Cone electroretinogram amplitude growth with light adaptation in patients with retinitis pigmentosa

It has been hypothesized that the increase in the b-wave during light adaptation is directly related to the level of cone malfunction in patients with retinitis pigmentosa (RP). Because this hypothesis has important bearing on the mechanism for the increase in the electroretinogram (ERG), we examined the increase in the amplitude of the cone ERG during light adaptation in patients with typical RP.

Cone ERGs were recorded to Ganzfeld white flash stimuli in the presence of white background illumination in 51 RP patients and in 27 normal subjects.

In the normals, the increase in the b-wave amplitude during light adaptation ranged from 14-92% of the dark-adapted amplitude. All RP patients showed an amplitude increase that ranged from 5 to 100% of the baseline amplitude. This increase was not significantly different from that of the normals (p = 0.71, unpaired t-test). The baseline amplitudes and the increase in the relative amplitude were weakly correlated in the RP patients (r = 0.31; p = 0.029). No significant difference was observed in the amplitude increase between patients with near normal b-wave implicit times and those with delayed times (p = 0.17, unpaired t-test). Changes of the b-wave implicit time were not significantly different from those in the controls.

These findings that the changes in the cone ERG with light adaptation in the RP patients were very similar to those in normal subjects do not support the proposed hypothesis that the increase in the b-wave amplitude during light adaptation was directly related to the level of cone malfunction.