A Tissue Change After Suprachoroidal-Transretinal Stimulation with High Electrical Current in Rabbits - Artificial Sight: Basic Research, Biomedical Engineering, and Clinical Advances - page 330

layers, especially the nerve fiber layer, than in the outer layers including RPE
or photoreceptors.

Discussion

The Safety of Electrical Injection Limit of STS Method
We have reported previously that EEPs can be elicited from rabbits by the same
100m Pt-wire electrode with relatively small electricity, by a 0.5-ms-duration
biphasic pulse stimulation with a current as small as 100A [2].
We also studied the safety of continuous 1-hour electrical stimulation (anodic
first, duration; 0.5msec, frequency; 20 Hz) by STS with a range of current
between 0.5 and 1mA. And we were not able to find any tissue change with
that electrical current.
This study investigated the safe limit with continuous 1-hour stimulation. That
was 1mA with 100m electrode, and 1.5mA with 200m electrode from result
of funduscopycal change.
There was a wide safety range between current to elicit EEP 100A and to
make tissue damage (1.5mA).
Previous studies have been reported about the cause and the threshold of
electrode-induced neural damage. And one of main causes is thought to be
electrode surface electrolysis or pH change, so the charge per phase or charge
densities are used dominantly to describe a threshold [4]. This law may be correct
in direct retinal stimulation, but in our non-direct retinal stimulation it is not
clear whether the charge also regulates the retinal damage. In recently, some
papers of epi- or sub-retinal electrode, a safe current of stimulation describes
threshold as the other unit (e.g. W/cm2 or Voltage).
Next, the histological safe injection limit in our study was calculated by wattage
or electrical charge density (using a simple formula: P

I T, where
I - current, V - voltage, T - duration time, P - electrical power, Q - electrical charge).
For 100m electrode, a safe current of 1mA (11 V was necessary to
inject between active and return electrode) corresponded to 093W/cm 2
or 2119C/cm 2 , and harmful current of 1.5mA (14 V) corresponded to
178W/cm 2 or 3178C/cm 2 .
For 200m electrode, safety 1.5mA (12 V) corresponded to 057W/cm 2
or 1194C/cm 2 , and harmful 2mA (17 V) corresponded to 108W/cm 2

= IV Q =

or

1592C/cm 2 .
Reports by other groups concerning retinal (tissue) damage after electrical
stimulation indicated that an epi-retinal type heater probe with 25W/cm 2
caused immediate change of canine retina [5], and recent report about chronic
stimulation by epi-retinal type electrode showed that threshold charge density
of canine retina was 01mC/cm 2 [6] and by sub-retinal type electrode with
2.5 V caused acute change of rabbit retina [7]. In the study of brain tissue,
800C/cm 2 50C/phase (electrode; 65 10 − 6 cm 2 ) was the safety limit with ×