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Identifier

000373965

Title

Η ποιότητα ειδώλου στη περιφέρεια μετά απο διαθλαστικές επεμβάσεις και ο ρόλος τους στα οπτικά πεδία

Alternative Title

Visual fields after refractive surgery

Author

Δεμέστιχα, Ν. Αντωνία

Thesis advisor

Γκίνης, Χαρίλαος

Reviewer

Κυμιώνης, Γεώργιος

Abstract

Purpose : The aim of this prospective study was to evaluate the sensitivity in the peripheral visual field in patients undergoing photorefractive keratectomy for the correction of myopia. Materials and methods: Thirty two eyes of eighteen patients (13 women and 5 men) aged between 22 and 34 years (mean ±SD age 29,09 ± 3,39 years) were evaluated preoperatively and at the 3 months postoperative interval following PRK for the correction of myopia (mean ±SD spherical equivalent refraction ‐2.75 ± 0.97D) at an optical zone of 6.5mm with the Allegretto 400 Excimer laser. The study was approved by the Institutional Review Board. Evaluation of the optical fields was made using the MEDMOND M700 visual fields analyzer using a full threshold procedure with stimulus size: Size III (26 arcmin) and Background Illumination: 10cd/m2 . Measurements were performed under cycloplegia . For the central degrees of the optical field [1⁰‐22⁰] the refracted error was completely corrected . The marked cycloplegic effect necessiated the use of a+3D auxillary lenses to cover the distance of the stimulus (33cm) Methods ‐ data analysis: The thresholds in 140 individual test locations over 40 degrees were grouped in 7 regions(3⁰,6⁰,10⁰,15⁰,22⁰,30⁰,40⁰) and also into a central[0⁰ ‐22⁰] and a peripheral region (22⁰ ‐40⁰]. The sensitivities for locations within each region were averaged to generate a mean sensitivity(MS). The preoperative and postoperative MS values were calculated, and the difference between them was determined. The 2‐tailed t test was used for statistical analysis and the null hypothesis was that there is no difference between the preoperative and postoperative mean sensitivity. An investigation of the effect of uncorrected refractive errors upon a peripheral visual field examination was made preoperative to determine if this could be a factor affecting the normal perimetric profile derived by automatic threshold perimetry. 12 Results: The detection threshold value exhibited a decrease ranging from 0.5 to 9 DB (mean ±SD threshold decrement 2.64518 ±1.85 DB) for the periphery of the optical field [30 to 40 degrees]. This change was statistically significant (p= 3.97912* ). In particular in peripheral 40⁰ of visual field the threshold MS changed with a depression of 3.86258 ±2.65DB (p=2.51619* ) .No statistically significant difference was observed for the central visual field [0‐22 degrees].The results of this study demonstrate that the effect of defocus on threshold retinal sensitivities were negligible at 30⁰ or more from fixation. Modeling the postoperative eye: It is well documented that in a healthy and intact eye visual performance in the periphery is mainly governed by neuronal rather than optical factors. The purpose of this model was to employ optical calculations of the PSF in the periphery of post PRK eyes and to establish a relationship between optical deterioration and the associated change in sensitivity. We employed a wide angle schematic eye model (Escudero‐ Navarro) where the anterior surface of the cornea was modified according to the ablation profile suggested by the parabolic approximation of the Munnerlyn’s equation with a properly approximated transition zone. The model was implemented and the wavefront aberration (for each field angle) was calculated in ZEMAX. The change upon the fraction of the PSF (calculated in MATLAB) that remained within a circle having diameter equal the corresponding size of the receptive field at each angle was used to determine the associated change in sensitivity. Conclusion: The loss of peripheral visual fields following refractive surgery may be attributed to the deteriorated retinal image quality in the peripheral visual field. In clinical practice, a visual field loss may occur due to disease or disorders of the eye, optic nerve, or brain. For example in case of glaucoma, where the threshold in the midperiphery is evaluated, it is important to take into account whether the patient 13 has undergone a refractive surgery in the past.

Language

Greek

Subject

Ophthalmology

Visual fields

Οπτικά πεδία

Issue date

2010-03-23

Collection

School/Department--School of Medicine--Department of Medicine--Post-graduate theses

Type of Work--Post-graduate theses

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