PARTICIPANTS AND METHODS

Participants

All infants born before 32 completed weeks in 1991–92 in the eight hospitals within the Liverpool postal districts were ascertained. Those who died before discharge from hospital or whose mothers were not resident within a Liverpool postal district at the time of birth were excluded.

Initial contact was made with the family doctor through the child’s hospital paediatrician to ascertain current health status and school placement. The parents of those children who were alive and attending mainstream schools were then approached to seek consent for their child to take part in the study. The individual children’s schools were then contacted to arrange assessment visits, and to request that the class teacher choose the child of same sex and first language in the class whose birthday was closest to that of the index child. The parents of that control child were then approached with information about the study, and consent for their child to participate was sought. Most children were tested at their schools, although a few were tested at the Institute of Child Health, Royal Liverpool Children’s Hospital, at their parents request. The study protocol was approved by the local research ethics committee.

Vision tests

Visual acuity was assessed using a standard Snellen chart. Monocular visual acuity was assessed for each eye at a distance of 6 m. A score of 6/9–6/6 is considered normal.¹⁸ The Snellen chart was used because of its ease of use in the school situation, although better and more precise tests are now available. It was noted if children wore spectacles. A cover test was performed to detect the presence of strabismus. If it was present, it was noted to be latent or manifest, the laterality observed and whether it was alternating. Stereopsis was determined using the TNO random dot test.¹⁹ This has been designed as a screening tool for school age children. The test plates viewed through red-green spectacles can identify retinal disparities ranging between 15 and 480 seconds of arc; 120 seconds of arc is recommended as a pass/fail criterion by the test authors. Contrast sensitivity was assessed using the Cambridge low contrast gratings.²⁰ This is a simple and quick screening test, and is suitable for use with subjects with normal Snellen visual acuity. The gratings on the test measure contrast sensitivity at only one spatial frequency of 4 cycles/degree. No normative data are published for this test. Children with an acuity of worse than 6/9 were excluded from this test. Scores of 120 or less were considered abnormal.

All the visual tests were applied by a single observer (L F-H).

Other test instruments used

Integration of visual and motor abilities was assessed using the Developmental test of visual-motor integration (VMI).²¹ It consists of 27 geometric forms which increase in complexity and are in a developmental sequence. There is a one point scoring system for each shape copied by the child. Scores are standardised for age. Standard scores have a mean of 100 and a standard deviation of 15.

Fine and gross motor skills were assessed using age band 2 of the Movement assessment battery for children (Movement ABC).²² The test comprises eight items, divided into four subsections: manual dexterity, ball skills, and static and dynamic balance. The scoring system for each item is from 0 to 5 ranging from no impairment to severe impairment. The scores for each item are added and converted into centiles. A score between the 5th and 15th centile for age is considered “borderline” impairment, and at or below the 5th centile “definitely impaired”.

General intelligence was measured using the Wechsler intelligence scale for children UK (WISC III UK).²³ Total, verbal, and performance scores were calculated.

The occipitofrontal head circumference was measured in a standard manner.

Perinatal data

Perinatal data were abstracted from the clinical case records at the eight hospitals using a standard proforma. The results of cranial ultrasound examinations were recorded, and the most extensive degree of periventricular haemorrhage (PVH) and periventricular leucomalacia (PVL) observed was recorded. Infants weighing less than 1500 g were routinely screened in the neonatal period and later for ROP according to established national guidelines.²⁴ The most extensive ROP observed defined by zone and stage was recorded for each eye.

Statistical analysis of data was carried out using parametric or non-parametric tests depending on the distribution of the data.

RESULTS

Of a potential cohort of 382 preterm children identified, 33 had moved out of the area or could not be traced, six were traced but failed to attend appointments for testing, 18 had died, 29 were attending special schools, and 16 parents refused permission for their children to be tested. In addition to these 280 children, 210 term controls were also recruited. In 70 cases, a control was not obtained, or the parents of the control selected would not agree to testing. Time constraints at school meant that a few children did not complete all the tests. The index children comprised 151 (53.9%) boys and 129 (46.1%) girls. The term controls comprised 112 (53.3%) boys and 98 (46.7%) girls. The mean gestational age for the index group was 29.8 (range 23–32) weeks and the mean birth weight 1467 (range 512–2860) g. Nearly a quarter (21.4%) were ⩽ 28 weeks gestation, and 3.6% ⩽ 24 weeks. Half (50.4%) weighed < 1500 g at birth and 14.6% ⩽ 1000 g. There were 215 singleton births, 56 twins, and nine triplets. The mean age at testing was 89.8 (range 82–101) months for the index children and 89.9 (range 72–107) months for the controls.

All but one of the preterm children (279) and all 210 controls received full visual screening. One preterm child could not complete the visual tests because of an acute eye infection. Of the 143 preterm children who had weighed less than 1500 g at birth (very low birthweight), 115 (80.4%) had been screened for ROP.

Visual acuity

Thirty six (12.8%) of the preterm and nine (4.3%) of the term control children wore spectacles (p = 0.002). Table 1 shows Snellen visual acuity. Significantly poorer acuities were seen for either eye in the preterm children (p = 0.007 left, p = 0.002 right). Eighteen (6.5%) of preterm children had a visual acuity at 6 m of less than 6/9 in one or both eyes (14 left, 9 right). Three term children had poor acuity (two left, two right; p = 0.01).

Strabismus

Strabismus was detected in 38 (13.6%) preterm and three (1.4%) term control children (p < 0.001). Strabismus was manifest in 18 preterm and one control child. Latent strabismus was found in 20 preterm and two term children.

Stereopsis

Using 120 seconds of arc on the TNO as a pass/fail criterion, 115 (41%) of preterm and 26 (12.4%) term control children failed the test. Absent stereopsis was seen in 46 (16.5%) preterm and eight (3.8%) term control children (p < 0.001).

Contrast sensitivity

The median contrast sensitivity score was significantly lower at 190 and 210 for the left and right eye respectively for the preterm than for either eye in the term controls (both median score 250, p < 0.001). Using a score of < 150 to represent significant impairment, 24 (8.6%) and 13 (4.6%) preterm children were impaired for their left and right eyes respectively, compared with nine (4.2%) and 11 (5.2%) for the term control children. These differences were not significant.

When the frequency of all ophthalmic impairments were compared between preterm children with a birth weight of greater than 1500 g and those of lower birth weight, no significant differences were found.

Relation between ophthalmic impairments and other functions

Significant differences in visual perception, motor impairment, and intelligence were seen between preterm children with and without ophthalmic impairments (table 2). Visual perception was significantly poorer in children with poor acuity, stereopsis, or strabismus. Overall intelligence was lower in children with poorer contrast sensitivity, stereopsis, or acuity, but this was due to poorer performance intelligence quotient (IQ), there being no significant differences in verbal IQ. Motor impairment was seen more often in children with poor acuity, stereopsis, contrast sensitivity, and strabismus.

Neonatal cranial ultrasound findings and visual impairments

Periventricular haemorrhage (PVH) was evident as subependymal haemorrhage in 11 (3.9%), intraventricular haemorrhage in 17 (6.1%), and parenchymal haemorrhage in three (1.1%). Cystic periventricular leucomalacia (PVL) was noted in three (1.1%). Grade of PVH was significantly related to the stage of retinopathy in those infants who had been screened in the neonatal period (χ² = 20.8, p < 0.013). Strabismus, poor visual acuity, absent stereopsis, and low contrast sensitivity were unrelated to the maximum extent of PVH or PVL determined on neonatal cranial ultrasound scans.

Retinopathy of prematurity

Of the 115 preterm infants weighing less than 1500 g who were screened for ROP, 31 (27%) had stage 1, 20 (17.4%) had stage 2, and eight (7%) had stage 3 disease. No infants had plus disease or required treatment in this cohort. Of the 59 infants with retinopathy, the disease was present in zone 2 in 42 (71%) and zone 3 in 17 (29%) and occupied a median of 4 (interquartile range 3–7) clock hours. Stage of retinopathy was significantly related to gestation (χ² = 55, p < 0.001). Stage of retinopathy was also significantly related to the wearing of glasses (χ² for trend 6.7, p = 0.01).

Moderate retinopathy (stage 3) was associated with significantly poorer scores on the Movement ABC and total and verbal IQ, but these differences were of borderline significance after standardising for gestational age (table 3). Moderate retinopathy was significantly associated with impaired acuity, but not lack of stereopsis, strabismus, or reduced contrast sensitivity in this cohort (table 4).

Occipitofrontal head circumference

The preterm group had a mean (SD) occipitofrontal head circumference of 51.5 (1.9) cm compared with 52.5 (1.6) cm for the term controls (p < 0.001). Significant correlations between occipitofrontal head circumference and visual, motor, and cognitive outcomes were found for all children and the preterm group alone (table 5).

DISCUSSION

Poorer visual function was observed in the preterm infants than controls on all measures used except for contrast sensitivity. Three times as many preterm children wore glasses, three to four times as many had poor visual acuity or stereopsis, and nearly 10 times as many had strabismus.

Comparison of overall visual defects with other studies is problematic because of differences in the measures used and selection criteria. Previous estimates have been between 15% and 79% for abnormalities detected by standard visual screening.^{1,4–6,25–27} It is easier to compare individual measures. Monocular visual acuity was poorer than 6/9 in 6.5% in either or both eyes in this study compared with 17% in a hospital based study on very low birthweight infants,¹ 37% in a cohort of children at 5 years born at less than 30 weeks gestation,¹⁴ and 10.5% and 9.5% in studies of very low birthweight infants in which binocular acuity only was tested.²⁶ Geographical studies published recently have used more sensitive measures of acuity, but have reported poor acuity in similar proportions of their study cohorts.^4–6 Some 12.8% of the children in this study wore glasses for myopia or hypermetropia compared with 10–27%, and higher stages of ROP were associated with a greater likelihood of wearing glasses as previously reported.^4–6,8 Strabismus was present in 13.6%. Other studies have reported rates of 9.5–25%, with a positive association with ROP and PVH and PVL.^{1,3–6,12,15,26–28} This association was not seen in this cohort probably because children with major disability were excluded.

More subtle tests of visual function such as contrast sensitivity and stereopsis showed higher rates of abnormalities in the preterm group as previously described.^7,29 A contrast sensitivity < 150 in one or both eyes was found in 9.7%, but it is difficult to compare this finding with other studies because of differences in measurement technique. Stereopsis was impaired (< 120 seconds of arc) in 43.3% and absent in 17.4% of the preterm group. This is similar to previous findings.^1,3,29

The rates of ophthalmic deficits in this study are lower than in some earlier ones, although these differences are probably due to cohort selection. This cohort was selected by gestation and geographical area rather than by hospital, and may contain fewer growth restricted infants. Recent developments in neonatal management such as the extensive use of antenatal steroid prophylaxis and postnatal surfactant may have reduced the severity of disorders such as PVH and retinopathy, which would impact on later visual development. More importantly, this cohort excludes children who are not attending mainstream school, including all those with major disabilities such as cerebral palsy which are often associated with strabismus. On the other hand, survival of infants of 26 weeks and less has become much more common, and these children are likely to have a poorer prognosis. Comparison of infants with birth weights of above and below 1500 g showed no significant differences in visual outcomes, despite the significantly greater risk for PVH and ROP in the smaller babies. The actual rate of stages of ROP in those over 1500 g is unknown, however, as they were not screened. In those screened for retinopathy, the stage of ROP did not appear to relate to developmental outcomes when gestational age was allowed for. Neither did retinopathy appear to be significantly related to visual outcomes except for acuity. This may be because there were no severe cases of ROP in the cohort, or because modern screening and treatment programmes have reduced the risk of ROP affecting later vision. Previous studies have emphasised that it is mainly the severest grades of ROP involving zones 1 and 2 that affect later vision.⁸

It is of particular interest to know if the visual defects observed are likely to be related to poor school performance. In the preterm group, visual outcomes were significantly related to measures of intelligence, minor motor disability, and visual perception, with the exception of verbal IQ (table 2). The strongest associations were between visual outcomes and motor impairment as measured by the Movement ABC and performance IQ. Such associations have been described before and thought to be due to either a common neurological insult such as PVL or the direct effect of the visual defect on functional motor abilities. Ultrasound evidence of PVH and PVL was not associated with adverse visual outcomes in this cohort. This may have been because of the relatively low incidence of PVH/PVL seen because of exclusion of children with major disabilities, or because the causative lesion cannot be identified by neonatal cranial ultrasound scanning. The association of poor acuity and low contrast sensitivity with minor motor impairment would suggest that a diffuse cortical lesion such as defective myelination of the cerebrum may be responsible. Longitudinal magnetic resonance imaging studies in the preterm have shown that this does occur in preterm infants as they mature, and is associated with a smaller brain.^30,31 The preterm group had significantly smaller brains at age 7 years than controls, and brain size correlated with a wide range of visual, motor, and cognitive outcomes (table 5). This would suggest that in common with IQ and minor motor impairment, visual disabilities in the preterm, particularly subtle ones such as impaired stereopsis and contrast sensitivity, in the absence of major disability, relate to poorer postnatal brain growth rather than ROP and PVH/PVL.