Functional Measurements of Retinal Phototoxicity using Photopigment Densitometry and Adaptive Optics

Recent advances in high-resolution retinal imaging have led to the discovery of retinal changes caused by light exposures below previously published damage thresholds. These effects were discovered by imaging the retinal pigment epithelium (RPE) with adaptive optics (AO). The changes observed were a transient decrease in RPE autofluorescence (AF reduction) and a disorganization of RPE autofluorescence (RPE disruption). The origins of these changes are not fully understood and their functional consequences have not been previously investigated. Understanding these phenomena is critical for the field of ophthalmic imaging; yet, research to date has been limited to techniques that measure retinal structure and provide little information about function. Photopigment densitometry is a measure of retinal function that can be implemented in a reflectance imaging system and has been used to study the retina for over 60 years. With the advent of near diffraction-limited ophthalmoscopes, which can resolve individual photoreceptors, it is now possible to apply this technique to investigations requiring high spatial resolution. This thesis describes a study that combined high-resolution retinal imaging with photopigment densitometry to investigate the functional consequences of AF reduction and RPE disruption. An adaptive optics scanning laser ophthalmoscope was adapted to measure the density and regeneration rate of the photopigment rhodopsin. Rhodopsin kinetics were measured before and after a series of radiant exposures that caused various degrees of RPE disruption. No measurable change in rhodopsin recovery rate was found at any exposure level and RPE disruption was found to be visible at exposure levels that did not produce a significant reduction in rhodopsin density. However, such a reduction in rhodopsin density was measured at the highest exposure levels. Additionally, a new effect caused by near-infrared (NIR) illumination was discovered: a decrease in infrared autofluorescence (IRAF) measured after exposure to NIR illumination at levels well below recommended limits. Because many retinal imaging systems rely on NIR illumination to avoid potentially harmful exposures at shorter wavelengths, understanding the source of this IRAF reduction is important for both scientific and clinical imaging. This thesis provides the first description of IRAF reduction, as well as an examination of its basic properties.