Morphometric Analysis of Acutely Stimulated Optic Nerves in Normal–Sighted Dogs With an Implanted Active Epiretinal Array

Abstract

Abstract: : Purpose: Microelectronic retinal implants have shown increasing promise in restoring useful vision in patients with degenerative diseases. This study morphometrically measures axonal counts in normal–sighted dog optic nerves after acute electrical stimulation by an epiretinal array. Methods: Seven eyes from four normal–sighted dogs were used in this study. In three dogs, the right eye was implanted with an epiretinal array, stimulated for four hours, and followed for 30 days before euthanasia. Immediately before sacrifice, the same epiretinal array implantation procedure was applied to the left eye with stimulation for four hours. For the remaining dog, there was no implantation to the left eye prior to sacrifice. All seven eyes were enucleated at terminal surgery. Optic nerves were fixed in half–strength Karnovsky's and embedded into epon blocks. Sections were cut at 1.5 µ m on an ultramicrotome and stained with p–phenylenediamine (PPD), highlighting the myelin of each axon. Morphometric analysis was performed on light microscopic images captured via a Spot II digital camera with a Zeiss Standard microscope. For each optic nerve, myelinated axonal counts were obtained in 32 sectors mapped across its area at a magnification of 1,250x. Results: The optic nerves of retinas that were acutely stimulated and followed for 30 days showed a trend towards higher total axon counts compared to the optic nerves of the opposite eye, which were stimulated immediately before sacrifice (average number of axons in the optic nerve = 138,643 for acute stimulation with 30–day follow–up vs. 119,438 for acute stimulation with no follow–up). The optic nerve of the control eye, which did not undergo an epiretinal array implantation procedure, had a total axon count of 122,064, which was not statistically different from the electrically stimulated eyes. Conclusions: This preliminary study suggests that acute stimulation of the retina via an epiretinal array may lead to higher total axon counts in the respective optic nerve. It will be interesting to see whether this effect holds up after more eyes are evaluated. If so, it may reflect the electrical stimulation giving rise to an increase in the diameter of smaller axons, which would otherwise be missed by morphometry. Electrical stimulation of the retina may have certain remodeling effects on the RGC axons.

Keywords: neuro-ophthalmology: optic nerve • retina • plasticity