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Mark Humayun, MD; Roy Propst, PhD; Eugene de Juan, Jr, MD; Keith McCormick; Dyson Hickingbotham

Arch Ophthalmol. 1994;112(1):110-116.

Abstract
Background
Retinitis pigmentosa with attendant photoreceptor loss can cause a profound visual handicap. We have postulated that an intraocular prosthesis that could electrically stimulate the inner retina might provide vision to some of these patients. For such a prosthesis to be feasible, electrical stimulation of the inner retina must elicit a focal retinal response. The stimulating current densities required to elicit such a response must not result in irreversible toxic reactions at the electrode-tissue interface.

Methods
To test the feasibility of this approach, we used bipolar platinum wire electrodes to electrically stimulate the inner retinal surface in bullfrog eyecup preparations and, using similar methods, we electrically stimulated rabbit eyes after injecting intravenous sodium iodate (40 mg/kg), a retinal pigment epithelial toxin with secondary effects on the photoreceptors.

Results
Surface electrical stimulation of the inner retina in normal eyes and in eyes with outer retinal degeneration can elicit a localized retinal response. The threshold stimulating currents resulted in charge densities of 2.98 microcoulombs per square centimeter (bullfrog), 8.92 µC/cm² (normal rabbit), and 11.9 µC/cm² (rabbit retinas with outer retinal degenerations). These charge densities are within the previously delineated safe limits for long-term electrical stimulation of neural tissue using platinum microelectrodes (100 µC/cm2).

Conclusion
Multifocal electrical stimulation of the retina might be a viable approach to provide some vision to patients who have profound visual loss due to outer retinal degenerations.

Author Affiliations
From the Duke University Eye Center, Durham, NC (Dr Humayun and Mr Hickingbotham); the Department of Biomedical Engineering, University of North Carolina, Chapel Hill (Dr Propst and Mr McCormick); and The Wilmer Ophthalmological Institute, The Johns Hopkins University, Baltimore, Md (Dr de Juan).

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