Retinal electrophysiological characteristics of the myopic eye

Abstract

Introduction: Myopia is a common refractive error in the Chinese population. About 50% of the children in Hong Kong are myopic. Anatomically, myopia is usually characterized by an increase in axial length of the eyeball, leading to an optical focus in front of the retina. Although myopia can be simply corrected by optical aids to solve the vision problem, the elongated eyeball has been found to influence the retinal structure and physiology. Clinically, high myopes are at greater risk of developing retino-choroidal degeneration. Numerous studies have demonstrated reduced and delayed multifocal electroretinogram (mfERG) response in myopic adults. About 40% delayed mfERG response in myopic adults has been attributed to the effect of both refractive error and axial length, and the remaining variance of implicit time has been proposed to be related to attenuation of inner retinal function. In contrast, the mfERG response in myopic children is only delayed without significant change in amplitude. The reasons of underlying difference in retinal function between children and adults with myopia are still not clear. It has been shown that the retina can detect defocus signals locally in chicks. Inducing optical defocus in different retinal regions has profound effects on the compensatory response of the whole globe. However, there is still a lack of knowledge on the regional retinal activity in the presence of positive and negative optical defocus. In this study, we aimed to investigate the retinal function in myopic eyes of both children and young adults. In addition, we also studied the changes of retinal activity to the defocus signals in different regions. Objective: 1. To investigate the changes in adaptive circuitry of the inner retina in myopic adults by using the global flash mfERG at different contrast levels (Experiment 1). 2. To compare the retinal functions of myopic children versus young adults using global flash mfERG (Experiment 2). 3. To examine the retinal electrophysiological changes during myopia progression over a 1-year period in children (Experiment 3). 4. To study the effect of positive and negative optical defocus on changes of electrical response as a function of retinal region in adults (Experiment 4). Methods: The mfERG measured with conventional stimulation mainly reflects the activity of the outer retina. The global flash mfERG, which incorporates the global flash screen response within conventional stimulation, can enhance inner retinal activity, in addition to outer retinal activity. So, this special paradigm of mfERG recording was used in this study. There are two components, the direct component (DC) and the induced component (IC), which reflect the activity from outer retina and inner retina respectively, recorded in the global flash mfERG paradigm. In Experiment 1, fifty-four adults (aged from 19 to 29 years) with various magnitudes of refractive error received the global flash mfERG at different levels of contrast, i.e. 29%, 49%, 65% and 96%. Cycloplegic subjective refraction and axial length were measured. Hierarchical multiple regression models were used to evaluate the effect of refractive error and the combined effects of refractive error and axial length on the mfERG responses. In Experiment 2, fifty-two children (aged from 9 to 14 years) and nineteen young adults (aged from 21 to 28 years) with refractive errors ranging from plano to -5.50 D were recruited for the global flash mfERG at both 49% and 96% contrasts. Refraction and axial length were measured. The analyses were the same as for Experiment 1. In Experiment 3, twenty-six children (aged from 9 to 13 years) received the global flash mfERG at both 49% and 96% contrasts and refraction in two visits 1-year apart. Pearson{174}s correlation was used to study the association between change in refraction and change in mfERG response in different retinal regions over the 1-year period. In Experiment 4, twenty-three subjects (aged from 19 to 25 years) with normal ocular health were recruited for global flash mfERG measures at 96% contrast under the condition of control (in-focus), positive defocus (+2 D and +4 D) and negative defocus (-2 D and -4 D) conditions. Repeated-measures ANOVA was used to investigate the effect of defocus on the mfERG response in different retinal regions. Results: In Experiment 1, myopic adults had a significant reduction in the paracentral DC amplitudes for both 29% and 49% contrasts and in the paracentral IC amplitudes at all contrasts measured. The peripheral IC amplitudes for 49% contrast were also reduced. Refractive error explained about 14% and 16% of the reduction in paracentral DC and IC amplitudes respectively, but axial length could not account for further change in either paracentral DC or IC amplitude in the hierarchical regression models used. Neither refractive error nor axial length contributed to any change in implicit time for either DC or IC response. In Experiment 2, myopic children had a significant reduction in central DC amplitude at 96% contrast and unaffected IC responses at both contrasts for all regions. In contrast, myopic adults showed a significant reduction in paracentral IC amplitudes at 49% contrast but not at 96% contrast. The DC amplitudes at both contrasts of all regions examined were virtually unaffected. Implicit times for DC and IC responses were unaffected for either group. In Experiment 3, children with progressing myopia showed significant reduction of central DC and IC amplitudes, and mild attenuation of paracentral DC and IC amplitudes at 49% contrast as myopia progressed. In Experiment 4, the mfERG responses were found to have more significant changes in the paracentral retinal region than in the central region under defocused conditions. The paracentral DC amplitudes showed a significant reduction under negative defocused conditions. In contrast, the paracentral IC amplitudes showed a significant increment under positive defocused conditions. Interestingly, the central IC response showed significant reduction in amplitude only to negative defocus, while the response increased in amplitude to positive defocus. However, the DC and IC implicit times were virtually unchanged under defocused conditions. Conclusions: This study shows that the effect of myopia mainly affected the inner retinal function in myopes. Retinal function was generally unaffected in myopic children, except for the outer retinal function in the central region. As myopia progressed, the inner retinal function from central to paracentral regions was reduced, especially in the central region. The retinal function in myopic adults differed from myopic children in terms of regions and retinal components being affected. The inner retinal function from paracentral to mid-peripheral regions was significantly impaired in myopic adults, whereas the outer retinal function in these regions was only mildly reduced due to myopia. There was also a progressive change of retinal impairment from central to mid-peripheral regions from children to adults with myopia. Moreover, paracentral retina in the human eye reacted more strongly to optical defocus than central retina did; paracentral retina also differentiated the sign of defocus. Therefore, we speculate that the regional deterioration in retinal function in adults with myopia is probably related to the effect of peripheral defocus on the myopic eye growth.