Monday, August 28, 2006

Section 6-D: Peripheral vision (visual field testing)

Visual fields measure both central and peripheral vision. Central vision (or visual acuity is used for fine-detail tasks such as reading, recognizing faces, and watching television. Peripheral vision is more important for navigating through obstacles in the environment. Early vision loss from glaucoma generally affects peripheral vision. The early peripheral vision loss is not commonly noticed by patients because central vision is usually spared in the early stages. Therefore, it is important to assess the visual fields to detect glaucoma early.

The systematic measurement of visual fields is referred to as perimetry. Patients keep one eye fixed on a target directly forward, while the other eye is covered. Next a test object is presented to the test eye at various positions. Patients signal when they see these objects, allowing their visual field to be mapped. Areas in which visual stimuli are not perceived are plotted to indicate the location of visual field defects (blindspot or scotoma).

In glaucoma patients, visual fields are assessed using special devices (perimeters) that allow a systematic mapping of blindspots. Patients are placed in front of a bowl-shaped screen and are instructed to indicate when they see test lights that are projected onto various positions on the bowl screen. Some perimeters are computerized and project lights at fixed positions on the screen (Humphrey’s field analyzer, Carl Zeiss Meditec, Thornwood, NY, Figure 6-10), while other perimeters are manual and project moving lights on the screen (Goldmann perimeter, Haag Streit AG, Mason, Ohio Figure 6-11). Patients indicate that they see a target by pressing a button with a finger. The output from a Humphrey’s field analyzer is a computer generated plot of the central 24 degrees (Figure 6-12), while the output of the Goldmann perimeter is a manual plotted diagram (Figure 6-13).

Figure 6-10. Humphrey’s field analyzer. The Humphrey’s field analyzer is a commonly used automated perimeter. A computer projects test lights of varying brightness at different positions on the target screen. The visual field is determined by the patient’s ability to detect the presence of the test lights.
Figure 6-11. Goldmann perimeter. An examiner uses the Goldmann perimeter to present the patient with moving test lights of varying size and brightness are used to map the visual field.

Figure 6-12. Humphrey’s field analyzer visual field reports. These plots represent visual field graphically. The center of the field is at the intersection of the axes. Shading represents areas of vision loss that is proportional to the darkness of the shading. A. Normal visual field (left eye). There is a normal blindspot where the optic nerve enters the eye shown by the arrow. B. Glaucomatous visual field (right eye). There is moderate vision loss in the superior aspect of the visual field in an arching pattern (arcuate defect) that extends from the normal blindspot (indicated with an arrow). C. Glaucomatous visual field left eye. There is significant vision loss in the inferior and nasal (towards the nose) aspect of the visual field (nasal step) as well as an early inferior arcuate defect.

Figure 6-13. Goldmann perimeter visual field reports. The center of the visual field is represented by the intersection of the lines on the grid. Patients are presented with targets of increasing size and brightness. The areas in which these targets can be seen are encircled with colored markings to depict the visual field. Focal areas in which the targets cannot be seen are indicated with color shading using the same color scheme. A. Normal visual field. A normal Goldmann visual field (right eye) consists of concentric circular markings. The circular markings (called isopters) indicate that larger, brighter targets can be seen farther into the periphery (blue isopter) than the smaller, dimmer targets (red isopter). The normal blindspot is shown with an arrow. B. Glaucomatous visual field. There is an arc-shaped loss of superior visual field (arcuate defect) that extends from the normal blindspot (indicated with an arrow). This is a typical pattern of vision loss in glaucoma that spares central vision (marked with “OK”).

Visual field testing is painless, but requires a high degree of concentration throughout the test. For accurate field measurements, it is important for patients to keep their eye pointed straight ahead at a fixation target while the test lights are presented at various positions on the screen. Visual field testing may take from 5 to 20 minutes per eye depending on type of perimeter and the degree of visual field loss.

Although perimetry provides reliable measurements of visual fields, some variations in the visual fields can occur that are NOT associated with glaucoma. Consequently, it is often necessary to repeat examinations to confirm the presence of a visual field defect or its progression.


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