Electrical stimulation — a therapeutic strategy for retinal and optic nerve disease?

Abstract

Since Galvani’s experiments with frog legs in 1791, we know that neural tissue can be readily excited by electrical currents. Doctors have utilized the therapeutic potential of electricity ever since, with such lasting achievements as cardioversion and defibrillation. In ophthalmology, one of the earliest scientific mentions of electrical currents is from Henri Dor in 1873 [1]. He used complicated machines for the treatment of “amblyopia and amauroses”, “retinochoroiditis with pigment infiltration”, “glaucoma”, and “white optic atrophy” — following a tendency of a technology minded era when electrical currents were praised for all kinds of ailments. His experiments and studies, however, fell into oblivion in the following decades at the beginning of the twentieth century, with giant progress of scientific medical practice in many other areas. Electrical stimulation of the visual system was re-discovered in the 1970s for elicitation of visual percepts, or phosphenes, by supra-threshold stimulation [2]. These experiments constituted the basis for retinal implants which today allow patients to recognize letters and shapes in laboratory and natural settings [3, 4]. During the course of these trials, the therapeutic potential of subthreshold electrical stimulation was detected in 2004 by Chow [5] in patients carrying an inactive subretinal prosthesis which produced only sub-threshold currents. His patients experienced amelioration of their residual vision even in retinal areas far from the implant. This effect was attributed to the release of neurotrophic factors, and various groups worldwide started to explore the therapeutic potential in animal experiments and in human trials. For practical reasons — such as ease of use and availability — application of currents through corneal electrodes has been widely adopted since then, coining the term transcorneal electrical stimulation (TES). Various types of contact lens-electrodes or DTL-electrodes deliver currents to counter-electrodes, usually integrated in the corneal electrode or on the periorbital skin, to ensure good transretinal currents. More than 20 peer-reviewed publications in PubMed in the last 5 years are evidence for this renewed interest in electrical stimulation of ocular tissue.