ it:
For every 0.50 diopters of astigmatism correction, substitute 0.25 diopters of spherical (ie ‘regular’ myopia) correction.
This one is not a practice I would put in the first chapter of a book about correction myopia. Combining the fact that we are generalizing here, with me having no input on your case, knowing nothing about the circumstance, this is a sweeping, and somewhat eyebrow-raising approach to simplifying your prescription. Though, if the aforementioned conditions are met, you might see success in doing this.
If you are progressing but very slowly, not aggressively, often forgetting to really keep yourself at the distance limit, adding a lot of change to your prescription won’t make your day-to-day vision experience any more pleasant. That’s to say, you won’t benefit from it – so use this #endmyopia trick with caution.
Also, if you do decide to pursue this trick, the most you want to ‘trade’ at a time, is -1.50D of astigmatism correction (ie. adding no more than -0.75D of sperical, while taking out up to -1.50D of astigmatism). From experience, more than this is a jump that doesn’t end up providing a greater return on your effort-investment. It is already very aggressive, and suited only for those who are working on rapidly reducing their reliance on prescriptions altogether.
If you make this change, keep a log, and introduce the change slowly, as usual – no correction for the first 15 minutes outdoors, then using your new normalized prescription for 45 minutes, then taking a break from distance vision altogether (for the first 3-5 days, depending on your feel when using the changed prescription). You want to avoid wearing your previous normalized prescription in the same day, prior to the new one. We want a ‘clean slate’ as far as your visual cortex is concerned, which is much easier to accept of the correction is preceded not by another, more familiar correction, but just uncorrected distance vision.
This is a big installment, and really worthy of more of a chapter. Until another time, when I might have the time to expand on this, I hope this provides you with some helpful insights. As always, use the forum for questions.