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Home > Archive > Lasik Eyes Surgery > November 2004 > Pupil size matters in lasik
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Pupil size matters in lasik
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| http://www.eyeworld.org/Oct04/1004_p82.html
Point: Pupil size matters in LASIK
by James Salz, M.D.
Editors' note: The following is an excerpt of a debate presented at
the recent ASCRS•ASOA Symposium as part of the Cornea Society's
Symposium program, "Kera-net LIVE."
The etiology of night vision complaints after refractive surgery is
certainly multifactorial.
The amount of correction is important, as well as ablation diameter,
centration of the ablation, the blend zone, corneal haze, and striae.
All of these things are possible factors in patients that are unhappy
with the quality of their night vision.
My position has always been that the large-pupil patient in scotopic
conditions is potentially a very important factor in predicting or in
causing night vision complaints.
The studies
There have been numerous studies as of late that have demonstrated the
idea that pupil size can affect night vision in LASIK patients.
In an earlier version of a paper by Steve Schallhorn (M.D., U.S. Naval
Medical Center, San Diego) — in which they compare the incidence of
severe complaints or moderate to severe complaints of nighttime glare,
and pre-operatively you see patients with large pupils — some 20% of
them already had these complaints. At one month, it was 40%. In fact,
at one month, it was a statistically significant difference.
By three months, in this final study, it showed that there wasn't any
statistical significant difference in large-pupil patients versus
medium-pupil patients. The same basic numbers held true also of their
incidence of halos.
I believe a problem with this study is that of the 100 patients, only
about 75 were in attendance at the final follow-up at which time they
determined their endpoints.
Only one patient had a pupil greater than 7 mm, and that particular
patient actually had very low complaints.
I have no patients in my practice in this range — 5 mm to 5.5 mm —
that have high glare or halo scores or complain about their vision.
I'm not sure what the etiology of that is, but Jack Holladay (M.D.,
Baylor college of Medicine, Houston) has looked at some of the
topography on these patients and said some of them have decentered
ablations.
In another study, this one by Scott MacRae (M.D., university of
Rochester Medical Center, Rochester, N.Y), he correlates wavefront
aperture diameter with pre-op higher-order Root Mean Square (RMS)
values. He determined that as soon as you get above 6 mm, the RMS
scores for higher-order aberrations dramatically increase, up to 7 mm.
You can see a significant difference between 7 mm and 5 mm, for
example.
But does this, in fact, happen in real life?
Topography would show that for a patient with a small, 5.5-mm ablation
with one blend, the pupil is larger at night and would result in
complaints about night vision and glare.
Now, can we correlate that with higher-order aberrations?
In fact, this specific patient does have a spherical aberration number
of 1.6, is very unhappy with the quality of vision, and elected not to
have surgery with the second eye.
In her untreated eye, she started with the spherical aberration of
0.7, and with a contact lens on her eye, her spherical aberration is
0.1. She's asymptomatic in this eye with a contact lens, and I would
not recommend a treatment on this patient, because I can't predict
whether she's going to have trouble.
Ezra Maguen (M.D., American Eye Institute, Los Angeles) performed a
study in our post-op wavefront-based treatments that had CustomCornea
(Alcon, Fort Worth, Texas) ablation in 34 eyes. As soon as the pupil
size increased, the post-op spherical aberration numbers increased as
well. This has been shown in multiple studies.
The first correlation study in the literature — published by Ronald
Krueger (M.D., Cleveland Clinic Foundation, Cole Eye Institute, Ohio)
in Ophthalmology earlier this year — shows a strong correlation of
night vision symptoms with one or more higher-order aberrations and
scotopic pupil size, and a strong correlation of scotopic pupil size
and glare and starbursts.
Predicting ahead of time
How do you predict this problem ahead of time?
I had one patient who was -10 in each eye with pupils of almost 8. He
had no symptoms related to his LASIK surgery, despite his large pupils
and his high correction. If we do a wavefront analysis on him, he has
a very high spherical aberration number. His pupil is 6.5 mm, a 1.07,
and yet, he doesn't have any significant complaints.
Another example of this was a 41-year-old TV executive with 6.5-mm
pupils who had not a high correction (-5/-2), had a large treatment
zone, and was miserable with his night vision symptoms. There is no
way to predict this.
He has a well-centered ablation, but works in a dark editing room
indoors and couldn't function very well. He uses a combination of
medications that includes Alphagan (brimonidine, Allergan, Irvine,
Calif.) to control his symptoms.
If we look at his wavefront studies, with a pupil of 6.5 mm, he has a
spherical aberration number of only 0.7.
We actually retreated him with an Alcon Custom retreatment and reduced
his spherical aberration from 1 to 0.36, and coma also went down in
both eyes, and he is happy now with his vision.
Warnings about pupil size
It is not logical to assume that patients with large pupils have no
more risk of glare, because more light enters their eye. Higher-order
aberrations increase with increasing pupil size, but not all patients
with higher-order aberrations are symptomatic.
Multiple experts and laser companies have warned us about patients'
pupil size. Studies in the literature, which I believe fail to
correlate this, have design flaws. The recent study by Dr. Krueger now
proves this.
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