Improvement of Visual Acuity Following Refractive Surgery for

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					Improvement of Visual Acuity Following
Refractive Surgery for Myopia and Myopic
Elisa Vuori, MD; Timo M.T. Tervo, MD, PhD; Martti V.A. Holopainen, PhD; Juha M. Holopainen, MD, PhD

                                                                           yopia is a refractive disorder in which the image
                                                                           of a distant object is formed anterior to the retina.
PURPOSE: To test the hypothesis that anisometropic
                                                                           It occurs when the refractive power of the eye is
adults without significant amblyopia suffer from mild           greater than the length of the eye and this may happen when
visual impairment probably due to aniseikonia, which           the eye has a greater refractive power, a longer axial length,
might be improved by corneal refractive surgery.               or a combination of these two factors. However, most myopic
                                                               eyes are longer than emmetropic eyes.1 Myopia is a leading
METHODS: Fifty-seven patients presenting with myopic           cause of vision impairment worldwide. Anisometropia caus-
anisometropia 3.25 diopters (D) and 174 myopic con-
trols appropriate for refractive surgery were included. Pho-
                                                               ing aniseikonia is a significant risk factor for the development
torefractive keratectomy (PRK) or LASIK was performed          of amblyopia.2 Patients with anisometropic amblyopia show
on 57 anisometropic eyes. As 43 of the 174 myopic              significant interocular differences in their contour detection
control patients had bilateral surgery, PRK or LASIK was       thresholds,3 suggesting that the processing of ocular input to
performed on 217 myopic control eyes. Best spectacle-          the visual cortex is impaired. Although the tolerance of aniso-
corrected visual acuity (BSCVA), refraction, and refractive
correction were measured preoperatively and at 1, 3, 5
                                                               metropia varies considerably between patients, it is generally
to 7, 8 to 13, and 25 months following surgery.                agreed that a difference in refractive power exceeding 3.00
                                                               diopters (D) will cause fusion problems, and these patients
RESULTS: Preoperative mean spherical equivalent                often lack binocular vision.
was 7.20 2.40 D for anisometropic patients and                    Spectacles or contact lenses can be used for correction of
  6.40 1.90 D for myopic patients. At 8 to 13 months           the defocus and image formation. Alternatively, refractive
postoperatively, when 23 (40%) anisometropic eyes and
94 (43%) myopic eyes were examined, the mean spher-
                                                               errors can be treated by refractive surgery such as LASIK,
ical equivalent refractions were 0.80 1.60 D and               photorefractive keratectomy (PRK), laser epithelial keratomi-
  0.30 0.60 D, respectively. Preoperatively, the mean          leusis (LASEK), or with phakic intraocular lenses. Extensive
BSCVA on a logMAR scale was 0.0143 0.0572                      literature exists on the effect of myopic refractive surgery
(Snellen 0.98 0.12) in the anisometropic group
and 0.0136 0.0361 (Snellen 1.04 0.09) in the
                                                               From the Department of Ophthalmology, University of Helsinki, Helsinki
control group (P=.001). Eight to 13 months postop-             (Vuori, Tervo, J.M. Holopainen), and Häme Polytechnic, University of Applied
eratively, these values were 0.0076 0.0659 (Snel-              Sciences, Valkeakoski (M.V.A. Holopainen), Finland.
len 1.03 0.15) and 0.0495 0.0692 (Snellen
1.13 0.18) and this difference remained statistically          Financial support for this study was provided by Helsinki University
significant (P=.012). For the myopic patients, the im-          Central Hospital, the Finnish Eye Foundation, the Instrumentarium Science
provement in BSCVA reached almost maximum at 3                 Foundation, the Ella and Georg Ehrnrooth Foundation, and the University of
                                                               Helsinki, Helsinki, Finland.
months, and this improvement was found to be highly
significant 3 months after surgery (P=.001). The im-            The authors have no financial or proprietary interest in the materials pre-
provement in BSCVA was significantly slower for aniso-          sented herein.
metropic patients and became statistically significant          The authors thank Raymond A. Applegate, OD, PhD, Director of the Visual
only after 8 to 13 months postoperatively (P=.041).            Optics Institute at the College of Optometry, University of Houston, Houston,
                                                               Tex for his comments on the manuscript prior to submission, and Olli
CONCLUSIONS: Anisometropia reduces visual acuity in            Kumpulainen for retrieving the data.
the more myopic eye and can be at least partially re-          Correspondence: Juha M. Holopainen, MD, PhD, Dept of Ophthalmology,
versed by refractive correction. The slower improvement        University of Helsinki, PO Box 220, 00029 HUS, Finland. Tel: 358 9 471 77197;
in BSCVA for anisometropic patients suggests plastic           Fax: 358 9 471 73162; E-mail:
changes in the visual cortex following refractive surgery.
[J Refract Surg. 2007;23:447-455.]                             Received: May 8, 2006
                                                               Accepted: September 13, 2006
                                                               Posted online: March 1, 2007

Journal of Refractive Surgery Volume 23 May 2007                                                                                        447
Improvement of Visual Acuity After Refractive Surgery/Vuori et al

on visual acuity.4-21 A priori best spectacle-corrected      (mean preoperative anisometropia 4.80 1.40 D, range:
visual acuity (BSCVA) would be expected to slightly          3.25 to 10.00 D) were included in the study. Thirty pa-
improve after refractive surgery because of image en-        tients were excluded because of lack of follow-up data
largement inherent in correcting myopia at the corneal       on their visual performance pre- or postoperatively or
rather than the spectacle plane.16 Some clinical stud-       because of postoperative complications, which strong-
ies, however, report improvement in BSCVA,7-9 where-         ly affected visual performance.
as others have reported reduction11,12 or no change fol-        Bilateral refractive surgery was performed on 33
lowing surgery.4-6,13-15,20                                  patients and the more myopic eye was included in
   Only two studies have reported the effect of refrac-      our analyses. Twenty-four patients had unilateral sur-
tive surgery on anisometropic adult patients. Maden          gery during our data collection. Some patients having
et al18 followed anisometropic adult patients for 12         unilateral surgery had the other eye operated later.
months. In their study, BSCVA increased in 7 (35%)           Of the operated eyes, 27 were right and 30 were left.
eyes, remained the same in 12 (60%) eyes, and de-            Forty-four (77%) eyes were operated using PRK and 13
creased in 1 (5%) eye. Unfortunately, the study lacked       (23%) using LASIK. Conventional methods (spectacles
statistical analysis. We did not find statistically signifi-   and contact lenses) had subjectively been insufficient.
cant improvement in BSCVA in a small retrospective           Because patients were unsatisfied with their refractive
series of 11 adult anisometropic patients.21 Recently,       correction of the more myopic eye, at least some degree
Paysse et al9 demonstrated the long-term benefit of PRK       of visual deprivation was expected. In accordance with
on visual acuity and stereopsis in children with aniso-      this, BSCVA was worse (P=.08, paired samples t test),
metropic amblyopia.                                          albeit weakly, in the more myopic eye compared to the
   Our previous study21 concerning PRK for anisome-          contralateral eye. Amblyopic eyes (BSCVA 20/40)
tropia suffered from two main drawbacks. The number          were excluded from the study.
of patients was low (n=11) and a control group was not          Patients were examined before PRK/LASIK and 1,
included. In this study, we hypothesize that anisome-        3, 5 to 7, 8 to 13, and 25 14 months postoperatively.
tropic patients suffer from mild visual impairment and       Some patients had more than one postoperative fol-
have a higher risk for development of amblyopia. We          low-up evaluation during the periods 5 to 7 months
hypothesize that this visual impairment can be reversed      and 8 to 13 months, and in these cases, data from the
by refractive surgery. If this were the case, BSCVA of       visit with the highest BSCVA were included in the
the anisometropic eyes would be worse compared to            analysis. The value of BSCVA was rounded off to the
simple myopic eyes with the same spherical equiva-           closest full decimal on the logMAR scale when listing
lent and also worse compared to the patient’s con-           the preoperative BSCVA values. For statistical analy-
tralateral eye. The improvement in BSCVA following           sis of the data, BSCVA was measured on a Snellen
refractive surgery should be significantly slower for         visual acuity chart to the closest 0.05.
anisometropic patients compared to myopes because
of reorganization of the visual cortex. In nonanisome-       STUDY CONTROLS
tropic myopes, the visual input of both eyes is initially       The control group comprised 174 patients (124 fe-
in equilibrium, and improvement in BSCVA would be            males and 50 males; mean age 31 7.3 years, range:
more rapid in these patients.                                16 to 52 years). Forty-three patients underwent bilat-
                                                             eral surgery, therefore 217 eyes were included. Some
              PATIENTS AND METHODS                           patients having unilateral surgery during our data
   The Ethical Review Committee of Helsinki Univer-          collection time had the other eye operated earlier or
sity Eye Hospital approved the research plan, and the        after data collection had ended. Similar to the an-
study followed the tenets of the Declaration of Helsinki.    isometropic group, data were collected before PRK/
Patients were operated at the Helsinki University            LASIK and at 1, 3, 5 to 7, 8 to 13, and 26 14 months
Eye Hospital between November 1999 and April                 postoperatively. Because some myopic patients had
2002. Patients in the control group were operated            more than one follow-up evaluation during the time
at the same hospital between September 1995 and              intervals 5 to 7 months and 8 to 13 months, we in-
March 2004.                                                  cluded data from the follow-up with the highest
                                                             BSCVA in the analysis.
  Eighty-seven patients were enrolled, and of these,         REFRACTIVE CORRECTION
57 patients (42 women and 15 men; mean age 33 8.1               The mean spherical equivalent of attempted correc-
years, range: 18 to 58 years) with myopic anisometropia      tion was 7.00 2.40 D (range: 3.125 to 13.375 D) for

                                                   Improvement of Visual Acuity After Refractive Surgery/Vuori et al

anisometropic patients and 6.30 1.90 D (range: 2.90           refraction ranged from 4.60 to 2.00 D, with a mean
to 12.90 D) for myopic patients. Photorefractive kera-        value of 0.80 1.60 D. Eight (35%) eyes were within
tectomy and LASIK were performed as previously de-              0.50 D and 13 (57%) eyes were within 1.00 D of the
scribed22-25 by four highly experienced surgeons.             attempted correction. In the control group, the mean
   Of the anisometropic patients, 13 were treated using       refraction was 0.30 0.60 D (range: 3.00 to 1.25 D).
LASIK (5 patients by VISX 20/20B and 8 patients by            Sixty-nine (73%) eyes were within 0.50 D and 86
VISX Star S2 [VISX Inc, Santa Clara, Calif]). The re-         (91%) eyes were within 1.00 D of the attempted cor-
maining 44 anisometropic patients were treated using          rection. The attempted versus achieved correction of
PRK (21 patients by NIDEK EC-5000 [NIDEK Co Ltd,              the spherical equivalent of 31 eyes and 96 eyes 5 to 7
Gamagori, Japan], 19 patients by VISX 20/20B, and 4           months postoperatively and of 23 eyes and 94 eyes 8 to
patients by VISX Star S2).                                    13 months postoperatively is shown in Figure 1.
   Of the myopic group, 93 eyes were treated using
LASIK (7 eyes by NIDEK EC-5000, 4 eyes by VISX                VISUAL ACUITY OUTCOME
20/20B, 76 eyes by VISX Star S2, and 6 eyes by VISX              The effect of refractive surgery on visual acuity
Star S4). The remainder of the control group (124 eyes)       was followed for 25 14 months in the anisometropic
was treated using PRK (38 eyes by NIDEK EC-5000, 36           group and 26 14 months in the control group. The
eyes by VISX 20/20B, 47 eyes by VISX Star S2, and 3           mean preoperative BSCVA of the operated eyes was
eyes by VISX Star S4).                                        significantly higher for myopic patients compared to
                                                              anisometropic patients (P=.001, independent sam-
CLINICAL EXAMINATION                                          ples t test), suggesting that the anisometropic patients
   Patients were examined preoperatively and at 1, 3, 5       showed some degree of anisometropic visual suppres-
to 7, 8 to 13, and mean 25 months postoperatively. Best       sion. This difference in visual acuity between myopic
spectacle-corrected visual acuity, manifest refraction,       and anisometropic patients was sustained throughout
and corneal slit-lamp microscopy were evaluated.              follow-up although the statistical significance became
                                                              weaker (Table 1).
MATHEMATICAL ANALYSIS OF VISUAL ACUITY DATA                      The preoperative BSCVA in the anisometropic
   Several mathematical models were used to evaluate          group in the eye to be operated on was 1.25 (20/16)
the improvement of BCSVA.26 Estimated variances and           in 5 eyes, 1.0 (20/20) in 44 eyes, and 0.8 (20/25) in 6
R2 (coefficient of determination) were used to evalu-          eyes, with poorer values measured for 2 eyes. Of these
ate fit of the models (Mathematica 4.1.1; Wolfram Re-          8 patients whose preoperative BSCVA was 20/20, 4
search Inc, Champaign, Ill). From the models, Gomp-           were mildly amblyopic, ie, these patients presented
ertz and logistic functions resulted in lowest estimated      with at least two Snellen line interocular differences
variances, but due to Occam’s razor principle27 (ie, the      in BSCVA. None of the patients were amblyopic by the
simplest model consistent with the data), the logistic        definition of BSCVA 20/40. The average preopera-
model was chosen to represent the data.                       tive BSCVA in the eyes included in the analysis was
                                                                0.0143 0.0572 on a logMAR scale.
STATISTICAL ANALYSIS                                             Three months after refractive surgery when 45
   Pre- and postoperative levels of visual acuity in an-      (79%) of 57 eyes were examined, 12 (27%) eyes gained
isometropic and myopic patients were compared with            1 Snellen line, 1 (2%) eye gained 2 lines, none lost 1
the independent samples t test (SPSS version 12.0.1;          Snellen line, and 2 (4%) eyes lost 2 lines. Thirty (67%)
SPSS Inc, Chicago, Ill). Changes in intra-group visual        eyes showed no changes. Five to 7 months after surgery
acuity were tested with a paired samples t test (SPSS         when 31 (54%) of 57 eyes were examined, 8 (26%) eyes
version 12.0.1). A P value .05 was considered statisti-       gained 1 Snellen line, 1 (3%) eye gained 2 lines, and
cally significant.                                             4 (13%) eyes lost 1 Snellen line. Eighteen (58%) eyes
                                                              showed no changes. Eight to 13 months after refractive
                         RESULTS                              surgery when 23 (40%) of 57 eyes were examined, 5
                                                              (22%) eyes gained 1 Snellen line, 1 (4%) eye gained 2
REFRACTIVE OUTCOME                                            lines, 1 (4%) eye gained 3 lines, and 2 (9%) eyes lost
   Preoperative mean spherical equivalent manifest re-        1 Snellen line and none lost 2 lines. Fourteen (61%)
fraction was 7.20 2.40 D (range: 13.375 to 3.25 D)            eyes showed no changes. Twenty-five months after re-
in the anisometropic eyes and 6.40 1.90 D (range:             fractive surgery when 30 (53%) of 57 eyes were exam-
  13.375 to 3.125 D) in the control eyes. In the an-          ined, 12 (40%) eyes gained 1 Snellen line, 1 (3%) eye
isometropic group 8 to 13 months postoperatively, the         gained 2 lines, 1 (3%) eye gained 3 lines, and 2 (7%)

Journal of Refractive Surgery Volume 23 May 2007                                                                  449
Improvement of Visual Acuity After Refractive Surgery/Vuori et al

                                                                                                                    Figure 1. Deviation from attempted spheri-
                                                                                                                    cal equivalent refractive correction A) 5 to
                                                                                                                    7 months and B) 25 months after refractive
                                                                                                                    surgery performed for myopic anisometropia
                                                                                                                    (▲) and myopia (●).



                                                                            TABLE 1
            Statistical Analysis of Best Spectacle-corrected Visual Acuity at Different
                 Postoperative Times in Patients With Anisometropia and Myopia
                            Preoperative               1 Month               3 Months            5 to 7 Months         8 to 13 Months            25 Months
  No. of eyes
      Anisometropic               57                      37                     45                     31                     23                    30
      Myopic                     217                     141                     78                     96                     94                    100
  Mean BSCVA
  logMAR (Snellen)
      Anisometropic        0.0143 0.0572           0.0200 0.0712         0.0003 0.0805          0.0022 0.0757          0.0076 0.0659          0.0296 0.0529
                            (0.98 0.12)             (0.97 0.14)            (1.02 0.16)            (1.02 0.17)            (1.03 0.15)            (1.08 0.13)
      Myopic              0.0136 0.0361           0.0149 0.0672          0.0416 0.0637          0.0479 0.0635          0.0495 0.0692          0.0596 0.0620
                            (1.04 0.09)             (1.05 0.16)            (1.11 0.16)            (1.13 0.17)            (1.13 0.18)            (1.16 0.17)
  P Value*                       .001                    .010                   .004                   .004                   .010                  .012
  BSCVA = best spectacle-corrected visual acuity
  A P value (independent samples t test) .05 implies that BSCVA remains better in myopic patients throughout the follow-up. Note that the significance becomes
  weaker during the follow-up.

                                                   Improvement of Visual Acuity After Refractive Surgery/Vuori et al

                                                                                      Figure 2. Changes in best spectacle-cor-
                                                                                      rected visual acuity following PRK or LASIK
                                                                                      in A) myopic anisometropic patients and B)
                                                                                      myopic patients. Number of eyes shown in
                                                                                      parentheses above the bars.
                                                                     3 months
                                                                     5-7 months
                                                                     8-13 months
                                                                     25 months


                                                                       3 months
                                                                       5-7 months
                                                                       8-13 months
                                                                       25 months


eyes lost 1 Snellen line. Fourteen (47%) eyes showed          2 lines, and 3 (3%) eyes lost 1 Snellen line. Fifty-four
no changes. The above data are illustrated in Figure 2A.      (56%) eyes showed no changes. Eight to 13 months af-
One patient who was considered mildly amblyopic pre-          ter refractive surgery when 94 (43%) eyes were exam-
operatively gained 3 lines at both 8 to 13 months and         ined, 37 (39%) eyes gained 1 Snellen line, 6 (6%) eyes
25 months postoperatively. Of the 3 other mildly am-          gained 2 lines, 2 (2%) eyes lost 1 Snellen line, and 2
blyopic patients, 2 patients gained 2 Snellen lines at        (2%) eyes lost 2 lines. Forty-seven (50%) eyes showed
last follow-up.                                               no changes. Twenty-six months after refractive surgery
   Preoperative BSCVA in the myopic group in the eye          when 100 (46%) eyes were examined, 38 (38%) eyes
to be operated was 1.6 (20/12.5) in 1 eye, 1.25 (20/16)       gained 1 Snellen line, 5 (5%) eyes gained 2 lines, and
in 35 eyes, and 1.0 (20/20) in 181 eyes. Poorer values        4 (4%) eyes lost 1 Snellen line. Fifty-three (53%) eyes
were not measured for any eye. The average preopera-          showed no changes (Fig 2B).
tive BSCVA of the myopic eyes included in the anal-              No statistically significant association was found
yses was 0.0136 0.0361 on a logMAR scale. Three               between preoperative anisometropia, astigmatism, or
months after refractive surgery when 78 (36%) eyes            age of the patients to the final gain or loss in BSCVA
were examined, 27 (35%) eyes gained 1 Snellen line,           (data not shown).
none of the eyes gained 2 lines, 3 (4%) eyes lost 1 Snel-        For myopic eyes, a significant improvement in
len line, and 2 (3%) eyes lost 2 lines. Forty-six (59%)       BSCVA was evident at 3 months postoperatively
eyes showed no changes. Five to 7 months after refrac-        (P=.001, paired samples t test) (Table 2) and the im-
tive surgery when 96 (44%) eyes were examined, 35             provement reached maximum at 3 months (Fig 3);
(36%) eyes gained 1 Snellen line, 4 (4%) eyes gained          thereafter only a modest increase was evident. The im-

Journal of Refractive Surgery Volume 23 May 2007                                                                            451
Improvement of Visual Acuity After Refractive Surgery/Vuori et al

                                                                           TABLE 2
       Analysis of Best Spectacle-corrected Visual Acuity Following Refractive Surgery
                         in Patients With Anisometropia and Myopia
                                                                                         Preoperative to
                                              1 month                3 months             5 to 7 months         8 to 13 months            25 months
      No. of eyes examined (%)
       Anisometropic                           37 (65)                45 (79)                31 (54)                 23 (43)                30 (53)
       Myopic                                 141 (65)                78 (36)                96 (44)                 94 (43)                100 (46)
      Mean difference in BSCVA
       Anisometropic                           0.0118                 0.0187                  0.0243                 0.0360                 0.0397
       Myopic                                  0.0016                 0.0233                  0.0346                 0.0389                 0.0479
      P value*
       Anisometropic                            .430                    .094                   .050                   .041                    .007
       Myopic                                   .772                    .001                   .001                   .001                    .001
      BSCVA = best spectacle-corrected visual acuity.
      *Paired samples t test.
      Note. Among the anisometropic patients, improvement in BSCVA became evident only at 8 months postoperatively whereas in myopic patients, BSCVA
      improved 3 months after surgery and remains significantly higher compared to preoperative BSCVA.

                                                                                   provement continued beyond 24 months (see Fig 3).
                                                                                   The improvement in BSCVA became statistically sig-
                                                                                   nificant at 8 to 13 months (P=.041, paired samples t
                                                                                   test) (Table 2). A mathematic model was sought to de-
                                                                                   scribe the above data sets. Several mathematical func-
                                                                                   tions were fitted to the data sets as described above. A
                                                                                   logistic model (y=a/(1 e(b-cx))) yielded the best fit. The
                                                                                   model describes both data sets with estimated varianc-
                                                                                   es being as low as 0.0003 and 0.0002 for anisometropic
                                                                                   and myopic eyes, respectively. By extrapolating the
                                                                                   results, the asymptote of the logistic function is 1.15
                                                                                   (0.061 on a logMAR scale) for myopic eyes, whereas
                                                                                   for anisometropic eyes the asymptote is 1.09 (0.037 on
                                                                                   a logMAR scale).

                                                                                      Several studies regarding changes in BSCVA after
                                                                                   refractive surgery have been performed. Some of these
                                                                                   report improvement of BSCVA,7-9 whereas others have
Figure 3. Best spectacle-corrected visual acuity (BSCVA) values measured           reported reduction11,12 or no change4-6,13-15,20 following
as a function of time. The solid lines represents a logistic mathematical          surgery. The improvement may be attributed to retinal
model (y=a/(1 e(b-cx))) fitted to the data sets. The upper solid line repre-       image enlargement, whereas reduction may be due to
sents myopic patients and the lower anisometropic patients. The mean               decentration of the corneal ablation or increased high-
BSCVA standard deviation is represented preoperatively and 1, 3, 5 to
                                                                                   er order aberrations.28-31 In a large cohort, these effects
7, 8 to 13, and 25 months following surgery. The estimated variances are
0.0003 and 0.0002 for anisometropic and myopic patients, respectively.             should be similar in myopic and myopic anisometropic
                                                                                   patients following refractive surgery.
provement in BSCVA for anisometropic eyes behaved                                     We studied the improvement of visual acuity in
differently than that for myopic eyes: the slope of the                            myopic and anisometropic patients in a prospective
curve was much less steep and it appeared that the im-                             series. We hypothesized, based on our previous re-

                                                   Improvement of Visual Acuity After Refractive Surgery/Vuori et al

sults,21 that anisometropic patients had mild sensory         EC-5000, 2 (20%) with the VISX 20/20B, and 4 (40%)
deprivation due to aniseikonia and that refractive sur-       with the VISX S2. In the 8 myopic patients, 2 (25%)
gery (either PRK or LASIK) could reverse this. Our            were treated with the NIDEK EC-5000, 1 (13%) with
results demonstrate that for myopic patients the im-          the VISX 20/20B, and 5 (63%) with the VISX S2. Ac-
provement in BSCVA could result, at least partly, from        cordingly, the groups seem to be comparable and may
image enlargement.16 This is supported by the finding          not explain the observed differences in the attempted
that visual acuity increases rapidly following surgery by     versus achieved scattergrams. At present, we have no
approximately 1/2 Snellen line. According to Applegate        explanation to this observed difference.
and Howland,16 correcting 6.30 D myopia from spec-               The improvement of BSCVA following surgery
tacle to corneal plane yields improvement in BSCVA            in anisometropic patients is slow, as is evident from
by approximately two letters equaling 1/3 Snellen             Figure 3. Corneal wound healing following refractive
line. For anisometropic patients of this study, image         surgery is initially rapid, but reorganization of the cor-
enlargement should improve visual acuity by 1/3 Snel-         neal stroma continues for at least 3 months.34 In ac-
len line. These results are in good agreement with the        cordance with this, the BSCVA in the myopic group
measured improvements of BSCVA. Importantly, the              improved up to 3 to 6 months (see Fig 3). However,
two groups showed significant differences in the time          for anisometropic patients, the improvement seems to
needed to gain the maximum improvement in BSCVA,              continue much longer (see Fig 3). The corneal restora-
suggesting that plastic changes for anisometropic pa-         tion should be similar in both groups and thus differ-
tients in the visual cortex are taking place. For myopic      ences in wound healing are not likely to explain these
patients, approximate maximum BSCVA is reached in             results. Interestingly, it has been shown that the visual
3 months in agreement with previous results showing           acuity improves slowly (up to 10 years) after treatment
that the refraction stabilizes 3 to 6 months postopera-       of macula-off retinal detachments.35 Such slow im-
tively.32,33 However, for anisometropic patients, BSCVA       provement suggests that plastic changes in the visual
continues to improve for over 24 months. Finally, it          cortex are involved and may also provide explanation
is feasible to suggest that the improvement in BSCVA          for the observed slow improvement in BSCVA for an-
in both groups is not derived solely from enlarged or         isometropic patients after corneal refractive surgery.
improved optical images, as optical aberrations and ir-          No treatment is currently available for adults with
regular astigmatism will increase after refractive sur-       anisometropic or strabismic amblyopia. Juvenile am-
gery.28,31 An intriguing finding in this study was that        blyopia is primarily treated with eye patching forcing
three of four mildly amblyopic patients gained at least       the “lazy eye” to function. This treatment is not feasible
two Snellen lines, suggesting that mild adulthood vi-         in adults, and the efficiency of patching is negatively
sual impairment can be reversed by refractive surgery.        correlated with age at treatment.36 The visual deficien-
   This study suffers from a few drawbacks. First, the        cies are thought to be irreversible after the first decade
number of drop-outs was relatively large and second, a        of life, by which time the developmental maturation
variety of laser instruments can be considered as a con-      window has been terminated. The loss of vision is
founding factor. Third, we could not perform subgroup         thought to result from abnormal operation of the neu-
(ie, PRK vs LASIK and comparison between different            ronal network within the primary visual cortex.37 Rep-
laser instruments) analysis due to small sample size in       etition of certain visual tasks has provided evidence
some subgroups. Finally, some possible confounding            that perceptual learning can improve the adult visual
factors such as suppression and the effect of perform-        system with significant improvement also in BSCVA.38
ing surgery on the dominant versus non-dominant eye           Our results suggest that similar plasticity changes in
were not tested.                                              the visual cortex may take place after refractive sur-
   A curious finding in this study can be seen in Fig-         gery and after restoring the visual input balance arising
ure 1: significantly more anisometropic patients did           from both eyes. These results may implicate that spe-
not achieve the intended correction than myopic pa-           cific neuronal network connections can be activated by
tients. Those patients whose achieved correction was          correcting optical abnormalities at the corneal plane
     1.00 D of attempted correction were analyzed sepa-       improving visual performance. Levi and Polat39 sug-
rately. For instance, 8 to 13 months postoperatively, 10      gested that in amblyopic patients undergoing repeti-
(43%) anisometropic patients and 8 (8.5%) myopic pa-          tive visual tasks, at least two specific plastic processes
tients were found. Of these 10 anisometropic patients,        were taking place: 1) a slow process involving neural
3 (30%) had LASIK whereas 4 (50%) of 8 myopic pa-             modifications occurring between training sessions, and
tients were treated by this method. Of the 10 anisome-        2) a rapid process taking place within a single training
tropic patients, 4 (40%) were treated with the NIDEK          session. We suggest that corneal refractive surgery per-

Journal of Refractive Surgery Volume 23 May 2007                                                                    453
Improvement of Visual Acuity After Refractive Surgery/Vuori et al

                                                                             atomileusis and photorefractive keratectomy for 2.50 to 8.00
formed for anisometropia may activate a type of slow                         diopters of myopia. Ophthalmology. 1999;106:447-457.
neural plasticity improving visual performance. This
                                                                         15. Malecaze FJ, Hulin H, Bierer P, Fournie P, Grandjean H, Thal-
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