Pupil Size and LASIK: Studies and Articles

Key medical studies and journal articles regarding the role of pupil size and night vision disturbances after LASIK:

Theoretical Analysis of the Effect of Pupil Size, Initial Myopic Level, and Optical Zone on Quality of Vision After Corneal Refractive Surgery
Journal of Refractive Surgery December 2012 - Volume 28 · Issue 12: 901-905
Aixa Alarcón, PhD; Manuel Rubiño, PhD; Francisco Pérez-Ocón, PhD; José R. Jiménez, PhD

Abstract

PURPOSE:
To analyze the theoretical effect that pupil size, optical zone, and initial myopic level have on the final retinal image after corneal refractive surgery.

METHODS:
A schematic myopic eye model corrected by the Munnerlyn formula was used to analyze the optical quality of the final retinal image. Root-mean-square radius spot and modulation transfer function were calculated by ray tracing to evaluate retinal image quality.

RESULTS:
Pupil size had a negative effect on the retinal image only when it was greater than the diameter of the optical zone. In addition, the greater the initial myopic level, the more the pupil size affected image quality. Thus, a clear dependence exists between the initial myopic level and effect that the pupil size can have on the retinal image after laser refractive surgery.

CONCLUSIONS:
Pupil size may be a risk factor for night vision disturbances, but only when it is larger than the theoretical optical zones utilized in this study. Its effect depends not only on the optical zone size, but also on the initial myopic level. Therefore, this relationship should be taken into account during patient selection for refractive surgery.

From the full text:

Our results show that the retinal image was negatively affected by pupil size only when the pupil size was larger than the diameter of the optical zone. Also, we found that this effect depended on the initial myopic level. The greater the initial myopic level, the more the pupil size affected retinal image quality. These results partly explain the discrepancies found among different clinical studies in which the effect of pupil size on retinal image quality was evaluated under night vision conditions. In most of these studies, the initial myopic level was not taken into account.

Pop and Payette11 reported 795 patients with myopia up to -9.75 D, with a 5.5- to 6.5-mm optical zone and a mean pupil size measured under scotopic conditions of 6.6 +/- 1.1 mm. They found that the attempted degree of spherical correction and the optical zone size, among other factors, were major risk factors related to night vision complaints, whereas pupil size was not a factor. Although their study reported a significant relationship between the initial myopic level and night vision disturbances, they did not take into account this relationship when evaluating the effect of the pupil. This could explain the fact that they found no relationship between pupil size and night vision complaints, even for large pupils (range: 3 to 9 mm). Another explanation for this, reflected in our results, may be that the mean diameter of the pupil under scotopic conditions was not large enough compared to the optical zone size. A similar scenario can be found in the studies by Schallhorn et al and Lee et al.

Dark-Adapted Pupil Diameter as a Function of Age Measured with the NeurOptics Pupillometer.
J Refract Surg. 2011 Mar;27(3):202-7. doi: 10.3928/1081597X-20100511-01. Epub 2010 May 17.
Bradley JC, Bentley KC, Mughal AI, Bodhireddy H, Brown SM.

Abstract
PURPOSE: To measure the dark-adapted pupil diameter of normal research participants in their second through ninth decades of life using the NeurOptics pupillometer (Neuroptics Inc).

METHODS: Individuals aged 18 to 80 years with no history of eye disease or injury, intraocular surgery, or use of systemic antihistamines or opiates were recruited. After 2 minutes of adaptation at 1 lux illumination, the right dark-adapted pupil diameter was measured using the NeurOptics pupillometer, with accommodation controlled by distance fixation. The NeurOptics pupillometer reported a mean dark-adapted pupil diameter and a standard deviation of the mean, which were analyzed as a function of age-decade.

RESULTS: Two-hundred sixty-three individuals participated. For participants aged 18 to 19 years (n=6), the mean dark-adapted pupil diameter was 6.85 mm (range: 5.6 to 7.5 mm); 20 to 29 years (n=66), 7.33 mm (range: 5.7 to 8.8 mm); 30 to 39 years (n=50), 6.64 mm (range: 5.3 to 8.7 mm); 40 to 49 years (n=51), 6.15 mm (range: 4.5 to 8.2 mm); 50 to 59 years (n=50), 5.77 mm (range: 4.4 to 7.2 mm); 60 to 69 years (n=30), 5.58 mm (range: 3.5 to 7.5 mm); 70 to 79 years (n=6), 5.17 mm (range: 4.6 to 6.0 mm); and 80 years (n=4), 4.85 mm (range: 4.1 to 5.3 mm). These values were consistent with studies using infrared photography. The standard deviation was >0.1 mm in 10 (3.8%) participants, all of whom were younger than 55 years.

CONCLUSIONS: The dark-adapted pupil diameter is an important clinical variable when planning refractive surgery. Surgeons can compare a patient's dark-adapted pupil diameter with the results of this population study to identify outlier measurements, which may be erroneous, and repeat testing prior to surgery.

Excerpt: Although geometric optics supports the psychovisual importance of the emmetropic optical zone diameter relative to the dark-adapted pupil diameter, for a period of time the refractive surgery literature held that the dark-adapted pupil diameter was an unimportant clinical variable. Despite this idea, which was well received as it increased the number of patients considered suitable refractive surgery candidates, ophthalmologists continued to measure the dark-adapted pupil diameter and new devices for this purpose were developed, marketed, and compared. Opinion on the importance of the dark-adapted pupil diameter is currently polarized, but some excimer laser manufacturers now warn patients in their product labeling about vision quality loss with large low-light pupils; the US Food and Drug Administration also warns the public of this possibility. In our opinion, if ophthalmologists measure the dark-adapted pupil diameter they should do so accurately. This requires a correct testing protocol and an accurate pupillometer. From the quality-of-practice and medicolegal perspectives, a cursory dark-adapted pupil diameter measurement is more dangerous than no measurement at all.

Am J Ophthalmol. 2010 Jul;150(1):97-109.
Effect of pupil size on corneal aberrations before and after standard laser in situ keratomileusis, custom laser in situ keratomileusis, and corneal refractive therapy.
Queirós A, Villa-Collar C, González-Méijome JM, Jorge J, Gutiérrez AR.

PURPOSE: To evaluate the effect of changing the pupil size on the corneal first-surface higher-order aberrations induced by different refractive treatments: standard laser in situ keratomileusis (LASIK), custom LASIK, and corneal refractive therapy.

DESIGN: Observational study.

METHODS: Eighty-one right eyes from patients with a mean age of 29.94 +/- 7.5 years, of which 50 were female (61.7%), were analyzed retrospectively at the Clínica Oftalmológica NovoVision, Madrid, Spain. Corneal videokeratographic data were used to obtain corneal first-surface higher-order aberrations for aperture diameters from 3 to 8 mm using the Vol-CT software (Sarver & Associates, Inc). Total root mean square (RMS) and RMS for third- to sixth-order Zernike polynomials as well as spherical-like, coma-like, secondary astigmatism, and spherical plus coma-like variables were calculated.

RESULTS: We verified an increase in the higher-order aberration total RMS after treatments of 0.014 +/- 0.025 microm, 0.019 +/- 0.027 microm, and 0.018 +/- 0.031 microm for standard LASIK, custom LASIK, and corneal refractive therapy, respectively, for 3-mm pupil diameter. For the 8-mm aperture diameter, changes in total RMS increased by a factor of 50 compared with the variation for the 3-mm diameter up to 0.744 +/- 0.731 microm, 0.493 +/- 0.794 microm, and 0.973 +/- 1.055 microm for standard LASIK, custom LASIK, and corneal refractive therapy, respectively.

CONCLUSIONS: The 3 techniques increase the wavefront aberrations of the cornea and change the relative contribution of coma-like and spherical-like aberrations. For a large aperture (> 5 mm), corneal refractive therapy induces more spherical-like aberrations than standard and custom LASIK. However, no clinically or statistically significant differences existed for narrower apertures. Standard and custom LASIK did not display statistically significant differences regarding higher-order aberrations.

J Refract Surg. 2010 Feb 134-143.
Comparison of the Colvard, Procyon, and Neuroptics Pupillometers for Measuring Pupil Diameter Under Low Ambient Illumination.
Schallenberg et al.

Excerpts:

Recently, measurement of the low light pupil diameter has become more important in ophthalmologic practice because of modern refractive surgery. Pupil diameter plays an important role because some postoperative complaints such as halos, glare, ghosting, poor contrast sensitivity, and monocular double vision are associated with large pupils. Under low ambient illumination, it is possible that pupil diameter becomes larger than the functional optical zone and light passes through the ametropic ring where the light is scattered by the cornea. Therefore, exact measurement of the pupil diameter is essential before refractive surgery to avoid such postoperative problems and to preoperatively determine if a patient is suitable for refractive surgery.

Although numerous articles compare various pupillometers using different techniques, most are difficult to interpret because most investigators failed to properly measure ambient illumination, did not use a consistent and adequate dark adaptation protocol, did not use the same light levels for each pupillometer, or used data compiled from clinical records of refractive surgery patients in which numerous individuals performed the testing.

Low light-adapted pupil diameter is strongly influenced by age. The average age of our patient cohort was 25.7 years (range: 18 to 45 years). Based on laboratory data using infrared photography, we would expect a mean pupil diameter of approximately 7.0 mm at less than 1 lux.

To avoid patient complaints such as halos, glare, ghosting, monocular double vision, and loss of contrast sensitivity, the exact measurement of pupil size is essential prior to refractive surgery. Measurements with a negative bias (pupil diameter is measured smaller than actual) have a greater surgical risk for a patient than measurements with a positive bias (pupil diameter is measured larger than actual). In a young, fully dark-adapted cohort such as ours, very few patients should have a pupil diameter smaller than 6.0 mm at 0.04 lux.

Am J Ophthalmol. 1994 Mar 15;117(3):394-8.
Keratorefractive surgery, success, and the public health.
Maguire LJ.

Excerpts:

"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.

Excerpts:

"In 1993, Roberts and Koester [12] 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. [13], 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 [23], a total of 129 eyes were evaluated with the Larson glarometer [24]. 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.

Abstract:

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.

Abstract:

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.

Abstract:

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.

Abstract:

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.

Abstract:

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.

Mayo Clinic Article

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.

Excerpts:

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.


More medical studies about pupil size and LASIK »