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M. D. Crossland, H. M. P. Dunbar, G. S. Rubin; Fixation Stability Measurement Using the MP1 Microperimeter: A Method to Improve the Quantification of Results. Invest. Ophthalmol. Vis. Sci. 2008;49(13):1507.
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© ARVO (1962-2015); The Authors (2016-present)
The Nidek microperimeter (MP1, Nidek Instruments, Italy) is a recently available clinical perimetry device which also assesses fixation stability. The MP1 expresses fixation stability in three ways: (i) as a fixation score (stable, relatively unstable, or unstable); (ii) as the proportion of fixations falling within the central 2º of retina; and (iii) as the proportion of fixation points falling within the central 4º of retina. In most published research, fixation stability is quantified by calculating a bivariate contour ellipse area (BCEA) which encompasses 68% of fixation points. Here we compare these four methods of fixation assessment (Fixation Score, Central 2º, Central 4º, BCEA) by correlating them to various parameters of reading known to be related to fixation stability.
Twenty-three people with age-related macular disease were assessed. Eye position was recorded at 25Hz using the MP1 whilst patients performed a perimetry task. Fixation score, central 2º and central 4º values were obtained from the MP1’s inbuilt software. BCEA values were calculated from raw fixation data extracted from the MP1. Reading speed was assessed using MNREAD, Rapid Serial Visual Presentation (RSVP) and EUREAD tests.
No relationship was revealed between any reading parameters and the MP1 fixation score, the Central 2º value or the Central 4º value (p>0.1 for all comparisons). In contrast, there was a significant relationship between fixation stability assessed using the BCEA technique and RSVP reading speed (r=0.59, p<0.01), EUREAD reading error rate (r=0.66, p<0.01), and MNREAD peak reading speed (r=0.55, p<0.05).
Using the software supplied with the MP1 does not adequately quantify fixation stability in people with age-related macular disease. However, a straightforward post-hoc analysis of the raw fixation data from the MP1 can be applied to detect subtle yet important differences in fixation stability between subjects. We suggest that this technique is used in studies where change in fixation stability is of importance, such as in clinical trials.
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