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LASIK complication - patients lose contrast sensitivity
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| What We Should Really Tell LASIK Patients
Jack Holladay, MD, MSEE, FACS Houston
http://www.revophth.com/1999/May_ar...RPE9f5lasik.htm
Small pupils or not, patients will lose some contrast sensitivity. All
surgeons know that, done properly, LASIK and PRK can markedly improve
uncorrected Snellen acuity. But there is a flip side to these
procedures of which surgeons and their patients are completely unaware.
Because excimer laser manufacturers were unaware of the true
physiologic shape of the human cornea, both procedures reduce contrast
sensitivity in low-light conditions. In this article, I'll explain the
problem, how it impacts patients, and what you can do to minimize the
problems associated with it. I'll also discuss what excimer companies
are working on to reduce, or eliminate, these concerns.
Excimers assume all corneas look like this, a sphere. Unfortunately,
that's not the case. Function follows form. To understand the problem,
it's important to understand the impact of the cornea's shape on the
light that passes through it. The natural shape of most corneas is
what's called prolate. A prolate surface is shaped something like the
head of a bullet. It is steep curvature in the center, but flatter
toward the periphery. As the cornea flattens, its dioptric power
decreases. The net effect is that central and peripheral light rays all
focus at a single point inside the eye, regardless of pupil size.
In a small percentage of the population, the cornea is spherical. In
these patients, the focus of rays shifts depending on pupil size. In
the daytime, when the pupil is 3 mm or smaller, most of the rays focus
at one point. At night, when the pupil is dilated, the peripheral rays
focus at a point anterior to that of the central cornea. This results
in night myopia; the optical property is called spherical aberration.
The problem with all excimer lasers on the market today is twofold.
First, the engineers assumed that the cornea is spherical rather than
prolate. Second, they assumed that their job was to reshape a
relatively steep sphere into a relatively flat sphere, rather than to
reshape a steep prolate into a flatter prolate.
As a result, excimer lasers actually reshape prolate corneas into what
is known as an oblate. An oblate is shaped like the cross-section of a
hamburger bun, flatter in the center but steeper in the periphery .
This shape is actually optically worse than a sphere, because now the
peripheral rays are bent even more powerfully than in the periphery of
a sphere, causing even more pronounced spherical aberration when the
pupil dilates. Unlike the haloes that result from a too-small optical
zone, this problem affects every patient who undergoes an excimer laser
procedure to some extent. Like the halo problem, the oblate dilemma is
worst for patients whose pupils dilate widely at night.
Oblation in practice
We recently tested this concept on seven patients before and after they
underwent bilateral LASIK. We first tested the patients at 98 percent
contrast, which is the level of the letters on a Snellen chart. Then,
we decreased the contrast to 13 percent, which is roughly equivalent to
reading a faded newspaper. Finally, we tested each person's contrast
threshold. The contrast threshold test involves showing the patient a
20/200 size letter, then decreasing the contrast to the lowest possible
level at which he can still read it. To determine the extent of the
pupil's role in any qualitative vision problems, we performed these
tests under conditions of high and medium brightness, and in the dark
(pupil size averaged 6 mm).
You can best visualize the concepts of oblate and prolate with an
ellipse. The top and bottom of the ellipse above and to the left is
oblate;
the sides are prolate.
On the first day postoperatively, though high contrast acuity stayed
relatively stable in medium and high brightness levels, it decreased a
full line in the dark. Thirteen percent contrast acuity dropped 1.4
lines in dark conditions.
At one and six months, high-contrast acuity in medium and high
brightness was back to normal, and even slightly exceeded baseline
levels. This is most likely due to the reduction in minification from
the thicker high minus spectacles the patients wore preoperatively. In
darkness, however, high-contrast acuity was still slightly below
baseline, by approximately a quarter of a line, at six months. At the
lower contrast level, 13 percent, acuity was even worse, almost an
entire line below preoperative levels. Also, at the six-month mark, the
average contrast threshold in dark conditions was 2.3 lines poorer than
at baseline, though it returned to baseline under medium and high
brightness levels.
It's important to remember that these patients' pupils did not dilate
outside the ablation zone. Patients with pupils that do dilate this
much could be expected to have even lower contrast sensitivity.
The present
Right now, practicing surgeons can do little to alter the ablation
pattern of their excimer lasers. The manufacturers will have to address
that in the days and months to come. Right now, our one, best recourse
is good, effective patient screening and counseling. The first step is
to make sure you take an accurate measurement of pupil size. As I
mentioned, the larger the pupil, the more optical aberration patients
will experience from the oblate cornea effect. Large-pupil patients are
also much more likely to see haloes at night, because their pupil
exceeds the ablation zone.
To measure the pupil, I simply darken the room and then aim a penlight
covered by a cobalt blue filter from the temporal side of the eye
towards the nose. Blue light is also very inefficient at causing
pupillary reflex. I then measure the pupil with a pupil gauge
manufactured by Asico. I have recently begun to use the Colvard
infrared Pupillometer. If the patient appears to dilate widely under
dim lighting, I use this pupillometer, which is much more accurate and
requires no visible light.
If the patient is a large scotopic pupil, I emphasize that the quality
of vision during night driving will be poorer and that supplementary
spectacles may be necessary. I also inform the patient of the
possibility of haloes around lights at night. Patients who wear contact
lenses often are already familiar with haloes because the optical zones
of their contact lenses are also too small for their pupils. If the
patient is not bothered by these haloes, I tell them the haloes from
LASIK probably will be very similar and should not bother them either.
If the patient seems concerned about the haloes and states he or she
has never seen them, then more discussion is required. Remember that
high correction and astigmatic correction both can effectively reduce
the size of the optical zone. This will intensify the halo effect.
In the oblate cornea, the powers in the periphery become stronger
sooner causing spherical aberration. In a prolate cornea, the lack of
stronger powers in the periphery means no spherical aberration.
The future
Since the oblate cornea is a creation of the laser itself, only the
laser companies can ultimately do away with it. Here are some of the
developments underway:
New beam profiles. Companies are already working on revising their beam
profiles to leave the cornea in a prolate shape. This change should be
relatively simple for both broad-beam and scanning lasers to effect.
Unfortunately, the new pattern will require the removal of 20 to 25
percent more tissue. This will lower the maximum amount of refractive
error that surgeons can correct with a flatter and prolate cornea. For
example, if -12 D is approximately the limit now, it will probably drop
to -8 D if the surgeon wants an anatomical prolate cornea.
Adjustable parameters. I believe new software developments will allow
surgeons to enter all the vital patient characteristics into a laser's
computer-pupil size, corneal thickness, corneal asphericity and
intended correction- and then optimize the correction. For example, if
someone has an error slightly above the theoretical maximum of -8 D,
the laser may "know" to make the cornea slightly less prolate, or to
use a smaller optical zone, depending on the patient's pupil size.
Though contrast sensitivity loss is undesirable, especially in a
procedure designed to improve vision, the effects are not debilitating,
and many patients are very willing to make the trade-off. We have a
responsibility to inform patients of this potential problem, however.
We also have a responsibility to continue to push laser manufacturers
to improve their software so that we can reduce these adverse effects.
-----------------------------------------------------------------------------------------
Dr. Holladay, who is in private practice with the Houston Eye
Associates and is the McNeese Professor of Ophthalmology at the
University of Texas Medical School in Houston.
| |
|
| In article <1136224950.671755.170570@f14g2000cwb.googlegroups.com>,
"Eye" <eyetooamdamaged@yahoo.com> wrote:
> The future
> Since the oblate cornea is a creation of the laser itself, only the
> laser companies can ultimately do away with it. Here are some of the
> developments underway:
> New beam profiles. Companies are already working on revising their beam
> profiles to leave the cornea in a prolate shape. This change should be
> relatively simple for both broad-beam and scanning lasers to effect.
> Unfortunately, the new pattern will require the removal of 20 to 25
> percent more tissue. This will lower the maximum amount of refractive
> error that surgeons can correct with a flatter and prolate cornea. For
> example, if -12 D is approximately the limit now, it will probably drop
> to -8 D if the surgeon wants an anatomical prolate cornea.
> Adjustable parameters. I believe new software developments will allow
> surgeons to enter all the vital patient characteristics into a laser's
> computer-pupil size, corneal thickness, corneal asphericity and
> intended correction- and then optimize the correction. For example, if
> someone has an error slightly above the theoretical maximum of -8 D,
> the laser may "know" to make the cornea slightly less prolate, or to
> use a smaller optical zone, depending on the patient's pupil size.
> Though contrast sensitivity loss is undesirable, especially in a
> procedure designed to improve vision, the effects are not debilitating,
> and many patients are very willing to make the trade-off. We have a
> responsibility to inform patients of this potential problem, however.
>
> We also have a responsibility to continue to push laser manufacturers
> to improve their software so that we can reduce these adverse effects.
This was published in 1999. What is the current state now...that's
what's important. Have the issues raised in the is article been
addressed?
--
~RT
| |
| Glenn - USAEyes.org 2006-01-02, 6:07 pm |
| Wavefront-guided ablations did not exist in 1999. That is undoubtedly
why "Eye" is using a six year old report.
On the whole, contrast sensitivity is reduced after all excimer laser
assisted refractive surgery techniques, however not in every case and
the change does not necessarily reflect a degradation in vision
function to below normal levels.
In some cases contrast sensitivity is increase after refractive
surgery, however an increase is not reliably predictable.
Glenn Hagele
Executive Director
USAEyes.org
"Consider and Choose With Confidence"
Email to glenn dot hagele at usaeyes dot org
http://www.USAEyes.org
http://www.ComplicatedEyes.org
I am not a doctor.
| |
| Marshall Cosme 2006-01-09, 1:02 am |
| http://www.321recipes.com/aspartame.html
Please excuse the interruption , but I thought you may be interested to know
what may be causing all kinds of health problems for yourself or for your
loved ones. I can't seem to get it through to my wife's brain about this
vital health issue, Now she is always saying she is having trouble with her
vision. Gee, I wonder why? I am so pissed off right now, I really want to
slap some sense into her. She has been feeding this poison to my four
children for years after I asked her not too, so you can see why I said what
I said previously, but I would never hit my wife. Why are people ignore what
science brings to are attention about health issues. I am really upset, but
I hope this helps you....since I can't get through to my wife of 16
years.....!!!!!!!!!!!!!!!!
"Eye" <eyetooamdamaged@yahoo.com> wrote in message
news:1136224950.671755.170570@f14g2000cwb.googlegroups.com...
> What We Should Really Tell LASIK Patients
> Jack Holladay, MD, MSEE, FACS Houston
>
> http://www.revophth.com/1999/May_ar...RPE9f5lasik.htm
>
> Small pupils or not, patients will lose some contrast sensitivity. All
> surgeons know that, done properly, LASIK and PRK can markedly improve
> uncorrected Snellen acuity. But there is a flip side to these
> procedures of which surgeons and their patients are completely unaware.
> Because excimer laser manufacturers were unaware of the true
> physiologic shape of the human cornea, both procedures reduce contrast
> sensitivity in low-light conditions. In this article, I'll explain the
> problem, how it impacts patients, and what you can do to minimize the
> problems associated with it. I'll also discuss what excimer companies
> are working on to reduce, or eliminate, these concerns.
> Excimers assume all corneas look like this, a sphere. Unfortunately,
> that's not the case. Function follows form. To understand the problem,
> it's important to understand the impact of the cornea's shape on the
> light that passes through it. The natural shape of most corneas is
> what's called prolate. A prolate surface is shaped something like the
> head of a bullet. It is steep curvature in the center, but flatter
> toward the periphery. As the cornea flattens, its dioptric power
> decreases. The net effect is that central and peripheral light rays all
> focus at a single point inside the eye, regardless of pupil size.
> In a small percentage of the population, the cornea is spherical. In
> these patients, the focus of rays shifts depending on pupil size. In
> the daytime, when the pupil is 3 mm or smaller, most of the rays focus
> at one point. At night, when the pupil is dilated, the peripheral rays
> focus at a point anterior to that of the central cornea. This results
> in night myopia; the optical property is called spherical aberration.
>
> The problem with all excimer lasers on the market today is twofold.
> First, the engineers assumed that the cornea is spherical rather than
> prolate. Second, they assumed that their job was to reshape a
> relatively steep sphere into a relatively flat sphere, rather than to
> reshape a steep prolate into a flatter prolate.
> As a result, excimer lasers actually reshape prolate corneas into what
> is known as an oblate. An oblate is shaped like the cross-section of a
> hamburger bun, flatter in the center but steeper in the periphery .
> This shape is actually optically worse than a sphere, because now the
> peripheral rays are bent even more powerfully than in the periphery of
> a sphere, causing even more pronounced spherical aberration when the
> pupil dilates. Unlike the haloes that result from a too-small optical
> zone, this problem affects every patient who undergoes an excimer laser
> procedure to some extent. Like the halo problem, the oblate dilemma is
> worst for patients whose pupils dilate widely at night.
>
> Oblation in practice
> We recently tested this concept on seven patients before and after they
> underwent bilateral LASIK. We first tested the patients at 98 percent
> contrast, which is the level of the letters on a Snellen chart. Then,
> we decreased the contrast to 13 percent, which is roughly equivalent to
> reading a faded newspaper. Finally, we tested each person's contrast
> threshold. The contrast threshold test involves showing the patient a
> 20/200 size letter, then decreasing the contrast to the lowest possible
> level at which he can still read it. To determine the extent of the
> pupil's role in any qualitative vision problems, we performed these
> tests under conditions of high and medium brightness, and in the dark
> (pupil size averaged 6 mm).
>
> You can best visualize the concepts of oblate and prolate with an
> ellipse. The top and bottom of the ellipse above and to the left is
> oblate;
> the sides are prolate.
>
> On the first day postoperatively, though high contrast acuity stayed
> relatively stable in medium and high brightness levels, it decreased a
> full line in the dark. Thirteen percent contrast acuity dropped 1.4
> lines in dark conditions.
> At one and six months, high-contrast acuity in medium and high
> brightness was back to normal, and even slightly exceeded baseline
> levels. This is most likely due to the reduction in minification from
> the thicker high minus spectacles the patients wore preoperatively. In
> darkness, however, high-contrast acuity was still slightly below
> baseline, by approximately a quarter of a line, at six months. At the
> lower contrast level, 13 percent, acuity was even worse, almost an
> entire line below preoperative levels. Also, at the six-month mark, the
> average contrast threshold in dark conditions was 2.3 lines poorer than
> at baseline, though it returned to baseline under medium and high
> brightness levels.
> It's important to remember that these patients' pupils did not dilate
> outside the ablation zone. Patients with pupils that do dilate this
> much could be expected to have even lower contrast sensitivity.
>
> The present
> Right now, practicing surgeons can do little to alter the ablation
> pattern of their excimer lasers. The manufacturers will have to address
> that in the days and months to come. Right now, our one, best recourse
> is good, effective patient screening and counseling. The first step is
> to make sure you take an accurate measurement of pupil size. As I
> mentioned, the larger the pupil, the more optical aberration patients
> will experience from the oblate cornea effect. Large-pupil patients are
> also much more likely to see haloes at night, because their pupil
> exceeds the ablation zone.
> To measure the pupil, I simply darken the room and then aim a penlight
> covered by a cobalt blue filter from the temporal side of the eye
> towards the nose. Blue light is also very inefficient at causing
> pupillary reflex. I then measure the pupil with a pupil gauge
> manufactured by Asico. I have recently begun to use the Colvard
> infrared Pupillometer. If the patient appears to dilate widely under
> dim lighting, I use this pupillometer, which is much more accurate and
> requires no visible light.
> If the patient is a large scotopic pupil, I emphasize that the quality
> of vision during night driving will be poorer and that supplementary
> spectacles may be necessary. I also inform the patient of the
> possibility of haloes around lights at night. Patients who wear contact
> lenses often are already familiar with haloes because the optical zones
> of their contact lenses are also too small for their pupils. If the
> patient is not bothered by these haloes, I tell them the haloes from
> LASIK probably will be very similar and should not bother them either.
> If the patient seems concerned about the haloes and states he or she
> has never seen them, then more discussion is required. Remember that
> high correction and astigmatic correction both can effectively reduce
> the size of the optical zone. This will intensify the halo effect.
>
> In the oblate cornea, the powers in the periphery become stronger
> sooner causing spherical aberration. In a prolate cornea, the lack of
> stronger powers in the periphery means no spherical aberration.
>
> The future
> Since the oblate cornea is a creation of the laser itself, only the
> laser companies can ultimately do away with it. Here are some of the
> developments underway:
> New beam profiles. Companies are already working on revising their beam
> profiles to leave the cornea in a prolate shape. This change should be
> relatively simple for both broad-beam and scanning lasers to effect.
> Unfortunately, the new pattern will require the removal of 20 to 25
> percent more tissue. This will lower the maximum amount of refractive
> error that surgeons can correct with a flatter and prolate cornea. For
> example, if -12 D is approximately the limit now, it will probably drop
> to -8 D if the surgeon wants an anatomical prolate cornea.
> Adjustable parameters. I believe new software developments will allow
> surgeons to enter all the vital patient characteristics into a laser's
> computer-pupil size, corneal thickness, corneal asphericity and
> intended correction- and then optimize the correction. For example, if
> someone has an error slightly above the theoretical maximum of -8 D,
> the laser may "know" to make the cornea slightly less prolate, or to
> use a smaller optical zone, depending on the patient's pupil size.
> Though contrast sensitivity loss is undesirable, especially in a
> procedure designed to improve vision, the effects are not debilitating,
> and many patients are very willing to make the trade-off. We have a
> responsibility to inform patients of this potential problem, however.
>
> We also have a responsibility to continue to push laser manufacturers
> to improve their software so that we can reduce these adverse effects.
> -----------------------------------------------------------------------------------------
>
> Dr. Holladay, who is in private practice with the Houston Eye
> Associates and is the McNeese Professor of Ophthalmology at the
> university of Texas Medical School in Houston.
>
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