Test results can often also suggest where in the visual pathway a particular probl may be located. Probls might include; night vision/ night driving difficulties, photophobia, high light levels preferred when reading, and difficulties adapting to disparate light levels.Grigg said visually-evoked potentials (VEP) could be useful for conditions involving the inner retina, specifically the retinal nerve fibre layer and ganglion cell layer, while electroretinogram (ERG) was relevant to the inner retinal layers, from the inner plexiform layer, to the rods and cones.Electro-oculography (EOG) is most relevant to the outer retinal layers, beginning at the outer limiting mbrane and finishing at the retinal pigment epithelium. He then presented the typical ERGs for scotopic and photopic light states for the normal eye to be contrasted later with abnormal findings. They were followed by rod and cone-specific tests, light-adapted, 30 Hz flicker, and flash stimuli.Flash ERG was described as a mass response by the retina, which consists of 92 million rods and 4.6 million cones. The latter is packed 200,000 to the square mm at the fovea, accounting for 20% of the total cone population.Grigg also described the more complex pattern ERG (PERG – 15 and 30° fields), multifocal ERG (mfERG), and full-field ERG (FFERG) tests, and finished his formal presentation with an overview of the particular assessments or differentiations each was used for, e.g. retinopathy versus maculopathy versus neuropathy and central versus peripheral vision loss.In one of his case presentations that followed, he revealed that the combination of electrophysiology tests required for some conditions can take up to two hours to complete. Finally, he explained that ocular electrophysiology has also found uses in genetic testing and treatment, although it becomes a challenge when multiple genetic probls are suspected or present.
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Associate Professor John Grigg is Head of the Discipline of Ophthalmology at The University of Sydney’s Save Sight Institute. He has clinical responsibilities at Sydney Eye Hospital including inherited eye disease and glaucoma clinics as well as at The Children’s Hospital, Westmead, Sydney. His main areas of research are in genetic eye disease and electrophysiology of the visual syst. |
