USAEyes Lasik grant application.
How to Choose
A Lasik Doctor
Find Certified Surgeon
50 Tough Lasik Questions
Ask Lasik Expert
Lasik Q&A Forum
Top Articles
What is Lasik?
Lasik Cost
Lasik Results
Wavefront Custom iLasik
Bladeless Lasik
Lasik Patient Survey
Lasik Alternative
Top Articles
Monovision Lasik
PRK, LASEK, Epi-Lasik
RLE Lens Exchange
Lasik Groupon
$1,500 Off Lasik

Image of Lasik doctors certification logo.

This website is accredited by Health On the Net Foundation. Click to verify.   The website complies with the HONcode standard for trustworthy health information: verify here.

Lasik Halo and Starburst; Large Pupil Size Importance

Night vision halo and starburst problems relating to Lasik, Bladeless Lasik, PRK, LASEK, Epi-Lasik, etc.

Profile image of woman with large dilated pupils.  
The size of the naturally dilated pupil may relate to halo or starburst effects after Lasik eye surgery, but not predictably.  

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"

Normal Night Vision
Starburst Night Vision
Halo Night Vision

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.

Personalized Answers

If this article did not fully answer your questions, use our free Ask Lasik Expert patient forum.

Recent Corneal Central Island Medical Journal Articles...

Related Articles

Functional outcome and patient satisfaction after laser in situ keratomileusis for correction of myopia and myopic astigmatism.

Middle East Afr J Ophthalmol. 2015 Jan-Mar;22(1):108-14

Authors: Bamashmus MA, Hubaish K, Alawad M, Alakhlee H

PURPOSE: The purpose was to evaluate subjective quality of vision and patient satisfaction after laser in situ keratomileusis (LASIK) for myopia and myopic astigmatism.
PATIENTS AND METHODS: A self-administered patient questionnaire consisting 29 items was prospectively administered to LASIK patients at the Yemen Magrabi Hospital. Seven scales covering specific aspects of the quality of vision were formulated including; global satisfaction; quality of uncorrected and corrected vision; quality of night vision; glare; daytime driving and; night driving. Main outcome measures were responses to individual questions and scale scores and correlations with clinical parameters. The scoring scale ranged from 1 (dissatisfied) to 3 (very satisfied) and was stratified in the following manner: 1-1.65 = dissatisfied; 1.66-2.33 = satisfied and; 2.33-3 = very satisfied. Data at 6 months postoperatively are reported.
RESULTS: This study sample was comprised of 200 patients (122 females: 78 males) ranging in age from 18 to 46 years old. The preoperative myopic sphere was - 3.50 ± 1.70 D and myopic astigmatism was 0.90 ± 0.82 D. There were 96% of eyes within ± 1.00 D of the targeted correction. Postoperatively, the uncorrected visual acuity was 20/40 or better in 99% of eyes. The mean score for the overall satisfaction was 2.64 ± 0.8. A total of 98.5% of patients was satisfied or very satisfied with their surgery, 98.5% considered their main goal for surgery was achieved. Satisfaction with uncorrected vision was 2.5 ± 0.50. The main score for glare was 1.98 ± 0.7 at night. Night driving was rated more difficult preoperatively by 6.2%, whereas 79% had less difficulty driving at night.
CONCLUSION: Patient satisfaction with uncorrected vision after LASIK for myopia and myopic astigmatism appears to be excellent and is related to the residual refractive error postoperatively.

PMID: 25624684 [PubMed - in process]


Last updated

"Consider and Choose With Confidence"TM

A few of the communities where Lasik doctors are certified by USAEyes :