Visual Acuity Increase After Laser Refractive Surgery Uzeyir Erdem, MD, Orkun Muftuoglu MD Gulhane M

1
Visual Acuity Increase After Laser Refractive
Surgery Uzeyir Erdem, MD, Orkun Muftuoglu
MDGulhane Military Medical Faculty Ankara-Türkiye
P186 Refractive

• INTRODUCTION
• The primary goal of laser refractive surgery is
  to increase uncorrected visual acuity (UCVA) by
  correcting the refractive errors. Although best
  corrected visual acuity (BSCVA) usually remains
  same, it is sometimes possible to observe an
  increase in BSCVA, after laser refractive
  surgery.1-16 Laser in situ keratomileusis (LASIK)
  is the most popular laser refractive surgery
  technique for the correction of different
  refractive errors.17 In the abscence of other
  visible signs of eye disease, a reduction of the
  BSCVA to less than 20/30 in one eye, or as a
  2-line difference between the two eyes, is
  defined as amblyopia.18 Many studies have
  reported significant improvement in postoperative
  BSCVA in amblyopic eyes, after laser refractive
  surgery recently. 19-28
• The reasons of BCVA increase after laser
  refractive surgery remain to be elucidated..
  Better correction of optical errors or
  improvement in neurosensorial function are
  possible contributing factors to the improvement
  in BSCVA.29,30 Changes in pupil diameter, corneal
  asphericity and aberrations may affect the
  optical quality of the eye and consequently the
  quality of the retinal image. 31,32,2.
• In this study, we evaluated the pre- and
  postoperative vectorial refractive data, pupil
  diameter, corneal asphericity, and aberrations of
  amblyopic and normal patients with increased
  BSCVA
• METHODS
• This prospective study comprised 72 eyes of 43
  consecutive patients (18 men and 25 women) whose
  best corrected visual acuity (BCVA) increased out
  of 850 eyes of 480 patients who underwent laser
  refractive surgery, between October 2003 and June
  2005, in Refractive Surgery Department of Gulhane
  Military Medical Faculty Eye Clinics. A detailed
  ocular examination was performed including
  manifest and cycloplegic refractions,
  keratometry, slitlamp evaluation, tonometry,
  mydriatic fundoscopy preoperatively. Criteria for
  inclusion in the study were age greater than 18
  years, documented stable refraction for 1 year
  before surgery, the availability of preoperative
  and 6-month postoperative optical path difference
  scan (OPD Scan, Nidek Co. Gamagori, Japan) maps
  with no missing data points within the central
  6.0-mm zone, no intraoperative or postoperative
  complications, and no retreatments. Exclusion
  criteria were previous refractive surgery,
  evidence of keratoconus or keratoconus suspect as
  evidenced by corneal topography, active ocular or
  systemic disease likely to affect corneal wound
  healing, pregnant and nursing. All procedures
  performed by the same excimer laser and
  microkeratome, the same LASIK surgical technique.
  All laser refractive surgeries were performed
  using the Nidek EC-5000 CX II excimer laser with
  an eye tracker (200 Hz) and the Final Fit
  software (NAVEX system). The Nidek MK 2000
  microkeratome used to create a corneal flap of
  9.5 mm in diameter and 130 µm in thickness with a
  nasal hinge.
• The mean optical zone (OZ) was 5.83 0.69 mm and
  the mean transitional zone (TZ) was 8.72 0.45
  mm. Optimized ashperical transitional zone
  software (CATz, OPD CAT or OATz, Nidek Co,
  Gamagori, Japan) was used for the ablation of
  myopic eyes.. (Nidek Navex system has 7 profile
  options for the ablation of transition zone to
  obtain more prolate ablation. Profiles 4-6 were
  used for myopic ablations). Chayets bitoric
  ablation nomogram was used for the correction of
  mixt astigmatism. Wide optical zone and
  transitional zone for correction of hiperopia or
  hyperopic astigmatism (6,5/7 OZ, 9.5 mm TZ).
  Postoperative treatment consisted of topical
  ofloxacin 0.3 and prednisolone acetate 1.
  Patients were seen 1 day, 1 week, and 1, 2, and 6
  months postoperatively.
• Optical Path Difference (OPD)-scan to evalute
  mesopic and photopic pupil diameter, corneal
  topography (corneal power (K), difference between
  K2 and K1 (dK), asphericity (Q-value, derived
  from the 6.0 mm diameter region for a given
  cornea), optical path refraction (RMS-3 mm zone
  and RMS-5 mm zone in diopters) and wavefront
  aberrations, preoperatively and at their last
  visit.
• The vectorial method proposed by Thibos and
  Horner was used to convert any spherocylindrical
  refractive error.1,33 The difference between
  preoperative and postoperative value of each
  parameter was defined as ?data
  datapostoperative - datapreoperative.
• All statistical data were analyzed using the SPSS
  11.0 (SPSS Inc, USA) statistical software.
  Normality of data in each group was confirmed by
  the normal probability plots. Paired t-test for
  parametric data and Wilcoxon signed ranks test
  for non-parametric data were used to analyze data
  before and after laser refractive surgery, for
  each parameter. Mann-Whitney u test and
  Kruskal-Wallis test were used to compare
  parameters between groups. Multiple comparison
  tests was used to compare difference between
  parameters of more than two, when there is a
  significance at Kruskal-Wallis test.34 The
  association between ?BSCVA and the clinical
  outcomes were tested using Spearman rank
  correlation and regression analysis. Statistical
  differences were considered significant when
  Plt.05.
• RESULTS
• The mean age of the 43 patients (18 men and 25
  women) was 30.8 years 7.2 (SD) (range 21 to 53
  years). Twenty-two eyes of 17 patients were
  amblyopic and 50 eyes of 26 patients were normal.
  Classification of eyes depending on their
  refraction and presence of amblyopia are
  demonstrated in table 1. Ten eyes of 10 patients
  (31.6) had anisometropic amblyopia, 3 eyes of 2
  patients had ammetropic amblyopia, 2 eyes of 1
  patient had meridional amblyopia, 2 eyes of 2
  patients had ammetropic and anisometropic
  amblyopia and 5 eyes of 3 patients had
  anisometropic and meridional amblyopia.
• Vision The mean BSCVA was 0.17 0.15 (range
  -0.04 to 0.58) before and 0.01 0.13 (range
  0.20 to 0.40), after surgery. The mean UCVA was
  0.99 0.45 (range 0.06 to 1.80) before and 0.12
  0.21 (range -0.17 to 1.00), after surgery.
  There was a statistically significant increase in
  BSCVA (Plt.001) and UCVA (Plt.001), 6 months after
  surgery.

DISCUSSION The reported rates of BSCVA increase
after laser refractive surgery are between 0 -
67 , in previous reports.4-15 This variability
is possibly due to differences between studies
with regard to the patient groups, preoperative
refractions, techniques, microkeratomes and
lasers. However, most of these studies intended
to report the efficacy and safety of the
procedure. Therefore, the magnitude of BSCVA
increase and its reasons were not investigated.
Maldonado-Bas compared 300 consecutive myopic
eyes depending on the magnitude of refraction and
observed that BSCVA increase was prominent in
eyes with higher preoperative refraction.4 The
results of studies that reported an increase in
BSCVA after LASIK, in pediatric amblyopic
patients, were often suboptimal, as the patients
were under anesthesia and unable to fixate.35,36
Although, it is commonly accepted as amblyopia
preventive therapy is effective until 8 to 11
years of age3,19,23, there are reports of BSCVA
increase after LASIK, in autofixating adolescent
and adult amblyopic patients.20 Lanza et al.
reported results of photorefractive keratectomy
in 38 eyes of 36 adult amblyopic patients
comprised mostly (30 of 38 eyes) of compound
myopic astigmatism and only 2 hyperopes. Although
not statistically significant, they observed
tendencey of greater BSCVA improvement in younger
patients, in eyes that were closer to emmetropia,
and in eyes that have better astigmatic
correction.6 We found a statistically
significant correlation between ?BSCVA and
preoperative BSCVA. This implies that patients
with worse preoperative BSCVA have more reserve
visual capacity. We could not find significant
correlation between the increase in BSCVA and
age however, there was a tendency of greater
BSCVA increase in younger patients. Although we
could not observe a significant correlation
between increase in BSCVA and the amount of
correction (?B, ?TWA, ?K), there was significant
correlations between ?BSCVA and ?B, ?TWA, ?K, in
myopes and significant correlation between ?BSCVA
and ?TWA, in hyperopes. However, there was no
such a correlation in mixt astigmats. This
implies that BSCVA increase is related with the
magnitude of refractive/aberration correction in
myopes and aberration correction in hyperopes,
but not in mixt astigmats. We could not find any
significant correlation between increase in BSCVA
and better astigmatic correction (?J45, ?J0, ?dK)
in all eyes and also in each refraction
group. Pupil diameter may affect the vision by
changing the depth of focus.1,19,37 Previous
reports of LASIK could not find a difference in
pupil diameter, after LASIK.38 Accordingly, in
our study, we could not find a significant change
in mesopic or photopic pupil diameter before and
after laser refractive surgery. These results
show that pupil diameter does not have a
significant effect on BSCVA increase, after
LASIK. The corneal asphericity (Q-value) in the
conic equation has been used to describe the
corneal suface.39 Myopic, hyperopic and
astigmatic ablation profiles are different
regarding the pattern and effect on corneal
asphericity.1,40,41 In our study, the
preoperative asphericity of the cornea was worse
in mixt astigmats than myopes and hyperopes.
However, the asphericity was better after bitoric
astigmatic correction. Similar to previous
reports, the asphericity was worse after myopic
and hyperopic corrections.43,44 This shows that
the laser refractive surgery corrects the corneal
refractive errors and improves asphericity, in
mixt astigmats. Contrary to that, the laser
refractive surgery adds aberrations to cornea to
counteract against refractive errors mostly due
to axial length, therefore worsening the corneal
asphericity, in myopes and hyperopes.45,46 There
was no significant correlation between ?BSCVA and
?Q and postoperative Q, in all eyes and also in
refraction groups. This implies that although
Q-value is usefull in evaluationg the differences
between refraction groups, it does not give an
idea about the amount of BSCVA increase. Previous
studies found a significant increase in HOAs
after LASIK.47-54 Similarly in our study, we
found a significant increase in HOAs after LASIK,
in eyes with BSCVA increase. Besides, as the TWAs
decrease, the mean HOA/TWA ratio increased to 38
postoperatively, compared to 9 preoperatively.
This suggests that increase in HOAs have low
impact on BSCVA improvement, after LASIK.
Although, Applegate et al.55 found a high inverse
correlation between high levels of HOAs and
visual performance, the true clinical
significance of HOAs and their role in refractive
surgery is not fully understood. 55-64 Joslin et
al.65 reported that corneal refractive therapy
with contact lenses significantly increased HOAs,
whereas it had no significant effect on visual
acuity. On the other hand, visual acuity
measurements may not be sensitive enough to
demonstrate the impact of HOAs on visual acuity.
Also, development of new metrics may better
correlate with clinical measures of visual
performance.58,66 Although there was a
significant increase in HOAs after surgery, in
all refraction groups, have the greatest increase
in HOAs was observed in hyperopic followed by
high myopic, moderate myopic and astigmatic
corrections. No significant T3F, T4F and TSA
induction observed after bitoric astigmatic
correction whereas, all zernike terms (TSA
increase had a low significance), except T3F,
significantly increased after moderate myopic
all zernike terms significantly increased after
high myopic and all zernike terms, except T4F,
significantly increased after hyperopic
corrections. Also, the least increase in RMS-3
and only decrease in RMS-5 were observed after
bitoric astigmatic correction. Differences in HOA
induction among refraction groups can be
explained with the asphericity change bitoric
mixt astigmatic correction had the best
postoperative asphericity and least HOA
induction, followed by moderate myopic, high
myopic and hyperopic corrections. Previous
studies also found a greater change in HOAs in
hyperopes compared to myopes.67,68 The steeper
slope of the hyperopic ablation profile with a
smaller transition zone, compared with the myopic
profile, two transition points in myopic
treatments compared to three transition points in
hyperopic LASIK,69,70 more difficult centration
in hyperopic eyes due to higher incidence of
larger angle kappas are suggested reasons of
higher HOA induction in hyperopes.71 Many studies
reported that myopic or hyperopic LASIK induces
SA, but in different directions. 42,44-50, 67 It
was proposed that BSCVA increase after refractive
surgery in myopic amblyopes occurs due to relief
from spherical aberrations.58,6 Conversely, in
our study, TSA increased significantly after
myopic and hyperopic surgery, both in normals and
amblyopes. However, the TSA induction after LASIK
was much more prominent in hyperopes and was even
higher than high myopes. In addition, TSA
induction after surgery in moderate myopics had a
very low significance (P 0.54). Expanding and
using a concave ablation at the transition zone
with the NIDEK-OATZ software may explain why we
observed less SA induction, after myopic
ablation. There was increase in TC in all
refraction groups, and possibly is the reason why
we observed significant HOA induction in all
refraction groups after LASIK, in our study.
Increase of TC in all ablation patterns suggest
that TC induction is due to a common reason such
as creation of lamellar flap or laser ablation
itself. Previous studies also found significant
coma induction after hyperopic and myopic
LASIK.67,72 There were no significant
correlation between ?BSCVA and ?HOAs or other
Zernike terms (TC, T3F, T4F, and HiA) except TSA.
However, there was a significant correleation
between the amount of BSCVA increase and less TSA
induction, and there were tendencies between
increase in BSCVA and less HOA, T3F, and HiA
induction, after surgery. This suggests that TSA
may have an impact on visual acuity more than the
other zernike terms. Increase in TC and T4F did
not seem to be related with improvement in BSCVA.
There were no correlation with BSCVA increase and
HOAs and other Zernike terms, in refraction
groups. Sakatani et al.19 reported the results of
LASIK in 21 eyes of 19 adult patients with
amblyopia and found that the postoperative BCVA
was better than preoperatively in 42.8 of eyes.
They stated that amblyopic patients with myopia
showed a statistically significant improvement in
postoperative BSCVA. Some other reports also
proposed that BSCVA increase in amblyopes is seen
particularly in myopic patients. 6,20 Better
correction of ametropia, elimination of
aberrations occuring from the use of spectacle
lenses in myopic eyes and a real recovery from
amblyopia are suggested reasons of BCVA increase
after myopic LASIK in ambliyopic
patients.6,19-21. BSCVA increase was higher in
bitoric mixt astigmatic correction followed by
high myopic, moderate myopic and hyperopic
corrections. Better asphericity and less HOA
induction might explain why we observed a higher
BSCVA increase in patients after astigmatic
correction and worse asphericity and higher HOA
induction might explain why we observed less
BSCVA increase after hyperopic correction.
Previous studies reported that increase in BSCVA
is very common after bitoric astigmatic LASIK
1,58 The BSCVA increase were explained by the
fact that the optical compensation for high
astigmatism produces a larger distortion of the
image than that caused by a lens compensates for
smaller degrees of astigmatism.1 Previous reports
indicated that amblyopes may have visual
capability of increase, after laser refractive
surgery.6,19 Increase in BSCVA that is observed
in both amblyopes and normals in our study
suggests that they both have reserve capability
of vision that can increase after laser
refractive surgery. We also could not find
significant differences between amblyopic and
normal groups with regard to the amount of BSCVA
increase, change in corneal asphericity and pupil
diameter, decrease in TWAs, induction of HOAs and
Zernike terms. This implies that the optical
mechanisms of BSCVA increase that is observed
after laser refractive surgery may be similar in
both groups. It is believed that laser refractive
surgery increases BSCVA primarly by correcting
the optical errors. However, further studies are
needed to clarify whether this reserve capability
is predominantly optical or neurosensorial. The
devices or techniques used in the various studies
for the measurement of ocular aberrations are not
identical.59,60 Optical Path Difference Scan,
relatively a new method to detect optical
refractive status, has different software
versions. One study reported that it has a good
accuracy to measure wavefront aberrations,
whereas another found low.72 Unlike
Hartmann-Shack systems, the OPD-Scan uses dynamic
retinoscopy to measure aberrations, which makes
it unique for measuring the refractive power map
over the entire central pupil. Also, it has an
advantage of performing the pupil, refractive,
topographic, and wavefront measurements all in
once. However, further studies are needed to
evaluate its accuracy and reliability,
particularly in the long term73,74. We evaluated
the optical factors in eyes with increase in
BSCVA, after LASIK. These optical factors should
also be compared with the results of refractive
surgery without BSCVA increase. Also, our results
should be compared with the results of wavefront
guided laser refractive surgery. Previous studies
reported that the rate of BSCVA increase after
hyperopic correction is very low. Similarly in
our study, the number of hyperopic patients was
limited due to low rate of BSCVA increase after
hyperopic correction and exclusion of the eyes
with strabismus, which is frequent among
hyperopes.

• Conclusion
• Our study showed that, the eyes have worse
  preoperative BSCVA have more reserve capability
  of vision increase. In myopes, the amount of
  BSCVA correction is related with the magnitude of
  correction. Although HOAs seems to have a low
  impact on vision increase, different HOA
  induction patterns among refraction groups and
  less TSA induction seems to be related with BSCVA
  increase. Bitoric mixt astigmatic correction has
  the highest BSCVA with better corneal asphericity
  and the least HOA induction, whereas hyperopic
  correction has the least BSCVA increase with the
  highest HOA induction. Amblyopes and normal
  patients have similar optical properties after
  surgery suggesting similar optical mechanisms of
  BSCVA increase among these groups. Further study
  is needed to analyze the relationship between the
  optical properties of eyes and different visual
  performance measurement techniques.
• References
• Albarran-Diego C, Munoz G, Montes-Mico R, Alio
  JL. Bitoric laser in situ keratomileusis for
  astigmatism. J Cataract Refract Surg 2004
  3014711478
• Chayet AS, Montes M, Gomez L, et al. Bitoric
  laser in situ keratomileusis for the correction
  of simple myopic and mixed astigmatism.
  Ophthalmology 2001 108 303-308.
• Rueda L, Pineda-Fernandez A, Huang D, Nur J.
  Laser in situ keratomileusis for mixed and simple
  myopic astigmatism with the Nidek EC-5000 laser.
  J Refract Surg 2002 18234-238.
• Maldonado-Bas A, Onnis R. Results of laser in
  situ keratomileusis in different degrees of
  myopia. Ophthalmology 1998 105606611.
• Seward MS, Oral D, Bowman RW,et al. Comparison of
  LADARVision and Visx Star S3 laser in situ
  keratomileusis outcomes in myopia and hyperopia J
  Cataract Refract Surg 2003 2923512357
• Lanza M, Rosa N, Capasso L, et al. Can we utilize
  photorefractive keratectomy to improve visual
  acuity in adult amblyopic eyes? Ophthalmology
  200511216841691
• Gimbel HV, Van Westenbrugge JA, Penno EEA, et al.
  Simultaneous Bilateral Laser In Situ
  Keratomileusis. Ophthalmology 199910614611468
• El-Maghraby A, Salah T, Waring III GO, et al.
  Randomized bilateral comparison of excimer laser
  in situ keratomileusis and photorefractive
  keratectomy for 2.50 to 8.00 diopters of myopia.
  Ophthalmology 1999 106447457
• El Danasoury MA, El Maghraby A, Klyce SD, Mehrez
  K. Comparison of photorefractive keratectomy with
  excimer laser in situ keratomileusis in
  correcting low myopia (from -2.00 to -5.50
  diopters) a randomized study. Ophthalmology
  1999 106411420
• Balazsi G, Mullie M, Lasswell L, et al. J
  Cataract Refract Surg 2001 2719421951
• Shaikh NM, Manche EE. Laser in situ
  keratomileusis for myopia and compound myopic
  astigmatism using the Technolas 217 scanning-spot
  laser J Cataract Refract Surg 2002 28485490
• El-Agha MSH, Bowman RW, Cavanagh D, McCulley JP.
  Comparison of photorefractive keratectomy and
  laser in situ keratomileusis for the treatment of
  compound hyperopic astigmatism. J Cataract
  Refract Surg 2003 29900907