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M.K. Menz, M.D. Menz, E.E. Sutter; The Effect of Fixation Instability on the Multifocal ERG . Invest. Ophthalmol. Vis. Sci. 2003;44(13):2699.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To examine the effects of relatively large amounts of fixation instability on the multifocal electroretinogram (mfERG) in normal subjects. Methods: MfERGs were recorded with voluntary eye movements that simulated involuntary fixation instability. Two normal subjects were tested. Stimulation and analysis were performed with the VERIS 4 Science system using a 103 element scaled stimulus array. Two methods of voluntary eye movements were employed to simulate fixation instability. A rotation pattern experiment consists of moving fixation every 2 seconds between the ends of a fixation cross in a clockwise direction. A star pattern experiment required moving fixation every 2 seconds between the center and the ends of the fixation cross, so that 50% of the time fixation was at the center. Three different sizes of the fixation target cross were used: 2, 4 and 6 degree diameter. Results: In both methods of simulating fixation instability, the response of the central hexagon had a greatly reduced amplitude, between 54% and 80% of control depending on subject and experiment for the 6 degree fixation cross. There was no significant effect in the center hexagon waveform with the 2 degree cross experiments. Latency changes in the positive peaks of ring averages were less than 1.0 ms. However, the latency of N2 of the center hexagon waveform was reduced by as much as 5 ms with unstable fixation (6 degree cross). Normally, the late components of the first order waveforms are delayed toward the center. As expected these changes become greatly reduced. In the 4 degree experiments, the optic disc was still well defined, while in the 6 degree case the depth of depression was reduced. Spatial averaging applied to the control condition of correct fixation does not simulate the effects of unstable fixation. Conclusion: The results suggest that if fixation is maintained within the center hexagon, the overall topography is unaffected. Greater fixation instability affected the results, primarily in areas with steep response gradients such as near the central peak. We thus conclude that the detection of small scotomata becomes more difficult with unstable fiuxation. For patients with poor fixation stability, the use of a fundus monitoring system is desirable. However, at the stimulus resolution used in our experiments (scaled 103 element stimulus array) an eye camera that permits detection of eye movements of 2 degrees or less provides adequate control for fixation stability.
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