Low light problems such as glare and halo occur in some 3-5% of patients
after PRK or LASIK. The 3 commonest reasons for this are:
1. Ammetropia - not being
"zero" refraction. A bit of residual short sight will show up at
night when the pupil is bigger. This problem is corrected by wearing weak
glasses when needed. e.g. for night driving.
2. Spherical Aberration: see the pages on
spherical aberration and lasik
complications on night vision for more details on this. Spherical aberration
is more common in high corrections.
3. Pupil size: Patients with
bigger pupils are more likely to have low light problems. As we get older our
pupils normally get smaller and give us a bigger depth of field. This is one of
the few things about getting older that is to our advantage!
Below are opposing articles on pupil size by 2 American surgeons on pupil size:
Although I think that pupil size is important, I tend to agree more with Sam
Omar than James Salz
Point/Counterpoint:
Is Pupil Size Important in Refractive Surgery? (Part 1)
Pupil
Size Is Overrated.
BY
SAM OMAR, MD
A debate is raging within
the electronic forums of refractive surgery that I would like to address. How
important is pupil size in modern keratorefractive surgery? Physicians are
taking two basic positions. Some believe that the LASIK treatment optical zone
must at least match that of the scotopic pupil. For example, if a patient with a
refractive error of -7.0 D has a 7.5-mm pupil and desires LASIK or PRK, the
surgeon should try to match the optical zone to the scotopic pupil (7.5 mm).
This surgical plan (optical zone size = scotopic pupil size) grows out of
conclusions drawn from PRK studies in the early 1990s that noted that patients
with 6-mm optical zones logged fewer subjective halo and glare complaints than
patients with 4- to 5-mm optical zones. This theory of increasing optical zone
size to minimize subjective complaints of night vision does have a scientific
basis. I believe it is a gross mistake, however, to draw major conclusions from
studies that utilized first-generation excimer laser technology and techniques.
Refractive surgeons bolster this surgical argument with case reports of patients
who have large pupils and severely debilitated night vision after undergoing
LASIK or PRK with optical zones smaller than their scotopic pupil. Furthermore,
surgeons are now using wavefront devices to support patients’ complaints of
postoperative night vision symptoms. Unfortunately, wavefront technology only
measures physical aberrations, not patient perceptions which may be affected by
the Stiles-Crawford effect and other factors. Those using modern excimer lasers
have been able to expand optical zones and remove greater tissue volume in order
to match the optical zone size to the scotopic pupil. For many highly myopic
patients who lack adequate preoperative corneal thickness for LASIK, PRK/LASEK
may be substituted for deeper (higher volume) ablations. It would be unwise,
however, to perform higher volume/deeper ablations without well-designed
clinical investigations into the long-term effects and potential benefits of
such off-label treatments. My experience using larger ablation zones matched to
scotopic pupil size has led me to the following findings.
SURPRISING CONCLUSIONS
As a refractive surgeon, I believed much of what our fellow ophthalmologists
said about pupil size. I carefully implemented recommendations on matching
optical zone to pupil size, only to be surprised with the following clinical
conclusions:
1. Variable optical zones
result in multiple nomograms and decrease the probability of achieving
emmetropia.
2. Increased optical zones are extremely inefficient, removing up to 40% more
corneal tissue.
3. Incidence of subjective night vision complaints due to larger optical zones
does not appear to be appreciably less compared to that with smaller optical
zones (6.0 mm) in patients matched for pupil size and level of myopic
correction.
CAUSES OF NIGHTTIME VISION SYMPTOMS
It is far easier to attribute a patient’s subjective visual complaint to
obvious explanations (large pupil, smaller optical zone) than to a more complex
and subtle explanation. Utilizing four separate excimer laser platforms over the
past 4 years, it is clear to me that the following (in descending order) are the
most likely causes of chronic postoperative night vision complaints:
Residual ametropia. Halos
are basically myopic, blurred circles surrounding point sources of light. If the
patient is even mildly myopic and/or astigmatic, the residual ametropia coupled
with a nighttime myopic shift can certainly elicit complaints.
Magnitude of correction.
Patients with higher levels of correction have a higher probability of nighttime
visual complaints. Theory points to increased postoperative higher-order
wavefront aberrations. These patients usually demonstrate larger degrees of
residual ametropia (lower-order aberrations), greater degrees of treatment
decentratio
n, and a greater degree
of technical difficulty required in performing the ablation and keratotomy.
Optical zone decentration. Even when the optical zone is equal to or greater
than the scotopic pupil, a decentration greater than 0.5 mm can produce
significant visual complaints. Unfortunately, high myopes with large pupils
present a greater technical challenge in terms of treatment centration and
patient fixation.
Optical zone drift. This
phenomenon, characterized first by initial optical zone centration, then eye
movement as the ablation proceeds, and finally resulting in a drift in the
optical zone, is an underdiagnosed cause of decreased UCVA, BSCVA, and night
vision issues.
Flap striae. Higher
myopic ablations create a greater curvature disparity between the flap and
stromal bed. If the flap is excessively hydrated and not tightly stretched over
the treatment bed at the time of surgery, unwanted flap microstriae occur, which
can be a significant source of night vision complaints.
Multizone ablations. In
my experience, there is a higher probability of night vision complaints with
multizone ablations. Unfortunately, some excimer laser platforms default above a
certain dioptric level to a multizone algorithm. These place an even greater
emphasis on treatment centration and are associated with a higher incidence of
postoperative night vision complaints.
Many of the factors
described above are interrelated and can be combined to provide a working
hypothesis to explain many patients’ postoperative night vision complaints. A
significant percentage of postoperative night vision complaints may be related
to the lamellar aspect of the surgery. In a study comparing LASIK to PRK, Mihai
Pop, MD, concluded that LASIK patients were twice as likely to experience halos
postoperatively.1 Considering the above list, it is obvious that highly myopic
patients are at the greatest risk for postoperative visual complaints. In my
experience, patients treated with a broad-beam excimer laser, with a 6.0-mm
single zone myopic sphere or 5.0-mm X 6.5-mm single zone myopic cylinder,
demonstrate a 3% incidence of mild to moderate nighttime visual symptoms (halos,
glare, starbursts, and/or decreased contrast sensitivity).
A MOOT POINT?
In the past 3 years, investigations utilizing modern excimer platforms have
focused on the effect of pupil size on nighttime visual complaints. In separate
studies using the EC-5000 excimer laser (NIDEK Co., Ltd, Fremont, CA), the
Technolas 217 laser (Bausch & Lomb, San Dimas, CA), and the Star S2 excimer
laser (VISX, Inc., Santa Clara, CA), researchers have concluded that, when
utilizing modern optical zones, pupil size is not correlated with postoperative
complaints of glare and halos.2-6 Some of these researchers have also noted that
an increase in postoperative night vision symptoms appears to be correlated to
the attempted level of spherical equivalent correction. If there is still any
doubt that modern lasers with optical zones of 5.5 to 6.5 mm (associated with
blends) have virtually eliminated ablation algorithms as the source of
subjective nighttime visual complaints, I will briefly review a study described
by Mark J. Cohen, MD, (president of the Canadian Association of University
Professors of Refractive Surgery).6 The purpose of the study was to address the
Canadian public’s raised awareness of night vision problems due to LASIK. Dr.
Cohen noted that 35% of patients preoperatively reported symptoms of glare,
halos, and starbursts, and this percentage remained constant postoperatively.
Utilizing a modern excimer platform, LASIK was performed on 26,000 myopic eyes.
In reviewing subjective symptoms and related functional problems
postoperatively, only six patients had a significant night vision complaint that
was not correctable with glasses.
With modern excimer
lasers and techniques, pupil size has become a moot point. It is unlikely that
surgeons can eradicate all night vision complaints, and it is inadvisable to
attempt to do so by removing greater volumes of precious corneal tissue by
dogmatically trying to match the optical zone to the scotopic pupil size.
Continuously improving surgical equipment and techniques, practicing clear
physician-patient communication, and obtaining detailed surgical informed
consent including computerized night vision simulations (that duplicates night
vision problems such as halo, glare, starbursts, ghosting, and contrast
reduction) can significantly decrease the number of patients with nighttime
visual complaints.
Sam Omar, MD, is from Advanced Vision Institute in Orlando, Florida. Dr. Omar
may be reached at (407) 389-0800; omar_eye@yahoo.com
1. Pop M, Payette Y:
Photorefractive keratectomy versus laser in situ keratomileusis: A
control-matched study. Ophthalmology 107:251-257, 2000
2. Pop M: Fall ISRS Symposium, Nov 2000, Dallas, TX
3. Schumer J: Pupil protocol: A prospective analysis of LASIK using varied
treatment sizes based on dark-adapted pupil size. Fall ISRS Symposium, Nov 2001,
New Orleans, LA
4. Haw W, Manche E: Effect of preoperative pupil measurements on glare, halos,
and visual function after photoastigmatic refractive keratectomy. J Cataract
Refract Surg 27:907-916, 2001
5. Shallhorn S: Pupil size irrelevant to post-LASIK complaints. Ocular Surgery
News, Vol. 20, No. 6, March 15, 2002
6. Cohen MJ: Night vision symptoms: A preventable complication. Refractive
Eyecare May 2001, Vol 5, No. 5 (suppl)
Point/Counterpoint:
Is Pupil Size Important in Refractive Surgery? (Part 2)
Pupil Size
Is Critical.
BY
JAMES SALZ, MD
Some physicians
believe that because excimer lasers have
become so sophisticated and their ablations are so smooth, that pupil size is no
longer an issue in refractive treatments. I cannot agree with this opinion,
however, because I have seen many patients for whom pupil size does matter.
Although I have not conducted a formal study pertaining to pupil size, I have
treated many refractive surgery patients over the past 10 years who are
miserable because they have good vision, but high incidences of night time glare
and halos. Patients who have small pupils are usually highly satisfied with
their results, but some patients with 6.5- to 8-mm pupils are truly unhappy. A
few of these patients are so incapacitated that they are unable to drive at
night. In addition, I have observed in a dark
refracting lane that many patients’ complaints disappear or are significantly
minimized when a penlight is directed into their nontested eye, thus
constricting the pupil in the tested eye through the consensual light reflex.
These same patients often respond well to
Alphagan (Allergan, Inc., Irvine, CA), which reduces pupillary dilation in the
dark as first described by Jay McDonald, MD. Measuring pupil size and informing
patients with large pupils that they may be at increased risk of experiencing an
increase in night glare and halos, especially when
they require corrections over -4.0 D, is very important in evaluating patients
for refractive surgery. This is especially true if the laser being utilized
cannot ablate larger optical zones and blend zones.
DECIPHERING THE DATA
A large pupil does not guarantee that a
patient will experience any difficulty with night time vision—we have all had
patients whom we expect will experience trouble, but do not. There is a highly
variable patient response in refractive surgery. Nevertheless, as surgeons, we
must not trivialize the risk of patient outcomes. A few studies have determined
pupil size to be unimportant; a recent study published by Stephen Schallhorn,
MD, stated that pupil size is perhaps not as significant as previously thought.
In that study, Dr. Schallhorn correlated patients’ good results with their
amount of correction and pupil size having higher incidences of night time halos
and glare. I agree that a patient with a 7-mm pupil and a prescription of -3.0 D
is far less likely to be symptomatic than a patient with a 7-mm pupil and a
refraction of 7.0 D. The more tissue the surgeon must remove, the greater the
patient’s risk of halos and glare. Edward Manche, MD, and Mihai Pop, MD, also
conducted recent studies that show that pupil size did not correlate very well
with patients’ symptoms, but those studies were conducted with 60 to 80 eyes.
If a study were conducted involving thousands of refractive surgery patients,
some would emerge who were truly unhappy with their results.
THE IMPORTANCE OF MEASUREMENTS
It does not matter how, but pupil size
must be measured. I believe that the best method is with an infrared device,
such as the Colvard Pupillometer (OASIS Medical, Inc., Glendora, CA), the
Pupilscan, or the Procyon P2000SA pupillometer (Keeler Instruments, Inc.,
Broomall, PA). In my practice, we bring our patients into a completely dark room
and measure their pupil size in the least amount of light possible. This setting
tests patients’ maximum pupil size (such as in a dark, moonless night). We
then try to match the ablation diameter of their pupil whenever possible. If the
patient has a 7-mm pupil, I always ablate a 7-mm zone. I may even ablate an 8-mm
zone using the LADARVision Excimer Laser System (Alcon Laboratories, Fort Worth,
TX). We have treated some patients now that we would not have treated 2 years
ago when we could only create 6.5-mm ablation zones. For the most part, these
patients are doing well. Some say they experience mild glare, but they are
not incapacitated by it. I believe that our job as surgeons is to first measure
pupil size. If the patient has an unusually large pupil, regardless of their
refraction, I feel that we must inform the patient that he or she is at a
potentially increased risk of glare.
NIGHT TIME SYMPTOMS AND ABERRATIONS
Most patients who complain of night time
problems have a greater incidence of spherical aberrations, and surgeons can now
measure these aberrations using wavefront technology. Other types of aberrations
such as coma, in addition to spherical aberration, contribute to symptoms.
Wavefront technology can determine net RMS values for these aberrations, and
patients who have many complaints usually have high readings. Interestingly, if
the surgeon constricts the patient’s pupil and then re-measures the wavefront,
all the patient’s aberrations disappear. The reason is that although the
cornea still contains the abnormality, the pupil is now smaller than the
peripheral area where the aberrations allow light to enter through the retina.
Therefore, the pupil blocks any erroneous light waves from reaching the retina,
and patients do not experience the effects of their higher-order aberration. Any
surgeon performing wavefront studies can see that patients’ aberrations—coma
as well as spherical—are highly correlated to pupil size, and that reducing it
eliminates these symptoms.
CASE IN POINT
I recently treated a case that
specifically illustrates my point. This male patient had no negative ocular
symptoms when his pupils were around 5 mm, but experienced significant glare and
ghosting when they were larger than 6 mm. At pupil measurements of 5.5 mm, this
patient shows very low spherical aberration and coma reading. At 6.5 mm,
however, wavefront maps show that both of these values increase considerably.
This also correlates with his symptoms, and therefore proves the importance of
pupil size. The patient was -4.0 -1.50 X 115 preoperatively, and his pupils
measured 6.5 mm. He underwent a 6.5-mm ablation with a 1.0-mm blend zone on the
LADARVision laser (Alcon Surgical, Fort Worth, TX). His postoperative UCVA was
20/20, and he had a well-centered ablation on topography. His pupils are now 5
to 5.5 mm on Alphagan, which provides significant relief of his symptoms.
PATIENT MANAGEMENT
Although I know some ophthalmologists
feel otherwise, I believe that as surgeons, we have a responsibility to warn
patients of their potential risks. I have a personal philosophy with borderline
patients. First, I never treat both eyes simultaneously, particularly patients
with larger pupils. I tell them that I believe they will most likely have good
outcomes, but I do not know that definitively. For that reason, I will treat
their nondominant eye first. I create the largest zone I safely can, based on
the amount of correction the patient requires (you cannot always perform an 8-mm
ablation zone if the cornea is too thin or if the patient has a high
correction). At times, I will recommend PRK or LASEK if corneal thickness does
not permit a larger ablation diameter.
Once the surgery
is complete, I tell my patients to live with their results for as long as they
choose—a few days, a week, or longer, and I tell them to specifically go out
at night and compare the vision of the treated, nondominant eye with
the eye that is still wearing the contact lens. If the patient tells me that the
lasered eye is much worse than their contact lens eye, then I will not treat the
other eye until those symptoms improve, as they do in many patients. Patients
may be symptomatic for the first few weeks, and then they adjust. There is a
tendency for most to improve, but not all, and some have never had the second
eye treated because they feel there is a huge difference in the quality of
vision between the treated and untreated eye. For this reason, I feel it is a
mistake to treat both eyes at once in these potentially high-risk patients.
HELP IN A BOTTLE
One treatment that we can now offer
patients suffering from night time symptoms, in addition to larger optical
zones, is the use of Alphagan. Dr. MacDonald shared in an online forum
approximately 2 years ago that Alphagan prevents the pupil from dilating in the
dark. Since then, Alphagan has proven very helpful for some patients suffering night time
symptoms. Patients self-administer one drop about 30 minutes before sundown, and
their pupil decreases 1 to 2 mm from its normal size. Alphagan can dramatically
decrease patients’ incidence of night glare when used in this manner.
INFORMED CONSENT
I believe that pupil size will always
matter in refractive surgery, regardless of what technological advancements we
make. It will always be important to correlate patients’ symptoms with their
pupil size and to try to make their ablation consistently as large and as safe
as possible from a thickness standpoint.
Even more importantly, patients should have a proper, informed consent form that
advises them that they are at potential risk for night time symptoms. Some
surgeons believe that they should not have to inform patients of this risk,
but I feel those at higher risk, especially those
with larger pupils, have the right to know.
James Salz, MD, practices at the American
Eye Institute in Los Angeles, California. He is a consultant to both VISX and
Alcon LADARVision. Dr. Salz may be reached at (323) 653-3800; jjsalzeye@aol.com
There
is another nice article by James Salz showing the importance of spherical
aberration. This is found at:
http://www.crstoday.com/03_archive/0702/crst0702_151.html
