The rapid N-wave as a potentially useful measure of the photopic negative response.

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.