Functional involvement of cone photoreceptors in advanced glaucoma: a multifocal electroretinogram study

The purpose of the study is (1) to demonstrate the anatomical variation of cone photoreceptor density across normal retina as a sectoral amplitude asymmetry of photopic multifocal electroretinogram (mfERG) and (2) to study the potential presence of sequential or differential, functional cone photoreceptor damage in glaucoma using this amplitude asymmetry. A 37-Block scaled mfERG was recorded from 22 controls and 27 glaucoma subjects. The N1 and P1 amplitudes of averaged responses from corresponding zones nasal and temporal to fovea were analyzed for asymmetry in controls and glaucoma subjects. Amplitude asymmetry was demonstrable for both N1 (p < 0.001) and P1 (p < 0.001) parameters in control subjects. Although this amplitude asymmetry was preserved in glaucoma subjects with moderate field defects, it was not demonstrable in patients with advanced field defects. The anatomical variation in cone photoreceptor distribution across normal retina is demonstrated as an amplitude asymmetry in first order kernel responses of mfERG. The cone photoreceptors in the region nasal to fovea appear to be affected only in advanced glaucoma possibly suggesting that photoreceptors could follow a sequential damage like the overlying neuroretinal rim in glaucoma.