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Angela M Brown, Delwin T. Lindsey, Edward Kopidlansky, Michael R. Stenger; Contrast threshold of the preterm infant. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):4392.
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© ARVO (1962-2015); The Authors (2016-present)
Preterm infants are at risk for many blinding disorders, but there is no body of normative data available to support the development of methods of testing their visual function. Particularly, it is not known whether the contrast sensitivity of preterm infants is good enough to allow visual acuity to be measured using printed gratings of ~0.9 contrast. For example, if median preterm infant contrast sensitivity were 0.9, acuity measurement would be impossible for half the infants, and it would probably be difficult for the remainder. Here, we measured the contrast sensitivity of preterm infants in the Neonatal Intensive Care Unit (NICU) to determine whether their contrast sensitivity is high enough in the neonatal period to support visual acuity measurement.
Healthy, awake, preterm infants (born 33.5 SD=1.4 wks gestational age, tested at 34-38 wks corrected gestational age, avg=36 wks SD=1 wk; N=11) were tested in the NICU just before or after feeding. Stimuli were 31x61 cm gray cards with 3-cycle, 0.05 cy/deg vertical square-wave gratings in the center. The tester placed each card along the infant’s line of sight, then moved it laterally, observing the presence or absence of fixation-and-following behavior. The stimulus was “seen” if the infant attempted to refixate the grating. After presentation of a 0.96-contrast “easy” stimulus, testing at contrasts 0.50, 0.71, and 0.96 was under the method of constant stimuli (MCS), with the tester unaware of the contrast values or their order during presentation. The tester made a yes-no decision on each card, and infant contrast threshold was the lowest contrast judged to be “seen”.
The main challenge in testing preterm infants is finding them awake for long enough to test them, and all the infants in this study were alert and awake. All of them saw at least the first “easy” stimulus, and all but one saw at least the 0.96 contrast stimulus during MCS testing. The average contrast threshold was 0.705 (SEM = 0.055), and 8/11 (72%) of the infants had contrast thresholds at or better than 0.71. When we combined the present results with previous data on full-term infants, neonatal contrast threshold improved with age (r=-0.414, p<0.005) at an overall rate of 0.042 contrast improvement per week.
The contrast sensitivity of most preterm infants can be measured starting by age 34 weeks. The contrast thresholds of many infants were good enough to allow visual acuity measurement.
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