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L. Naycheva, A. Schatz, T. Röck, G. Willmann, K. U. Bartz-Schmidt, E. Zrenner, F. Gekeler; Comparison of Electrically Evoked Phosphene Thresholds in Healthy Subjects and Patients With Retinal Diseases. Invest. Ophthalmol. Vis. Sci. 2010;51(13):1058.
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To evaluate electrically evoked phosphene thresholds (EPT) in healthy subjects and patients with retinal diseases over a range of frequencies.
115 eyes of 115 individuals (healthy subjects (n=20) and patients with retinitis pigmentosa (RP, n=29), Stargardt macular dystrophy (STG, n=14), retinal artery occlusion (RAO, n=20), non-arteritic anterior ischemic optic neuropathy (NAION, n=15) and primary open-angle glaucoma (POAG, n=17) were enrolled in the study. Phosphene thresholds evoked by transcorneal electrical stimulation (TES) using DTL electrodes were determined at 3 Hz, 6 Hz, 9 Hz, 20 Hz, 40 Hz, 60 Hz and 80 Hz with 10 ms biphasic pulses; an alternative forced choice-method was used. After analysis of variance (one-way ANOVA) the Dunnett’s test was used to compare the groups’ means against the mean of the control group (=healthy) for all frequencies. Statistical analyses were performed with commercial software (JMP IN, SAS Institute, Cary, NC).
In all individuals (except 3 patients with RAO, who had no phosphene perception despite stimulation > 4 mA) thresholds were safely determined without side effects.Subjects reported a homogenous, central phosphene. The mean ± SD values for EPT in healthy subjects were at 20 Hz 0.062 ± 0.038 mA, at 40 Hz 0.084 ± 0.053 mA, at 60 Hz 0.098 ± 0.061 mA, at 80 Hz 0.101 ± 0.061mA, at 3 Hz 0.109 ± 0.076 mA, at 6 Hz 0.097 ± 0.060 mA and at 9 Hz 0.088 ± 0.056 mA. In all groups with retinal diseases and across all frequencies the EPT were significantly increased compared to the control group, except STG vs healthy at 20 Hz (p = 0.09) and at 40 Hz (p = 0.17). We found the highest increase of EPT in the RAO group vs healthy (at 3 Hz, 6 Hz, 9 Hz, 60 Hz, 80 Hz: p < 0.0001 and at 20 Hz: p = 0.0008 and at 40 Hz: p = 0.0002), however RAO also exhibited the largest variance (e.g. at 20 Hz mean ± SD 0.988 ± 1.142 mA). The EPT at all frequencies are lowest in healthy subjects and highest in RAO, mid-range in increasing order were the EPT in STG, POAG and NAION (e.g. at 20 Hz mean ± SD in healthy: 0.062 ± 0.038 mA, STG: 0.102 ± 0.097 mA, POAG: 0.127 ± 0.09 mA, NAION: 0.244 ± 0.126 mA, RP: 0.371 ± 0.223 mA, RAO: 0.988 ± 1.142 mA).
While determination of EPT is already applied for examination of eyes that are to undergo implantation of a retinal prosthesis, electrically evoked phosphenes could also be a valuable additional method for elucidating retinal disease processes, especially when other ophthalmological tests like visual field, full-field ERG or multifocal ERG cannot yield unambiguous results.
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