152 C. D. Luu and A. W. L. Chia
Assessment of Retinal Function
By using our knowledge of the origin of various components of the electroretinography response, we are able to assess the effect of myopia on different retinal layers and regions in turn.
Outer retinal (photoreceptor) function
The ERG a-wave response amplitude is reduced in adults with myopia,9,10 indicating the presence of abnormal outer retinal (photoreceptors) function in adult myopic eyes. The relationship between ERG amplitude and severity of myopia is best described by a linear function. The amplitude of the ERG a-wave is positively correlated with the severity of myopia9 and inversely correlated with the axial length.10,11
The affect of myopia on the function of each individual cone type was investigated by Yamamoto et al.12 using specialized ERG technique. In their study, the ERG was recorded with chromatic stimuli obtained with various colored filters. The results showed that short- (S), medium- (M), and long-wavelength (L) sensitive cone response amplitudes decreased with increasing myopia, however, a more significant correlation was detected between the L,M-cone response amplitude and refractive error than between the S-cone response amplitude and refractive error.12 These findings suggest that the L,M-cones are possibly more affected than the S-cones in myopia.
Post-receptoral (bipolar cell) and retinal transmission function
Many studies have shown that the ERG b-wave response amplitude is decreased with increasing myopia.9–11 Similar to that of the ERG a-wave response, the b-wave response amplitude is positively correlated with the severity of myopia9 and inversely associated with the axial length.10,11
The interpretation of the b-wave reduction in myopia is not as straightforward as that of the a-wave. Although the ERG b-wave amplitude has been reported to be consistently reduced in eyes with myopia, this does not necessarily imply that there is a presence of abnormal transmission between the outer and middle retina, or post-receptoral dysfunction. This is because reduction of the a-wave amplitude will cause a proportionate reduction of the b-wave amplitude. In order to examine
153 Retinal Function
the retinal transmission abnormality, a b-/a-wave amplitude ratio should be investigated.
Perlman et al. reported that all myopic eyes in their study had subnormal ERG b-wave amplitude, but normal b-/a-wave amplitude ratios suggest that myopic eyes have normal signal transmission in the retina.10,13 However, Pallin et al.9 reported that the higher the myopia, the smaller the b-/a-wave amplitude ratio, although these ratios were within the normal range. Pallin et al. believed that signal transmission in the retina tends to be somewhat reduced in an eye with high myopia.
Inner retinal function
Abnormal oscillatory potentials (OPs) and retinal adaptation in eyes with myopia have been reported in a number of studies.11,14 Chen et al.15 investigated retinal adaptation in eyes with myopia using a global flash paradigm of the mfERG. They showed that retinal adaptation varied with the degree of myopia. The abnormal OP and retinal adaptation are thought to be linked to the hypothesis that dopamine may play a role in the development of myopia.
It has been shown in animal models of myopia that dopamine, a neurotransmitter released by the inner retina, is associated with the development of myopia.16,17 Dopamine is an important chemical messenger for amacrine cell and retinal ganglion cell processing and is involved in the process of retinal luminance adaptation.18 Retinal amacrine cells have also been shown to play an important role in the modulation and control of ocular growth.19–22 The retinal oscillatory potentials (OPs) of the ERG reflect the amacrine cell function, hence, abnormal OPs in myopia have been suggested to be related to the changes in dopamine level within the inner retina associated with myopia. However, the presence of outer retinal dysfunction, which is commonly seen in eyes with myopia, can cause an apparent abnormal inner retinal ERG parameter, including the OPs. Therefore, the association between abnormal OPs and myopia needs to be interpreted with caution, because it is possible that the abnormal OPs were due to the presence of abnormal outer retinal function.
Macular function in myopic retina
The macular function of eyes with myopia has been investigated by a number of groups using the multifocal ERG (mfERG).23–27 There was