The luminance-response function of the brief flash full-field photopic electroretinogram (ERG) rises to a peak before falling to a sub-maximal plateau -- the 'photopic hill'. The combination of on- and off-responses inherent in the brief flash photopic ERG suggests that this luminance-response function could be modelled by the sum of a Gaussian function and a logistic growth function. Photopic ERGs to a luminance series of brief flashes against three different background luminances recorded from seven healthy adults showed the characteristic 'photopic hill' function for b-wave amplitudes which were satisfactorily fitted with the sum of a Gaussian curve and a logistic growth curve. As background luminance increased, both components shifted to the right on the luminance axis. The Gaussian component increased in amplitude while the logistic growth function component decreased in amplitude. The luminance-response function of a complete congenital stationary night blindness patient had almost no logistic growth component.
The purpose of the study is to characterise retinal function using light-adapted electroretinograms (ERGs) in a series of young children with ONH, congenital dysplasia of retinal ganglion cells. ERGs were recorded with chloral hydrate sedation in 27 children with ONH (18 with bilateral and 9 with unilateral ONH, age 4-35 months) and an adult reference population (n = 12). Stimuli included ISCEV standard flash, oscillatory potentials (OPs) and standard flicker as well as a light-adapted luminance-response series (photopic hill). The disc diameter to disc macula (DD:DM) ratio was measured from fundus photographs. The results are eyes with ONH, classified by DD:DM, were severe (0.35, n = 9), all had prolonged ERG implicit times and smaller i-waves than those of adults. Eyes with moderate or severe ONH also had smaller amplitudes for OPs and flicker ERGs and required stronger flashes to obtain the peak b-wave amplitude. Abnormalities of the photopic hill were a common but inconsistent feature of ONH and were not indicative of ONH severity. Abnormalities of the photopic hill of the ERG suggest that some cases of ONH may have retinal dysfunction with specific deficits in the ON or OFF pathways of the retina. ONH is a complex and heterogeneous condition that may involve dysfunction distal to the retinal ganglion cells.
Optic disc diameter, observation of disc pallor, VEP and PERG testing in infancy are useful for establishing the visual prognosis at 5 years of age in children with ONH. This is consistent with the notion that these parameters are related to the anatomic and functional preservation of retinal ganglion cells.
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