Correlation between Optical Coherence Tomography and Photopic Negative Response of Flash Electroretinography in Ganglion Cell Complex Assessment in Glaucoma Patients

Abstract

G laucoma is a multifactorial disease that is characterized by loss of RGCs that leads to adistinctive optic neuropathy and associated VF loss, where elevated IOP is considered the principal risk factor for the development and progression of glaucomatous neuropathy. Diagnosis of glaucoma is based upon detecting structural and functional glaucomatous changes. Structural changes are detected using slit lamp funduscopic examination or by observing for changes on fundus stereophotographs, and also by using various imaging modalities, especially OCT. Functional changes can be detected by SAP. Optical coherence tomography is an objective means to obtain high-resolution topographic images and measurements of the ONH, peripapillary RNFL, and macular GCC for assessing glaucomatous damage. Standard automated perimetry is the most widely used method to assess the visual deficit in glaucoma, where it detects functional changes in patients with suspected disease and monitors the progression of disease in those with established glaucoma. Presence of characteristic visual field defects can confirm the diagnosis, but as many as 30% to 50% of RGCs may be lost before defects are detectable by SAP. Thus, there can be considerable permanent structural loss before any functional changes are detected. In addition, Visual field tests are inherently subjective, so there can be significant variability on repeated testing, even in stable cases. The ERG recorded to a full-field flash stimulus contains an initial negative deflection, the a-wave that originates from photoreceptors, followed by a positive deflection, the b-wave that originates from Muller cells and Bipolar cells. The ISCEV extended protocol describes an extension to the ERG Standard, namely the PhNR of the light adapted flash ERG. The PhNR is a slow negative-going wave after the b-wave that provides information about the function of RGCs and their axons