Variability of peripheral test points, central test points and visual field indices in the Medmont M700 perimeter

Abstract

Purpose: The Medmont M700 automated perimeter is widely used in many countries, but literature regarding its repeatability, and comparisons with the Humphrey Field Analyser (HFA), are scarce. We therefore intend to create some clinically applicable confidence intervals for the M700 Overall and Pattern Defect indices, and compare these with similar HFA descriptors. We will also examine the variance of points of equal decibel values in the central and peripheral fields of the same M700 visual field tests.

Methods: Twenty four glaucoma patients with varying degrees of glaucomatous field loss were enrolled in the study, and twenty one patients (forty eyes) had usable results for the study. A Central 30° and a Macula (10°) test using the M700 (version 3.9.7) fast threshold strategy was performed on each eye on the same day. To determine retest variability, the tests were repeated one week later at the same time of day, which minimised the possibility of circadian variations affecting the results.

Results: For sensitivities of 0 - 5 dB and 25 - 35 dB, points in the outer 20° of the Central 30° test showed similar retest variance to points of equal decibel value in the central 10° of the same test. For sensitivities of 5 - 25 dB, points in the outer 20° of the Central 30° test showed slightly lower variance than points of equal decibel value in the central 10° of the same test. The M700 OD index appears to be quite different to the Mean Deviation (MD) index in the HFA when grading glaucoma severity, and significantly different to the Average Defect index in earlier Medmont programs. For Overall Defect (OD), the 95% confidence interval (CI₉₅) was ± 2.1 dB. The CI₉₅ for Pattern Defect (PD) varied with glaucoma severity: for PD < 2.8, CI₉₅ = (± 1.25 dB), for PD 2.81 to ≤ 5.7 CI₉₅ = (± 1.14 dB) and for PD > 5.7 CI₉₅ = (± 3.1 dB).

Conclusions: In glaucoma patients, for points of equal decibel values, variance does not seem to increase with increasing eccentricity with the M700 stimulus layout. OD values obtained with the M700 did not appear to correlate well with the amount of visual field loss, and should not be directly compared to MD values obtained with the HFA. As a result, the M700 OD value should not be used to stage glaucomatous field loss in the same way as the MD figure is used with the HFA. PD values in the M700 appeared to correlate well with the degree of glaucomatous field loss.