City Research Online

Abstract

The purpose of this project was to investigate the central visual field by automated static perimetry in a group of healthy myopic individuals with peripapillary crescents and tigroid fundus changes only. The appearance of the optic nerve head and surrounding structures were used to classify subjects into groups for the analysis of field data. In addition, the dimensions of features of the optic nerve head and peripapillary crescents were evaluated.

The study population comprised 122 young, healthy volunteers between the ages of 18.5-35.4 years, free from any ocular or systemic condition, with refractive errors ranging from +4.00D to -25.75D. A young subject population was deliberately chosen to avoid confounding the results with age-related changes affecting the visual field, for example crystalline lens opacities.

The central visual field was examined using two different programs of the Humphrey Field Analyzer. A combination of these two programs yields a test point resolution of 4.2° within the central 30° field. Optimum experimental design for this field examination procedure was established following a preliminary study of serial visual fields in normals, with due regard to the effects of learning on the visual field. A customised program was configured on the Humphrey Field Analyzer to examine the blind spot region with a test point resolution of 1.4°.

A ray tracing program was devised to determine the true area of the intra- and peripapillary optic nerve head features from photographic slides of the ocular fundi taken with the Carl Zeiss Jena fundus camera. For this program the magnification of the fundus camera employed and that of the human eye are required. The former was determined experimentally using a model eye. Differing methods for estimating the latter are presented.

Visual field analysis showed a decline in the differential light sensitivity in myopes with peripapillary changes only. These field changes become more pronounced with increases in the degree of myopia, axial length and area of the peripapillary crescent. The field region most affected appeared to be the superior hemifield, particularly the upper- temporal quadrant. Enlargement of the blind spot occurred in subjects with relatively larger peripapillary crescents, who were inclined to have longer axial lengths and more myopia. Results suggest that the sensitivity decline of the central field occurs in subjects with axial lengths above 26mm and more than 5D of myopia, whereas BS enlargement appears to occur in subjects with axial lengths above 28mm and more than 10D of myopia.

The areas of the optic disc, neuroretinal rim and peripapillary crescent increase with increasing axial length and increasing myopic refraction. A log transformation of the optic disc parameters showed them to increase linearly with axial length and ocular refraction, apart from cup area. Marked inter-individual variations exist for the areas of the intra- and peri-papillary optic disc structures. Significant correlations exist between disc area and neuroretinal rim area, and between peripapillary crescent area and disc area.

Knowledge of typical sensitivity values with automated static perimetry in myopia may help in the differential diagnosis of diseases, including glaucoma, which cause visual field abnormalities. Knowledge of the dimensions of the optic nerve head and peripapillary crescents may aid in the diagnosis of pathologically disturbed optic discs.

Publication Type:	Thesis (Doctoral)
Subjects:	R Medicine > RZ Other systems of medicine
Departments:	School of Health & Psychological Sciences > Optometry & Visual Sciences School of Health & Psychological Sciences > School of Health & Psychological Sciences Doctoral Theses Doctoral Theses

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