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(Investigative Ophthalmology and Visual Science. 2006;47:5412-5422.)
© 2006 by The Association for Research in Vision and Ophthalmology, Inc.
doi:10.1167/iovs.05-1564
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Contrast Acuity in Cataracts of Different Morphology and Association to Self-Reported Visual Function

Eva Stifter, Stefan Sacu, Arnulf Thaler, and Herbert Weghaupt

From the Department of Ophthalmology, Medical University of Vienna, Vienna, Austria.


   Abstract
Top
 Abstract
Patients and Methods
 Results
 Discussion
 References
 
PURPOSE. To evaluate the relationship between contrast acuity at declining contrast levels and the type and density of lens opacity in cataract.

METHODS. Contrast acuity at declining contrast levels was determined with the Holladay Contrast Acuity Test, in relation to the type and density of age-related cataract in 180 patients with bilateral cataract and 20 control subjects with normal macular function. Cataracts were graded according to the Lens Opacities Classification System (LOCS) III of nuclear color (NC), nuclear opalescence (NO), cortical (C), and posterior subcapsular (P) cataract. Best-corrected visual acuity and near contrast acuity were determined in randomized order monocularly in both eyes. Visual difficulties in everyday life were evaluated, using the VF-14 questionnaire and the Cataract Symptom Score.

RESULTS. The contrast-dependent effect of cataract on contrast acuity was statistically significant (P < 0.001; two-way ANOVA). In the comparison of early, intermediate, and advanced nuclear, nuclear-cortical, and posterior subcapsular cataracts (PSCs), significantly reduced contrast acuity scores were found for the PSC groups (P < 0.001). Comparison of nuclear and nuclear-cortical cataracts showed the contrast acuity scores to be comparable at all contrast levels (P > 0.05). High correlation coefficients were found between the LOCS III P score and the contrast acuity measurements (r = 0.77–0.84; P < 0.001). In contrast, the correlation coefficients of the NO, NC, and C scores were considerably lower (r = 0.45–0.66; P < 0.001). High correlation coefficients were also found between the contrast acuity measurements and self-reported functional vision.

CONCLUSIONS. The statistically significant, contrast-dependent effect of cataract on contrast acuity supports the clinical relevance of recording visual acuity at low contrast levels in patients with age-related cataract. Particularly, the severity of PSC has a strong influence on the impairment of contrast acuity. Contrast acuity corresponded closely to the self-reported visual difficulties in everyday life.

Contrast acuity measured at declining contrast levels has been shown in recent clinical studies to represent a more sensitive method of testing visual function than high-contrast visual acuity (VA) measurements.1 2 3 4 5 6 7 At reduced contrast levels, many sensory disorders may cause relevant visual impairment possibly missed in high-contrast tests.1 8 9 10 11 Therefore, vision under reduced light or contrast conditions should be considered for estimating real-life visual performance.1 12 13

As a standardized procedure, high-contrast VA measurements are based on the ability to discriminate standardized optotypes of logarithmically decreasing print size.14 15 16 Even though VA is the main outcome variable of many clinical studies, high-contrast black optotypes on white background cannot represent real-world vision. This may be one of the reasons why self-reported functional vision reflects patients’ satisfaction with visual performance better than high-contrast VA alone.17 18 19 With good clinical validity and reliability, several visual performance tests and questionnaires (e.g., the VF-14, the Activities of Daily Vision Scale or the Vision-Related Quality of Life [VR-QOL] questionnaires), showed significantly higher correlations with functional vision than high-contrast distance VA.17 18 19 20

For example, in the widely used VF-14 questionnaire, six questions are dealing with contrast-dependent activities of daily life.12 Particularly, when considering day and night driving, contrast sensitivity is often used in clinical studies for evaluating visual function, because impairment in real-world visual performance is better predicted by a contrast-sensitivity test than by high-contrast VA measurements.21 22 In addition, a contrast acuity test may provide more information about the quality of vision, for example after cataract surgery, even with respect to different intraocular lenses.7 23 24 Consequently, a standardized, highly reproducible, and reliable contrast acuity test may also be used to evaluate clinically relevant differences caused by different morphologic types of lens opacities.

Because cataract morphologies directly influence the optical quality of the retinal image, clinically relevant differences between nuclear, nuclear-cortical, and posterior subcapsular cataracts (PSCs) have been observed for the functional vision VF-14 score and reading performance in previous studies: both were significantly reduced in patients with PSCs.25 26 27

In the present study, contrast acuity was tested at declining contrast levels by using the standardized Holladay Contrast Acuity Test to evaluate whether there are contrast-dependent effects of cataractous lens opacities on contrast vision. The relationship between the impairment in contrast acuity and cataract density was evaluated for all contrast levels and for all four cataract grading scores: nuclear color (NC), nuclear opalescence (NO), cortical (C), and posterior subcapsular (P) lens opacity.


   Patients and Methods
Top
 Abstract
 Patients and Methods
Results
 Discussion
 References
 
The present study was conducted at the Department of Ophthalmology, Medical University of Vienna, Austria, according to the tenets of the Declaration of Helsinki.

One-hundred-eighty patients with bilateral cataract and 20 normal-sighted control subjects (mean age ± SD: 66.9 ± 9.2 years; 400 eyes) were included in the study. Exclusion criteria were a history of ocular disease, intraocular surgery, laser treatment, glaucoma, diabetic retinopathy, amblyopia, and age-related macular degeneration. In the control group, only participants without ocular disease and full VA (logMAR 0.0 or better) were included.

Cataract Classification and Grading
Cataracts were categorized and graded at the slit lamp and classified as nuclear opalescence (NO: 0.1–6.9), nuclear color (NC: 0.1–6.9), cortical (C: 0.1–5.9), and posterior subcapsular (P: 0.1–5.9) cataract, using the Lens Opacities Classification System (LOCS) III.28 The grading was performed by the same experienced examiner. The cataract grading scores of both eyes were carefully checked as to whether they were within the boundary values of the different cataract groups (Table 1) . Patients with different cataract morphologies in the two eyes were excluded from the study.


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  		TABLE 1. The Boundary Values of the Cataract Groups and the Control Group, According to the Lens Opacities Classification System (LOCS) III

 
As an exclusion criterion for the control group, the presence of cataract was defined as greater than grade 1.5 nuclear opalescence or color or greater than grade 1 cortical or subcapsular opacity (Table 1) .

For each cataract patient, the eyes were classified as the patient’s better or worse eye according to the best-corrected monocular distance VA. All statistical analyses were performed for the better and worse eyes separately.

Distance VA
After a full objective and subjective refraction, the best-corrected distance VA was determined monocularly using ETDRS charts (Early Treatment Diabetic Retinopathy Study chart; Precision Vision; Bloomington, IL) in logMAR (logarithm of the minimal angle of resolution) at a test distance of 4 m.15 All tests were performed at a constant luminance of 80 to 90 cd/m2.

Chart 1 and 2 were used for testing the right and left eye, respectively, and chart R was used for refraction. The features of the charts were five high-contrast Sloan letters in each of the 14 lines—lines of equal difficulty and a geometric progression of letter size (and thus, an arithmetic progression of the logarithm of minimum angle of resolution) from line to line. The patients were prevented from seeing charts 1 and 2 until refraction was completed and acuity testing began. The patients were told that the charts have letters only, that they were allowed to guess, and that they should read slowly to achieve the best identification of each letter. The testing did not proceed until the patient had given a definite response and was stopped when the patients made three or more errors on one line. Each correctly read letter was counted for assessing best-corrected VA. The examiner was blinded with respect to the contrast testing and the cataract grading.

Contrast Acuity Testing
Contrast acuity was assessed in a standardized procedure using the Holladay Contrast Acuity Test at five different contrast levels ranging from 100% to 6.25% contrast.1 All tests were performed monocularly with the patients’ optimal correction for near vision at a testing distance of 40 cm, which was checked for every chart before and continuously during the test with a measuring tape. Contrast acuity was determined for each contrast level, starting from 100% contrast, followed by the 50%, 25%, 12.5%, and 6.25% contrast levels. Each contrast chart consisted of 16 lines of logarithmically decreasing print size, with five letters of equal legibility in each line. Two different but equivalent sets of contrast charts were used for testing the control subjects’ right and left eyes, respectively the cataract patients’ better and worse eyes, in randomized order. The total number of correctly read letters was counted for each chart separately. The final contrast acuity score for each contrast level was recorded in logMAR to provide comparable units for visual and contrast acuity.1 The maximum score on each chart was logMAR –0.2 representing 80 correctly read letters (16 lines with five letters). The tests were stopped when the patients made three or more errors on one line. All tests were performed at a standard background luminance of 80 to 90 cd/m2.1 The examiner was blinded to the cataract type and density, as well as the patients’ VA and functional impairments.

The 100% contrast acuity scores were then used to equate the state of progression for each cataract type, discriminating early (logMAR –0.2–0.1), intermediate (logMAR 0.12–0.4), and advanced progression (logMAR ≥ 0.42).

Functional Vision
The subjectively perceived visual impairments in everyday life were evaluated by using a German version of the VF-14 questionnaire to determine the ability to perform specific vision-dependent activities.12 29 30 The VF-14 scores were calculated, resulting in a possible final score ranging between 0 (unable to do any of the applicable activities because of vision) and 100 (able to do all applicable activities without difficulties). In addition, the patients were asked to quantify their subjectively perceived impairments in distance and near vision in everyday life, ranging from 0 (best) to 10 (worst visual distortion possible), for both their better and worse eyes separately.

Cataract Symptom Score
All patients were asked to quantify in each eye the impact of the most common cataract symptoms, according to the Cataract Symptom Score: (1) double or distorted vision; (2) seeing glare, halo, or rings around light; (3) blurry vision; (4) colors looking different than they used to, disturbing the patient; (5) worsening of vision within the past month; and (6) disturbing brightness.12 31 For each of the symptoms the patient was asked to grade severity from 0 (the patient does not have the symptom or is not bothered by it) to 3 (very bothered by the symptom), resulting in a possible score ranging from 0 (not bothered by any of the symptoms) to 18 (very bothered by all the symptoms).

Statistical Analysis
All statistical analyses were performed for the better and worse eyes separately. Descriptive and inferential data analyses were performed (SPSS, ver. 12.0 for Windows, Chicago, IL). To search for intergroup differences in distance VA, the VF-14 score, the Cataract Symptom Score, and the subjective visual impairment scores, analyses of variance (ANOVA) were performed using group as the fixed factor. To analyze the contrast-dependent effect of cataract on contrast acuity, a two-way ANOVA was performed, using "group" as the fixed between-subjects factor and the contrast levels as the within-subject factor. The contrast acuity scores for the better and worse eyes were the dependent variables. Whenever significant differences were observed in using ANOVA, two-sample t-tests were subsequently performed among all pairs of groups to search for significant differences between the groups. For nonparametric data, the Mann-Whitney test was applied. Considering the interrelationships between the different types of opacity, partial correlation coefficients and the corresponding probabilities were calculated for each of the lens-grading scores (NC, NO, C, and P) and the contrast acuity scores at the 100%, 50%, 25%, 12.5%, and 6.25% contrast levels. The probabilities were all corrected for multiple testing, applying the Bonferroni-Holm step-down test ({alpha} = 0.05).32 33


   Results
Top
 Abstract
 Patients and Methods
 Results
Discussion
 References
 
The patients’ mean age and demographic characteristics are shown in Table 2 . Age was comparable in the early nuclear, nuclear-cortical, PSC groups, and the control group, as well as in the intermediate- and advanced-cataract groups (Table 2 ; P > 0.05).


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  		TABLE 2. Sociodemographic Characteristics and Best-Corrected Distance VA of the Study Groups

 
No significant differences were found in the demographic characteristics sex, years of education, and civil status between the cataract groups (Table 2 ; P > 0.05).

Contrast-Dependent Influence of Cataract on Contrast Acuity
The contrast-dependent effect of cataract on contrast acuity was statistically significant for both the better (P < 0.001, two-way ANOVA; F-ratio, 137; df = 54) and worse eyes (P < 0.001, two-way ANOVA; F-ratio, 104; df = 54).

In the subsequent analyses searching for statistically significant differences between the pairs of cataract groups, contrast acuity scores of the early nuclear and early nuclear-cortical cataract groups were comparable to those of the normal-sighted control group at all contrast levels (Figs. 1a 1b ; Table 3 ; P > 0.05). In contrast, patients with early PSC had significantly reduced contrast acuity scores at decreasing contrast levels (Figs. 1a 1b ; Table 3 ), although the best corrected distance VA and the 100% contrast acuity score were both statistically comparable to the nuclear and nuclear-cortical cataract groups (Tables 2 3 ; P > 0.05).


Figure 1
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  		FIGURE 1. Contrast acuity scores at declining contrast levels in patients with early cataract: in the (a) better and (b) worse eyes; intermediate cataracts: in the (c) better and (d) worse eyes; and advanced cataracts: in the (e) better and (f) worse eyes.

 

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  		TABLE 3. Contrast Acuity Scores at Declining Contrast Levels in Different Types of Cataract

 
A comparison of the intermediate-cataract groups showed contrast acuity to be statistically comparable in the nuclear and the nuclear-cortical cataracts, whereas intermediate PSCs significantly reduced contrast acuity at decreasing contrast levels (Figs. 1c 1d ; Table 3 ).

The advanced-cataract groups showed no significant differences between advanced nuclear and nuclear-cortical cataracts, for any of the tested contrast levels (Table 3 ; Figs. 1e 1f ). However, the 50%, 25%, 12.5%, and 6.25% contrast acuity scores were all significantly lower in the patients with advanced pure PSC and those with mixed nuclear-cortical-PSC (P < 0.001; Table 3 ; Figs. 1e 1f ). The latter two groups were statistically comparable in all visual parameters (Table 2 and 3 ; P > 0.05), indicating that it was the PSC that caused the functionally relevant impairment.

High-Contrast Distance VA
No significant intergroup differences were observed in high-contrast distance VA between the three early-cataract groups, in both the better and worse eyes, as well as between the intermediate- and advanced-cataract groups (Table 2 ; P > 0.05). Comparison of high-contrast distance VA measurements with the contrast acuity at the 100% contrast level showed no significant differences, neither for the better (P = 0.51) nor for the worse (P = 0.61) eyes.

Statistically significant correlation coefficients were observed between all contrast acuity scores and the high-contrast VA (r = 0.38–0.49; P < 0.001).

Functional Vision
In early cataracts, no significant differences were found between pure nuclear and mixed nuclear-cortical cataract groups for the functional vision VF-14 score. In contrast, the VF-14 score was significantly impaired in the early PSC group (P < 0.001). Comparing the correlation between the VF-14 score and the 100% contrast acuity, with respect to the 6.25% contrast acuity, we found a close relationship between self-reported functional vision and contrast acuity in patients with PSC, for both the better and worse eyes (r = –0.8–0.88; P < 0.001; Fig. 2 ).


Figure 2
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  		FIGURE 2. Self-reported functional vision in pure nuclear cataracts (n = 54) in relation to (a) 100% contrast acuity (better eyes: r = –0.56; P < 0.001; worse eyes: r = –0.52; P < 0.001) and (b) 6.25% contrast acuity (better eyes: r = –0.68; P < 0.001; worse eyes: r = –0.64; P < 0.001). Functional vision in nuclear-cortical cataracts (n = 54) in relation to (c) 100% contrast acuity (better eyes: r = –0.46; P < 0.001; worse eyes: –0.52; P < 0.001) and (d) 6.25% contrast acuity (better eyes: r = –0.59; P < 0.001; worse eyes: r = –0.57; P < 0.001). Self-reported functional vision in pure PSCs (n = 55) in relation to (e) 100% contrast acuity (better eyes: r = –0.8; P < 0.001; worse eyes: r = –0.82; P < 0.001) and (f) 6.25% contrast acuity (better eyes: r = –0.88; P < 0.001; worse eyes: r = –0.87; P < 0.001). Similar results were found for the Cataract Symptoms Score.

 
Comparable results were also found for the Cataract Symptoms Scores, which were significantly higher in patients with PSC than in the other cataract groups (P < 0.001). This finding was even statistically significant in the early-cataract groups with 100% contrast acuities ≤0.1, indicating that the patients felt significantly more disturbed by the typical cataract symptoms in case of small posterior subcapsular lens opacities.

A comparison of the intermediate- and advanced-cataract groups showed significant impairments also in the groups with pure or mixed PSC, indicated by both the VF-14 score (P < 0.001) and the Cataract Symptoms Scores (P < 0.001).

Regarding subjectively perceived overall visual impairment, which was separately rated for distance and near vision, higher impairments were observed in the PSC groups (P < 0.001). Comparison of the scores for near and distance vision showed that self-reported visual impairment was significantly higher in near vision, even in the early-PSC groups (P < 0.001).

Partial Correlation Coefficients between Cataract Grading and Contrast Acuity Scores at Declining Contrast Levels
In all cataract types, strong correlations were found between the 100% and 6.25% contrast acuity scores, in both the better and worse eyes (Fig. 3) .


Figure 3
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  		FIGURE 3. Relationship between 100% contrast acuity and 6.25% contrast acuity in (a) pure nuclear cataracts (n = 54), (b) nuclear-cortical cataracts (n = 54), and (c) PSCs (n = 55).

 
High partial correlation coefficients were observed between the contrast acuity scores at the decreasing contrast levels and the LOCS III P score, ranging from r = 0.8 at the 100% contrast level to r = 0.83 at the 6.25% contrast level in the patients’ better eyes; comparable results were found for the patients’ worse eyes (Figs. 4c 4d ; P < 0.001). In contrast, considerably lower correlation coefficients were found for the NO and NC scores, with r ranging from 0.49 to 0.66 (Figs. 4a 4b ; P < 0.001). For the C score, the partial correlation coefficients ranged from 0.45 to 0.52 (P < 0.001; data not shown).


Figure 4
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  		FIGURE 4. Relationship between the contrast acuity scores and the LOCS III NO score at (a) the 100% contrast level (n = 200; better eyes: r = 0.65; P < 0.001; worse eyes: r = 0.63; P < 0.001) and (b) the 6.25% contrast level (n = 200; better eyes: r = 0.66; P < 0.001; worse eyes: r = 0.66; P < 0.001). Similar results were found for the NC score. The relationship between the contrast acuity scores and the LOCS III P score at (c) the 100% contrast level (n = 200; better eyes: r = 0.8; P < 0.001; worse eyes: r = 0.77; P < 0.001) and (d) the 6.25% contrast level (n = 200; better eyes: r = 0.83; P < 0.001; worse eyes: r = 0.84; P < 0.001).

 

   Discussion
Top
 Abstract
 Patients and Methods
 Results
 Discussion
References
 
The contrast-dependent effect of cataract on contrast acuity was found to be statistically significant, supporting the clinical relevance of recording VA at low-contrast levels in patients with age-related cataract. The interaction of group by contrast level provided the relevant information about the contrast-dependent effect of cataract on contrast acuity. Particularly, the severity of PSC showed a strong influence on the impairment of contrast acuity. Even early posterior subcapsular lens opacities caused statistically significant impairment in the contrast acuity measurements at reduced contrast levels (P < 0.001; two-way ANOVA; Table 3 ). This statistically significant reduction of letter contrast sensitivity can be considered to be functionally relevant for the patients with cataract, because the self-reported VF-14 score and the subjective Cataract Symptom Score were significantly impaired in the early-, intermediate-, and advanced-PSC groups, when compared with nuclear and nuclear-cortical cataract groups of similar VA. Consequently, contrast acuity tests may provide clinically relevant information about functional visual performance.

This is in agreement with a recent study showing that late PSC causes the greatest reduction in VA.34 Early-grade PSC and cortical cataract also causes significant reduction in contrast sensitivity at intermediate and high spatial frequencies, but late-grade cataract reduces contrast sensitivity across all spatial frequencies.34 Eyes with early nuclear or cortical cataract cause no loss of contrast sensitivity at the lowest spatial frequency, whereas eyes with PSC show contrast sensitivity loss at low spatial frequencies.35 In nuclear and cortical cataracts with a logMAR VA of < 0.5, no loss of contrast sensitivity occurs at the lowest spatial frequency (1 cyc/deg).35 In PSC, low spatial frequency contrast sensitivity loss has been observed but was unrelated to VA.35 Therefore, it has been concluded that contrast and glare sensitivity measurements are a useful tool for assessing visual function in patients with PSC.

One explanation for this significant influence of posterior subcapsular lens opacities on contrast and glare sensitivity is that cataract is known to increase intraocular light-scattering, reducing retinal image contrast.36 37 38 Analysis of the intraocular light-scattering in patients with age-related cataract showed that mean forward scatter was in the upper range with PSC, when compared with nuclear and cortical cataracts.36 In contrast, the mean backscatter was largest for nuclear cataracts, intermediate for PSC, and almost zero for cortical cataracts.36 Although the ratios between forward scatter and back scatter have been shown to vary considerably among individuals, these differences, depending on cataract morphology, can be considered to influence contrast and glare disabilities.

As approved in previous clinical studies,1 2 6 7 two aspects of vision were united in the Holladay Contrast Acuity Test, both of which are very important for visual functioning in everyday life: small letters corresponding with the visual perception required for reading performance and the gradually reduced contrast levels.1 The relevance of both can be seen in the VF-14 questionnaire, where five questions consider reading under different contrast conditions to be one of the most important visual abilities in daily life.12 25 This may also explain the close relationship between self-perceived visual function and contrast acuity measurements, both of which clearly revealed functional impairment in patients with PSC in the present study.

Small-letter contrast sensitivity has been found to be a more sensitive measure of early cataract than are VA and large-letter contrast sensitivity.39 In early cataracts, Elliott and Situ39 observed in 18 patients with cataract who had predominantly pure nuclear cataract type and a mean VA of –0.01 logMAR that large letter contrast sensitivity was often not reduced when compared with age-matched normal subjects, providing only limited information.39

To evaluate the functional benefits of new cataract surgery techniques and intraocular lens designs, the assessment of functional deficits appears to be of increasing clinical interest.23 24 The patients’ subjectively perceived functional impairment may reveal relevant discrepancies in the pre- and postoperative evaluation. In the present study, the VF-14 score was used to evaluate functional impairment with respect to the ability to perform vision-dependent activities in daily life. The overall subjective visual impairment was assessed for both distance and near vision separately. In evaluating the impact of lens opacities on the most common cataract symptoms, we included the Cataract Symptoms Score. Consequently, we were able to determine that the influence of cataract on contrast vision was functionally relevant for the patients. Besides the significantly impaired contrast acuity measurements, patients with early PSC (LOCS III P score: 1–2.9) experienced functional impairment and optical disturbance in their everyday life, long before distance VA or 100% contrast acuity were reduced. In contrast, functional impairment in early nuclear and nuclear-cortical cataracts was minimal.

The nuclear and nuclear-cortical cataract groups both achieved significantly higher contrast acuity scores (Table 3 ; Fig. 1 ) and the self-reported functional vision scores were all found to be statistically comparable. These results are in agreement with a previous study in which functional vision was evaluated in different types of cataract. All parameters of functional vision including the VF-14 score and the overall amount of trouble and satisfaction with vision were statistically comparable in patients with nuclear and nuclear-cortical cataracts, but there was significant impairment in patients with PSC.25 In addition, reading acuity and particularly reading speed measurements were significantly impaired in patients with PSC.26 27

A reason for the comparably strong effect of PSC on reading and contrast acuity may be that the typical morphology of posterior subcapsular lens opacities obscures the eye’s nodal point and may result in central visual loss. The effect of the PSC gradually increased with the LOCS III P score, resulting in high partial correlation coefficients (Fig. 4) . This effect also increased at declining contrast levels, because of the contrast-dependent effect of cataract on acuity measurements.

Consequently, contrast acuity tests may provide clinically relevant information about the functional visual performance and may be included in the clinical evaluation of cataract patients when we want to evaluate functionally relevant visual impairment in the absence of high-contrast VA loss.


   Footnotes
 
Submitted for publication December 8, 2005; revised April 18 and August 13, 2006; accepted October 9, 2006.

Disclosure: E. Stifter, None; S. Sacu, None; A. Thaler, None; H. Weghaupt, None

The publication costs of this article were defrayed in part by page charge payment. This article must therefore be marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.

Corresponding author: Eva Stifter, Department of Ophthalmology, Medical University of Vienna, Waehringerguertel 18–20, A-1090 Vienna, Austria; eva.stifter{at}meduniwien.ac.at.


   References
Top
 Abstract
 Patients and Methods
 Results
 Discussion
 References
 

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