Ocular biometry in preterm infants: implications for estimation of retinal illuminance.

PURPOSE Retinal illuminance varies with stimulus luminance, pupil size, eye size, and transmissivity of the ocular media. There are few published data to inform estimates of retinal illuminance in early infancy, and no concurrent studies of pupil size and ocular length have been described. The goals were to document simultaneously the measured ocular parameters in growing preterm infants and to estimate the potential errors associated with using either stimulus luminance or troland value as a proxy for retinal illuminance in this population. METHODS Ocular biometry including diameters of the eye, vitreous chamber depth (VCD) and dilated pupil diameter was performed on 111 occasions in 33 preterm infants aged between 30 and 55 weeks' postmenstrual age. RESULTS Eye size increased rapidly between 30 and 55 postmenstrual weeks and was comparable to that of term-born infants. The ratio of dilated pupil area to VCD(2) was highly variable. Retinal illuminance of the infant eye compared with adult eyes was underestimated by both stimulus luminance and troland values. CONCLUSIONS Stimulus luminance and troland values cannot be used to infer retinal illuminance when comparing eyes of markedly differing sizes or transmissivities. Error in estimating retinal illuminance in prematurely born infants is inevitable because of uncertainty regarding media transmissivity, but this discrepancy can be minimized by using directly measured pupil diameter and data presented herein for eye size in this population.