The rapid N-wave as a potentially useful measure of the photopic negative response
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ORIGINAL RESEARCH ARTICLE
The rapid N-wave as a potentially useful measure of the photopic negative response Brandon H. Pham
. Jeffrey L. Goldberg
. Michael F. Marmor
Received: 18 November 2019 / Accepted: 30 April 2020 Ó Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract Purpose The photopic negative response (PhNR) correlates with ganglion cell function and has previously been examined as an indicator of glaucomatous optic nerve damage. However, it is a prolonged response that is measured against baseline, and its clinical utility has been limited by extensive variability, poor repeatability, and baseline instability. We have observed a distinct brief negative wave (‘‘Nwave’’) commonly present within the slow PhNR trough, which may provide practical and analytic advantages as a clinical measure. Methods We reviewed data from an interventional trial of 59 glaucoma patients who had 4 exams over an 8-month period. The PhNR was recorded with standard ISCEV stimuli (1 Hz and in some cases 4 Hz stimulation), and N-waves were measured manually, relative to return to baseline. Results N-waves, when present, could be measured easily despite shifting baselines and a degree of background noise. The PhNR median amplitude
B. H. Pham Stanford University School of Medicine, 291 Campus Drive, Li Ka Shing Building, Stanford, CA 94305, USA J. L. Goldberg M. F. Marmor (&) Department of Ophthalmology, Byers Eye Institute at Stanford, 2452 Watson Court, Palo Alto, CA 94303, USA e-mail: [email protected]
centered around 18 lV, while the N-wave median centered around 7 lV, with a distribution of responses skewed toward low or zero amplitudes. Conclusions The N-wave appears to be a component of the longer PhNR, though its exact origin and significance remain unclear. As a rapid waveform that is independent of baseline, the N-wave is in many ways easier to measure accurately than the slower PhNR, which is highly dependent on baseline stability. The N-wave may prove useful clinically if further studies can optimize its stimulation, show its behavior in normal individuals and find correlation with markers of optic nerve disease. Keywords Photopic negative response Electroretinogram Glaucoma
Introduction Glaucoma and other optic nerve (ON) disorders are challenging to follow clinically, because few methods measure retinal ganglion cell (RGC) function specifically and accurately. Visual fields, optic disk observation, and nerve fiber layer measurements are complicated by subjectivity or reliability and their imperfect correlation using current measures suggests differing elements of structural and functional progression in disease. Although not used in common
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clinical practice for glaucoma, electrophysiologic measures of RGC function may provide additional insight into disease status or progression. The photopic negative response (PhNR) is a slow negative-going wave after the cone b-wave of the electroretinogram (ERG) that originates in RGCs
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