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A. Wood, A. Binns, T. Margrain; Evaluation of a Modified ERG Photostress Technique. Invest. Ophthalmol. Vis. Sci. 2009;50(13):4513.
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The ‘ERG Photostress Test’ is an objective method of assessing photoreceptor recovery following a period of intense light adaptation. The technique has previously been shown to discriminate between people with age-related maculopathy (ARM) and healthy controls. The current protocol utilises a 2 minute equilibrium photobleach, which is time consuming and can be uncomfortable for some patients. This investigation compared the results obtained following an equilibrium bleach with those following a ‘photoflash’ that bleached a similar amount of photopigment.
25 healthy participants took part in the study. ERGs were obtained using an active DTL fibre and contralateral DTL reference electrode, in response to a focal (20°) amber (peak output 595nm) stimulus flickering at 41Hz. Signals were amplified, bandpass filtered (1-100Hz) and averaged with a Medelec Synergy EP system. Flicker ERGs were recorded before, and for 5 minutes after a period of intense light adaptation that bleached approximately 90% of cone photopigment. The long duration bleach was provided by a tungsten halogen bulb and the photoflash by a Metz Mecablitz 76 MZ-5 flashgun; in both cases participants were protected from excessive IR and UV radiation by the appropriate filters. Diffusing filters ensured even illumination. Participants were randomly assigned to one of two groups determining whether the photoflash or equilibrium bleach was used first. A 10 minute break between tests prevented any carry over effects. Fourier analysis was used to extract the first harmonic from ERGs recorded after each type of bleach and amplitude recovery curves were plotted and modelled with a single exponential function using a least squares paradigm.
The exponential model provided a good fit to recovery data obtained using both techniques and time constants of recovery ranged from 29 to 290s, with a large overlap between groups. The mean recovery time constants for the long and short duration flashes were 129±78s and 107±73s respectively.
Recovery following long and short duration bleaches was comparable in healthy adults. However, before concluding that the bleaching sources are interchangeable it is necessary to extend the evaluation to people with ARM.
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