Reproducibility Of Visual Electrophysiology Recordings Between Laboratories: The Importance Of Regular Calibration

Abstract

Purpose: : ISCEV (International Society of Clinical Electrophysiology of Vision) provides standards for visual electrophysiology. The purpose of which is to standardise electrical records for ease of comparison across laboratories. In the past the standards had a wide range of settings to accommodate several different (branded) systems with their own preset values. Improvement in the flexibility that manufacturers now give the electrophysiologist in terms of stimuli selection now means a more stringent standard can be presented and realistically adhered to.This study aims to compare the Electroretinograms (ERGs recorded at three different EDT (Electrodiagnostic Testing) laboratories from the Merseyside region in the UK.

Methods: : ERGs were recorded five times at Royal Liverpool University Hospital (RLUH) to establish repeatability, and once at the Alder Hey and Walton laboratories. Calibration of the stimulus was conducted on the day for Alder Hey & Walton laboratories and was taken from a routine calibration 4 months prior for RLUH. Testing was conducted with gold leaf electrodes, testing included dim flash (0.01cd.s/m²) in the dark, standard flash (3.0 cd.s/m²) in the dark (SFD), oscillatory potentials in the dark (3.0 cd.s/m²), standard flash (3.0 cd.s/m²) in the light (34 cd/m²) (SFL) and 30Hz Flicker (3.0 cd.s/m²) in the light (34 cd/m²). Dark adaptation was 15 minutes.

Results: : The repeatability for each ERG parameter was examined using the CV (coefficient of variation). ERG amplitude CV was under 25% for all subjects for all tests. The CV for latency for SFL was less than 5% for all subjects. The variation in ERG results between sites was carried out using Friedman analysis which revealed modest evidence (p=0.022) to reject the null hypothesis for the latency parameter of the SFL at Alder Hey and RLUH, with medians of 31.5ms and 29 ms respectively. At this point a calibration revealed the output from the ganzfeld from the RLUH was reduced. Subsequently, the flash tube was cleaned and the system re-calibrated. Further data collection produced median values of 32ms and 31ms at Alder Hey and RLUH respectively.

Conclusions: : The repeatability of the ERG at the RLUH site is within clinically acceptable limits. Comparable data was obtained across the laboratories when luminance was correct despite using LED stimulators at two sites and xenon flash tubes at the other. Visual electrophysiologists must be vigilant about the output of their equipment and calibration must be done frequently particularly when involved in monitoring pathology.

Keywords: electroretinography: non-clinical • clinical laboratory testing