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F. Gekeler, M. Ottinger, A. Messias, E. Zrenner; Phosphenes From Electrical Stimulation With DTL–Electrodes in Various Retinal Diseases: Chronaxie and Rheobase . Invest. Ophthalmol. Vis. Sci. 2006;47(13):3193.
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© ARVO (1962-2015); The Authors (2016-present)
To develop an easy and reliable method to elicit phosphenes from electrical stimulation in eyes which are to undergo implantation of sub– or epiretinal prostheses.
WConjunctival DTL–electrodes as in standard clinical electroretinography were used to elicit phosphenes in different volunteer groups: normals (n=15), retinitis pigmentosa (RP, n=10, near blind or blind), glaucoma (n=7, varying visual field loss), deep amblyopia (n=4). Gold cup skin electrodes near the lateral canthus served as reference. We recorded stimulation thresholds for pulse lengths of 0.05, 0.075, 0.1, 0.3, 0.5, 0.75, 1, 2.5, 5, 25, and 50 ms provided by a neuro–stimulator using an alternative forced choice method multiple times. Patients' perceptions were recorded by a short questionnaire.
Chronaxie, the duration of a stimulus having a strength of twice the threshold minimum strength (rheobase) required for excitation was measured with high repeatability in all subgroups and with minimal discomfort (slight muscle fibrillation in the 3 shortest pulse lengths at supra–threshold currents). In the normals rheobase and chronaxie were 0.06±0.02 mA and 7.53±2.88 ms, in RP– patients 0.54 ± 0.38 mA and 5.68 ± 4.99, in glaucoma 0.12±0.10 mA and 12.45±4.88ms, in amblyopia 0.06±0.02 mA and 9.05±0.97 ms, respectively. Almost all patients reported a white flash in the center of the visual field. Interestingly, in the glaucoma patients the stimulus location showed no correlation with the visual field loss. In the retinitis Pigmentosa patients an increase was found in chronaxie and rheobase with duration of blindness.
DTL–electrodes and an alternative forced choice method allow easy and reliable definition of electrical excitability of the retina and of optic nerve transmission in normals and patients with degenerative retinal disease and thereby provide an important criterion for the suitability of patients for electrical retinal prostheses. The correlation of thresholds in different diseases and location of retinal or optic nerve damage can contribute to the definition of the exact neuronal layer of retinal electrical stimulation.
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