| Abstract |
It has been observed that the human eye can adapt when a change of refraction occurs. Many studies report visual improvement in myopes after sustained periods without correction [Pesudovs & Brennan (1993), Mon-Williams (1998), Rosenfield & Sini George (2004)]. This thesis examines the effect of temporal and sustained blur, induced by positive defocusing lenses, on visual acuity (V.A) in emmetropic and myopic observers using two different logMAR charts: the modified ETDRS chart developed in the University of Crete based on letter recognition and the Landolt C chart, based on gap detection. The aim was: a) to consider the effect of spherical defocus on two different charts and to determine whether myopes and emmetropes were differently affected by blur b) to evaluate possible differences in blur adaptation between emmetropes and myopes and c) to examine possible differences between the dominant and the non-dominant eye of the participants.
The subjects included in the study consisted of 13 emmetropic and 13 myopic healthy volunteers (52 eyes). Pupils were not dilated. The age group of the subjects ranged between 21 and 38 years (mean age 26,8 ± 3,5). The myopic subjects had a spherical equivalent refractive error ranging between –0,75 and –5,00 D and the emmetropic group ranged between -0,63 and +0,50 D. All participants had 0,00 logMAR or better visual acuity at least in one eye and they were screened to exclude astigmatism > 1,50 DC or any ocular disease. Right and Left eyes were tested separately with both charts. Using an autorefractor (Huvitz HRK 7000), the refraction was measured objectively at the beginning and at the end of the experiment which included three sessions. 1st: V.A assessment of all subjects wearing their best subjective refraction (determined by subjective measurement). 2nd: Myopes and emmetropes were examined under the condition of myopic defocus imposed by +2.00 D lenses. V.A was measured using the letter and the Landolt C logMAR charts. 3rd: The participants underwent a 60 min adaptation period through the +2.00 D lens. Subjects were instructed to view a film of their interest, displayed on a screen from approximately 6 m. Near tasks were not allowed during the procedure.
The influence of refractive blur produced a significant reduction in the V.A of both eyes of all subjects. The reduction for the letter chart was on average 0,74 logMAR while for Landolt C chart was 0,81 logMAR. Separating emmetropes and myopes, a decrease of
0,81 logMAR and 0,66, respectively, was found in letter recognition. The reduction for the Landolt C chart was 0,86 for emmetropes and 0,77 logMAR for myopes. Focalizing on myopes and trying to correlate the reduction in V.A with their spherical equivalent there was a trend for: a) higher reduction in lower myopes and b) higher reduction in Landolt C chart.
A significant improvement was found in V.A of both eyes when measured after a 60 minute adapting period. Mean improvement for all subjects was 0,08 logMAR for the modified letter chart and 0,11 logMAR for Landolt C chart. Emmetropes represented a mean improvement of 0,09 logMAR while myopes 0,10 logMAR. For the myopic subjects it seemed than there is a linear tense between V.A and refractive error indicating a) larger increase for higher refractive errors and b) larger increase for the Landolt C chart. Objective measurements before and after the adaptation showed no significant change in refractive error. Finally, there was not statistically difference between the dominant and the non-dominanat eye.
In conclusion, the study demonstrated that emmetropes are more affected, compared to myopes, by positive spherical defocus. In low vision conditions, ETDRS letter chart and Landolt C chart were not comparable since gap detection was more difficult than letter identification. Although prolonged exposure to blur for 60 minutes showed improvement in V.A of both refractive groups, myopes, and especially those with high refractive errors, exhibited better ability to adapt to blur than emmetropes. The adaptation was more pronounced for the Landolt C chart.
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Μελέτη της προσαρμογής ατη θόλωση σε μύωπες και εμμέτρωπες : σύγκριση δύο πινάκων μέτρησης οπτικής οξύτητας
