laser in situ keratomileusis ( lasik ) has a high rate of improving uncorrected snellen visual acuity ( 1 ) , however it can degrade the quality of vision , resulting in reports of reduced night vision clarity , glare , and halos ( 2 ) .
the contrast sensitivity test more effectively evaluates the visual quality over a range of sizes and daytime contrast levels ( 3 ) and is necessary to assess the visual performance in refractive surgery patients ( 4 ) .
the increased higher order aberrations induced after lasik is one reason for the reduced contrast sensitivity ( 5 , 6 ) .
the contrast sensitivity after wavefront - guided lasik using the ablation depth based on the individual higher order aberrations compared with standard lasik after surgery showed a significantly improvement ( 7 ) .
the reduction in the contrast sensitivity was greater for higher amounts of myopia in standard lasik ( 8) . however , previous studies for contrast sensitivity after wavefront - guided lasik did not include high myopia patients .
the aim of this study was to evaluate the contrast sensitivity , glare sensitivity and total high order aberrations after wavefront - guided lasik in both moderate and high myopia patients .
the high myopia group included 25 eyes of 14 patients with -7.78~-6.17d ( table 1 ) .
the uncorrected visual acuity , spherical equivalent , contrast sensitivity and glare sensitivity were measured preoperatively , and at 1 week , 1 month and 2 months postoperatively on patients undergoing wavefront - guided lasik .
the ocular total higher order aberration ( hoa ) was measured at 2 months postoperatively .
were performed using a hartmann - shack aberrometer ( wavescan , visx , sunnyvale , ca , u.s.a . ) and visx star s4 ( visx ) excimer laser , respectively . in all patients ,
the individual aberrations gained including the higher order aberrations by wavescan was programmed into the star s4 laser to create a customized treatment . a flap thickness of 130 m was created using a moria m2 microkeratome ( moria , france ) .
after surgery , ofloxacin 0.3% ( samil pharm , seoul , korea ) and fluorometholone 0.1% ( samil pharm ) were prescribed 4 times daily beginning one day after surgery for a one week . the contrast sensitivity and glare
were examined using a visual capacity analyzer ( vca , l2 informatique , france ) with landolt rings as the optotypes in a darkened room with the monitor as the only light source .
a standard 15-inch computer monitor was used , and the horizontal distance between the monitor and the eyes was 1 m. there were five spatial frequencies , each with 20 levels of 0.1 - 100% contrast : 3 , 4.8 , 7.5 , 12 , 19 ( cycle / degree ; cpd ) .
the monitor illumination for the day and nighttime contrast sensitivity testing were 100 cd / m and 30 cd / m , respectively .
the measured levels were calculated as the log units , and the minimum contrast levels were recorded .
the nighttime glare test used a vca attaching light source ( sb99 , l2 informatique , france ) of 500,000 cd / m during 60 sec .
if the patients did not read the landolt ring in the monitor as a result of a glare disturbance , the observer run the decrease button of vca , which was automatically was recorded .
the pupil diameter was measured at the daytime ( 220 lux ) and nighttime condition ( 5 lux ) using a pupillometer ( colvard pupillometer , oasis medical , u.s.a . ) . the hartmann - shack wavefront analyzer ( wavescan , visx )
was used to measure the ocular wavefront aberrations for a 4 mm and 6 mm entrance pupil .
the magnitudes of the coefficients of the zernike polynomials are represented as the root mean square ( rms ; in microns ) and were used to show any ocular wavefront aberrations . from the contrast sensitivity data obtained using this system , the area under the log contrast sensitivity function ( aulcsf ) was calculated according to the method reported by applegate et al . ( 9 ) .
the log of the contrast sensitivity was plotted as a function of the log of the spatial frequency , and the third - order polynomials were fitted to the log spatial frequency limits of 0.48 ( corresponding to 3 cpd ) and 1.28 ( 19 cpd ) .
the resulting value was defined as the aulscf , which is a single quantity used to characterize the overall visual performance of the eye .
an independent t - test , analysis of variances ( anova ) , chi - square test and pearson correlation analysis using spss software ( spss inc . , chicago , il , u.s.a . ) were used .
age , gender and preoperative pupil diameter in the daytime and nighttime did not differ between the two groups ( p>0.05 ) ( table 1 ) .
table 2 shows time course of the changes in the uncorrected visual acuity ( ucva ) .
a ucva of 20/20 or better was achieved by 96.0% in those with moderate myopia and by 94.1% in those with high myopia 2 months after surgery .
1 shows the daytime contrast sensitivity at all spatial frequencies after wavefront - guided lasik .
the contrast sensitivity increased at 7.5 cpd at 1 month after wavefront - guided lasik in the moderate myopia group ( p=0.018 ) .
2 shows the nighttime contrast sensitivity at all spatial frequencies after wavefront - guided lasik .
the contrast sensitivity increased at 7.5 cpd 2 month after wavefront - guided lasik in the moderate myopia group ( p=0.005 ) .
all other frequencies were not significantly different after wavefront - guided lasik both in the daytime and nighttime ( p>0.05 ) .
the contrast sensitivity at all spatial frequencies did not differ in both groups ( p>0.05 , fig . 3 , 4 ) .
table 3 shows correlation analysis between the amounts of myopia and postoperative 2 months contrast sensitivity at all spatial frequencies in the daytime and nighttime .
no significant correlation was found between the ablation depth by wavefront - guided lasik and the contrast sensitivity ( p>0.05 , table 3 ) . in all patients with moderate myopia and high myopia , no correlation was found between the photopic pupil diameter and the daytime contrast sensitivity .
the scotopic pupil diameter showed no correlation with the nighttime contrast sensitivity ( p>0.05 , table 4 ) .
the reduction in glare sensitivity was observed at 1 week and 1 month after surgery .
however , this was recovered at 2 months postoperatively and the difference between the groups was not significantly different in the glare test ( table 5 ) . for all patients ,
the aulcsf in the daytime did correlate with the total hoa in the 4 mm entrance pupil ( r=-0.071 , p=0.612 ) .
the correlation between the aulcsf in the nighttime and the total hoa in the 6 mm entrance pupil was not significant ( r=-0.176 , p=0.260 ) ( fig .
the visual acuity had improved after laser refractive surgery but most patients have reported blurring and glare symptoms ( 8) . their vision is also susceptible to the changes in illumination and contrast ( 2 , 10 ) .
therefore , a contrast sensitivity test is needed to more accurately and objectively evaluate the visual function ( 5 , 11 ) .
this study was designed to evaluate the visual quality with a contrast sensitivity and glare test after wavefront - guided lasik .
the visual acuity test determines the ability to resolve small details at a high contrast ( 12 ) .
it does not mean that an individual performs normally on all visual tasks , and the acuity is a poor predictor of the visual performance in certain daily perceptual tasks such as face recognition ( 12 ) .
on the other hand , contrast sensitivity discriminates the luminance differences between a material or an area in a space ( 13 ) . as a sine - wave grating system ,
the contrast sensitivity is 3 or 5 times more sensitive than the letter acuity ( 13 ) .
this study used the vca , which assesses overall visual function with difference modes ( 8) .
( 8) reported that the reliability coefficient ranged from 89.1% to 99.8% under maximum background luminance . under 3 cd / m background luminance ,
some studies have reported decreased low - contrast sensitivity after lasik ( 10 , 13 - 22 ) .
chan et al . ( 4 ) related the contrast sensitivity reduction after lasik to some optical factors ( high order aberration ) ( 5 , 6 ) .
eighty eight percent of the contrast sensitivity measurements improved 1 month after the wavefront - guided lasik based on the individual high order aberration ablation pattern ( 7 ) .
however , previous studies did not include the high myopia patients . in standard lasik ,
the reduction in the contrast sensitivity was greater for correction of higher amounts of myopia ( 1 ) . in this study
pop and payette ( 23 ) reported that the aulcsf did not correlate with the total hoa ( r=-0.11 ) after lasik . using a cutoff point of a total hoa of either 0.5 , 0.6 , or 0.7 m , the independent t - test showed that the aulcsf did not differ between the lower and higher total hoa groups .
these supports the hypothesis proposed in earlier studies in that a low wavefront aberration does not completely fit the entire visual performance ( 11 , 24 ) . in this study ,
the total hoa after wavefront - guided lasik in most patients was < 0.5 m , and there was no correlation between the aulcsf and the total hoa .
a larger pupil causes a spurious resolution in an optically aberrant system and the pupil size is theoretically important in determining the optical quality of the retinal image and the visual performance ( 25 , 26 ) . however , previous studies reported that a low correlation between the contrast sensitivity and the glare symptoms after lasik and the pupil size ( 27 )
. this study also found no significant correlation between the contrast sensitivity and the pupil size in both the daytime and nighttime conditions .
however , the number of cases in our study limited scotopic pupil was not larger than 7.0 mm
. a large sample size with a wider range of pupil sizes will be necessary to confirm previous reports showing that the visual performance may demonstrate a decline in function related to the clearance zone compromised by a large pupil diameter ( 27 ) .
el danasoury ( 28 ) found , using a questionnaire , that 49% of eyes reported glare after lasik with an optical zone of 5.5 mm .
several studies reported night - driving difficulties and glare ranging from 2% to 55.6% ( 29 ) . the score of the nighttime glare symptoms with 75 lux after standard lasik was 1.481.16 and 2.161.11 in those with moderate and high myopia , respectively ( 27 ) . in all cases ,
the glare symptom was recovered 2 months after lasik and there was no significant difference between the groups . in conclusion ,
the reduction in the contrast sensitivity and glare was not caused after the wavefront - guided lasik based on the individual higher order aberration in both moderate myopia and high myopia groups .
 this study evaluated the visual quality after wavefront - guided laser in situ keratomileusis ( lasik ) for treating myopia . 
 thirty - two eyes with moderate myopia ( -5.78~-2.17d ) and 25 eyes with high myopia ( -7.78~-6.17d ) were prospectively reviewed . 
 the contrast sensitivity ( cs ) , glare and the total higher order aberrations ( hoa ) were measured before and 1 week , 1 month and 2 months after lasik . 
 the pupil diameter was measured at day- and night - time illumination . 
 the cs and glare at all spatial frequencies were not reduced after wavefront - guided lasik ( p<0.05 ) and the difference between the moderate and high myopia group was not significant . 
 no significant correlation was found between the amounts of myopia and the postoperative cs ( p>0.05 ) . 
 the area under the log contrast sensitivity function ( aulcsf ) showed no correlation with the total hoa ( r2=-0.071 , p=0.612 , between the daytime aulcsf and the total hoa with a 4 mm entrance pupil , r2=-0.176 , p=0.260 , between the nighttime aulcsf and the total hoa with a 6 mm entrance pupil ) . 
 there was no decrease in cs and glare after wavefront - guided lasik for myopia . in conclusion , 
 wavefront - guided lasik based on the individual ablation patterns is a good option for refractive surgery to improve the visual quality in both moderate and high myopia cases . 
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION