Five eyes of five normal subjects with a mean age of 39.8 ±10.47 years (SD; range, 25– 52) with no known ocular disease or visual defect and five eyes of five patients with glaucoma with a mean age of 67.8 ±10.75 years (range, 52–78) were selected for this study. In accordance with the tenets of the Declaration of Helsinki, each subject gave voluntary, written consent to participate after being fully informed of the purposes of the study. The study was approved by Legacy Health System’s institutional review board for the protection of human subjects. All participants were experienced in automated perimetry and were required to demonstrate reliable baseline visual field results with few false-positive errors, false-negative errors, or fixation losses.
Eligibility for normal subjects was based on three criteria (applied to both eyes): (1) normal visual fields, as determined by a within normal limits glaucoma hemifield test (GHT) result and P > 0.05 for PSD and MD; (2) intraocular pressure <21 mm Hg; and (3) normal optic disc appearance (determined by a previous full clinical eye examination). In addition to fulfilling these standard criteria, all the normal subjects had to have no history of systemic diseases or of taking medication known to affect vision.
Inclusion requirements for patients with glaucoma were based on a previous clinical diagnosis of glaucoma, an
outside normal limits result on the GHT, and the presence of glaucomatous optic neuropathy in one or both eyes. Participants with glaucoma were selected on the basis of the severity of the disease. Glaucomatous field loss of patients with primary open-angle glaucoma was used to classify each individual’s disease as mild (
n = 2), moderate (
n = 1), or severe (
n = 2), according to the criteria of Hodapp et al.
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The Humphrey Field Analyzer II (HFA II) M750 (Carl Zeiss Meditec) was used to conduct 24-2 SITA-S visual field tests in all subjects. SITA strategies are adaptive among perimetry techniques, in that the algorithm constantly uses newly received data to recalculate thresholds throughout the test.
Five nominal false-positive error frequencies were selected for evaluation in this study: 0%, 5%, 10%, 20%, and 33%. The perimetry test operator generated aperiodic, randomly spaced responses at a predetermined mean frequency by pressing the response button while the patients completed the tests as usual. Calculations for these erratic responses were based on the knowledge that the HFA II Full Threshold testing strategy presents a stimulus to the patient at a mean period of approximately once every 2 seconds. We calculated how often an erroneous response must be made to produce a false-positive reading (i.e., for 33% error frequency, an erroneous response should be introduced, on average, every 6 seconds). We then introduced random fluctuation to the length of this period to determine when each of these responses should be introduced. False-response events were calculated for each nominal error frequency and then randomly introduced during the testing. To reduce bias, patients were not informed of the error frequency chosen by the test operator for each test run and were instructed to respond normally, despite any responses generated by the perimetrist. Subjects and the perimetrist shared the same response button throughout the testing, and the response alarm on the HFA II apparatus remained intact. A total of 25 monocular SITA-S visual field examinations were conducted in each normal patient, five at each of the four predetermined error frequencies and five baselines with no introduced false responses. The latter served to ensure the testing reliability of each patient. Nine SITA-S visual field tests were conducted in the glaucoma subjects (three each at 0%, 20%, and 33% false-positive error frequency). All visual fields were obtained using the appropriate near refractive error correction for each patient. Testing sessions were held within a 1-month period and did not exceed 1 hour. A minimum 10-minute break was given between each test during a session. In addition, 40 SITA-S tests (10 at each of the 5%, 10%, 20%, and 33% error rates) were conducted without a subject present. These tests were conducted to serve as a reference comparison for the internal reporting accuracy of the software and can be used to simulate an eye with complete vision loss.
Commercial software (Statistical Package for Social Sciences [SPSS] ver. 13.0, SPSS, Chicago, IL; SigmaPlot version 8.0, Systat Software Inc., Point Richmond, CA; and Prism 4, Graphpad Software Inc., San Diego, CA) was used to conduct statistical analyses and construct graphic representations.