DEVELOPMENT OF ELECTRORETINOGRAM AND ROD PHOTOTRANSDUCTION RESPONSE IN HUMAN INFANTS

Purpose. To describe and analyze developmental change from birth throu gh maturity of human electroretinogram (ERG) response, especially in t erms of rod phototransduction as represented in the ERG a-wave. Method s. Electroretinograms were recorded from 16 human infants from 5 to 27 0 days of age, two children 1.9 and 3.4 years of age, and 13 older sub jects between 10 and 43 years of age. A range of full-field, white-lig ht flashes up to intensities sufficient to saturate a-wave and b- wave amplitudes and a-wave rate-of-rise was used. The a-wave leading edge, the a-wave and b-wave amplitudes, and the b/a-wave ratio at maximum i ntensity were analyzed using a model of the activation steps of the G- protein phototransduction cascade. This model, applied to a-waves, pro vides three parameters interpretable in terms of rod phototransduction : a(max) (a-wave maximum amplitude, proportional to circulating dark c urrent), A' (estimated constant of transduction amplification), and t' (eff) (sum of brief delays associated with the cascade steps). Results . Both a(max) and b(max) (maximum b-wave amplitude) increased rapidly from birth. b(max) reached apparently mature values by approximately 6 months, but a(max), and thus (b/a)(max) (b/a ratio at maximum intensi ty), did not reach mature values until sometime after the third or fou rth year. Similarly, A' was immature at birth at approximately 25% to 50% of adult levels at intensities below rate-of-rise saturation. For the youngest infants, rate-of-rise saturation appeared to occur at low er effective isomerizations per rod compared to that of the adult Foll owing a time course similar to that of a(max), full maturity for A' pr obably was not reached before 5 years of age. Conclusions. Results fro m the a-wave analysis are consistent with immaturities in the rod phot oreceptors early in life. The difference from those of the adult may b e explained by lower neonatal concentrations in one or more of the tra nsduction substrates, decreased outer segment length, and, possibly, d ecreased density of some membrane proteins mediating the cationic dark current. Early adultlike b-wave amplitudes suggest early maturity for the inner retinal elements (rod bipolar and Muller cells) underlying b-wave response, compared to the photoreceptors.