What is a refraction?
(Posted Apr 2003)
What’s the point?
If you have "refractive error", that is, you have short sight (myopia) or long sight (hyperopia) and/or astigmatism, the refraction will decide how much and what your glasses prescription should be. Another way to say this is it measures certain problems with your visual acuity, which is how clearly you see things.
How is it done?
Well, first they might measure you on an auto-refractor. That’s a machine that measures your acuity automatically, that is, without asking you whether you can actually see what the machine says you should be able to say. But generally speaking they will rely on a manual refraction, using an eye chart (the one with the big E on top) and sets of lenses in varying strengths, as follows: You stare at the eye chart, which is usually projected onto a wall using a mirror to get it set up at just the right distance. Then you are asked to compare what you see through one set of lenses after another. Usually this is done through a mechanism they swing in front of your face that has all the lenses inside it and they move things around inside it, but we know there are still some eye doctors who may just have individual lenses in a big box and they’ll hold a couple of lenses in front of your eyes at a time. You will usually remember this as the exam where they ask what the lowest line is that you can read, and then they ask "Better 1, or better 2?" over and over.
What does it tell them (and you) about your vision?
UCVA: First, without using any lenses they can establish your uncorrected visual acuity (UCVA), or how your vision is without any help from lenses. This is measured in terms of 20/something, where 20/20 is normal. Usually it takes some days after surgery for your vision to settle down, so this isn’t expected to be perfect right off the bat, but within a couple of weeks, your UCVA will tell you whether the surgery accomplished what it set out to do, more or less, which is to reduce your refractive error (myopia, hyperopia, astigmatism).
BCVA: Second, using the lenses, they can establish your best-corrected visual acuity (BCVA, also called Best Spectacle Corrected Visual Acuity or BSCVA), which means what’s the best you can possibly achieve in terms of eye-chart reading ability. This also is measured in terms of 20/something. If you cannot reach 20/20 BCVA, you are generally considered to have lost some vision (that is, assuming you had 20/20 BCVA before surgery — and if you didn’t have that or pretty close to it, they probably should not have been giving you surgery anyway). For example, if your new BCVA is 20/25, you have "lost one line of BCVA"; if it is 20/30, you have "lost two lines of BCVA and so on.
Glasses prescription: Third, using the lenses, they can establish what prescription gets you the best vision that you can possibly achieve with glasses. The prescription contains three parts: correction for short sight or long sight (called sphere), measured in dioptres; correction for astigmatism (called cylinder), also measured in dioptres; and a number in degrees indicating the axis of astigmatism, and it might look something like this: OD —1.50 —0.75 x180 OS —0.50 —1.00 x160, where OD is the right eye and OS is the left eye.
What does it NOT tell them (and you) about your vision?
Basically, the refraction is purely descriptive, not diagnostic, and it is descriptive of only one aspect of your vision — your acuity. Some of the things it does not answer include: (1) If you have lost some BCVA, it won’t tell them why, just that it’s happened. (2) If you have fluctuating vision, which is a complication of LVC, it will not tell them much of anything useful because your refraction may vary greatly from one day to the next or even from one hour to the next, or even during the refraction itself. (3) It will not measure any other vision disturbances, whether loss of contrast sensitivity, ghosting, night vision problems or anything else. (4) It will not tell them how much of your vision problem might be due to a tear film disorder.
What does it tell them (and you) about your surgery and/or your healing?
Before your surgery they measured your (a) myopia or hyperopia and (b) astigmatism, if you had any. They then programmed the laser to correct both. to correct your short sight (myopia) or long sight (hyperopia) and, if you had any, your astigmatism.
Correct correction: Ideally, you would get to 0.00 both for the myopia/hyperopia part of your prescription and for the astigmatism part. If that happened, then you are now "plano", and that is all the refraction tells then. Otherwise, there are a number of further possibilities:
Overcorrection: First, if you used to be myopic (short-sighted, with a prescription that starts with a minus) and are now hyperopic (long-sighted, with a prescription that starts with a plus), you have been overcorrected. They overshot the target and put you the other side of zero — instead of short sight you’ve got long sight. This condition can also be referred to as induced hyperopia. A small amount of this, that lasts just a few days, is normal. (Most people are deliberately overcorrected a little bit, because they are expected to regress, that is, become more myopic.) A large amount of overcorrection, or one that persists for weeks and months, is not normal. — Second, the same thing applies more or less in reverse if you started out hyperopic (long-sighted) and have become and stayed myopic (short-sighted). You have been overcorrected, also referred to as induced myopia. — Third, what about astigmatism? If you didn’t have any, and now have some, or now have more than you did, I suppose it would be polite to call this an overcorrection of sorts, otherwise it is induced astigmatism. — Finally, all three of these overcorrection possibilities can easily be determined in a refraction, unless you have other conditions which make a refraction unreliable.
Undercorrection: If you started with a certain refractive error, and still have some (though probably a lot less than you originally had), you have probably been under-corrected, meaning they fell short of the target. You might have been under-corrected for myopia or hyperopia, and/or astigmatism. Whatever the leftover refractive error is that you have, it is properly referred to as residual, that is, leftover short sight is residual myopia, leftover long sight is residual hyperopia, and leftover astigmatism is, you guessed it, residual astigmatism.
Note on undercorrections and overcorrections: Now, your refractive error is determined in the refraction, however, what could complicate things here is regression (see below). But basically, whatever your prescription was within a few days of surgery should establish what your correction state is — i.e. did the surgery hit the target, did it overshoot over undershoot. What happens after that is another story.
Regression: If after surgery you had a certain prescription, but after that you gradually slid backwards towards your original prescription, that is called regression and will be measured in each successive refraction exam. I know people who have regressed anywhere from half a dioptre to 8 dioptres of myopia (i.e. prescription of —8.00). I know people who regressed steadily for months, and others who had a stable refraction for months or years and then suddenly began to regress. Of course, whatever myopia would have occurred naturally is still going to occur whether you have laser eye surgery or not (the two are unrelated), but there are certain instances where it clearly seems to have some relationship to the surgery.
I’ve been told I’m "plano". What does it mean?
Plano means that you have no prescription. You have 0.00 prescription. You are 20/20 uncorrected. Your UCVA is 20/20. (All different ways of saying the same thing.)
Plano is a good thing. Only, if your vision is crap, you probably won’t get too excited about it. But whatever your problem is, the refraction isn’t going to tell you anything else about it so you need to look further — topographies and Wavefront scans.
I’ve been told my problem is residual astigmatism (or induced astigmatism). Will the refraction show this?
Well, the answer to that is yes if they were talking about true astigmatism and the answer is no if, as usually seems to be the case, they are talking about induced irregular astigmatism which sounds suspiciously like refractive error but isn’t.
Unfortunately, "astigmatism" has become a greatly misused term with respect to patients of laser vision correction who have induced vision problems. Astigmatism is a refractive error — like short sight and long sight, it’s something you may have had that the surgery was trying to correct.
If the surgery did NOT correct all of your astigmatism, the leftover astigmatism is residual astigmatism. On the other hand, if they somehow managed to give you more astigmatism than you had in the first place, that is induced astigmatism. Either way, you simply have some astigmatism and it should be correctable with glasses. If it is NOT correctable with glasses, that’s probably a good clue that your doctor was either misusing, or abbreviating, the term. (If any doctors are reading this, first of all, shame on you as this is really for patients. But since you’re here, if you ever wondered why some of us get upset with you over what seem to be euphemistic terms for conditions that are not in fact correctable, this is a good example. What have you got to say for yourselves?)
So about that "irregular astigmatism": No, it will NOT show in a refraction. It should be apparent in your topographies and will certainly show in your Wavefront scans.
Manifest vs. cycloplegic refraction
"Manifest" is just a fancy way of saying "the way it is". A manifest refraction is where they plunk you in front of the machine and refract you. What you see is what you get.
A "cycloplegic" refraction is where they first insert a cycloplegic agent like cyclogyl in your eyes, and then do the refraction. That’s an eyedrop which dilates your pupils (i.e. makes the black part at the centre of your eye larger). When they use a cycloplegic agent, it paralyses your "auto-focus" muscles connected to the lens of your eye, so that you can’t use it to adjust your focus at different distances (a process known as accommodation). Normally, your results from a cycloplegic refraction ought to be pretty much the same as what they are in a manifest refraction, but if they are noticeably different, that would be a good clue that you’ve got some kind of muscular thing going on that shouldn’t be going on, like accommodative spasm.