Patients with large pupils are not good candidates for LASIK. Patients with pupil diameters larger than the laser optical zone (not including the blend zone) may suffer from permanent, debilitating visual aberrations such as starbursts, halos, multiples images and loss of contrast sensitivity at night after LASIK.
Read from the FDA LASIK web site:
Your doctor should screen you for the following conditions or indicators of risk:
Large pupils. Make sure this evaluation is done in a dark room. Although anyone may have large pupils, younger patients and patients on certain medications may be particularly prone to having large pupils under dim lighting conditions. This can cause symptoms such as glare, halos, starbursts, and ghost images (double vision) after surgery. In some patients these symptoms may be debilitating. For example, a patient may no longer be able to drive a car at night or in certain weather conditions, such as fog. Source
Under "Know What Makes You A Poor Candidate": Pupil size - are your pupils extra large in dim conditions? Source
Generally, a dark-adapted pupil diameter of greater than 6.5 millimeters is considered large. High myopia in combination with large pupils is double-trouble.
There is abundant evidence in the medical literature demonstrating the importance of pupil size in refractive surgery. A compilation of peer-reviewed medical studies which establish the importance of pupil size can be found at this link:
The American Academy of Ophthalmology (AAO), American Society of Cataract and Refractive Surgery (ASCRS), Federal Trade Commission (FTC), Food and Drug Administration (FDA), and Ophthalmic Mutual Insurance Company (OMIC) have issued statements regarding pupil size in refractive surgery.
One argument that patients hear from the LASIK industry is that pupil size alone does not predict who will experience night vision disturbances. There are actually two factors involved – pupil size and laser optical zone. Pupil size/optical zone mismatch, termed “negative clearance”, is a result of an effective optical zone smaller than the maximum pupil diameter at night. A patient should never consent to surgery where the fully treated area is smaller than the dark-adapted pupil. LASIK surgeons frequently state that the cause of night vision disturbances is multifactorial. Factors involved in night vision disturbances include quality and centration of the ablation, residual refractive error, and negative clearance. High myopia results in smaller effective optical zones which create greater negative clearance. It is well known that all corneal aberrations increase with increasing pupil size. To say that pupil size is not a factor in night vision disturbances is frankly absurd. Negative clearance is a preventable cause of night vision disturbances with proper patient screening.
Laser manufacturers, LASIK providers, and LASIK industry representatives who receive money to promote LASIK have a financial incentive to coverup the importance of pupil size. If you receive or locate information that minimizes or downplays the importance of pupil size, consider the source. Is the information provided by a LASIK surgeon who has a financial interest in patients' decision to have LASIK? Is the information published on a website that promotes LASIK or refers patients to LASIK surgeons? Do not be deceived by reckless surgeons or fake patient advocates who downplay the importance of pupil size.
Why does pupil size matter?
The pupil controls the amount of light entering the eye similar to the aperture of a camera. The diameter of the cornea that receives the full intended correction by the laser should be at least as large as the patient's pupils in dim light.
The standard optical zone for LASIK surgery is 6.0 - 6.5 millimeters. Use of larger optical zones place patients at higher risk for development of the vision-threatening complication post-LASIK ectasia. Most lasers are not FDA-approved for optical zones larger than 6.5 millimeters. The ablation zone may include an additional blend zone which should not be considered when determining the size of the effective optical zone. In a study published in 2004, dark-adapted pupil sizes of candidates for refractive surgery were found to range from 4.3 to 8.9 millimeters with an average of 6.5 millimeters (see abstract below). Based on these findings, a large percentage of patients should be disqualified for LASIK.
The effective optical zone (also called "functional optical zone") is the fully corrected area of the cornea, after healing, as determined by topography. The effective optical zone of the laser treatment should cover the entire diameter of the entrance pupil in dim light. If the pupil dilates larger than the effective optical zone, unfocused light rays will pass through uncorrected cornea and the resulting image will be distorted. The greater the disparity between the effective optical zone and pupil size, the more severe the visual disturbances.
The risk for night-time visual disturbances is further increased for patients with high myopia due to smaller effective optical zones associated with deeper ablations. If the ablation zone of the laser treatment is decentered, even an adequate effective optical zone for the pupil size may not cover the entire pupil diameter.
More information about the correlation between pupil size and night vision disturbances (spherical aberrations) can be found at the following link:
Pupil Size Lawsuits
There have been several LASIK medical malpractice lawsuits based on pupil size. Unfortunately, these cases are challenging for the plantiff due to phony "expert" witnesses -- hired guns for the defense who are paid to give bogus testimony about pupil size. A true "expert" knows that pupil size is a critical factor in quality of vision after LASIK. When a pupil size case makes it to trial, and a jury sees all the evidence, these cases can be won. Here are two pupil size cases that caught the attention of the LASIK industry:
LASIK surgeons will use junk science in their attempt to defend against patients with large pupils who file lawsuits. It is important to understand flaws in the methodology of medical studies published by phony expert witnesses. Read more:
Ask yourself if any medical study published by a well-paid defense expert witness is reliable, or just phony science with the intent of covering up the importance of pupil size to protect surgeons from liability. If a LASIK surgeon's reputation as a defense expert witness is based on his testimony that pupil size doesn't matter, what motivation would he possibly have to publish or quote medical studies that proves he's wrong? All good scientists know that a study can be designed to draw any desired conclusion, and if you fail to control for all the known variables, the truth will be obscured. Look beyond the small handful of articles based on junk science which fails to find a correlation between pupil size and night vision disturbances. The medical literature is full of authoritative studies which demonstrate that pupil size is predictive of quality of vision after LASIK.
Importance of Accurate Pupil Measurement
Pupils should be measured in a dark room after allowing the patient's eyes to become accustomed to the darkness (this is known as "dark-adaption"). For more about accurate pupil measurements, see:
Jack Holladay, MD: "Accurate pupillometry is an essential part of the evaluation for refractive surgery. With reports of halos and glare following refractive surgery on many of the prime-time news shows, pupillometry has become one of the preoperative tests that patients expect. It is very clear from the published and anecdotal reports of nighttime glare and halos that a large pupil is the predominant factor leading to these problems."
Source: Review of Ophthalmology, Vol. No: 9:03 Issue: 3/15/02, The High Cost of Inaccurate Pupillometry
Effective optical zone (EOZ), also known as functional optical zone (FOZ)
The goal of LASIK is to change the refractive power of the eye by changing the steepness of the cornea with a laser. Due to the risk of post-LASIK ectasia, the diameter of the cornea that is fully treated by the laser (called the optical zone) is generally limited to 6 - 6.5 millimeters. If the cornea was a flat piece of plastic, a 6 millimeter LASIK treatment would be sufficient for a patient whose pupils dilate in the dark to 6 millimeters. But the cornea is not flat, and it's not made of plastic. Factors that play a role in the effective optical zone after LASIK include the healing response of the cornea and the "cosine effect" or "radial compensation function". It is important to be aware of these factors and how they relate to night vision disturbances after LASIK. Learn more:
Articles regarding the role of pupil size and night vision disturbances after LASIK:
Am J Ophthalmol. 1994 Mar 15;117(3):394-8.
Keratorefractive surgery, success, and the public health.
"To avoid aberration in the center of the visual field, the cornea must be regular over the entrance pupil".
"When the cornea is irregular over the entrance pupil, the image generated by the cornea loses contrast and edge definition".
"The final result is that corneal irregularity from refractive surgery can cause optical degradation; and optical performance in the central field can change with pupil size".
"To avoid aberration in more peripheral portions of the visual field, the cornea must be regular over the cornea adjacent to the entrance pupil as well as over the entrance pupil itself."
"First, the pupil enlarges. As it does, aberration of central vision increases as more distorted paracentral cornea falls within the pupillary space."
"The problems with pupil-related aberration are further magnified by the reality that the Stiles-Crawford effect is negated in night vision."
"I hope the reader will understand how a patient may have clinically acceptable 20/20 visual acuity in the daytime and still suffer from clinically dangerous visual aberration at night if that patient's visual system must cope with an altered refractive error, increased glare, poorer contrast discrimination, and preferentially degraded peripheral vision. People die at night in motor vehicle accidents four times as frequently as they do during the day, and these figures are adjusted for miles driven. Night driving presents a hazardous visual experience to adults without aberration. When we discuss aberration at night we are considering a possible morbid effect of refractive surgery."
Curr Opin Ophthalmol. 2006 Aug;17(4):373-9.
Pupil size and corneal laser surgery.
Salz JJ, Trattler W.
"In 1993, Roberts and Koester  used an optical analysis computer program to study the effect of the optical zone with entrance pupils of 2–8mm. The conclusion was that ‘optical zone diameter must be at least as large as the entrance pupil diameter to preclude glare at the fovea, and larger than the entrance pupil to preclude parafoveal glare’. A similar conclusion was reached by Klonos et al. , who used a computer model. When scotopic pupil size is greater than 7mm, the ablation diameter often cannot be made larger than the pupil size as recommended by Roberts and Koester as well as Klonos et al., so we would expect patients with these larger pupil sizes who have laser vision surgery to have more NVCs. The fact that many of them eventually adapt to their situation, as in the Schallhorn et al. and Pop and Payette studies, should not lead us to the conclusion that their large scotopic pupil size is irrelevant and not a risk factor."
"In one study , a total of 129 eyes were evaluated with the Larson glarometer . Using the Colvard pupillometer in scotopic conditions, 79 eyes had pupil sizes of 6.0mm or smaller (average age of 42.6 mm) and 59 eyes had pupil sizes of 7.0mm or larger (average age of 32.7 years). Patients were tested with the Larson glarometer in scotopic conditions while wearing trial frames with their best-corrected vision to remove refractive error as a contribution to the starburst score. The results of this study (Fig. 2) revealed that patients with small pupils had small starburst scores whereas patients with large pupils reported large starburst scores. The size of the starburst was unaffected by preoperative refractive error in patients with small pupils, but starburst size increased in patients with large pupils with increasing preoperative refractive error. The results of this study revealed that patients with small pupils were unlikely to experience large starbursts after laser vision correction with conventional treatment on the VISX laser. Conversely, patients with large pupils were at increased risk of experiencing large starbursts, and the size of the starburst increased with higher preoperative levels of myopia."
Ophthalmology. 2004 Mar;111(3):447-53.
Wavefront analysis in post-LASIK eyes and its correlation with visual symptoms, refraction, and topography.
Chalita MR, Chavala S, Xu M, Krueger RR.
PURPOSE: To evaluate the information assessed with the LADARWave wavefront measurement device and correlate it with visual symptoms, refraction, and corneal topography in previously LASIK-treated eyes.
PARTICIPANTS: One hundred five eyes (58 patients) of individuals who underwent LASIK surgery were evaluated. DESIGN: Retrospective, noncomparative case series.
MAIN OUTCOME MEASURES: Complete ophthalmologic examination, corneal topography, and wavefront measurements were performed. Correlations were made between the examinations and symptoms.
METHODS: Wavefront measurements were assessed with the LADARWave device. Manifest, cycloplegic refraction, and topographic data were compared with wavefront refraction and higher order aberrations. Visual symptoms were correlated to higher order aberrations in 3 different pupil sizes (5-mm, 7-mm, and scotopic pupil size). Pearson's correlation coefficient and generalized estimating equations were used for statistical analysis.
RESULTS: In post-LASIK eyes, wavefront refraction components were poorly correlated to manifest and cycloplegic components. The comparison between manifest, cycloplegic, and wavefront refraction with total amount of higher order aberrations showed no strong correlation. The comparison between topography and manifest, cycloplegic, and wavefront refraction did not show strong correlation. Visual symptoms analysis showed correlation of double vision with total coma and with horizontal coma for the 5-mm and 7-mm pupil size; correlation between starburst and total coma for the 7-mm pupil size; and correlation of double vision with horizontal coma, glare with spherical aberrations and with total aberrations, and starburst with spherical aberrations for the scotopic pupil size. Scotopic pupil size had a positive association with starburst and a negative association with double vision.
CONCLUSIONS: The LADARWave wavefront measurement device is a valuable diagnostic tool in measuring refractive error with ocular aberrations in post-LASIK eyes. A strong correlation between visual symptoms and ocular aberrations, such as monocular diplopia with coma and starburst and glare with spherical aberration, suggest this device is valuable in diagnosing symptomatic LASIK-induced aberrations. Horizontal coma was correlated with double vision, whereas vertical coma was not.
Acta Ophthalmol Scand. 2004 Aug;82(4):454-60.
Pupil size and night vision disturbances after LASIK for myopia.
Helgesen A, Hjortdal J, Ehlers N.
PURPOSE: To examine whether standardized, preoperative evaluation of pupil sizes can predict the risk of night vision visual disturbances after bilateral laser in situ keratomileusis (LASIK) for myopia.
METHODS: A prospective study was carried out involving 46 patients who underwent bilateral LASIK for myopia. Pupil sizes were measured before surgery using an infrared pupillometer under standardized settings. Pre- and postoperative refraction and best spectacle-corrected visual acuity (BSCVA) were registered. At the 3-month follow-up visit, the patients completed a questionnaire regarding night vision pre- and postoperatively.
RESULTS: The mean bilateral, spherical equivalent refraction (SE) was - 8.76 D (range 6.32 to - 12.0 D) preoperatively, and - 1.69 D (range 0 to - 4.38 D) postoperatively. The mean bilateral BSCVA was not changed by the operations. We found a significant correlation between large scotopic pupil sizes and the impression of worsened night vision (p < 0.01). A significant correlation between gender (males) and subjectively reduced night vision postoperatively was also found (p < 0.05).
CONCLUSION: Large pupil size measured preoperatively is correlated with an increased frequency of subjectively experienced post-LASIK visual disturbances during scotopic conditions. We recommend preoperative evaluation of pupil size in all patients prior to LASIK surgery.
J Cataract Refract Surg. 2004 Nov;30(11):2336-43.
Effect of expanding the treatment zone of the Nidek EC-5000 laser on laser in situ keratomileusis outcomes.
Macsai MS, Stubbe K, Beck AP, Ravage ZB.
PURPOSE: To evaluate the effect of expanding the treatment zone of the Nidek EC-5000 laser on postoperative visual acuity as well as night glare and halos after laser in situ keratomileusis (LASIK) using 4 ablation zone diameters.
SETTING: Division of Ophthalmology, Evanston Northwestern Healthcare and Northwestern University Medical School, Glenview, Illinois, USA.
METHODS: This prospective study comprised 301 eyes of 154 consecutive patients who had LASIK in 1 or both eyes using the Nidek EC-5000 laser by 1 surgeon with experience in keratomileusis and excimer laser refractive surgery. A 6.5 mm optical zone was used with a transition zone 1.0 mm larger than the pupil under scotopic conditions (7.5, 8.0, 8.5, or 9.0 mm). Targeted correction was calculated according to a customized clinical nomogram. All patients were queried about glare and halos preoperatively and 3 months postoperatively using a questionnaire assigning numeric values to the degree of perceived visual disturbance (0 = no glare or halos, 1 = minimal, 2 = moderate, 3 = severe).
RESULTS: The baseline uncorrected visual acuity (UCVA) was 20/200 or worse in 293 eyes. The baseline best spectacle-corrected visual acuity was 20/20 or better. The mean preoperative refractive sphere was -6.33 diopters (D) +/- 2.80 (SD) (range -1.00 to -16.25 D) and the mean preoperative refractive cylinder, 0.86 +/- 0.83 D (range 0 to +3.25 D). Three months postoperatively, 78% of eyes had a UCVA of 20/20 and 99%, of 20/40 or better. Preoperatively, 94 eyes (31%) had glare and halos. At 3 months, glare, halos, or both were present in 19 eyes of 11 patients (6.3%) (P<.0001); in 14 eyes, patients reported less severe glare and halos postoperatively than preoperatively.
CONCLUSIONS: The use of a peripheral transition zone 1.0 mm larger than the pupil under scotopic conditions resulted in a low incidence of glare and halos postoperatively and did not adversely affect visual acuity. There was no increase in postoperative complications including corneal ectasia.
J Cataract Refract Surg. 2005 Dec;31(12):2272-80.
Influence of pupil and optical zone diameter on higher-order aberrations after wavefront-guided myopic LASIK.
Buhren J, Kuhne C, Kohnen T.
PURPOSE: To investigate the influence of pupil and optical zone (OZ) diameter on higher-order aberrations (HOAs) after myopic wavefront-guided laser in situ keratomileusis (LASIK).
METHODS: Twenty-seven myopic eyes of 19 patients were included. The mean preoperative spherical equivalent was -6.86 diopters (D) +/- 1.24 (SD) (range -4.25 to -9.5 D); the mean planned OZ diameter was 6.26 +/- 0.45 mm (range 5.7 to 7.1 mm). All patients had uneventful wavefront-guided LASIK (Zyoptix version 3.1, Bausch & Lomb) and an uncomplicated follow-up of 12 months. Wavefront measurements were performed with a Hartmann-Shack sensor in maximum mydriasis preoperatively and 12 months after LASIK. Wavefront errors were computed for pupil diameters (PDs) of 3.0, 3.5, 4.0, 5.0, 6.0, and 7.0 mm for the individual OZ diameter and for the individual mydriatic PD (7.93 +/- 0.46 mm). The impact of the relationship between pupil diameter and OZ diameter (fractional clearance [FC]) on HOA was described and quantified using curvilinear regression with a 4th-order polynomial fit.
RESULTS: There was a reproducible relationship between FC and the amount of induced HOA. The change in HOA root mean square and primary spherical aberration (Z(4)(0)) was significantly correlated with FC. If the OZ was 16.5% larger than the pupil (FC = 1.17), only half the amount of HOA was expected to be induced than if the OZ equaled the pupil. In contrast, an OZ that was 9% smaller than the pupil (FC = 0.91) resulted in an HOA induction 50% higher than at FC = 1.
CONCLUSION: The OZ zone to pupil ratio (fractional clearance) had a significant impact on HOA induction after wavefront-guided LASIK.
J Refract Surg. 2004 Jul-Aug;20(4):337-42.
Pupil size in refractive surgery candidates.
Netto MV, Ambrósio R Jr, Wilson SE.
PURPOSE: To assess pupil size measurements obtained under scotopic and mesopic conditions with the Procyon pupillometer and under photopic conditions with the Humphrey videokeratographer.
METHODS: The pupil sizes of 96 candidates for refractive surgery (192 eyes) were measured with the Procyon pupillometer PS2000 SA and the Humphrey Atlas 992 corneal topographer. Anisocoria and pupillary unrest were analyzed according to gender (two groups: 51 females and 45 males), age (five groups: 20 to 30 yr, 31 to 40 yr, 41 to 50 yr, 51 to 60 yr, older than 60 yr) and level of refraction (five groups: >-6.00 D SE, -6.00 to -3.00 D SE, -3.00 to 0 D SE, 0 to +2.50 D SE, +2.50 to +5.00 D SE).
RESULTS: The median value of pupil diameter measured with the Procyon pupillometer at the scotopic (0.04 lux), mesopic-low (0.4 lux), and mesopic-high (4 lux) levels of illumination were 6.54+/-0.88 mm; 5.62+/-0.95 mm, and 4.09+/-0.76 mm, respectively. The median pupil size with the Humphrey topographer was 3.65+/-0.62 mm. Pupillary unrest was highest at the mesopic-high level of illumination, with a median value of 0.31+/-0.34 mm. Median pupil size measured with both instruments at all light levels dropped significantly after the fifth decade of life (P<.05, ANOVA).
CONCLUSIONS: The Procyon pupillometer and Humphrey videokeratographer revealed an inverse correlation between the pupil size and the age, but no relationship with gender or level of refraction. The Procyon pupillometer provides an objective method for measuring pupil size at controlled light levels with a permanent printed record.
Mayo Clin Proc. 2001 Aug;76(8):823-9.
Making sense of refractive surgery in 2001: why, when, for whom, and by whom?
Mannis MJ, Segal WA, Darlington JK.
Abstract: Surgical alteration of the focusing or refractive properties of the eye has been performed on millions of patients. An array of procedures to correct myopia, hyperopia, astigmatism, and presbyopia have been introduced over the past 25 years with varying degrees of success. Improved technology has increased patient and physician satisfaction and enthusiasm. Currently available surgical procedures can be categorized as incisional, surface-altering, lamellar, and intraocular. The choice of procedure depends on individual patient indications and contraindications based on results of ocular examinations, eg, corneal pachymetry to measure corneal thickness, keratometry to measure the corneal curvature, basal tear secretory rate, and dark-adapted pupil size. The postoperative uncorrected visual acuity depends, in large part, on the quality of the preoperative evaluation and refraction. Before scheduling a patient for surgery, the ophthalmologist must ensure that the patient understands the potential risks of the procedure and has realistic expectations for the postoperative level and quality of uncorrected visual acuity. Postoperative complications include corneal flap displacement, undercorrection and overcorrection, and epithelial ingrowth under the corneal flap and inflammatory keratitis. Postoperative dry eye, infection, and inflammation are usually treated medically. Ongoing technological innovations to customize the surgical approach to an individual patient's eye continue to improve outcomes.
Determination of pupil size in a darkened room is essential in the preoperative evaluation to identify patients who may be at risk of glare and halos after surgery. Patients with dark-adapted large pupils should be warned of the higher risk of postoperative visual distortion or glare in dark illumination."
"Medical contraindications to LASIK include a cornea that is too thin or eyes with corneal ectasia in association with irregular astigmatism. Ocular disease, including active collagen vascular disease, rheumatoid arthritis, central panstromal corneal scars, and active herpetic keratitis, contraindicates LASIK in most instances. Relative contraindications are moderate to severe dry eye, cataracts, severe diabetes, advanced or unstable glaucoma, severe anterior basement membrane dystrophy, and pupil size greater than the maximum available ablation optical zone of the laser to be used."
Several vision changes have been described following LASIK, including halos, glare, and ghosting of images. These visual changes occur most often in patients who have pupils that are larger than the ablation zone.