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| The size of the naturally dilated pupil may relate to halo
or starburst effects after Lasik eye surgery, but not predictably. |
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The size of the patient's naturally dilated pupils may be very important because of the possibility of Lasik night vision problems including halos, starbursts, and other undesirable effects. Laser assisted
refractive surgery techniques including conventional and wavefront Lasik, Bladeless Lasik, PRK, LASEK, and Epi-Lasik for myopia are discussed here, however the issues regarding pupil
size and night vision quality may be important to all refractive
surgery techniques, including RLE, P-IOL, CK and all refractive errors including myopia, hyperopia, and astigmatism.
When they occur, large pupil size related night vision problems can
range from a minor nuisance to transient debilitation, including
the inability to drive at night.
There are three major factors to consider with laser assisted
refractive surgery for myopia.
There are many other factors that are considered regarding
this issue, but we will focus on these primary three.
Not All Is "Seen"
It is important to remember that not all light reaches the retina and is "seen". The iris restricts, causing the large pupil to be smaller, reducing the
light reaching the retina, and thereby restricts the area on the cornea through which the light travels that reaches the retina
and is seen. In bright light the iris makes the big pupil smaller, allowing
less light and only light through a small central area of the cornea
to reach the retina. Most humans have optimal vision with about
a 3.5mm pupil size. In low light, the iris enlarges the pupil, allowing
light through the outer edges of the cornea through the large pupil, reaching
the retina.
Full Correction To A Transition Zone
With myopic correction, a laser removes tissue in the center
of the cornea. The more tissue that is removed, the greater the
refractive correction. When the laser ablates the tissue in the
cornea it does not simply drill a straight hole in the middle with
the full refractive correction. Normally, the central area of the
laser's ablation fully corrects the refractive error and the ablation
pattern gradually slopes out and up to the surface of the cornea.
The center is the optical ablation zone and the slope to the surface
of the cornea is the transition zone.
Visualize the ablation for myopia like the shape of the inside
of a soup bowl. The bottom of the bowl has full correction. The
rim of the bowl has no correction. The slope between the bottom
and the rim has a gradually changing amount of correction from full
to none.
Not All Treatment Zones Provided Full Correction
For an example, let us say that 4.00 diopters of myopia needs to be corrected with a 6.0mm optical
ablation zone and a transition out to 8.0mm. Across the center and
out to 6.0mm you have a full 4.00 diopters of correction. At 8.0mm
out you have zero correction. Following this transition, from center
to 6.0mm diameter out you have your full 4.00 diopters of correction.
At 6.5mm out you have 3.00 diopters of correction. At 7.0mm out
you have 2.00 diopters of correction, at 7.5mm out you have 1.00
diopters of correction and at 8.0mm out you have no correction.
At night or in dark rooms, the iris opens the size of the pupil.
If the pupil is large enough, light passing through the undercorrected
transition zone reaches the retina. In our example, if the pupil
is 6.5mm wide, an outer ring of about 0.5mm is providing only up
to 3.00 diopter of correction. Light passing through that outer
0.5mm will not focus well on the retina. If your pupil enlarges
to 7.5mm, then you have as little as 1.00 diopter of correction
at the outer edge when you really need a full 4.00 diopter of correction.
The light at this outermost edge will be very poorly focused on
the retina. This would very likely provide a halo effect around
sources of light and other night vision difficulties. The greater
the ratio of correction to transition zone size, the greater the
probability and severity of visual disruption.
In addition to the gradual undercorrection though the transition
zone, the angle of the transition zone can cause light to refract at odd angles. Light can essentially bounce off the
transition zone and cause light scatter, glare, or other problems.
Most Desirable, But Not Always Required
What is generally considered most desirable is for the size of
the optical ablation zone to be equal or larger than the naturally
dilated pupil in a low light environment and with a wide and smooth
transition zone. This provides the lowest probability of low light
vision problems.
It should be mentioned that the exact relevance of large pupil size
is not yet fully understood nor is it totally accepted as
a contributing factor by all ophthalmologists and researchers.
Pupil size alone is not a reliable predictor of night vision
problems. Some patients with high myopia, moderate ablation zones,
and very large pupils do not have problems in low light environments.
Why they do not is not fully understood. It is possible for a high
myope (deep ablation) with small pupils to experience halos. It
is possible for a low myope (shallow ablation) with large pupils
to not experience halos. There are other factors involved
that are not yet completely determined.
Bigger Zones, More Tissue Removal
It would seem that the quick answer to this problem would be
to simply use a larger ablation zone. Newer technology makes
this possible. Unfortunately, the physics of laser refractive correction
complicate this situation.
The amount of tissue that must be removed to effect the desired
refractive change is exponentially greater when the ablation zone
is enlarged. This is known as the Munnerlyn formula. As an example, with a 6.0mm ablation zone
many lasers require about 12 microns of tissue to be removed for each diopter of refractive error. If you are a 4.00 diopter myope, the total
amount of tissue to be removed would be 48 microns (4.00 X 12).
If you had a 7.0mm ablation zone the amount of tissue required to
be removed for each diopter of refractive change would be closer
to 18 microns. Now the total amount of tissue to be removed is 72
microns (4.00 X 18). This is just for the optical ablation zone.
Custom Wavefront Removes More Than Conventional
A custom wavefront-guided ablation will commonly require more
tissue removal than a conventional ablation. This is because
the wavefront-guided ablation is attempting to resolve higher order aberration issues as it resolve myopia, hyperopia,
and astigmatism.
Don't take these numbers as carved in stone. These ratios vary
greatly from laser to laser, the ratio of optical zone to transition
zone, the age, sex, and ethnic background of the patient, and even
depending upon the altitude and relative humidity where the surgery
is performed. This is just an example, but let's use this example
and go to the next calculation.
Enough Corneal Thickness To Remain Stable
If you are going to have Lasik, a flap will be required. The
flaps are usually about 120-180 microns thick. The thicker the flap
the more stable the flap. For continued corneal stability most doctors
want to leave at least 250 microns of healthy cornea untouched.
If the cornea becomes too thin and weak, it can bulge outward. This
bulging is called ectasia and is a problem you don't want to deal with.
Let us say you have a cornea that is 500 microns thick. Start
with your cornea (500 microns) and subtract the flap (180 microns)
then subtract the amount of tissue to be removed for a 4.00 diopter
myope with a 6.0mm ablation (48 microns) and you have 272 microns
remaining. That is enough to maintain stability and give you some
room for an enhancement if it becomes necessary. Now let's take
that same 500 micron thick cornea with a 7.00mm ablation zone. Start
with the cornea (500 microns) and subtract the flap (180 microns)
then subtract the amount of tissue to be removed with a 7.0mm ablation
(72 microns) and you have only 248 microns remaining. This is not enough to keep your cornea stable and there is no possibility
of an enhancement if you are undercorrected or regress.
The doctor can make a thinner flap, but that increases the possibility
of flap complications. Bladeless Lasik is Lasik with the flap created
by a femtosecond laser microkeratome rather than a mechanical
microkeratome with a metal blade. The laser microkeratome has greater
accuracy and can create a thinner flap with more predictability.
It may be that a thinner flap with Bladeless Lasik would be appropriate.
Surface Ablation Alternative
Lasik and Bladeless Lasik can be abandoned for a surface ablation
technique such as PRK or its cousins LASEK and Epi-Lasik, but with
a high refractive error these techniques tend to cause corneal
hazing. Surface ablation would probably be appropriate for the 4.00
diopter myope in our example, but may not be appropriate for everyone.
Some will argue that pupil size alone is not a predictor
of low light problems and is not relevant. While it is true
that not everyone with an ablation zone smaller than their dilated
pupil size will develop low light vision problems, this fact does
not make pupil size irrelevant.
Pupil Size Is An Unreliable Predictor of Problems
Do not think that big pupils automatically mean night vision
problems or that small pupils automatically mean no night vision
problems. Several studies have shown that pupil size alone is an
unreliable predictor of who will and who will not develop night
vision problems. Some people with large pupils and small ablation
zones do not develop night vision problems. This seems counterintuitive
when people who have night vision problems get relief by reducing
the size of their pupils, however there is a tremendous difference
between predicting night vision problems before they occur and treating
night vision problems after they exist.
This issue can be compared to drinking and driving. Drinking
and driving sounds like a very dangerous thing to do, but the majority
of people who drink at all, drink and drive. The difference
is that few drive while intoxicated. A glass of wine at dinner,
a beer during the game, or a cocktail at a gala affair does not
mean that the person is impaired and should not drive. There are
many factors that contribute to determine when it is safe to drink
and drive. That limit is different for different people and there
are a multitude of factors involved, such as body weight, number
of drinks, duration of time drinking, duration of time from last
drink, unique rates that individuals oxidize alcohol, etc. Drinking
and driving can be a manageable risk.
Pupil size is an issue where there are situations it is clearly
dangerous, situations it is clearly not problematic, situations
where it may or may not be problematic, and everyone's situation
is unique. Pupil size can be a manageable risk.
There is one point that cannot be challenge. If the optical ablation
zone is smaller than the naturally dilated pupil size, the patient
is being placed at an elevated risk for problems. That risk may
be mitigated by other issues and may be a very small elevation,
but the risk remains. This is an important issue that needs to be
discussed with a competent doctor for all the details relevant to
the individual.
Looking For Best Lasik Surgeon?
If you are ready to choose a doctor to be evaluated for conventional
or custom wavefront Lasik, Bladeless Lasik, PRK, or any refractive surgery procedure, we recommend you consider a doctor who has been evaluated and certified by the USAEyes nonprofit organization.
Locate a USAEyes Evaluated & Certified Lasik Doctor.
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