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Arunkumar Krishnan, Harold Bedell; Compensation of fixational instability by the NIDEK MP-1 micro-perimeter. Invest. Ophthalmol. Vis. Sci. 2013;54(15):5035.
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Micro-perimeters are used increasingly to assess patients with central field loss, because of the reported capability of these instruments to compensate for fixational eye movements and present test stimuli repeatably at an intended retinal location. In this study we investigated the precision of eye-movement compensation by the NIDEK MP-1, when the fixation instability of normal subjects is increased by changing the size of the fixation target.
Retinal sensitivity for horizontal (H) and vertical (V) traverses across one edge of the optic disc were assessed in one eye of 7 normal subjects, aged 25-35 years. Two fixation targets were chosen to promote ‘more’ and ‘less’ stable foveal fixation, respectively: a 1 deg cross and a 10 deg circle. A customized 15 x 2 element array of Goldmann size II test stimuli, each separated by 0.1 deg, was positioned horizontally across the temporal disc margin or vertically across the inferior or superior disc margin. An in-built 4-2-1 strategy determined the detection threshold for each stimulus element in the array. The MP-1 recorded the distribution of fixation positions during testing, from which the standard deviation (SD) of fixation was computed in both the H and V directions. Cumulative Gaussians were fit to the threshold profiles obtained during fixation on the cross and circle and the fitted SD for the circle fixation target was compared to the expected value based on an additive-variance model. Percent compensation during fixation on the circle target was defined as (expected SD - observed SD) / (expected SD).
Average fixational SDs increased from 0.20 to 0.65 deg (H) and 0.20 to 0.72 deg (V) for fixation on the cross and circle targets, respectively. Best-fit Gaussians to the perimetric profiles had SDs that were not significantly shallower during fixation on the circle compared to the cross (P = 0.11 [H]; P = 0.68 [V]). The average percent compensation for the increase in fixation instability on the circle target, was 91.7% (H) and 86.6% (V), corresponding to an increase in the SD of retinal image scatter of 2.2 (H) and 3.8 (V) min arc.
The NIDEK MP-1 compensates well for an increase in fixational instability in the horizontal and vertical directions. This compensation ensures that, even in subjects with unstable fixation, perimetric test stimuli are presented close to the intended retinal locations.
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