Multifocal Electroretinogram Study in Various Degrees of Myopia

Abstract

Myopia is a public health concern in many parts of the world where the prevalence of myopia has been reported to be as high as 80% in some parts. Myopia, also called near or short-sightedness, is a refractive defect of the eye in which collimated light produces image focus in front of the retina when accommodation is relaxed. The multifocal electroretinogram (mfERG) examines the retinal response of multiple loci within a short period of time (usually 5 to 10 minutes), which is similar to multiple measurements of focal ERGs. The mfERG measurement provides topographical responses of the retina. The multifocal electroretinogram (mfERG) has been applied to investigate retinal function in myopic eyes and altered mfERG responses were found in some studies; responses were reduced and delayed as the degree of myopia increased. The amplitude reduction was greatest towards the retinal periphery. In total, 80 eyes from 40 Egyptian subjects were enrolled in the study. Four study groups were established based on their refraction; group no.1 = emmetropic eyes, group no.2 = mildly myopic eyes with refraction ranging from -0.25 D to -3.00D, group no.3 = moderately myopic eyes with refraction ranging from -3.00D to -6.00D and group no.4= severely myopic eyes with refraction above -6.00D. The age of the subjects was ranging between 15 and 35 years. This study included the physiological form of myopia in which fundus showed no abnormality other than tigroid or tessellated fundus. All subjects had cylindrical corrections of less than 1.00D. All subjects were subjected to the following: History taking and thorough clinical examination including slit lamp examination, intraocular pressure measurement (using Goldmann applanation tonometer), autorefractometer reading, visual acuity, best corrected visual acuity (using Snellen chart), fundus examination (using indirect opthalmoscope), axial length measurement (using A-scan ultrasonography) and mfERG (using RETI-port/scan 21). Any subjects with pre-existing retinal diseases whether congenital or acquired especially DM, opaque media, glaucoma, previous eye trauma, previous retinal surgery, previous optic nerve disease as optic neuritis or ischemic optic neuropathy were excluded. In this study, we found that there was reduction in amplitude and prolongation of implicit times of mfERG in myopes as the refractive error increased and the amplitudes in the peripheral areas were more reduced. We found also that P1 implicit time of the mfERG response was increased in length in higher degrees of myopia and this was not explained by longer axial length. Multifocal ERG can differentiate between different degrees of myopia.