Validation of a measurement setup for simultaneous ERG recording and ocular current stimulation
Authors: Blum Maren‐Christina , Klee Sascha 
Published: December 2019

Purpose

Ocular current stimulation exhibits potential for the treatment of neurodegenerative ocular diseases. Until today, the underlying mechanism in the retinal cells remains subject of research. The measurement of an electroretinogram (ERG) simultaneous to a current stimulation could provide insights into the actual processes and effects of the retinal cells during a current stimulation. A novel measurement setup was designed to record the ERG response during a current stimulation on the eye. Test measurements have been conducted to proof the feasibility of the novel setup.

Methods

Six healthy volunteers (3m, 3f, 25.5 ± 2.8 years, one eye) have been stimulated with an anodal and a cathodal (randomized sequence) direct current stimulation (DCS) of 500 µA (DC‐stimulator MC, neuroCare Group GmbH, Munich, Germany) for 5 minutes with a break of 15 minutes between current stimulations. A cut‐sized rubber electrode placed around the eye and a square rubber electrode (3 cm × 3 cm) as return electrode placed at the ipsilateral temple served as stimulation electrodes. Before and during the DCS the electrophysiological answer (TheraPrax, neuroCare Group GmbH, Munich, Germany) of the retinal ganglion cells was measured using a pattern‐reversal stimulus (stimulus field:1°individual checks,16 whole; reversals per second:4; Michelson contrast:99%; mean luminance:186 cd/m2; recorded sweeps = 900; averaged sweeps = 600, red point in the middle). For that purpose, Ag/AgCl ring‐shaped skin‐electrodes have been placed at the lower eyelid (active), the ipsilateral earlobe (reference) and the forehead (ground). Additionally, an ERG without a visual stimulus in combination with and without a DCS was derived. This can rule out, that the results are phase‐synchronous technical artifacts similar to an averaged ERG response. For analysis, the absolute and percentage changes were evaluated and a confidence level analysis (95%confidence level) was performed.

Results

All characteristic amplitudes of a pattern‐reversal ERG (P50, N95), as well as the peak‐to‐peak difference (PPD) decreased during a cathodal DCS (P50/N95/PPD:‐15.88%/‐6.36%/‐14.30%). For anodal DCS (n = 5, technical problem in one volunteer, could be solved for further measurements) the P50 amplitude decreased while the N95 and the PPD amplitude increased (P50/N95/PPD:‐22.5%/27.93%/1.2%). The visible trends for decreasing during a cathodal and increasing during an anodal DCS as well as the difference between baseline and current measurement were not significant in the confidence level analysis (difference evaluation: cathodal P50/N95/PPD:[‐1.14 µV;0.318 µV]/[‐0.32 µV;0.96 µV]/[1.75 µV;0.29 µV]; anodal P50/N95/PPD:[‐1.14 µV;0.318 µV]/[‐0.32 µV;0.96 µV]/[‐1.75 µV;0.29 µV]). The ERG measurements without a visual stimulus showed no evaluable curves.

Conclusion

The novel developed measuring setup is feasible for recording an ERG during a DCS on the eye. Our results indicate changes in the ERG amplitudes during a DCS, which complies with the state of the art from stimulations of the visual cortex. Further studies with an increased number of participants will be provided in order to approve the indicative results.