How flat is flat in corneal K readings?

After being told repeatedly for a few months by my doctor and his
assistants that I have extremely flat corneas and therefore am
extremely limited as to which brands of contact lenses I can try, I
asked today for my K readings, and was told that they are 42/42 right,
42/42.5 left (in diopters). I've been scouring the web trying to find
out how this compares with an "average" reading, but haven't been able
to turn up anything. I'd be interested in knowing for example, even
anecdotally or as a wild guess, that 90% of corneas have K readings
between x and y.

I also attempted to correct my K readings based on the diameter of my
cornea (by measuring my HVID carefully while looking in a mirror using
some highly accurate scientific calipers--my doctor did not take such
a reading so I had to do it myself), using the formulas in the
following article:

http://www.clsa.info/ContinuingEducation/pdf/2Q2_01p7-11.pdf

If I measured accurately, my irises are about 0.9 mm larger than
average (average being 11.8mm), which means that my "equivalent K" is
46.50 left, 46.75 right. I'm not sure how significant a difference
that is in regard to selecting an appropriate base curve, but I am
basically doing these calculations to get a better handle on which
brands of lenses might be a good fit for my eyes, so even an estimate
would be helpful.

Any comments would be most appreciated.

Lothar

 

Aha, now we're getting somewhere. IMHO, the range of "normal" keratotomy
values is from - arguably, now - 42 to 46 give or take, with 44 about
average. While a value of 42 might cry out for a flatter base curve, I
have often found that a steeper lens is more comfortable on a larger cornea
because the average diameter lens does not adequately "bridge" the gap
between the edge of the cornea and the conjunctiva known as the limbus. I
might try to fit a larger lens in this case with a flatter base curve. I
have often found if useful to have a selection of lenses that go up to at
least 14.5mm in diameter or larger for patients with larger corneas. One
such disposable product is the B&L Soflens 66. This allows one to go
flatter in the base curve and still have a stable and comfortable fit.

The crux of the problem is that disposable lens manufacturers do not cater
to the tail ends of the normal distribution of eyes. I hope this helps.

DrG

 

Thanks, DrG, that was without question the most useful piece of advice
I have yet read on this very helpful newsgroup! I guess it helps when
people have all the relevant measurements to comment on.

A couple of comments...

That puts things so much better in perspective for me. Based on your
estimates (which I know are just a guess--I'd be interested to hear
other people's estimates too), it seems that I am on the flat end of
normal, but am not "unusually" flat.

Your comments seem to be in line with that article that I posted in my
last post. My "effective" K of 46.50 (after correcting for my cornea
diameter) actually seems to call for a steeper-than-normal base curve,
in spite of my flat corneas. This is assuming I was able to
accurately measure my HVID, which is not certain because I couldn't
actually touch my iris with the calipers due to their very sharp
points. I repeated the measurements several times though and all of
my measurements were within 0.4mm of my average reading of 12.7mm.

Thank you very much for that recommendation. I am currently trying
the original Acuvues, and they are extremely comfortable, although
with some fluctuation in my visual acuity on the computer. I don't
know if a change in the diameter or base curve will fix that, but I am
willing to experiment if my doctor is. The other lens that I am very
interested in trying is the Focus Night & Day, due to the high Dk
value (I want to be able to sleep in my lenses safely from time to
time), although I fear that any silicone hydrogels will tend to
accumulate more of the lipid deposits that have been causing me
trouble. Do you have any sense for whether those lenses might be a
good or poor fit for me? They come in BC 8.4 or 8.6, diameter 13.8.
I'm also interested in the 1-Day Acuvue, which comes in a BC 8.5 or
9.0, diameter 14.2.

Lothar

 

I see you're still on the "flat base curve" thing and now we have some
real data to look at. Your doctor is off base on this one. 42 is far
from "extremely flat" and is actual within the normal variance range of
42 to 46. In fact, many hyperopes are in the 41-43 range and myopes
43-46. So, you could indeed say that 90% of cornea curvatures fall
between 41 and 46.

I'd call extremely flat 37-39, flatter than average 39-42. And for
those who are interested, steeper than average 46-48 and extremely steep
48 and above. And most of those in that last group have keratoconus or
other cornea dystrophies.

You are measuring your cornea, by the way, not your iris, which is
located a couple of millimeters inside the eye.

Besides that, the relative change in curvature at the limbus (where the
cornea joins the sclera) plays a role in lens fit.

So as I've said before, the only true way to asses which lens would fit
appropriately is to examine the fit with the lens in place. As far as
lenses available to fit your "somewhat flat" and "somewhat large"
corneas, well there a litterally dozens of possibilities. You need to
get with a doctor who can do what's right for you.

--LB

--
Dr. Larry Bickford, O.D.
Family Practice Eye Health & Vision Care

The Eyecare Connection
http//www.eyecarecontacts.com
larrydoc at m a c.c o m

 

Yes, moderatly flat, not extremely flat, and not in the tail of the
distribution. However, an HVID of 12.7 is quite large, and I would always
be tempted to find a larger lens. Of course, if you want to sleep in
lenses, your choices are further limited. I believe that the Soflens 66 is
approved for overnight wear, but it does not have the DK of the Focus N&D.
Higher DK has proven to be better for overnight wear. However, it should
be mentioned that the rate of bacterial keratitis in patients who sleep in
conventional lenses such as Acuvue(non-silicone) is 1/500 per year,
confirmed by several studies.

Lipid spoilation can be a problem with silicone-hydrogel lenses. Good luck
in your quest.

DrG

 

Thanks, Larry, for another very useful response. I have a few
comments...

It is not so much that I am so concerned with flat base curves--it's
that my *doctor* is, and he is reluctant to let me try any lenses that
will not fit my eyes at least on paper. I am trying to gather enough
information to show him that steeper base curves might actually work
for me, although I'm not sure he will appreciate a patient telling him
what would work better for my eyes. He already seems pretty
frustrated with me for all the problems I have been experiencing (as
if it's my fault).

Thanks, that is very helpful. I found it interesting that everybody
in my doctor's office (the doctor himself and two of his assistants)
kept telling me that I had extremely flat corneas, but when I asked
for the K readings, she prefaced her answer by saying, "well, your
corneas actually aren't all *that* flat."

Your figures seem to support what DrG said. Based on both of your
answers, I conclude that my K readings of 42 are on the flat end of
normal, but not unusually flat.

Technically, what I was measuring, if I am not mistaken and was
measuring the edges accurately, was my HVID, or Horizontal Visible
Iris Diameter. My understanding (based on the article I quoted in the
original post) is that the cornea diameter is about 1mm larger than
this.

Believe me, I have not forgotten your previous advice, and I wish that
I could just try every lens on the market until I find the one that
works best for me. Unfortunately, this is not an option with my
doctor.

I agree with you. At my last visit, when my doctor was perplexed as
to why the RGP lenses didn't work for me, he did suggest that I go to
a particular contact lens specialist that he recommended. I had to
convince him to let me try the regular Acuvues before I left his
office (he said he would be "shocked" if they were any better for me),
but in fact these are by far the best lenses I have had in my eyes, in
terms of visual acuity, and especially comfort! They're not perfect
though--I still have inconsistent vision while working at my computer.

As far as changing doctors is concerned--there is one thing that is
holding me back from doing that just yet--the cost. I have already
paid $300 in fitting fees, plus my copays for my regular eye exams,
(not to mention the cost of the lenses and solutions themselves), and
am not especially enthusiastic of the idea of starting all over again
with fees to another doctor (and one who my doctor said is going to be
a lot more expensive than he is). I want to get as much mileage out
of my current doctor as I can before I switch to somebody new.
Perhaps when it's time for my next scheduled eye exam, I'll try this
other doctor.

Lothar

 

Thanks very much, DrG. I am aware of the risks of sleeping in my
lenses, but it was interesting to hear the actual statistic. I
actually don't plan to *routinely* sleep in any lens with a low Dk,
but I want something that is safe enough to do so occasionally if I
happen to fall asleep while wearing them, or if I end up someplace
where it was not convenient to bring my solutions with me for a night.

I actually slept in my Acuvues for the first time last night, mainly
to see if my eyes could tolerate them. I woke up feeling no sensation
whatsoever of anything being in my eyes (this was definitely not the
case with other lenses that I have tried), and my vision is perfectly
clear, with no lipid deposits that I can detect. So from the
perspective of comfort, fit, and tear chemistry compatibility, perhaps
I have found the lens that I am looking for (although my inconsistent
vision on the computer is slightly annoying). I'd sure like to find a
high-Dk lens that would fit my eyes though, and not deposit the heavy
lipid film that I have experienced.

Lothar