Utility in clinical practice of standard vs. high-intensity ERG a-waves.
OBJECTIVE
Standard ERG a-waves represent contributions from both photoreceptor and inner retinal cells, while the leading edge of the high-intensity a-wave is produced only by photoreceptors. This has raised questions about the value of the a-wave as an indicator of photoreceptor disease, and has led to suggestions for standardizing higher-intensity stimuli. Our objective was to compare the behavior of standard and high-intensity a-waves in clinical practice.


METHODS
Standard ISCEV (International Society for Clinical Electrophysiology of Vision) a-waves and high-intensity a-wave responses were recorded under scotopic and photopic conditions from normal subjects and from patients with photoreceptor dystrophies and other diseases.


RESULTS
The standard scotopic a-wave amplitude followed the high-intensity a-wave closely among patients with different diagnoses, and the results did not change significantly when cone a-waves were subtracted to isolate rod signals. The only exception was one patient with the enhanced S cone syndrome (ESCS) whose dark-adapted responses were cone-driven. Initial peak times clustered in a small range for both standard and high-intensity responses, and were not very sensitive to disease.


CONCLUSIONS
High-intensity a-waves can show photoreceptor characteristics directly, and may help analyze some rare disorders. However, in our study the amplitude of conventional scotopic a-waves mirrored that of the high-intensity responses quite closely over a wide range of patients. This suggests that for practical purposes even if it is not perfect, the standard ERG is an excellent indicator of photoreceptor disease.
Marcus
Mira
M
Department of Ophthalmology, Stanford University Medical Center, Room A-157, 300 Pasteur Drive, Stanford, CA 94305-5308, USA.
Cabael
Lorella
L
Marmor
Michael F
MF
D000221:Adaptation, Ocular; D003623:Dark Adaptation; D004596:Electroretinography / Q000379:methods* / Q000592:standards*; D006801:Humans; D010775:Photic Stimulation / Q000379:methods*; D017949:Retinal Cone Photoreceptor Cells; D012164:Retinal Diseases / Q000175:diagnosis* / Q000503:physiopathology; D017948:Retinal Rod Photoreceptor Cells