| Abstract |
Evaluation of visual acuity is the most common and simple method of assess the spatial frequency resolution. This is the purpose of using specific charts. The main reason for visual impairment is sphero- cylindrical refractive error.
This study is consisted of two parts. The purpose of the first part is to compare the visual acuity values that come up from the modified UoC charts (contain a set of letter recognizable by all European citizens) (Plainis et al, 2007) and the standard LogMAR/ETDRS charts in a population using the Cyrillic alphabet and to examine the reliability of the modified UoC charts. The purpose of the second part of this study is the evaluation of visual acuity and prevalence of refractive error in Greek and Bulgarian primary and secondary school children.
310 measurements from school students in a Bulgarian city (Stara Zagora), with a mean of age 12.5 (SD: 1.4) years used in order to compare the two charts. For every eye we have two measurements, one using the modified UoC chart and another using the standard LogMAR/ETDRS chart. To assess the test retest reliability of the UoC chart it was used the measurements from 213 school students from a school in Heraklio Crete, with a mean of age 11.8 (SD: 1.5) years. For every eye we have two different measurements using the modified UoC chart. To evaluate if there is a difference between the two charts standard method comparison measures (Bland-Altman analysis) is used. The coefficient ICC (intra class correlation coefficient) was used to assess the test- retest reliability of the UoC chart. Also Bland-Altman analysis was used in order to assess the difference between the measurements.
Analysis of the results showed that the modified UoC charts give higher visual acuity scores (the difference in mean visual acuity is about 0,02 log MAR, 1 letter difference) compared to the standard Log MAR/ETDRS chart. Bland-Altman
analysis revealed that 96.1% of the values for the right eye and 96.8% for the left eye lie between the limits of agreement.
The ICC is equal 0.984 for the right eye (CI: between 0,968 και 0,975) and 0.980 for the left eye (CI: between 0,950 και 0,970). Those values are very close to 1, so the modified UoC chart has very good test retest reliability. Moreover Bland-Altman analysis revealed that over 95% for both eyes lie between the limits of agreement.
All the above advocate that the visual acuity scores with the modified and the standard charts in a population using the Cyrillic alphabet are not statistical different and that the scores from the modified charts are valid. So modified UoC charts can be used for evaluating visual acuity in European citizens, because they use letters recognizable by all European citizens using the Latin, Greek, Cyrillic alphabets.
For the second part of the study, 932 children from schools in Greece (Heraklio) and Bulgaria (Stara Zagora) with a mean of age 12.7 (SD: 1.5) years (range 10-15 years) participate in the examinations. 616 were Greek (mean of age 12.7±1.5) and 310 Bulgarian (mean of age12.5±1.4). 464 were in primary (mean of age 11.40.8) and 462 were in secondary school (mean of age13.9±±0.8). Visual acuity was assessed with habitual refractive correction, using the modified UoC chart. Refractive error was measured using the auto refractometer PotecRK, Essilor.
Analysis of the results showed that there are a big enough percentage of school students who goes to school with visual impairment. 31.7% of Greek students and 21.9% of Bulgarian have decimal visual acuity <0.80, for at least one of the eyes (minimal impairment, WHO) (statistical significant difference, p<0.001).
From those who have visual acuity <0.80 46.7% of Greek students and 76.5% of Bulgarian doesn’t use refractive correction (statistical significant difference, p<0.001).
If criterion of visual impairment is decimal visual acuity <0.50, for at least one of the eyes (mild impairment, WHO) then 11.4% of Greek students and 4.8% of Bulgarian has decimal visual acuity <0.50, (statistical significant difference, p<0.001). From those who have visual acuity <0.50 44.3% of Greek students and 66.7% of Bulgarian doesn’t use refractive correction (no statistical significant difference, p>0.05).
Finally 37.9% of Greek students and 15.5% of Bulgarian showed myopic refractive error (>0.75D), while the respective percentages of cylindrical refractive error are 16.7% and 9.7%. Statistical test χ2 showed that the myopic prevalence depends on the country p<0.001 as also for cylindrical refractive error
Finally we can tell that a big enough percentage of school students go to school with visual impairment. Those percentages are high for Greece and Bulgaria. What is remarkable is the percentage of those that have visual impairment and they don’t use glasses.
Prevalence of refractive error in Greece and in students between 13-15 years is higher. The cause of increased values of myopia prevalence in Greek compared to Bulgarian students may arise in social economic. Also the increased values of myopia prevalence in secondary school students are a result of the different school needs.
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