2.5

Retinal Function

Chi D. Luu* and Audrey W.L. Chia†,‡

Introduction

Myopia, or short sightedness, is a condition where the refractive apparatus of the eye, the cornea, and lens, focus the image of distant objects in front of the retina.

Myopia is often associated with an increase in axial length, however, it is unclear whether anatomical changes as a result of axial elongation have any effect on retinal function. This chapter summarizes the existing literature on the effect of myopia on the function of various retinal components, including the photoreceptors, post-receptoral bipolar cells, and the inner retinal component. The central retinal function in myopia and its potential role for predicting the rate of myopia progression in children will also be discussed. This chapter will cover only those studies involving human subjects and that use electrophysiological techniques for the assessment of retinal function.

Electroretinography

Electroretinography is a technique used to study the physiology of both neuronal and non-neuronal cells in the retina. There are a number of electroretinography techniques, but only Ganzfeld electroretinography and the multifocal electroretinography will be described here, because they are the most commonly used as means to assess retinal function in myopic eyes.

*Centre for Eye Research Australia, The University of Melbourne, Royal Victorian Eye and Ear Hospital, Melbourne, Victoria, Australia. E-mail: chi_luu@!yahoo.com.au

†Singapore Eye Research Institute, Singapore.

‡Singapore National Eye Centre, Singapore.

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150 C. D. Luu and A. W. L. Chia

Ganzfeld electroretinography

Ganzfeld or full-field electroretinography (ERG) has been well-recognized as a useful and non-invasive tool for objective assessment of retinal function. The ERG response is a mass electro-retinal potential generated by various retinal cell types whose relative contributions depend on the stimulus properties and background adapting conditions. For example, under dark-adapted conditions, the ERG response to a bright flash stimulus manifests an a- and b-wave that primarily reflects the activity of photoreceptors (a mix of rods and cones) and depolarizing bipolar cells, respectively. The ERG response to a bright flash under light-adapted condition is derived from the activities of only cone photoreceptors and cone bipolar cells.

The standard protocol for full-field ERG as defined by the International Society for the Clinical Electrophysiology of Vision (ISCEV) consists of five responses (Fig. 1); scotopic response (derived from the rod system), maximal response (derived from rod and cone receptors and postreceptoral activities), oscillatory potentials (derived from inner retinal and amacrine cells), photopic single flash response (shaped by cone photoreceptors and post-receptoral activities), and response to 30 Hz flicker (cone bipolar cell function).1 Thus, the function of different layers

Figure 1. Five standard responses of the full-field ERG.

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of the retina can be assessed from the responses obtained, and the pattern of abnormality from these five responses enables us to identify the retinal site of the lesion. Because the ERG records a mass retinal potential, it is useful in diseases that affect the retina globally (e.g. retinitis pigmentosa), but it is not sensitive enough to detect those conditions associated with subtle or localized functional changes within the retina (e.g. macular degeneration).

Multifocal electroretinography

Unlike full-field ERG, multifocal ERG (mfERG) can record responses from more than 100 different retinal locations simultaneously and provide a detailed functional topography of the retina (Fig. 2).2,3 Thus, mfERG is more sensitive than full-field ERG in detecting a localized retinal dysfunction. The mfERG response waveform has three major components, namely N1 (first negative trough), P1 (first positive peak), and N2 (second negative trough). The clinical applications of mfERG have been widely recognized,4 and mfERG has been proven to be a sensitive technique for the early detection of retinal dysfunction in various conditions including diabetic retinopathy5–7 and retinal toxicity.8

Figure 2. Trace array response of the mfERG superimposition on a fundus photograph (left), demonstrating the functional topography feature of mfERG. The mfERG waveform was enlarged (right) to illustrate different components of the mfERG response.