Topography

Topography means a sort of relief map of your cornea. They are the scans where you see a big colourful circley-sort of thing with several different colours ranging from red to blue. The colours equate to different thicknesses of the cornea.

We have a real grudge about the fact that it can be so difficult to get straight answers about what your topographies MEAN. It simply can’t be all that hard. I mean, they have done these surgeries on how many millions of people? But, sadly, they don’t seem to have bothered to do any analysis at all of what makes for a GOOD topography, so now they don’t seem to agree with each other on rules about what constitutes a BAD topography (other than that no patient of theirs is likely to have one). Fortunately we patients have not been left altogether out to dry as there are many doctors who have experience with these problems and because they have been kind enough to spend time helping us, we tend to bombard them with topographies asking for advice.

You will learn very quickly, if you already haven’t, that you’d better get copies of your topographies if you want any help diagnosing your situation. There are people who are willing to help, but you’ve got to do your job by asking for copies.

Axial topography

We’re going to talk mostly about standard topography because it’s probably what will be most relevant to you. There are a number of topographers that fall into this category, with Humphrey Systems perhaps one of the most popular.

First, let’s talk about which of those colourful circles we’re interested in. On a Humphrey, it’s called an Axial Map. And we really hope you have Humphrey topos, but if you don’t, look for something called an Axial Map (or close to it) anyway.

Next, make sure you know which eye is which as they are labelled OD (right eye) and OS (left eye).

Next, let’s talk about resolution. There’s a key to one side of the scan showing what resolution was used. What you really want is one colour change for at least every _ dioptre, meaning, for example, that 42.0 is green and 43.0 is yellow and there’s a few discernible shades in between. Obviously, the greater the resolution, the less information the scan will give you. If resolution is set too high, everything is going to look beautiful. If it is set lower, you actually have a chance of seeing useful information. (Remember when you had to decide between a 2.0 megapixel and a 3.0 megapixel digital camera.... well, this is a lot more important.)

Now, down to business.

I’m taking a big risk in talking about topographies. Why? On the one hand, I’m not an expert. On the other hand, I’ve been told a lot of nonsense by people who should be experts. On the other hand, one or two experts whose opinions I value think it’s complicated stuff and are sometimes hesitant to pronounce judgement on certain kinds of things. On the other hand, one or two other experts whose scientific honesty I value think it is almost as simple as it ought to be and that there are actually some reasonable rules you can apply. On the other hand, when one has seen piles of topographies of people with horrible optical results from LASIK, presumably one ought to be able to identify certain common aspects contributing to the horribleness of their results. On the other hand, one never seems to see post-operative topographies from people with anywhere from acceptable to good to excellent results, because they’re simply so happy their doctor never bothered, and one wonders whether, if nobody’s analysing good topos, how does one know what good topos ought to look like? On the other hand...

For heaven’s sakes, how many hands can a person be expected to count on? Topos, tapas. I’m tired of debating with myself and will simply take the leap.

So, I’m going to take a stab at one or two basic points, and hopefully wherever I am wrong or incomplete, the medical community will rise up in wrath and pummel me with excellent, user-friendly scientific studies showing me the error of my ways in which case I will hasten to revise this text. Incidentally, at the time of this writing a trusted MD friend is engaged in research on this very topic and I am impatiently awaiting the results so that I can get a clearer view on what makes for good topos.

1. You should be looking at something that looks vaguely like a blue circle in a sea of red, with the bluish part extending for a certain way (across at least a couple of the boxes in the grid) then giving way to a progression of colours working their way up the rainbow: green, yellow, and orange, then red.
2. You should be able to identify your functional optical zone in there somewhere. The functional optical zone is the part that has been given full refractive error correction by the laser. The functional optical zone should be close to, but then again may not be anywhere near, the planned ablation zone as indicated on the operative report. That was the plan. This is the reality. We know people with FOZs as small as 2.5mm where their laser was programmed for 5.5. A little bit of this can be due to what’s known as ablation shrinkage, but the rest must be attributed to either what the laser did, or how badly the cornea was bothered by what the laser did.
3. Really small FOZs are Bad Things.
4. FOZs that are not all that small but are still smaller than your pupil size in dim light or dark are also Bad Things.
5. If you can’t identify your FOZ at all because there are so many colours swimming around right in the centre of the ablation, that is a Bad Thing.
6. Now, look at that blue circle (if there is one) or at any rate the central 6mm of your cornea. If it’s reasonably round, that means it had a centre. Try to identify the centre.
7. If the centre of the blue circle is not somewhere very near to the centre of your pupil, that is a very Bad Thing.
8. If the centre of the blue circle is perfectly aligned with the centre of your pupil, but there is a little X indicating your line of sight and that’s kind of offset from your pupil, and you had a high prescription, that might be a Bad Thing. (There is an industry argument over whether the ablation should be centred on the pupil or centred on the line of sight. Kind of important, like, boys, we need to decide whether we’re aiming to wing him or get him right in the heart.
9. Now, look at the central 6mm of your topography, and forget about whether it’s all the same colour (it probably isn’t or you wouldn’t be here). Do you see anything that looks like a bow tie, or a bug splat, or islands swimming around? That might not be a good thing. Now use the key that shows what fraction of a dioptre is represented by a colour. Are any of those fancy things more than 0.5 dioptre different in colour than the stuff around them? That indicates a Bad Thing.
10. Looking at that same central 6mm, use the key and find the minimum and maximum number of dioptres within that circle, and add up the different colours. If you are seeing 3 or more colours, and the minimum and maximum are 1.00 dioptre or more apart from each other, those are Bad Things. You should not have to be looking out at the world through all different kinds of refractive powered cornea. You might still be able to read an eye chart especially if the 1mm at the centre of your cornea is just the right power, but that doesn’t help too much with real life.
11. If your pupil size in dim light is greater than 6mm (which is probably the case: it is statistically true for half the population, and since you’ve got a bad result from surgery, it’s rather more likely to be true of you) look at the area between 6mm and however large your pupil is in dim light, and apply rules 9 and 10, and see if you find any more Bad Things.

By now, you are getting the idea. You ought to have a reasonably nice looking lasered area on your cornea that goes out somewhere at least close to where your pupil’s going to extend to. If you don’t, nobody should be surprised that you have some significant vision disturbances. If you do, you may still have plenty of reasons for vision disturbances when we get down to the dirty details. You’ll have to stay tuned for when The Lone Dog starts talking about that.