Microperimetry Reliability Assessed From Fixation Performance

Abstract

Purpose: Microperimetry provides an accurate assessment of central retinal sensitivity due to its fundus-tracking capability, but it has limited reliability indicators. One method currently employed, fixation loss, samples the optic nerve blind spot for positive responses; however, it is unclear if these responses arise from unintentional button presses or from tracking failure leading to stimuli misplacement. We investigated the relationship between blind spot scotoma positive responses (termed scotoma responses) and fixation.

Methods: Part 1 of the study involved a custom grid of 181 points centered on the optic nerve that was constructed to map physiological blind spots in primary and simulated eccentric fixation positions. Scotoma responses and the 63% and 95% fixation bivariate contour ellipse areas (BCEA63 and BCEA95) were analyzed. In Part 2, fixation data from controls and patients with retinal diseases (234 eyes from 118 patients) were collected.

Results: Part 1, a linear mixed model of 32 control participants, demonstrated significant (P < 0.001) correlation between scotoma responses and BCEA95. In Part 2, the upper 95% confidence intervals for BCEA95 were 3.7 deg2 for controls, 27.6 deg2 for choroideremia, 23.1 deg2 for typical rod-cone dystrophies, 21.4 deg2 for Stargardt disease, and 111.3 deg2 for age-related macular degeneration. Incorporating all pathology groups into an overall statistic resulted in an upper limit BCEA95 = 29.6 deg2.

Conclusions: Microperimetry reliability is significantly correlated to fixation performance, and BCEA95 provides a surrogate marker for test accuracy. Examinations of healthy individuals and patients with retinal disease are deemed unreliable if BCEA95 > 4 deg2 and BCEA95 > 30 deg2, respectively.

Translational relevance: Microperimetry reliability should be assessed using fixation performance as summarized by BCEA95 rather than the level of fixation losses.

Figure 1.

Healthy control example. (A) Custom 181-point grid pattern with 0.5° spacing. (B) Three consecutive examinations for an example participant: left, middle, and right with fixation on the target, fixation off of the target, and repeat fixation on the target, respectively. (C) Time-series fixation graph detailing eye position throughout testing. (D, left) Example participant result specifically highlighting the second (non-foveally centered) test only. (D, right) Corresponding eye positions throughout testing (cyan dots) with ellipse areas encompassing 63% and 95% of all points highlighted. (E) BCEA63 and BCEA95 areas and angle values.

Figure 2.

Healthy control example results. (A) A single right eye examination result demonstrating non-central position of optic nerve scotoma within a grid placed centrally on the anatomical optic nerve. In this case, the inferior temporal scotoma was likely due to the presence of a small band of peripapillary atrophy at the optic nerve border. The center of the scotoma was manually identified and is highlighted with a red star. (B) From this manually identified central scotoma point, the surrounding 45 points were isolated to assess the number of responses in this central region. These are defined as scotoma responses. In this case, there were zero scotoma responses.

Figure 3.

Linear mixed-model visualizations. (A) Visualization of the linear mixed regression model for a selection of participants, fitted with random intercepts as indicated by the colored lines. The black line represents the overall cohort mean regression line. (B) Linear mixed-model regression analysis demonstrating an almost perfect correlation between BCEA63 and BCEA95.

Figure 4.

Linear mixed-model regression model for the scotoma response percentage dependent outcome variable versus the independent predictor variables. (A) BCEA95. (B) Number of gaze spikes. The shaded regions represent the standard errors. P values, pseudo R², and conditional R² correlation coefficients and the equations of lines are displayed alongside each plot.

Figure 5.

Grouped fixation performance plots. Violin plots display the log BCEA95 values for the right eye (A) and left eye (B) for the control and pathology groups. The fovea of the STGD and AMD groups have a larger spread of fixation (larger BCEAs) than the more parafoveal/peripheral-affected choroideremia and RP groups.