Increment Threshold Functions in Retinopathy of Prematurity

PURPOSE: To assess scotopic background adaptation in subjects with a history of preterm birth and retinopathy of prematurity (ROP). Retinopathy of prematurity is known to have long-term effects on rod photoreceptor and rod mediated postreceptor retinal function. METHODS: Rod-mediated thresholds for detection of 3° diameter, 50 ms stimuli presented 20° from fixation were measured using a spatial forced choice method in 36 subjects (aged 9–17 years) with a history of preterm birth and 11 age similar term-born subjects. Thresholds were measured first in the dark-adapted condition and then in the presence of 6 steady background lights (−2.8 to +2.0 log scot td). A model of the increment threshold function was fit to each subject's thresholds to estimate the dark-adapted threshold (T(DA)) and the Eigengrau (A(0), the background that elevates threshold 0.3 log unit above T(DA)). RESULTS: In subjects with a history of severe ROP, both T(DA) and A(0) were significantly elevated relative to those in former preterms who never had ROP and term-born control subjects. Subjects who had mild ROP had normal T(DA) but elevated A(0). Neither T(DA) nor A(0) differed significantly between former preterms who never had ROP and term-born controls. CONCLUSIONS: The results suggest that in severe ROP, threshold is affected at a preadaptation site, possibly the rod outer segment. In mild ROP, changes in the Eigengrau may reflect increased intrinsic noise in the photoreceptor or postreceptor circuitry or both.