Increased phase lag of the fundamental harmonic component of the 30 Hz flicker ERG in Schubert-Bornschein complete type CSNB

The 30 Hz flicker electroretinogram (ERG) response was studied in seven patients with Schubert-Bornschein complete-type congenital stationary night blindness (SB-CSNB) by measuring conventional peak implicit times and by harmonic analysis. Responses were elicited with xenon photostrobe flashes. The fundamental flicker component showed a significant increased phase lag (P = 0.002), even though the peak implicit times were not delayed (P = 0.22). Although others have noted a “squared-off” flicker waveform in SB-CSNB, this is the first quantitative demonstration of a temporal abnormality in the flicker ERG and is the first report that a retinal disease can delay the fundamental flicker harmonic while the peak implicit time remains normal. Similarly, the phase of the flicker fundamental harmonic in monkey was also delayed after intravitreal 2-amino-4-phosphonobutyric acid (APB) that blocks the activity of depolarizing bipolar cells (DBC), whereas peak implicit time remained virtually unchanged. These observations, coupled with the normal amplitude and slope of the photopic a-wave (which was recorded under conditions that elicit a substantial contribution from hyperpolarizing bipolar cells) supports the hypothesis that the abnormality in the DBC pathway in SB-CSNB lies post-synaptic to the cones and not in the presynaptic glutamate release by cone photoreceptors.