Basic Principles of Automated Perimetry
In automated static threshold perimetry, the sensitivity of a patient’s central and peripheral vision is quantified using computerized algorithms to accurately ascertain the threshold of sensitivity at each location tested in the patient’s field of vision. At each location, stimuli of varying intensities are presented, the patient’s responses checked, and the differential light sensitivity measured.
Modern perimeters use the convention introduced with Goldmann perimetry to determine the target sizes for the stimuli. The stimulus sizes are numbered with Roman numerals I through V. Each stimulus covers a 4-fold-greater area, ranging from 0.25 mm2 for a Size I stimulus to 64 mm2 for a Size V stimulus. The most commonly used stimulus size for the Humphrey Field Analyzer (HFA; Carl Zeiss Meditec) is stimulus Size III, which corresponds to 4 mm2. Stimulus Size V may be employed for individuals with severe visual field loss or poor visual acuity.
Threshold strategies start by testing a single location in the field of view. If the stimulus is seen, subsequent stimuli at that location are dimmed a step at a time until no longer seen. If the initial stimulus is not seen, then subsequent presentations are made brighter in steps until the patient responds. This process can be repeated at the same location with reversal of the steps to confirm that the threshold of sensitivity has been accurately ascertained.
The HFA tests light intensities over 5 orders of magnitude, from 10,000 apostilbs (asb) to 0.1 asb. Every log order change in light intensity corresponds to 10 dB; the machine can measure sensitivities over a 50-dB range. Test locations at which a stimulus of 10,000 asb is not detected are assigned a value of 0 dB. It is important to note that this applies to the specific stimulus size that is being used. For example, a 0-dB location when testing with stimulus Size III means that the stimulus with maximum intensity of 10,000 asb and Size III was not seen. However, this does not imply that the location is totally blind. A larger stimulus in the same location, such as stimulus Size V, may be visible and detected by the patient. The threshold values reported for each location reflect the extent to which light can be dimmed (by a series of neutral density filters used in the perimeter) and still detected. For example, a value of 30 dB indicates that the stimulus can be dimmed 1000-fold, from 10,000 to 10 asb, and still be seen.
The state of light adaptation of the eye at the time of the visual field test influences luminance sensitivities. The HFA uses background lighting of 31.5 asb to saturate rod photoreceptors, producing photopic conditions in which cones are primarily tested.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.