Straylight values 1 month after laser in situ keratomileusis and photorefractive keratectomy

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1 ARTICLE Straylight values 1 month after laser in situ keratomileusis and photorefractive keratectomy Jeroen J.G. Beerthuizen, MD, FEBOphth, Luuk Franssen, MSc, Monika Landesz, MD, PhD, Thomas J.T.P. van den Berg, PhD PURPOSE: To compare straylight values before and 1 month after laser in situ keratomileusis (LASIK) and photorefractive keratectomy (PRK). SETTING: Private practice refractive surgery center, Delft, The Netherlands. METHODS: In a prospective nonrandomized study, straylight values of 21 patients (42 eyes) were measured using the van den Berg straylight meter (third generation) during intake sessions at a refractive surgery clinic. Of the 21 patients, 12 were scheduled for LASIK (6 patients, 12 eyes) or PRK (6 patients, 12 eyes). At the 1-month follow-up visit, straylight values were measured again in the same manner and compared with the preoperative straylight values. RESULTS: Overall, there was no statistically significant increase in straylight values compared with the preoperative values 1 month after LASIK or PRK (P >.05). Individual straylight values increased in some cases, however, and the values correlated well with decreased quality of vision and changes in the eye examination. CONCLUSION: Straylight values 1 month after LASIK or PRK did not increase on average, although individual straylight values increased in some cases. J Cataract Refract Surg 2007; 33: Q 2007 ASCRS and ESCRS Patient satisfaction after laser refractive surgery is not always guaranteed when visual acuity is 20/20. In particular, visual imperfections that become apparent or get worse at night can compromise visual function despite excellent visual acuity. Therefore, the term quality of vision might be more appropriate to use as an outcome measure for refractive surgery. Quality of vision includes, among other parameters, the degree of night-vision disturbances, which occur Accepted for publication January 17, From the Department of Ophthalmology, VU University Medical Center (Beerthuizen), and the Netherlands Ophthalmic Research Institute (Franssen, van den Berg), Amsterdam, and Visionclinics (Landesz), Delft, The Netherlands. Dr. van den Berg is an employee of the Royal Netherlands Academy of Arts and Sciences, which has a financial interest in the C-Quant straylight meter. None of the other authors have a financial or proprietary interest in any material or method mentioned. Corresponding author: Jeroen J.G. Beerthuizen, MD, FEBOphth, Department of Ophthalmology, VU University Medical Center, De Boelelaan 1117, 1081 HV, Amsterdam, The Netherlands. j.beerthuizen@vumc.nl. Q 2007 ASCRS and ESCRS Published by Elsevier Inc. relatively frequently and with a highly variable incidence. 1 This variability in incidence might be explained by the many definitions of night-vision disturbances and the many modifiers involved. 2 A reproducible clinical test that can objectively measure something that is related to 1 or more symptoms of subjective quality of vision is therefore needed. 3 One such test is the van den Berg straylight meter. The straylight meter measures forward light scatter and provides direct information about optical imperfections as the cause of glare disability. 4 Glare disability, a factor in the quality of vision, refers to a reduction in visual performance caused by a glare source, resulting in retinal contrast loss secondary to intraocular straylight. This occurs, for instance, when a person cannot see a car in front of him or her at night when confronted with the headlights of oncoming traffic. We were interested in determining whether laser refractive surgery induces an increase in intraocular straylight. Studies have shown both an increase 5 and no increase 6 in straylight values 1 month after photorefractive keratectomy (PRK). Theoretically, corneal wound healing (PRK) and flap-related optical imperfections (laser in situ keratomileusis [LASIK]) could lead to increased straylight values /07/$dsee front matter 779 doi: /j.jcrs

2 780 STRAYLIGHT VALUES AFTER LASIK AND PRK PATIENTS AND METHODS Twenty-one patients (42 eyes) who entered a private clinic for refractive surgery preassessment were enrolled in a prospective study. A full ophthalmologic examination was performed including uncorrected visual acuity (UCVA), manifest refraction, best spectacle-corrected visual acuity (BSCVA), cycloplegic refraction, pupillometry (mesopic), dilated fundoscopy, intraocular pressure (IOP), topography, pachymetry, and biomicroscopy. Straylight measurements were taken 4 times per eye, twice with the eye dilated and twice with the eye undilated. Of the 21 patients, 6 (12 eyes) were scheduled for LASIK and 6 (12 eyes) were scheduled for alcohol-assisted PRK. Choice of treatment was based on standard refractive surgery inclusion criteria and patient preference. The remaining 9 patients were excluded for refractive surgery or scheduled for phakic intraocular lens implantation. Laser in situ keratomileusis surgery was performed using a Hansatome microkeratome (Bausch & Lomb) and a 50 Hz flying-spot laser (Technolas 217Z, Bausch & Lomb). In the alcohol-assisted PRK group, the epithelium was removed after 30-seconds exposure to a 20% ethanol solution. The same excimer laser with the same software was used. One month after laser surgery, both groups were evaluated at the same clinic. Postoperative evaluations included UCVA, BSCVA, manifest refraction, IOP, and biomicroscopy. Patients were asked whether they experienced night-vision disturbances such as glare. Straylight measurements were again taken 4 times per eye, twice with the eye dilated and twice with the eye undilated. Three months postoperatively, visual acuity and biomicroscopy were reevaluated in the PRK group. Straylight Measurements Intraocular straylight was measured using the thirdgeneration van den Berg straylight meter. 7 The first 2 generations of this instrument measured straylight using the direct compensation method based on a flickering annular source of straylight. 8,9 This flickering annulus causes a perceptible straylight flicker in the center of this annulus. Counter-phase flickering light is then presented in that center; the subject can adjust the intensity of the light to eventually cancel the amount of straylight. Because some subjects found these measurements too difficult and time consuming, a new psychophysical approach was defined. In the third-generation straylight meter, the central test field is divided into 2 fields, 1 with counter-phase compensation light and 1 without counter-phase compensation light. The subject must choose which half flickers more strongly (2 alternative forced-choice procedure). A fixed number of stimuli is presented, which yields a controlled test duration. The subject s responses are analyzed by a maximum likelihood procedure, from which the straylight value and a reliability estimate of this value are calculated. 10 The thirdgeneration straylight meter has been used in 2400 subjects, producing a standard deviation of repeated measurements of 0.06 log units (van den Berg TJ, et al. IOVS 2005; 46:ARVO E-Abstract 4315). A commercially available version of the third-generation straylight meter, the C-Quant (Oculus), was recently introduced and found to give comparable results. 7 Straylight measurements were assessed for quality. Poorquality measurements were filtered out, yielding reliable straylight values. In an earlier study, 10 the parameter of expected standard deviation was developed as a qualitycontrol parameter. In the present study, a limit value of expected standard deviation of less than 0.10 was adopted. To check the actual standard deviation in the present study, every first measurement was compared to the second one, producing a standard deviation of repeated measurements of log units, virtually the same as in the earlier studies 7 (van den Berg TJ, et al. IOVS 2005; 46:ARVO E-Abstract 4315). Power Analysis Using Altman s nomogram, it was calculated that 8 eyes were required to obtain a 95% chance (power Z 0.95; b Z 0.05) of detecting a clinically relevant difference in log(s) of 0.10 log units (D Z 0.10) at the 5% level of significance (a Z 0.05) using the paired t test. In this calculation, a conservative value for the standard deviation of repeated measurements of 0.05 log units was used based on the fact that most measurements were an average of 2 measurements, each with an average standard deviation of repeated measurements of (as mentioned above). Based on this, it was concluded that a single repeat of each measurement was sufficient. RESULTS Both Groups The mean difference between preoperative and postoperative straylight measurements in the LASIK group and PRK group combined was G (SEM) in the undilated eye group and G in the dilated eye group. There was no statistically significant increase in straylight values after treatment (PO.05). However, individual increased straylight occurred in some cases. Laser In Situ Keratomileusis Group The UCVA was 20/20 or better in 8 eyes and 20/25 in 4 eyes. The BSCVA was unchanged in 5 eyes; 4 eyes gained 1 line and 3 eyes lost 1 line from baseline (1 eye from 20/20 to 20/25 and 2 eyes from 20/15 to 20/20). The mean spherical equivalent (SE) was 3.27 diopters (D) (range 1.00 to 4.90 D) preoperatively and 0.05 D (range 0.38 to C0.25 D) 1 month postoperatively. Straylight values did not increase on average (PO.05) (Figure 1). In Figure 1, eyes that had a straylight increase of more than 0.20 log units (P!.0005) are numbered 1 to 3. Number 4 refers to the only eye with a straylight increase between 0.15 log units and 0.20 log units (P!.006). Two patients reported nightvision disturbances, 1 of them in 1 eye only. Both patients had microstriae in the corneal flaps with good UCVA (20/20 or better). One had increased straylight in the symptomatic eye (Figure 1, eye 2), while the asymptomatic eye showed no increase. The other patient did not have increased straylight in either eye. Two asymptomatic patients had increased

3 STRAYLIGHT VALUES AFTER LASIK AND PRK 781 1,5 LASIK 1,5 PRK 1,4 1,4 1,3 1,3 log(s) postop 1,2 1,1 1 log(s) postop 1,2 1,1 1 0,9 undilated 0,8 dilated x=y 0,7 0,7 0,8 0,9 1 1,1 1,2 1,3 1,4 1,5 log(s) preop Figure 1. Preoperative versus postoperative straylight values in the LASIK group. Numbers 1 to 3 represent eyes with an increase in straylight of more than 0.20 log units (P!.0005). Number 4 represents the eye with an increase in straylight between 0.15 log units and 0.20 log units (P!.006). Data from dilated and undilated eyes are plotted with different markers. 0,9 0,8 undilated dilated x=y 0,7 0,7 0,8 0,9 1 1,1 1,2 1,3 1,4 1,5 log(s) preop Figure 2. Preoperative versus postoperative straylight values in the PRK group. Number 1 represents the eye with an increase in straylight of more than 0.20 log units (P!.0005). Number 2 represents the eye with an increase in straylight between 0.15 log units and 0.20 log units (P!.006). Data from dilated and undilated eyes are plotted with different markers. straylight values in 1 eye. One eye had microstriae in the flap (Figure 1, eye 1), while the fellow eye had no abnormalities on examination and no increase in straylight. The other eye had a significant amount of debris under the flap (Figure 1, eyes 3 and 4). Photorefractive Keratectomy Group The UCVA was 20/20 or better in 6 eyes, 20/25 in 3 eyes, 20/30 in 1 eye, 20/40 in 1 eye, and 20/60 in 1 eye. The BSCVA was unchanged in 6 eyes; 2 eyes gained 1 line and 4 eyes lost 1 line from baseline (from 20/15 to 20/20). The mean SE was 3.09 D (range 1.50 to 6.63 D) preoperatively and 0.19 D (range 1.00 to C0.63 D) 1 month postoperatively. As in the LASIK group, there were no statistically significant changes in straylight values (PO.05) (Figure 2). In Figure 2, the number 1 refers to the only eye with a straylight increase of more than 0.20 log units (P!.0005); 2 refers to the only eye with a straylight increase between 0.15 log units and 0.20 log units (P!.006). Two patients reported night-vision disturbances, 1 of them in 1 eye only. This patient had grade 1 haze and increased straylight in that eye (Figure 2, eye 2). The fellow asymptomatic eye showed grade 0 to 1 haze and no increased straylight. The UCVA in the symptomatic eye was lower (20/25) than in the asymptomatic eye (20/15). The other patient did not have increased straylight values or abnormalities on eye examination. The UCVA, however, was lower than desired (20/60 and 20/40). Furthermore, 1 eye with a straylight increase (Figure 2, eye 1) had slight haze, although it did not cause night-vision disturbances. Overall, 7 eyes had mild to grade 1 haze, with 2 having a straylight increase of more than 0.15 log units 1 month postoperatively. By the 3-month postoperative examination, haze had disappeared in 4 eyes and was mild in 3 eyes. DISCUSSION In this prospective nonrandomized study, no statistically significant increase in intraocular straylight occurred 1 month after LASIK or alcohol-assisted PRK. Animal studies have shown that corneal backscattering of light, which correlates with visible haze, is significantly stronger after PRK than after LASIK 11,12 and peaks 1 month after PRK. 13 In humans, haze peaks at approximately 2 months. 14 In our study group, haze was more apparent at 1 month and disappeared or decreased by 3 months postoperatively. Backscattering of light, however, does not equal forward scattering

4 782 STRAYLIGHT VALUES AFTER LASIK AND PRK of light, or intraocular straylight, which is the physical basis of glare disability. 15 Studies of intraocular straylight 1 month after PRK show both an increase 5 and no increase. 6 The patients in the study with increased straylight had significantly more haze (grade 2 and higher) than those in the study without increased straylight and those in our study (maximum grade 1 in both studies). These differences might be due to the use of a 5.0 mm ablation zone and a higher preoperative refraction in the first study. No peerreviewed studies of straylight 1 month after LASIK could be found in the literature. Straylight measurements were taken with undilated pupils and with pharmacologically dilated pupils. Preoperative versus postoperative straylight values did not show a statistically significant increase in either group, so all data could be pooled and analyzed together. However, straylight values in eyes with dilated pupils were higher both preoperatively and postoperatively, as expected. 16 Although straylight values did not increase on average, individual straylight increased in some cases. Two patients with microstriae in the flap and good UCVA (20/20 or better) reported night-vision disturbances after LASIK; 1 had increased straylight (O0.20 log units) in the symptomatic eye. Furthermore, 1 eye with microstriae had increased straylight (O0.20 log units) without night-vision disturbances. This led us to believe that microstriae might play a negative role in quality of vision after LASIK, although the number of eyes is too small to confirm that theory. It has been shown, however, that subtle microstriae can reduce contrast sensitivity despite normal visual acuity. 17 A well-recognized problem with assessing quality of vision and night-vision disturbances is the lack of generally accepted objective tests. 3 In our study, patients were asked whether they developed problems with nighttime vision after laser treatment. We then compared the outcomes with individual straylight measurements and an ophthalmologic examination, including preoperative and postoperative refractions. Two patients reported an increase in night-vision disturbances in 1 eye only. These symptomatic eyes had higher straylight values than the asymptomatic fellow eyes. On ocular examination, 1 eye had microstriae while the asymptomatic fellow eye did not. The other eye had more haze than the asymptomatic fellow eye. Two other patients with increased night-vision disturbances, however, did not have an increase in straylight values. One had microstriae in both flaps, and the other had a lower than targeted UCVA. Furthermore, 3 eyes had increased straylight without night-vision disturbances. One of the eyes had mild haze after PRK, 1 had microstriae, and 1 had a significant amount of debris under the flap. Overall, one third of eyes with changes in the ocular examination had increased straylight values (O0.15 log units), indicating that a slitlamp examination alone is not a good predictor of changes in intraocular straylight and the presence of glare disability. On the other hand, all eyes with individual increased straylight (O0.15 log units) had changes in the ocular examination, in particular microstriae in the LASIK group and haze in the PRK group. These changes did not always correlate with subjective changes in quality of vision. One reason might be that patients compare their quality of vision after laser correction with quality of vision while wearing contact lenses. This not only gives information about changes in the cornea, it also includes the effect of the contact lenses. Unpublished observations from van den Berg s laboratory showed that a contact lens induced straylight increase often occurs. Furthermore, we did not use a more precise subjective instrument, such as a questionnaire with a rating system, to assess nightvision disturbances. However, we found the straylight meter to be a useful test as it gives direct information about optical imperfections as the cause of nightvision disturbances and increased straylight values (O0.15 log units) correlated well with ocular examinations. In summary, straylight values 1 month after LASIK and PRK did not increase on average. Individual increased straylight values, however, were recorded, and they correlated with ocular examination findings, but not always with subjective symptoms. A larger series of patients is needed to ascertain the importance of individual increased straylight values. Furthermore, it would be interesting to determine whether refractive corrections higher than those in our study increase overall straylight after laser treatment. A longer follow-up after treatment to assess whether straylight values change over time should be included in future research. REFERENCES 1. Bailey MD, Mitchell GL, Dhaliwal DK, et al. Patient satisfaction and visual symptoms after laser in situ keratomileusis. Ophthalmology 2003; 110: Pop M, Payette Y. Risk factors for night vision complaints after LASIK for myopia. Ophthalmology 2004; 111: Fan-Paul NI, Li J, Sullivan Miller J, Florakis GJ. Night vision disturbances after corneal refractive surgery. Surv Ophthalmol 2002; 47: van den Berg TJTP. On the relation between glare and straylight. Doc Ophthalmol 1991; 78: Veraart HGN, van den Berg TJTP, Hennekes R, Adank AMJ. Stray light in photorefractive keratectomy for myopia. Doc Ophthalmol 1995; 90: Harrison JM, Tennant TB, Gwin MC, et al. 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