Impact of Natural Blind Spot Location on Perimetry

Abstract

Purpose : The natural blind spot location (NBSL) measured prior to the visual field (VF) by perimeters like the Humphrey Field Analyzer (HFA) is not considered for calculations of diagnostic parameters like pattern deviations (PD). We study the impact of NBSL on PD values of patients from 5 glaucoma practices.

Methods : The most recent reliable HFA VFs (fixation loss≤33%, false negative/positive rates≤20%, SITA Standard 24-2) were selected from each eye. To minimize possible effects of potentially existing glaucomatous VF loss, only VFs with mean deviations ≥-1dB, pattern standard deviations that are not flagged as abnormal and glaucoma hemifield tests within normal limits were included. As the HFA returns a NBSL of (15,-1) degree by default even if no blind spot test was performed, VFs with this NBSL were excluded as well. Distributions of NBSL frequency and spherical equivalent (SE) of refractive error for each NBSL were determined. In addition we compared PD values larger and smaller than the median distance as well as positive and negative angle between NBSL and fixation by t-tests.

Results : 12,751VFs were selected for data analysis. Fig. 1a illustrates the spatial distribution of NBSL relative to fixation. Average SE decreases with increasing distance from fixation (Fig. 1b), as expected by the well documented negative correlation between SE and ocular axial length. Fig. 2 shows the PD value difference effects with respect to NBSL relative to fixation (all eyes in right eye format). For NBSL distances larger than median distance 14.32, average PD values decrease (red) in the upper central (average difference for significant points (ADSP): -0.16 dB) and increase (blue) in the lower nasal VF region (ADSP: 0.13 dB). For angles in the direction of the upper hemifield relative to the median angle -8.13°, PD values decrease in lower nasal (ADSP: -0.10 dB) and increase in upper temporal VF areas (ADSP: 0.14 dB).

Conclusions : As effects of optic disc location relative to fovea on retinal nerve fiber trajectories were discussed in literatures, we demonstrate that the functional equivalent, NBSL relative to fixation, has a systematic effect on the spatial distribution of light sensitivity. NBSL distance and angle relative to fixation independently affect the distribution of PD values. As PD values are important diagnostic parameters for diseases like glaucoma, taking NBSL into consideration might improve the diagnosis of functional vision loss.

This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.

 

Fig. 1

 

Fig. 2