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J. Delbeke, M. Brelèn, I. Obeid, C. Veraart; An Intra–Orbital Alternative for the Optic Nerve Visual Prosthesis . Invest. Ophthalmol. Vis. Sci. 2005;46(13):1142.
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Purpose: In a first attempt to develop an optic nerve visual prosthesis the electrode was inserted around the free floating pre–chiasmatic segment of the optic nerve. Although technically more difficult, an intra–orbital implantation does not expose the brain. This alternative is therefore expected to limit possible hazards to the loss of an otherwise non–functional eye or optic nerve. Such an intra–orbital implantation has been performed and is being evaluated. Methods: This study fully complies with the declaration of Helsinki. Approval by an ethics committee as well as a written informed consent were obtained. A cuff electrode carrying eight platinum contacts and a stimulator with transcutaneous transmission system were implanted around the right optic nerve of a totally blind retinitis pigmentosa (autosomal recessive) patient. The optic nerve was proven to be at least partially functional by surface stimulation test. Results: A few weeks after implantation, a moderately palpable stimulation box under the parietal scalp was the only clinically perceptible mark left by the surgery. CT–scan images proved that the electrode was adequately placed. The electric field potentials generated by the stimulation pulses could be recorded as expected between a point just above the nasal end of the right eyebrow and the right mastoid. Surface stimulation yielded slightly lower thresholds after the operation than before it. Electrode impedance estimated from potential measurements on the constant current stimulator outputs were in the expected range of 2.5 to 3.5 kOhm. Phosphenes were collected two to three months after surgery. At that time, the skin thickness under the antenna had dropped from 7 to 4.9 mm. The strength–duration curve shows a chronaxy of about 190 µs for single pulses as well as for train stimuli. The rheobase for single stimuli is around 850 µA and drops to below 200 µA for trains of 16 pulses at 400 Hz. Conclusions: The electrode of the optic nerve visual prosthesis can be implanted intra–orbitally. However, in the orbit a dural sleeve shields the optic nerve and was not removed from under the electrode contacts. This probably explains the higher thresholds compared to the corresponding values obtained with intra–cranial stimulation. In the first few months after operation, inflammation, edema or other local tissue reaction could impede the stimulation at the level of the electrode and disturb the transcutaneous transmission at the level of the antenna.
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