- Email: [email protected]
Contrast sensitivity after wave front–guided LASIK1
Contrast Sensitivity after Wave Front–Guided LASIK Igor Kaiserman, MD, MSc,1 Rossen Hazarbassanov, MD,2 David Varssano, MD,3 Aharon Grinbaum, MD4 Purpose: To compare the effects on contrast sensitivity of wave front– guided (WFG) versus standard LASIK. Design: Prospective, nonrandomized, comparative clinical study. Participants: Twenty-four eyes of 13 consecutive patients (mean age, 25.2⫾8.4 years; spherical equivalent, ⫺0.5 to ⫺4.25 diopters [D]) treated with WFG LASIK (WaveLight-Allegretto scanning-spot laser and wave front analyzer) and 22 eyes of 12 consecutive patients (mean age, 28.4⫾9.1 years; spherical equivalent, ⫺0.75 to ⫺4.5 D) treated with standard LASIK (WaveLight-Allegretto scanning-spot laser). Methods: Best-corrected contrast sensitivity was measured before and 1 month after surgery in both the WFG LASIK group and the standard LASIK group. A sine-wave contrast sensitivity test (functional acuity contrast test) was used to measure contrast sensitivity at 5 spatial frequencies (1.5, 3, 6, 12, and 18 cycles/degree). We compared the LASIK-induced changes in contrast sensitivity in each groups at each spatial frequency. Main Outcome Measure: The effect on contrast sensitivity of WFG LASIK versus standard LASIK. Results: Uncorrected visual acuity of 20/20 or better was achieved by 72% of eyes treated with WFG LASIK and by 70% of the eyes treated with standard LASIK. One month after LASIK, 88% of the contrast sensitivity measurements improved in the WFG LASIK group, whereas in the standard LASIK group, only 40% of the contrast sensitivity measurements improved. The contrast sensitivity improvement was significantly larger in the WFG LASIK group at all spatial frequencies (P⬍0.05). The WFG LASIK patients had a negative correlation between the changes in contrast sensitivity and the preoperative refractive error. Conclusions: The ability of WFG LASIK to correct optical aberrations results in significantly improved contrast sensitivity compared with standard LASIK 1 month after surgery. Ophthalmology 2004;111:454 – 457 © 2004 by the American Academy of Ophthalmology.
Normal Snellen visual acuity tests measure the ability to identify progressively smaller, high-contrast letters. Although this may be adequate for quantifying refractive errors, it often fails to detect subtle vision loss. Real-world vision is not always high-contrast black and white. Rather, it consists of objects having a wide range of sizes viewed under a variety of visually degrading conditions such as fog, nighttime, bright sun, and so forth. Contrast sensitivity testing1,2 is a measure of sensitivity to degraded contrast for a range of spatial frequencies, and thus it more effectively evaluates visual quality over a range of sizes and daily life contrast levels. Although LASIK has a high rate of improving uncorOriginally received: March 4, 2003. Accepted: June 16, 2003.
Manuscript no. 230117.
1
Department of Ophthalmology, Hadassah University Hospital, Jerusalem, Israel.
2
American Lasers Medical Center, Rishon Le-Zion, Israel. Department of Ophthalmology, Tel-Aviv Sourasky Medical Center and Sackler School of Medicine, Tel-Aviv University, Tel-Aviv, Israel.
3
4
Department of Ophthalmology, Sheba Medical Center, Tel-Hashomer, Israel. Drs Kaiserman and Hazarbassanov contributed equally to this work. Correspondence and reprint requests to Igor Kaiserman, MD, MSc, Department of Ophthalmology, Hadassah University Hospital, P.O.B. 12000, IL-91120 Jerusalem, Israel. E-mail: [email protected].
454
© 2004 by the American Academy of Ophthalmology Published by Elsevier Inc.
rected Snellen visual acuity,3 previous studies have shown that it can degrade the quality of vision, resulting in reports of reduced night vision clarity,4 glare, and halos. On examination, those patients were found to have reduced contrast sensitivity.5–7 This degradation usually is transient and seems to resolve in 3 to 12 months.7 Chan et al7 reported that after LASIK, contrast sensitivity was depressed in all tested spatial frequencies (0.3–20.5 cycles per degree [c/deg]), especially in the low spatial frequencies (1.5 and 3.4 c/deg). Eventually, contrast sensitivity recovered in approximately 6 to 12 months. The reduction in contrast sensitivity was greater for higher amounts of myopia. One reason for reduced contrast sensitivity after LASIK is the increased high-order aberrations induced by the procedure.8 –10 To minimize this problem, the use of individual ablation patterns based on wave front aberrations have been introduced.11 Wave front– guided (WFG) LASIK includes measurement of the wave front aberrations with a wave front analyzer and mathematical transfer of the measured wave front aberrations into a customized ablation pattern to be performed by a scanning-spot excimer laser. Wave front– guided LASIK has been shown to reduce ocular aberrations.12 Thus, it should have a more favorable effect on contrast sensitivity. The purpose of this study was to compare the effects on contrast sensitivity of WFG LASIK versus standard LASIK. ISSN 0161-6420/04/$–see front matter doi:10.1016/j.ophtha.2003.06.017
Kaiserman et al 䡠 Contrast Sensitivity after Wave Front–Guided LASIK
Materials and Methods The study included 24 consecutive eyes of 13 patients (mean age ⫾ standard error [SE], 25.2⫾8.4 years) that were treated with WFG LASIK and 22 consecutive eyes of 12 patients (mean age ⫾ SE, 28.4⫾9.1 years) that were treated with standard LASIK during the same period by 2 surgeons (AG, DV) using the same laser system (WaveLight-Allegretto scanning-spot laser, WaveLight Laser Technologie AG, Erlangen, Germany). The study was approved by the institutional review board. The WFG LASIK patients had a mean preoperative spherical refraction of ⫺1.7⫾1.2 diopters (D; mean ⫾ SE) and a cylinder of ⫺1.1⫾0.9 D (spherical equivalent, ⫺0.5 to ⫺4.25 D), whereas the standard LASIK group had a mean preoperative spherical refraction of ⫺1.8⫾1.3 D and a cylinder of ⫺1.2⫾0.6 D (spherical equivalent, ⫺0.75 to ⫺4.5 D). The differences between the 2 groups were not statistically significant. Before surgery, a comprehensive eye examination was performed, including ocular and general health history, visual acuity, subjective refraction, corneal topography (Oculus keratograph; Oculus Optikgera¨te GmbH, Wetzlar, Germany), corneal pachymetry, pupil size, cycloplegic refraction, and an ocular health examination. Patients who wore soft contact lenses were asked to discontinue use 10 days before the examination. Wave front analysis was performed in all patients with a Tscherning aberrometer (WaveFront analyzer; WaveLight Laser Technologie). LASIK was performed under topical anesthesia (Benoxinate HCl 0.4%). A flap with a diameter of 9.5 mm and a thickness of 160 m was created with a superior hinge using the Hansatome microkeratome (Bausch & Lomb Surgical, Claremont, CA). The LASIK treatment was performed by a scanning-spot laser with a 0.95-mm spot size, a Gaussian-like spot profile, and a 200-Hz repetition rate (WaveLight-Allegretto scanning-spot laser). The eye tracking system had a response time of fewer than 6 milliseconds. Treatment area diameter ranged from 6.5 to 7.0 mm (optical zone), with a transition zone of 1.0 to 2.5 mm. In the WFG LASIK group, the information gained via the WaveFront analyzer was programmed into the WaveLight Allegretto laser to create a customized WFG LASIK treatment, whereas in the standard LASIK group, the manifest refraction was used to program the LASIK treatment (based on the manufacturer’s nomograms). After photoablation, the flap was repositioned and the interface was irrigated with balanced salt solution (Aqsia; Chauvin Opsia SA, Toulouse Labe`ge, France). Lomefloxacin 3 mg/ml 3 times daily and dexamethasone phosphate 0.1% 4 times daily were prescribed for 1 week. Best-corrected contrast sensitivity was measured within 7 days before surgery and at 1 month after surgery by a masked technician. Because contrast sensitivity is know to decrease with age, cataract, and other ocular and neural pathologic characteristics, we examined only the change in contrast sensitivity before and after LASIK. We used the functional acuity contrast test (Optec Vision Tester; Stereo Optical Co., Chicago, IL), which is a sine-wave contrast sensitivity test. It uses a set of 5 progressive sine-wave grating slides at spatial frequencies of 1.5, 3, 6, 12, and 18 c/deg. The slides are illuminated internally and are calibrated to assure a constant luminance of 85 cd/m2. At each spatial frequency, the patient is presented with slides of increasing contrast (contrast step, 0.15 log units, which means that there is a 50% loss or a 100% gain in contrast for any 2 contrast steps increase or decrease, respectively). The gratings are tapered into a gray background in such a way as to keep the mean retinal illumination constant. The grating patch size is 1.7° (more than the macula). The gratings are tilted ⫹15°, 0°, or ⫺15° (within the orientation bandwidth of the
Table 1. The Mean(⫾ SE) Preoperative Contrast Sensitivity (Log Units) of the Wave Front–Guided LASIK versus Standard LASIK Groups Spatial Frequency (cycles/ degree)
Wave Front–Guided LASIK Group
Standard LASIK Group
Statistical Significance
1.5 3 6 12 18
1.36 ⫾ 0.04 1.54 ⫾ 0.03 1.62 ⫾ 0.05 1.33 ⫾ 0.08 0.92 ⫾ 0.08
1.44 ⫾ 0.08 1.62 ⫾ 0.05 1.71 ⫾ 0.09 1.42 ⫾ 0.11 1.1 ⫾ 0.14
NS* NS NS NS NS
*The difference between the two groups was not statistically significant (Student’s t test, two-tailed).
visual channels), and the patient is asked to state the direction of the gratings. Preoperative and postoperative contrast sensitivity results were compared between the 2 groups at each spatial frequency using a statistical software (SPSS 10, SPSS Inc., Chicago, IL). All statistical analyses were performed on the contrast sensitivity expressed in log units.
Results Before surgery, the WFG LASIK group had a mean best-corrected visual acuity (BCVA) of 0.99⫾0.03 (mean ⫾ standard deviation), whereas the standard LASIK group had a mean BCVA of 0.98⫾0.04 (not statistically different). Table 1 presents the mean preoperative contrast sensitivity in the 2 groups. The difference between the groups was not statistically significant at any spatial frequency, although the standard LASIK group had somewhat better contrast sensitivity at all spatial frequencies. Uncorrected visual acuity and BCVA 1 month after surgery are presented in Table 2. An uncorrected visual acuity of 20/20 or better was achieved by 72% of eyes treated with WFG LASIK and by 70% of the eyes treated with standard LASIK. The difference between the groups was not statistically significant both for the uncorrected visual acuity and BCVA. Figure 1 presents the contrast sensitivity at all spatial frequencies after LASIK (1 month) versus before LASIK. Eighty-eight percent of the contrast sensitivity levels improved after LASIK in the WFG LASIK group, whereas in the standard LASIK group, only 40% improved. In the WFG LASIK group, 7.2% of the contrast sensitivity levels deteriorated after LASIK, compared with 38% in the standard LASIK group. On average, contrast sensitivity improved in the WFG LASIK group by 34.3⫾4.3% (mean ⫾ SE), Table 2. Distribution of Uncorrected and Best-corrected Visual Acuity at 1 Month after Surgery Uncorrected Visual Acuity
Best-Corrected Visual Acuity
Visual Acuity
Wave Front–Guided LASIK
Standard LASIK
Wave Front–Guided LASIK
Standard LASIK
ⱖ20/15 ⱖ20/20 ⱖ20/40
14.3% 72% 100%
0% 70% 90%
19% 91% 100%
4% 88% 100%
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Ophthalmology Volume 111, Number 3, March 2004
Figure 1. Contrast sensitivity after LASIK versus before LASIK at all spatial frequencies in wave front– guided LASIK (black circles, dotted regression line) and standard LASIK (white triangles, dashed regression line). The difference between the 2 regression lines is statistically significant (P⬍0.01).
whereas in the standard LASIK group, it improved only by 1.5⫾3.2% (Student’s t test, P ⫽ 0.0002). Figure 2 depicts the mean percentage of change in contrast sensitivity for each group at each spatial frequency. The difference between the WFG LASIK group and the standard LASIK group was statistically significant at all spatial frequencies except 6 c/deg. Figure 3 presents the change in contrast sensitivity as a function of the refractive error (spherical equivalent) before LASIK. The WFG LASIK patients had a statistically significant negative correlation (r ⫽ ⫺0.24; P ⫽ 0.05) between the change in contrast sensitivity and the magnitude of the refractive error, whereas for the standard LASIK patients, no such correlation was noted (r ⫽ ⫺0.02; P ⫽ 0.89).
Figure 2. Mean change in contrast sensitivity at the various spatial frequencies for the wave front– guided LASIK group (black circles) and the standard LASIK group (white circles). The difference between the 2 groups is statistically significant at all spatial frequencies except 6 cycles per degree (*P⬍ 0.05; **P⬍0.01).
456
Figure 3. LASIK-induced changes in contrast sensitivity as a function of the preoperative spherical equivalent in the wave front– guided LASIK group (black circles, solid regression line, y ⫽ 6.76x⫹46.79) and standard LASIK group (white triangles, dashed regression line, y ⫽ 0.55x⫹2.76). Dotted lines represent 95% confidence intervals. The difference between the 2 regression lines is statistically significant (P⬍0.01).
Discussion Pe´ rez-Santonja et al5 and Mutyala et al6 reported that LASIK causes short-term depression in contrast sensitivity at certain spatial frequencies and that recovery is complete in 1 to 6 months. Montes Mico and Charman13 reported that LASIK decreases contrast sensitivity at all spatial frequencies 1 month after surgery and that higher spatial frequencies (6 –18 c/deg) are decreased up to 3 months after surgery, again recovering at 6 to 12 months. Similarly, Nakamura et al14 reported that patients with less than ⫺6.0 D of myopia had a contrast sensitivity decrease of 15% 1 week after surgery that corrected or improved 1 month after surgery, whereas patients with more than ⫺6.0 D of myopia had a decrease of 15% that persisted up to 6 months after surgery. Others reported similar results with contrast sensitivity returning to normal within 3 to 6 months after surgery.7 We also found that, after 1 month, standard LASIK depressed contrast sensitivity slightly at the low spatial frequency (1.5 c/deg), it did not affect contrast sensitivity significantly at 3 and 18 c/deg, and it improved it slightly (although not statistically significantly) at the midrange spatial frequencies (6 and 12 c/deg; Fig 2). This improvement in contrast sensitivity may be explained by either the difference in the methods of measurement or by the improved accuracy of the scanning-spot narrow-beam laser system that we used. Because contrast sensitivity improves with time, its depression after LASIK probably is caused by optical aberrations. Marcos9 has shown that LASIK causes a decrease of the ocular modulation transfer function, which accounted for most of the decrease in contrast sensitivity. Because the ocular higher-order optical aberrations were shown to correlate with BCVA and low-contrast visual acuity (i.e., visual acuity at 13% contrast)15 one may expect that WFG LASIK, which corrects high-order aberrations, would have a favor-
Kaiserman et al 䡠 Contrast Sensitivity after Wave Front–Guided LASIK able effect on contrast sensitivity. Thus, we chose to compare WFG LASIK with standard LASIK 1 month after surgery. Recently, Mrochen et al,12 using a similar laser and wave front analyzer, showed that 3 months after surgery, WFG LASIK can improve BCVA by at least 1 line in 71% of patients and low-contrast visual acuity by at least 1 line in 25.8% of the patients. Using a more accurate method of contrast sensitivity assessment (grating acuity), we found that 88% of the contrast sensitivity measurements improved after WFG LASIK. Moreover, we have shown that a significantly larger number of contrast sensitivity measurements improved in patients treated with WFG LASIK compared with those treated with standard LASIK and that the mean improvement in contrast sensitivity was significantly larger in the WFG LASIK group at all spatial frequencies except 6 c/deg. Thus, our findings support the notion that the changes in contrast sensitivity after LASIK result from optical aberrations and therefore can be improved by WFG LASIK. We have also shown that in the WFG LASIK group, the improvement in contrast sensitivity was inversely related to the degree of myopia. This may be expected, because the higher the refractive error, the more corneal tissue must be ablated, creating a more oblate cornea. Because a more oblate cornea induces more spherical aberrations,16 it would be expected to affect contrast sensitivity adversely. In recent years, WFG LASIK has been gaining popularity because it promises not only to correct complicated cases with irregular corneas or decentered ablations,17 but also to achieve a better final visual acuity by correcting the highorder ocular aberrations.12 Despite the small group of patients involved in this study, we have shown that WFG LASIK is superior to standard LASIK in improving contrast sensitivity 1 month after surgery and thus should result in an improved quality of vision. Further studies involving larger groups of patients, longer follow-up periods, and various laser systems are necessary to evaluate fully the advantages and limitations of WFG LASIK.
3.
4. 5. 6. 7. 8.
9. 10.
11. 12. 13. 14. 15. 16.
References 1. Ginsburg AP, Evans DW, Cannon MW Jr, et al. Large-sample norms for contrast sensitivity. Am J Optom Physiol Opt 1984; 61:80 – 4. 2. Ginsburg AP, Cannon MW. Comparison of three methods for
17.
rapid determination of threshold contrast sensitivity. Invest Ophthalmol Vis Sci 1983;24:798 – 802. Sugar A, Rapuano CJ, Culbertson WW, et al. Laser in situ keratomileusis for myopia and astigmatism: safety and efficacy: a report by the American Academy of Ophthalmology. Ophthalmology 2002;109:175– 87. Miller AE, McCulley JP, Bowman RW, et al. Patient satisfaction after LASIK for myopia. CLAO J 2001;27:84 – 8. Perez-Santonja JJ, Sakla HF, Alio JL. Contrast sensitivity after laser in situ keratomileusis. J Cataract Refract Surg 1998;24:183–9. Mutyala S, McDonald MB, Scheinblum KA, et al. Contrast sensitivity evaluation after laser in situ keratomileusis. Ophthalmology 2000;107:1864 –7. Chan JW, Edwards MH, Woo GC, Woo VC. Contrast sensitivity after laser in situ keratomileusis: one-year follow-up. J Cataract Refract Surg 2002;28:1774 –9. Moreno-Barriuso E, Lloves JM, Marcos S, et al. Ocular aberrations before and after myopic corneal refractive surgery: LASIK-induced changes measured with laser ray tracing. Invest Ophthalmol Vis Sci 2001;42:1396 – 403. Marcos S. Aberrations and visual performance following standard laser vision correction. J Refract Surg 2001;17:S596 – 601. Marcos S, Barbero S, Llorente L, Merayo-Lloves J. Optical response to LASIK surgery for myopia from total and corneal aberration measurements. Invest Ophthalmol Vis Sci 2001;42: 3349 –56. MacRae S, Schwiegerling J, Snyder RW. Customized and low spherical aberration corneal ablation design. J Refract Surg 1999;15(2 suppl):S246 – 8. Mrochen M, Kaemmerer M, Seiler T. Clinical results of wavefront-guided laser in situ keratomileusis 3 months after surgery. J Cataract Refract Surg 2001;27:201–7. Montes Mico R, Charman WN. Choice of spatial frequency for contrast sensitivity evaluation after corneal refractive surgery. J Refract Surg 2001;17:646 –51. Nakamura K, Bissen-Miyajima H, Toda I, et al. Effect of laser in situ keratomileusis correction on contrast visual acuity. J Cataract Refract Surg 2001;27:357– 61. Seiler T, Kaemmerer M, Mierdel P, Krinke HE. Ocular optical aberrations after photorefractive keratectomy for myopia and myopic astigmatism. Arch Ophthalmol 2000;118:17–21. Holladay JT, Dudeja DR, Chang J. Functional vision and corneal changes after laser in situ keratomileusis determined by contrast sensitivity, glare testing, and corneal topography. J Cataract Refract Surg 1999;25:663–9. Mrochen M, Krueger RR, Bueeler M, Seiler T. Aberrationsensing and wavefront-guided laser in situ keratomileusis: management of decentered ablation. J Refract Surg 2002;18: 418 –29.
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Normal Snellen visual acuity tests measure the ability to identify progressively smaller, high-contrast letters. Although this may be adequate for quantifying refractive errors, it often fails to detect subtle vision loss. Real-world vision is not always high-contrast black and white. Rather, it consists of objects having a wide range of sizes viewed under a variety of visually degrading conditions such as fog, nighttime, bright sun, and so forth. Contrast sensitivity testing1,2 is a measure of sensitivity to degraded contrast for a range of spatial frequencies, and thus it more effectively evaluates visual quality over a range of sizes and daily life contrast levels. Although LASIK has a high rate of improving uncorOriginally received: March 4, 2003. Accepted: June 16, 2003.
Manuscript no. 230117.
1
Department of Ophthalmology, Hadassah University Hospital, Jerusalem, Israel.
2
American Lasers Medical Center, Rishon Le-Zion, Israel. Department of Ophthalmology, Tel-Aviv Sourasky Medical Center and Sackler School of Medicine, Tel-Aviv University, Tel-Aviv, Israel.
3
4
Department of Ophthalmology, Sheba Medical Center, Tel-Hashomer, Israel. Drs Kaiserman and Hazarbassanov contributed equally to this work. Correspondence and reprint requests to Igor Kaiserman, MD, MSc, Department of Ophthalmology, Hadassah University Hospital, P.O.B. 12000, IL-91120 Jerusalem, Israel. E-mail: [email protected].
454
© 2004 by the American Academy of Ophthalmology Published by Elsevier Inc.
rected Snellen visual acuity,3 previous studies have shown that it can degrade the quality of vision, resulting in reports of reduced night vision clarity,4 glare, and halos. On examination, those patients were found to have reduced contrast sensitivity.5–7 This degradation usually is transient and seems to resolve in 3 to 12 months.7 Chan et al7 reported that after LASIK, contrast sensitivity was depressed in all tested spatial frequencies (0.3–20.5 cycles per degree [c/deg]), especially in the low spatial frequencies (1.5 and 3.4 c/deg). Eventually, contrast sensitivity recovered in approximately 6 to 12 months. The reduction in contrast sensitivity was greater for higher amounts of myopia. One reason for reduced contrast sensitivity after LASIK is the increased high-order aberrations induced by the procedure.8 –10 To minimize this problem, the use of individual ablation patterns based on wave front aberrations have been introduced.11 Wave front– guided (WFG) LASIK includes measurement of the wave front aberrations with a wave front analyzer and mathematical transfer of the measured wave front aberrations into a customized ablation pattern to be performed by a scanning-spot excimer laser. Wave front– guided LASIK has been shown to reduce ocular aberrations.12 Thus, it should have a more favorable effect on contrast sensitivity. The purpose of this study was to compare the effects on contrast sensitivity of WFG LASIK versus standard LASIK. ISSN 0161-6420/04/$–see front matter doi:10.1016/j.ophtha.2003.06.017
Kaiserman et al 䡠 Contrast Sensitivity after Wave Front–Guided LASIK
Materials and Methods The study included 24 consecutive eyes of 13 patients (mean age ⫾ standard error [SE], 25.2⫾8.4 years) that were treated with WFG LASIK and 22 consecutive eyes of 12 patients (mean age ⫾ SE, 28.4⫾9.1 years) that were treated with standard LASIK during the same period by 2 surgeons (AG, DV) using the same laser system (WaveLight-Allegretto scanning-spot laser, WaveLight Laser Technologie AG, Erlangen, Germany). The study was approved by the institutional review board. The WFG LASIK patients had a mean preoperative spherical refraction of ⫺1.7⫾1.2 diopters (D; mean ⫾ SE) and a cylinder of ⫺1.1⫾0.9 D (spherical equivalent, ⫺0.5 to ⫺4.25 D), whereas the standard LASIK group had a mean preoperative spherical refraction of ⫺1.8⫾1.3 D and a cylinder of ⫺1.2⫾0.6 D (spherical equivalent, ⫺0.75 to ⫺4.5 D). The differences between the 2 groups were not statistically significant. Before surgery, a comprehensive eye examination was performed, including ocular and general health history, visual acuity, subjective refraction, corneal topography (Oculus keratograph; Oculus Optikgera¨te GmbH, Wetzlar, Germany), corneal pachymetry, pupil size, cycloplegic refraction, and an ocular health examination. Patients who wore soft contact lenses were asked to discontinue use 10 days before the examination. Wave front analysis was performed in all patients with a Tscherning aberrometer (WaveFront analyzer; WaveLight Laser Technologie). LASIK was performed under topical anesthesia (Benoxinate HCl 0.4%). A flap with a diameter of 9.5 mm and a thickness of 160 m was created with a superior hinge using the Hansatome microkeratome (Bausch & Lomb Surgical, Claremont, CA). The LASIK treatment was performed by a scanning-spot laser with a 0.95-mm spot size, a Gaussian-like spot profile, and a 200-Hz repetition rate (WaveLight-Allegretto scanning-spot laser). The eye tracking system had a response time of fewer than 6 milliseconds. Treatment area diameter ranged from 6.5 to 7.0 mm (optical zone), with a transition zone of 1.0 to 2.5 mm. In the WFG LASIK group, the information gained via the WaveFront analyzer was programmed into the WaveLight Allegretto laser to create a customized WFG LASIK treatment, whereas in the standard LASIK group, the manifest refraction was used to program the LASIK treatment (based on the manufacturer’s nomograms). After photoablation, the flap was repositioned and the interface was irrigated with balanced salt solution (Aqsia; Chauvin Opsia SA, Toulouse Labe`ge, France). Lomefloxacin 3 mg/ml 3 times daily and dexamethasone phosphate 0.1% 4 times daily were prescribed for 1 week. Best-corrected contrast sensitivity was measured within 7 days before surgery and at 1 month after surgery by a masked technician. Because contrast sensitivity is know to decrease with age, cataract, and other ocular and neural pathologic characteristics, we examined only the change in contrast sensitivity before and after LASIK. We used the functional acuity contrast test (Optec Vision Tester; Stereo Optical Co., Chicago, IL), which is a sine-wave contrast sensitivity test. It uses a set of 5 progressive sine-wave grating slides at spatial frequencies of 1.5, 3, 6, 12, and 18 c/deg. The slides are illuminated internally and are calibrated to assure a constant luminance of 85 cd/m2. At each spatial frequency, the patient is presented with slides of increasing contrast (contrast step, 0.15 log units, which means that there is a 50% loss or a 100% gain in contrast for any 2 contrast steps increase or decrease, respectively). The gratings are tapered into a gray background in such a way as to keep the mean retinal illumination constant. The grating patch size is 1.7° (more than the macula). The gratings are tilted ⫹15°, 0°, or ⫺15° (within the orientation bandwidth of the
Table 1. The Mean(⫾ SE) Preoperative Contrast Sensitivity (Log Units) of the Wave Front–Guided LASIK versus Standard LASIK Groups Spatial Frequency (cycles/ degree)
Wave Front–Guided LASIK Group
Standard LASIK Group
Statistical Significance
1.5 3 6 12 18
1.36 ⫾ 0.04 1.54 ⫾ 0.03 1.62 ⫾ 0.05 1.33 ⫾ 0.08 0.92 ⫾ 0.08
1.44 ⫾ 0.08 1.62 ⫾ 0.05 1.71 ⫾ 0.09 1.42 ⫾ 0.11 1.1 ⫾ 0.14
NS* NS NS NS NS
*The difference between the two groups was not statistically significant (Student’s t test, two-tailed).
visual channels), and the patient is asked to state the direction of the gratings. Preoperative and postoperative contrast sensitivity results were compared between the 2 groups at each spatial frequency using a statistical software (SPSS 10, SPSS Inc., Chicago, IL). All statistical analyses were performed on the contrast sensitivity expressed in log units.
Results Before surgery, the WFG LASIK group had a mean best-corrected visual acuity (BCVA) of 0.99⫾0.03 (mean ⫾ standard deviation), whereas the standard LASIK group had a mean BCVA of 0.98⫾0.04 (not statistically different). Table 1 presents the mean preoperative contrast sensitivity in the 2 groups. The difference between the groups was not statistically significant at any spatial frequency, although the standard LASIK group had somewhat better contrast sensitivity at all spatial frequencies. Uncorrected visual acuity and BCVA 1 month after surgery are presented in Table 2. An uncorrected visual acuity of 20/20 or better was achieved by 72% of eyes treated with WFG LASIK and by 70% of the eyes treated with standard LASIK. The difference between the groups was not statistically significant both for the uncorrected visual acuity and BCVA. Figure 1 presents the contrast sensitivity at all spatial frequencies after LASIK (1 month) versus before LASIK. Eighty-eight percent of the contrast sensitivity levels improved after LASIK in the WFG LASIK group, whereas in the standard LASIK group, only 40% improved. In the WFG LASIK group, 7.2% of the contrast sensitivity levels deteriorated after LASIK, compared with 38% in the standard LASIK group. On average, contrast sensitivity improved in the WFG LASIK group by 34.3⫾4.3% (mean ⫾ SE), Table 2. Distribution of Uncorrected and Best-corrected Visual Acuity at 1 Month after Surgery Uncorrected Visual Acuity
Best-Corrected Visual Acuity
Visual Acuity
Wave Front–Guided LASIK
Standard LASIK
Wave Front–Guided LASIK
Standard LASIK
ⱖ20/15 ⱖ20/20 ⱖ20/40
14.3% 72% 100%
0% 70% 90%
19% 91% 100%
4% 88% 100%
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Ophthalmology Volume 111, Number 3, March 2004
Figure 1. Contrast sensitivity after LASIK versus before LASIK at all spatial frequencies in wave front– guided LASIK (black circles, dotted regression line) and standard LASIK (white triangles, dashed regression line). The difference between the 2 regression lines is statistically significant (P⬍0.01).
whereas in the standard LASIK group, it improved only by 1.5⫾3.2% (Student’s t test, P ⫽ 0.0002). Figure 2 depicts the mean percentage of change in contrast sensitivity for each group at each spatial frequency. The difference between the WFG LASIK group and the standard LASIK group was statistically significant at all spatial frequencies except 6 c/deg. Figure 3 presents the change in contrast sensitivity as a function of the refractive error (spherical equivalent) before LASIK. The WFG LASIK patients had a statistically significant negative correlation (r ⫽ ⫺0.24; P ⫽ 0.05) between the change in contrast sensitivity and the magnitude of the refractive error, whereas for the standard LASIK patients, no such correlation was noted (r ⫽ ⫺0.02; P ⫽ 0.89).
Figure 2. Mean change in contrast sensitivity at the various spatial frequencies for the wave front– guided LASIK group (black circles) and the standard LASIK group (white circles). The difference between the 2 groups is statistically significant at all spatial frequencies except 6 cycles per degree (*P⬍ 0.05; **P⬍0.01).
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Figure 3. LASIK-induced changes in contrast sensitivity as a function of the preoperative spherical equivalent in the wave front– guided LASIK group (black circles, solid regression line, y ⫽ 6.76x⫹46.79) and standard LASIK group (white triangles, dashed regression line, y ⫽ 0.55x⫹2.76). Dotted lines represent 95% confidence intervals. The difference between the 2 regression lines is statistically significant (P⬍0.01).
Discussion Pe´ rez-Santonja et al5 and Mutyala et al6 reported that LASIK causes short-term depression in contrast sensitivity at certain spatial frequencies and that recovery is complete in 1 to 6 months. Montes Mico and Charman13 reported that LASIK decreases contrast sensitivity at all spatial frequencies 1 month after surgery and that higher spatial frequencies (6 –18 c/deg) are decreased up to 3 months after surgery, again recovering at 6 to 12 months. Similarly, Nakamura et al14 reported that patients with less than ⫺6.0 D of myopia had a contrast sensitivity decrease of 15% 1 week after surgery that corrected or improved 1 month after surgery, whereas patients with more than ⫺6.0 D of myopia had a decrease of 15% that persisted up to 6 months after surgery. Others reported similar results with contrast sensitivity returning to normal within 3 to 6 months after surgery.7 We also found that, after 1 month, standard LASIK depressed contrast sensitivity slightly at the low spatial frequency (1.5 c/deg), it did not affect contrast sensitivity significantly at 3 and 18 c/deg, and it improved it slightly (although not statistically significantly) at the midrange spatial frequencies (6 and 12 c/deg; Fig 2). This improvement in contrast sensitivity may be explained by either the difference in the methods of measurement or by the improved accuracy of the scanning-spot narrow-beam laser system that we used. Because contrast sensitivity improves with time, its depression after LASIK probably is caused by optical aberrations. Marcos9 has shown that LASIK causes a decrease of the ocular modulation transfer function, which accounted for most of the decrease in contrast sensitivity. Because the ocular higher-order optical aberrations were shown to correlate with BCVA and low-contrast visual acuity (i.e., visual acuity at 13% contrast)15 one may expect that WFG LASIK, which corrects high-order aberrations, would have a favor-
Kaiserman et al 䡠 Contrast Sensitivity after Wave Front–Guided LASIK able effect on contrast sensitivity. Thus, we chose to compare WFG LASIK with standard LASIK 1 month after surgery. Recently, Mrochen et al,12 using a similar laser and wave front analyzer, showed that 3 months after surgery, WFG LASIK can improve BCVA by at least 1 line in 71% of patients and low-contrast visual acuity by at least 1 line in 25.8% of the patients. Using a more accurate method of contrast sensitivity assessment (grating acuity), we found that 88% of the contrast sensitivity measurements improved after WFG LASIK. Moreover, we have shown that a significantly larger number of contrast sensitivity measurements improved in patients treated with WFG LASIK compared with those treated with standard LASIK and that the mean improvement in contrast sensitivity was significantly larger in the WFG LASIK group at all spatial frequencies except 6 c/deg. Thus, our findings support the notion that the changes in contrast sensitivity after LASIK result from optical aberrations and therefore can be improved by WFG LASIK. We have also shown that in the WFG LASIK group, the improvement in contrast sensitivity was inversely related to the degree of myopia. This may be expected, because the higher the refractive error, the more corneal tissue must be ablated, creating a more oblate cornea. Because a more oblate cornea induces more spherical aberrations,16 it would be expected to affect contrast sensitivity adversely. In recent years, WFG LASIK has been gaining popularity because it promises not only to correct complicated cases with irregular corneas or decentered ablations,17 but also to achieve a better final visual acuity by correcting the highorder ocular aberrations.12 Despite the small group of patients involved in this study, we have shown that WFG LASIK is superior to standard LASIK in improving contrast sensitivity 1 month after surgery and thus should result in an improved quality of vision. Further studies involving larger groups of patients, longer follow-up periods, and various laser systems are necessary to evaluate fully the advantages and limitations of WFG LASIK.
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References 1. Ginsburg AP, Evans DW, Cannon MW Jr, et al. Large-sample norms for contrast sensitivity. Am J Optom Physiol Opt 1984; 61:80 – 4. 2. Ginsburg AP, Cannon MW. Comparison of three methods for
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