Visual Fields in Glaucoma

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  • Topic: Visual field, Sound, Eye
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  • Published : January 13, 2013
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Visual
Fields
in
Glaucoma

Evaluation of the field of vision is the mainstay of the diagnosis and management of glaucoma. The use of manual perimetry for defining glaucomatous defects is an art that is highly dependent on the perimetrist's skill. With the advent of computers and automation, perimetry has changed from an art to a more quantitative science. Understanding glaucomatous abnormalities detected with both manual and automated techniques requires a thorough understanding of the normal and glaucomatous visual field and the factors that influence test results.

THE NORMAL VISUAL FIELD

The field of vision is defined as the area that is perceived simultaneously by a fixating eye. The limits of the normal field of vision are 60° into the superior field, 75° into the inferior field, 110° temporally, and 60° nasally (Fig. 1). Traquair,1 in his classic thesis, described an island of vision in a sea of darkness (Fig. 2). The island represents the perceived field of vision, and the sea of darkness is the surrounding areas that are not seen. In the light-adapted state, the island of vision has a steep central peak that corresponds to the fovea, the area of greatest retinal sensitivity. From the peak, the island slopes downward toward the periphery, which represents regions of diminishing retinal sensitivity. The physiologic blind spot corresponds to the area of optic nerve head. It is shown as a deep well to sea level 15° temporal to the peak of the island.

The contour of the island of vision relates to both the anatomy of the visual system and the level of retinal adaptation. The highest concentration of cones is in the fovea, and most of these cones project to their own ganglion cell. This one-to-one ratio between foveal cone and ganglion cell results in maximal resolution in the fovea. The sharp-peaked island of vision described by Traquair reflects the visual field in the light-adapted or photopic visual field. The contour of the island of vision changes greatly in the mesopic (twilight) and scotopic (dark adapted) states. As one proceeds from a photopic to a scotopic state, the overall retinal sensitivity increases as rod, rather than cone, vision predominates. In the dark-adapted island of vision, the contour is flatter than in the light-adapted state, and there is a central depression, rather than a central peak, in the area of the fovea. Thus, the level of retinal adaptation is crucial in defining the contour of the island of vision (Fig. 3).2

METHODS OF MEASURING THE
VISUAL FIELD

KINETIC PERIMETRY

In kinetic perimetry, a stimulus is moved from a nonseeing area of the visual field to a seeing area along a set meridian. The procedure is repeated with the use of the same stimulus along other meridians, usually spaced every 15°. In kinetic perimetry, one attempts to find locations in the visual field of equal retinal sensitivity. By joining these areas of equal sensitivity, an isopter is defined. The luminance and the size of the target are changed to plot other isopters. In kinetic perimetry, the island of vision is approached horizontally. Isopters can be considered the outline of horizontal slices of the island of vision (Fig. 4).3

STATIC PERIMETRY

In static perimetry, the size and location of the test target remain constant. The retinal sensitivity at a specific location is determined by varying the brightness of the test target. The shape of the island is defined by repeating the threshold measurement at various locations in the field of vision (Fig. 5).

MANUAL PERIMETRY: THE
GOLDMANN VISUAL FIELD

The Goldmann perimeter* is the most widely used instrument for manual perimetry. It is a calibrated bowl projection instrument with a background intensity of 31.5 apostilbs (asb), which is well within the photopic range. The size and intensity of targets can be varied to plot different isopters kinetically and determine local static thresholds.4 The stimuli used to plot an isopter...
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