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 (“N-wave”) 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 centered around 18 μV, while the N-wave median centered around 7 μV, 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.
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Funding
Stanford University School of Medicine, Medical Scholars Program: Brandon H Pham. Retina Research Foundation: Michael F Marmor. National Eye Institute P30-EY026877: Departmental award. Research to Prevent Blindness: Departmental award. Dompe Pharmaceuticals research grant: Jeffrey L Goldberg.
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All of the authors declare they have no conflicts of interest.
Research involving human participants and/or animals
This was a human retrospective record review, conducted in accordance with the Declaration of Helsinki. The study was approved by the Institutional Review Board at the Stanford University School of Medicine as meeting ethical standards.
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Approved by the Stanford IRB as a retrospective, observational review of electronic medical records with waiver of consent.
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Pham, B.H., Goldberg, J.L. & Marmor, M.F. The rapid N-wave as a potentially useful measure of the photopic negative response. Doc Ophthalmol 141, 253–257 (2020). https://doi.org/10.1007/s10633-020-09769-w
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DOI: https://doi.org/10.1007/s10633-020-09769-w