Electroretinographic Mapping of Retinal Function: Evaluation and Clinical Application

The conventional electroretinogram (ERG) is an electrophysiological examination that is used to assess the extent of retinal function within the eye. The test exploits the retina's ability to convert photons of light into electrical impulses. A flash stimulus is employed to evoke an electrical potential within the retinal cells (cones, rods and bipolar cells). These potentials, generated in the retina, are then recorded at the corneal surface of the eye using a contact lens or scleral electrode. The electrical potential generated possesses several distinct components that can be used to locate dysfunction from discrete layers of the retina. Although the ERG is used routinely in the diagnosis and monitoring of a wide range of retinal disorders its application is restricted because the diffuse stimulation of the retina evokes a global response thereby preventing the detection of localised abnormalities. A new technique has recently been developed, the Visual Evoked Response Imaging System (VERIS), which overcomes some of the shortfalls of the conventional electroretinogram. This new method allows functional mapping of the retina by the ERG. The technique enables simultaneous recording from a large number of retinal areas. Each area is independently stimulated in a sequence employing Pseudo Random Binary Sequences (PRBS). The sequences of stimulation are uncorrelated (achieved by temporal modulation of the sequence for each area) therefore the individual responses from different areas of the retina can be extracted. This thesis describes the evaluation of this system with regards to its potential within routine clinical practice. A number of investigations were performed to fulfil this evaluation. Possible factors that may influence the integrity of data obtained from the new technique were investigated and, where possible, minimised. These factors included quantifying the effects of filtering bandwidth, refractive errors, electrode type, response measurement method, luminance, contrast, dilation and the artefacts associated with poor patient shielding (from extraneous electromagnetic interference). The findings from these investigations were used to optimise the recovery of local ERG responses and established a protocol for future investigations. A custom software program was developed for analysis and interpretation of data. A study was undertaken to quantify the repeatability and reproducibility of the technique and to provide normative values. Finally these results were used to assess the techniques ability to objectively detect and quantify several retinal disorders. The investigations indicated that the system was of particular benefit in the assessment of local retinal pathology. However the system suffered a higher exclusion criteria than conventional electrophysiology and was unable, in the clinical setting, to identify retinal pathologies selectively affecting the ganglion cell layer. These factors limited the systems application within clinical practice. Current developments are aimed at improving the technique and establishing a routine clinical test with improved sensitivity and specificity that can be successfully applied to a wider population group