Behavioural Brain Research, 49 (1992) 115-122 9 1992 Elsevier Science Publishers B.V. All rights reserved. 0166-4328/92/$05.00

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BBR 01321

Visual field and grating acuity development in low-risk preterm infants during the first 21/2 years after term J. v a n H o f - v a n D u i n ~, D . J . H e e r s e m a n, F. G r o e n e n d a a l a, W. B a e r t s b a n d W . P . F . F e t t e r b a Department of Physiology I, and bDivision of Neonatology, Sophia Children's Hosp#al, Erasmus University Rotterdam, Rotterdam (Netherlands) (Received 7 August 1991) (Revised version received 12 December 1991) (Accepted 6 January 1992)

Key words: Grating acuity; Visual field; Preterm; Fullterm; Visual development; Infant; Low risk; Small for gestational age; Appropriate for gestational age

The effect of early visual experience on visual field size and grating acuity development was studied longitudinallyin 36 appropriate for gestational age (AGA) and 26 small for gestational age (SGA) low-risk preterm infants. These were selected out of 194 very low birth weight (VLBW) infants (birthweight < 1500 g) born in 1985 and 1986. Criteria for inclusion as low-risk were the absence of neurological, respiratory, circulatory and alimentary problems in the neonatal period; no retinopathy of prematurity and no evidence of abnormality on the neonatal cranial ultrasound scans. Binocular field sizes were assessed using kinetic arc perimetry. Binocular grating acuity was tested by means of the prototype version of the acuity card procedure. Results were compared with norms obtained in COlltrol fullterms in earlier studies. Infants were tested at 6 weeks, 3, 6, 9 and 12 months of age from the expected term date. Twenty-two of these infants were retested at 21[2 years of corrected age. Visual field size and visual acuity estimates of(both AGA and SGA) low-risk, VLBW preterms and control fullterms overlapped at all test ages, except for a slight but significantly faster development of the upper and the lower visual field at 6 weeks corrected age in the preterm group. These results indicate that for clinicalpurposes visual experience before the expected term date has not only no measurable effect on the normal development of behavioural acuity, but also no accelerating effect on the development of peripheral vision.

INTRODUCTION

Preterm infants are exposed to visual stimulation during a time in which this is not available to infants born after the normal 40 weeks gestation period. Since several visual behavioural responses have been reported from 25 weeks of gestation onwards 7'8"11'25, it may be assumed that preterm infants are susceptible to at least some of the visual stimulation provided. Therefore, visual development of preterm infants might be accelerated by this extra visual experience. Electrophysiological studies have given some evidence of accelerated visual development23'26. Behavioural studies of the development of pattern vision have up till now, however, given little indication of a stimulating effect of the early extrauterine influences on preterm infants, who had experienced a minimum of peri-

Correspondence: J. van Hof-van Duin, Department of Physiology I, Erasmus University Rotterdam, PO Box 1738, 3000 DR Rotterdam, The Netherlands.

natal p r o b l e m s 2A-t~176 Results from our earlier studies2t'3~ have confirmed the general opinion that grating acuity development ofpreterms, at least during the first year after term, lies within the range of that of fullterm infants of comparable conceptional, rather than postnatal age. This suggests that normal development of central vision as revealed by behavioural visual acuity, is primarily a maturational rather than an experience-dependent process. Little is known however, of the possible effect of preterm birth on the functional development of peripheral vision. This could be rather interesting, because the anatomical maturation of the peripheral retina precedes that of the central retina t6. Preliminary results on visual field size development31 from our laboratory suggested that the temporal visual fields of low risk preterm infants fell within the range of that of fullterms, if results were analysed according to corrected age. This conclusion was, however, based on assessment along the horizontal extent in only 20 low-risk preterms, and the variability was high. The aim of the present study was to investigate

116 whether peripheral and central vision are differentially or similarly influenced by early visual experience. This seemed also of clinical interest, since normative values are needed to allow early detection of possible visual impairments in preterms at risk for abnormal visual outcome. We studied visual field and binocular grating acuity development longitudinally in low-risk, very low birthweight (VLBW) preterm infants during the first year and (in some of them) at 2112years after term. Despite their birthweight of < 1,500 g, none of the 62 preterm infants, included in this study, had experienced serious perinatal problems. Results obtained in preterm infants were compared to those of fullterm infants who had reached the same corrected age, (i.e. age corrected for birth at 40 weeks gestation). Some of the results have been published previously 14'15'19'21'3~

SUBJECTS

Visual field size and binocular grating acuity were studied in 62 low-risk, very low birthweight (VLBW) preterm infants (25 boys and 37 girls) during the first year of corrected age. These were selected out of 194 VLBW infants born between August 1, 1985 and December 31, 1987 and hospitalized in the neonatal intensive care unit of the Sophia Children's Hospital in Rotterdam. The specific criteria for inclusion as low risk were: no indication of peri- or postnatal hypoxia; Apgar score > 6 at 5 min; mechanical ventilation for respiratory problems for no longer than 7 days after birth; no surgery or medical treatment for patent ductus arteriosus; no retinopathy of prematurity; no evidence of abnormality on the neonatal cranial ultrasound scans; no neonatal neurological abnormalities 2a and no neurological abnormalities at 1 year corrected age 27. All preterms were born between 4 and 13 weeks prior to term, the mean gestational duration was 31.4 weeks ( + 1.8 week). Assessment ofgestational age was based on mother's reports, evidence from ultrasound scans carried out during pregnancy and examination at birth. Birthweights ranged from 790 to 1;500 g (mean birth weight 1,241 + 172 g). Using growth charts of Usher and McLean 2s, 36 infants were classified as appropriate for gestational age (AGA) and 26 as small for gestational age (SGA). Although SGA often is considered a risk factor for impaired development 3, data obtained in SGA infants were included, since earlier studies from our department showed a similar incidence of visual deficits in at-risk SGA compared to AGA VLBW infants 13,34. Moreover, recent clinical studies in the Netherlands on the outcome of VLBW infants at 1 year

(Fetter, unpublished) and at 2 years of age, showed SGA infants (who survived the neonatal period), to have similar risks to develop handicaps as AGA infants 35. Infants were tested at 6 weeks, 3, 6, 9, and 12 months of corrected age, during outpatient visits to the Sophia Children's Hospital, where they came for developmental assessment. Postnatal ages at the time of testing ranged from 10 to 65 weeks; corrected ages (age corrected for the period of prematurity) ranged from 6 to 52 weeks. Twenty-two preterms (12AGA and 10 SGA) were retested at 21/z years of corrected age.

METHODS

Visual field size was assessed binocularly using kinetic arc perimetryZ~ The apparatus consisted of two 4-cm wide black metal strips, mounted perpendicularly to each other and bent to form 2 arcs, each with a radius of 40 cm. The perimeter was placed in front of a black curtain, concealing an observer, who could watch the child's eye and head movements through a peephole. The infant was held in the center of the arc perimeter, with the chin supported. During central fixation of a 6 ~ diameter white ball, an identical target was moved from the periphery towards the fixation point along one of the arcs of the perimeter at a velocity of around 3~ Eye and head movements towards the peripheral ball were used to estimate the outline of the visual field. Orthogonal half-meridia were tested in a pseudo-random succession. The median of at least three measurements along each half-meridian was used as an estimate of the visual field size along that half-meridian. Visual acuity was assessed by means of the acuity card procedure 18"21. This method is based on the inborn preference for a pattern over a uniform field. The infant was held in front of a uniform grey screen, containing on one side a black and white striped pattern and on the other side a 'blank', consisting of a very fine pattern assumed to be unresolvable by the infant. An observer watched the infant's eyes from behind the screen through a peephole located between the stimulus positions. Gratings of different stripe-widths were presented in ~]/-octave steps, while the left/right position of the pattern varied randomly from trial to trial. The observer, who was unaware of the left]right position of the grating stimulus, was required to judge the stimulus position on the basis of the infant's eye and head movements. Starting with a coarse grating well below the normal acuity age norm of the infant, cards with gratings of decreasing stripe widths were presented in rapid succession until the region of the acuity thresh-

117 old appeared to be reached. At this point, the infant was shown a grating near or at threshold in alternation with a grating ~/2 octave finer, and judged to be above threshold, at least 3 times each. The threshold of acuity was taken as the finest stripe-width for which the infant consistently responded correctly according to the observer, and was expressed as cycles per degree of visual angle (cy/deg). Infants up to 6 months of age were tested at a distance of 40 cm from the screen; the distance was increased to 57 cm for infants between 6 and 12 months. The number of trials ranged between 20 to 30; testing time was about 3 to 4 min. Results of both visual fields and visual acuity testing were compared with norms obtained in control fullterms in earlier studies 14'~5'2~

RESULTS

The values obtained for visual field size and visual acuity in 62 low-risk, VLBW preterms and control fullterm infants are shown in Fig. 1A and B, Fig. 2A and B, and Tables I-IV. The number of successfully tested preterms during the first year after term ranged from 46 to 55 at the various test-ages (see Table I). Some infants did not complete enough trials, while parents of others were unable to attend further follow-up assessments. Fig. 1 shows the visual field development along the 4 half-meridia during the first year after term. Preterm values + 1 S.D. are plotted according to either postnatal (Fig. 1A), or corrected (Fig. 1B) age, and are compared to those of fullterm infants according to corrected age. Visual field estimates obtained in 150 healthy fullterm infants and 25 fullterm 2.5-year-olds were used for the calculation of age norms ~4"~5'2~(see also Table II). Fullterms were only tested once, their age was calculated from the expected term age. Since field values wereassessed along 4 directions in each child, the 95 ~o confidence interval (mean value+ 1.96 S.D.) of full-

terms is indicated in Fig. 1A and IB, instead of the usual 9 0 ~ . When results obtained in preterm infants were plotted according to postnatal age (Fig. 1A), visual field sizes of low risk, VLBW preterms lagged behind that of fullterms up to at least 1 year of age. When corrected age was used (Fig. 1B and Table I), visual field sizes ofpreterms and fullterms overlapped, except for results obtained at 6 weeks corrected age along the vertical extent. At 6 weeks, both upper and lower field sizes of the preterms were significantly larger than those of fuliterm infants (sign test P

2 years after term.

The effect of early visual experience on visual field size and grating acuity development was studied longitudinally in 36 appropriate for gestational...
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