Influential factors of thresholds for electrically evoked potentials elicited by intraorbital electrical stimulation of the optic nerve in rabbit eyes

Monocular retinal degeneration induced by intravitreal injection of sodium iodate in rabbit eyes

PURPOSE

Our purpose was to evaluate the anatomical and functional changes in retinae of rabbit eyes following monocular intravitreal injection of sodium iodate (SI).

METHODS

Twenty albino rabbits were divided into four groups and underwent monocular intravitreal injection with four different doses of SI (0.1, 0.2, 0.4, and 0.8 mg). Before and for 28 days after injection, the eyes were examined using fundus photography, optical coherence tomography (OCT), and electroretinography (ERG). At postinjection days 2, 7, and 28, the eyes were enucleated and underwent histological examination.

RESULTS

On fundus examination, no distinct retinal changes were seen in any group except the 0.8-mg group, which showed chorioretinal vascular attenuation. In 0.1 and 0.2-mg groups, no significant anatomical changes were found except transient hyperreflective dots over the vitreoretinal interface on OCT. In 0.4 and 0.8-mg groups, disruption of the ellipsoid zone and diffuse retinal swelling were observed in the early period on OCT. In the 0.4-mg group, the outer retina was significantly destroyed at day 28, whereas the inner retina was relatively preserved. In the 0.8-mg group, the entire retina was destroyed irreversibly. The b-wave of ERG was reduced immediately in all groups, which recovered fully (0.1- and 0.2-mg groups), partially (0.4-mg group), or never (0.8-mg group). No structural or functional abnormalities were found in the fellow control eyes.

CONCLUSIONS

Retinal degeneration following intravitreal injection of SI appears to be dose dependent; retinal damage is reversible at low doses but irreversible at high doses. At a certain dose, the outer retina may be preferably ablated.

Electrically elicited visual evoked potentials in Argus II retinal implant wearers

PURPOSE

We characterized electrically elicited visual evoked potentials (eVEPs) in Argus II retinal implant wearers.

METHODS

eVEPs were recorded in four subjects, and analyzed by determining amplitude and latency of the first two positive peaks (P1 and P2). Subjects provided subjective feedback by rating the brightness and size of the phosphenes. We established eVEP input-output relationships, eVEP variability between and within subjects, the effect of stimulating different areas of the retina, and the maximal pulse rate to record eVEPs reliably.

RESULTS

eVEP waveforms had low signal-to-noise ratios, requiring long recording times and substantial signal processing. Waveforms varied between subjects, but showed good reproducibility and consistent parameter dependence within subjects. P2 amplitude overall was the most robust outcome measure and proved an accurate indicator of subjective threshold. Peak latencies showed small within-subject variability, yet their correlation with stimulus level and subjective rating were more variable than that of peak amplitudes. Pulse rates of up to (2)/3 Hz resulted in reliable eVEP recordings. Perceived phosphene brightness declined over time, as reflected in P1 amplitude, but not in P2 amplitude or peak latencies. Stimulating-electrode location significantly affected P1 and P2 amplitude and latency, but not subjective percepts.

CONCLUSIONS

While recording times and signal processing are more demanding than for standard visually evoked potential (VEP) recordings, the eVEP has proven to be a reliable tool to verify retinal implant functionality. eVEPs correlated with various stimulus parameters and with perceptual ratings. In view of these findings, eVEPs may become an important tool in functional investigations of retinal prostheses. (ClinicalTrials.gov number NCT00407602.) Dutch Abstract.