Purpose
To determine the safety and efficacy of transsclerally placed intrapapillary wire microelectrodes implanted chronically into
the optic nerve head of rabbit eyes.
Methods
Four platinum wire microelectrodes were passed through the sclera and implanted into the optic nerve head of five rabbit eyes
for 4–6 months. Color fundus photography, fluorescein angiography, electroretinograms (ERGs), and visually evoked potentials
(VEPs) were used to monitor the retina. Electrically evoked potentials (EEPs) were elicited by bipolar electrical stimulation
of the optic nerve axons by different combinations of the four electrodes immediately after the implantation and at 1-month
intervals thereafter. The effects of the chronic implantation of the electrodes on the morphology of the optic nerve were
evaluated by histological and immunohistochemical examinations at 4 and 6 months after the implantation.
Results
All of the electrodes remained stable in the implanted sites throughout the post-implantation period, except for one electrode
that had pulled out of the optic nerve head at 1 month after implantation. No intraocular infection, inflammation, or vitreoretinal
proliferation was observed in any eye. EEPs could be elicited from each pair of electrodes at all testing times. The mean
threshold currents (charge densities) to evoke EEPs increased from 19.3±9.2 μA (6.0±2.9 μC/cm2) on the implantation day to 78.8±31.9 μA (24.6±10.0 μC/cm2) at 1 month after implantation, but did not change significantly thereafter. The implicit time and amplitude of the a- and
b-waves of the ERGs and of P1 of the VEPs did not change significantly throughout the post-implantation period. Histological
evaluation of the optic nerve head revealed slight tissue encapsulations surrounding the electrode and increased expression
of glial fibrillary acidic protein near the surface of the optic nerve.
Conclusions
Implantation of transscleral intrapapillary microelectrodes appears to be safe and effective. These findings indicate that
the implantation of microelectrodes in the optic nerve head should be considered for an optic nerve-based prosthesis.
Keywords Electrically evoked potential - Electrical stimulation - Electrode - Optic nerve head - Histology - Visual prosthesis