A new approach to diagnosing eye diseases

Researchers have long tried to determine if non-functioning photoreceptors can be detected as dead pixels on a camera sensor, and so far it has not been possible. However, a new technique will allow rapid and non-invasive assessment of the physiological state of the retina, a potential change in the treatment of eye diseases.

For years, electroretinography (ERG) was the only advanced method to assess the physiological state of retinal function. In recent years, a new technique called optoretinography (ORG) has been developed. In a variation of this technique, optical coherence tomography (OCT), the physiological response of retinal photoreceptors to visible light is measured as a change in optical path length in the nanometer range.

Until now, the ability to study the retinal ORG response to scintillation light stimulation has been limited. However, researchers at the International Center for Translational Eye Research (ICTER) have developed the spatio-temporal optical coherence tomography (STOC-T) method, a variant of full-field OCT which they pioneered, to record retinal flicker optoretinograms.

The research was conducted by Slawomir Tomczewski, PhD, Piotr Wegrzyn, David Borycki, PhD, Egidijus Auksorius, PhD, Maciej Wojtkowski, PhD, and Andrea Curatolo, PhD, of ICTER, and the results have been published in the journal Express Biomedical Optics.1

How to study electrical signals from photoreceptors

Electrophysiological examinations of the eye (ERG) consist of a set of recordings of light-induced changes in functional currents generated in the retina, the part of the eye responsible for receiving visual stimuli. These currents are transmitted to the visual area of ​​the cerebral cortex, then to the oculomotor muscles. The study of these retinal currents is of particular interest for the diagnosis of ocular diseases. The most important parts of the retina are the light-sensitive photoreceptor cells, which are modified neurons called cones and rods. The human retina contains about 6 million cones and 100 million rods.

Rods are responsible for black and white vision. They are more numerous in the peripheral parts of the retina and absent in the central fovea. They contain the abundant light-sensitive protein rhodopsin, the biochemical transformations of which are responsible for the initiation of vision. The cones, on the other hand, are found mainly in the central part of the retina and are responsible for color vision. They contain pigments sensitive to the primary colors: blue, green and red. Located mainly in the macula, the cones are also responsible for visual acuity.

Assessing the quality of photoreceptors is crucial because their dysfunctions are linked to various eye diseases, some of which lead to blindness. ERG is the most common type of electrophysiological examination, recording the functional electrical potential created in the eyeball under the influence of light stimuli. The test itself consists of placing an electrode in the form of a contact lens or nylon thread saturated with a conductive agent (DTL electrode) on the eye and sending pulses of light from a light-emitting lamp red. Light stimuli include short flash (1-3 msec), double flash, or flickering flash (ERG flash); as well as a checkerboard pattern of lightening and darkening squares. A special device reads the voltage values ​​produced by these pulses.

Photoreceptors as dead pixels

Unfortunately, the ERG test cannot detect the conformational changes and faster photoreceptor contractions that result from exposure to light on the retina and thus propagate the visual process. However, these rapid physical changes, likely induced by the early stages of the phototransduction process, are accessible by optical investigation with ORG, which measures light-induced optical signals from photoreceptors.

Measurements of the retinal response to a scintillation stimulus have proven useful in analyzing retinal adaptation to light and differences in critical scintillation frequency (CFF) between the macula and the periphery. To date, a limited number of ORG studies using flicker have been reported in the literature. Of these, one ORG study measured the response of photoreceptors to a periodic stimulus limited to a single low frequency (5 Hz), while two more recent ORG studies measured either the response of dark-adapted photoreceptors between 1 and 6.6 Hz, which is the slow response of the inner plexiform layer between 1 and 50 Hz, respectively. It has not been possible to measure the rapid retinal ORG response at frequencies above 10 Hz, until now.

ICTER scientists pre-adapted the eyes of human volunteers to bright light for one minute, then stimulated the retina with visible light that shimmered at a constant frequency. They were thus able to record retinal ORG responses at frequencies up to 30 Hz. This protocol made it possible to determine changes in the optical thickness of photoreceptors on the order of a few nanometers in response to a visual signal at frequencies higher flicker rates than ever reported before. This state-of-the-art technology has made it possible to determine the spatial and temporal responses of the retinal photoreceptor, on time scales and frequencies correlated with the first stages of the phototransduction process, which is the vital engine of vision.

For the first time, tiny contractions and expansions of photoreceptors could be observed in a healthy human retina following light stimulation at frequencies up to 30 Hz, and these could be mapped to different areas of the retina. . This feat has not been achieved by anyone before. “Nor would it have been possible without the ICTER scientists’ previous research and development of the STOC-T technique, with its exquisite sub-nanometer displacement sensitivity, high phase stability and impressive volumetric acquisition, on the order of milliseconds,” said Dr. Andrea Curatolo, Head of the Image Guided Devices for Ophthalmic Care Laboratory at ICTER.

ICTER researchers have shown that statistically significant differences in photoreceptor optical path length modulation amplitudes in response to different flicker frequencies can be detected in light-adapted eyes with better signal-to-noise ratio. than in dark-adapted eyes. These results highlight the prospect of a more objective study of CFF changes across the retina, which could allow early detection of retinal degeneration and other photoreceptor abnormalities.

The ICTER team of scientists are already planning further research to advance understanding of the biological and medical implications of observed photoreceptor behavior Going through STOC-T, anticipating new insights into the basic science of vision and the development of novel biomarkers of early visual impairment.

Reference

Sławomir Tomczewski, Piotr Węgrzyn, Dawid Borycki, Egidijus Auksorius, Maciej Wojtkowski and Andrea Curatolo; Light-adapted flicker optoretinograms captured with a spatio-temporal optical coherence tomography (STOC-T) system. pages 2186-2201 (2022) doi: 10.1364/BOE.444567

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