Comparing the Nidek MP-1 and Humphrey field analyzer in normal subjects

Abstract

Purpose: To compare visual fields on the Nidek MP-1 to those obtained on the Humphrey field analyzer (HFA) in healthy volunteers and assess the effects of differences in stimulus parameters and testing strategies that may influence the interpretation of results in patients. A secondary aim was to establish MP-1 normative data to calculate the total deviation analyses and global indices analogous to those used by the HFA.

Methods: Fifty healthy volunteers (age 43.5 ± 13.9 years, range, 18 to 68 years) underwent repeat MP-1 and HFA visual field testing, using the 10-2 pattern. MP-1 data were converted to HFA equivalent dB units. Between instrument comparisons of HFA and MP-1 sensitivities, regression of sensitivity with age and examination duration were assessed. Test-retest variability was examined between visits.

Results: MP-1 (mean = 32.82 dB, SD = 1.92 dB) and HFA sensitivities (mean = 32.84 dB, SD = 1.83 dB) were not significantly different (p = 0.759). SD values for the HFA (range, 1.11 to 3.30 dB) were similar to the MP-1 (range, 0.14 to 2.75 dB). However, asymmetry comparisons between instruments showed significantly decreased superior rather than inferior retinal values for the MP-1. There was a small but significant difference (p = 0.004) in mean test duration between the MP-1 (mean = 6:11 min, SD = 1:49 min) and the HFA (mean = 5:14 min, SD = 0:42 min). There was also a difference in the decline of mean sensitivity with age, a decline of 0.1 and 0.4 dB per decade was noted in MP-1 and HFA sensitivity, respectively. Test-retest variability was similar between instruments. A small but non-significant increase in mean sensitivity at the second visit for both the MP-1 (p = 0.060) and HFA (p = 0.570) was found.

Conclusions: Both instruments showed similar variability and test-retest variability when results were compared using equivalent units. However, there are important differences in sensitivity values, stimulus parameters, and testing strategies that have to be taken into account when comparisons are made.

Figure 1

Mean difference in sensitivity values (in dB) between instruments. The HFA visual field is inverted superior-inferior to give the corresponding retinal view to the MP-1 and the MP-1 data was converted to HFA equivalent units. Negative values indicate lower MP-1 compared to HFA sensitivities and positive values indicate lower HFA values. Shading indicates that the difference was significant and non significant differences are unshaded.

Figure 2

Bland-Altman plot to show the difference in sensitivity values between instruments. The difference plotted along the ordinate shows the converted (HFA equivalent units) MP-1 sensitivities subtracted from the HFA sensitivities, for each stimulus location, for each patient. The relationship between the variability between instruments and magnitude of difference between instruments is shown. The horizontal solid line represents the mean difference and the dotted lines represent the 95% limits of agreement.

Figure 3

One standard deviation of the group mean sensitivity (in dB) shows the intratest interindividual variation in sensitivity at each stimulus location for the HFA (top) and the MP-1 (bottom). The HFA visual field is inverted superior-inferior to give the corresponding retinal view to the MP-1 and the MP-1 data was converted to HFA equivalent units.

Figure 4

Univariate linear regression of Mean Sensitivity (MS) as a function of age is shown by the scatterplot (left) for the MP-1 and the HFA. The 95% lower bound was predicted by the Bayesian model. The regression slopes at each stimulus location for the HFA (top right) and the MP-1 (bottom right) are shown and the MP-1 ceiling at 20dB is indicated by the dashed line. The HFA visual field is inverted superior-inferior to give the corresponding retinal view to the MP-1.

Figure 5

The stimulus locations of the 10-2 pattern for the MP-1 and HFA are plotted according to the coordinates for each location, in degrees. The HFA stimulus locations have fixed coordinates. The mean value for the MP-1 coordinates for normal subjects is presented. Standard error values were less than the scatterplot point size. The arrow indicates the distance between corresponding given stimulus location points of the HFA and the MP-1.

Figure 6

Test-retest variability for the HFA and MP-1 is shown by Bland-Altman plots. The Bland-Altman plots determine if there is a relationship between the variability and magnitude of sensitivity values between visits, for the HFA (A & C) and the MP-1 (B & D), for Mean Sensitivity (MS) (A & B) and pointwise sensitivity (C & D). The horizontal solid line represents the mean difference and the dotted lines represent the 95% limits of agreement. The ceiling effect due to the MP-1 is indicated by the gray lines in (B) and (D).

Figure 7

Mean Sensitivity (MS) as a function of test duration. MP-1 data was converted to HFA equivalent units.

Figure 8

Example of MP-1 maps from two normal subjects of different ages. The MP-1 local defect maps show the numerical deviations (in dB) to the age-matched normal sensitivities superimposed on the fundus image. These maps also indicate a classification of “suspect” or “relative scotoma” based on probability values derived from the existing MP-1 normative database, and a classification of “absolute scotoma” if the stimulus was not seen. MP-1 total deviation (TD) maps were generated from our own normative database, using the Bayesian model. The TD numerical maps indicate the deviations from the age-matched normal sensitivities (in dB) and the TD probability maps show probability defects. The two subjects in this example had probability defects on the local defect map, but both had no defects on the TD probability maps. A color verison of this figure is available online at www.optvissci.com.