Emmetropisation following preterm birth. (Clinical Science).

Background/aims: Even in the absence of retinopathy of prematurity (ROP), premature birth signals increased risk for abnormal refractive development. The present study examined the relation between clinical risk factors and refractive development among preterm infants without ROP.

Methods: Cycloplegic refraction was measured at birth, term, 6, 12, and 48 months corrected age in a cohort of 59 preterm infants. Detailed perinatal history and cranial ultrasound data were collected. 40 full term (plus or minus 2 weeks) subjects were tested at birth, 6, and 12 months old.

Results: Myopia and anisometropia were associated with prematurity (p<0.05). More variation in astigmatic axis was found among preterm infants (p<0.05) and a trend for more astigmatism (p<0.1). Emmetropisation occurred in the preterm infants so that at term age they did not differ from the fullterm group in astigmatism or anisometropia. However, preterm infants remained more myopic (less hyperopic) than the fullterm group at term (p<0.05) and those infants born <1500 g remained more anisometropic than their peers until 6 months (p<0.05). Infants with abnormal cranial ultrasound were at risk for higher hyperopia (p<0.05). Other clinical risk factors were not associated with differences in refractive development. At 4 years of age 19% of the preterm group had clinically significant refractive errors.

Conclusion: Preterm infants without ROP had high rates of refractive error. The early emmetropisation process differed from that of the fullterm group but neither clinical risk factors nor measures of early refractive error were predictive of refractive outcome at 4 years.

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High refractive errors are common in the neonatal period following fullterm and preterm birth. (1-12) Fullterm neonates commonly demonstrate high levels of hyperopia and astigmatism that reduce rapidly during the first year of life. (2) Ingram et al found this process, known as emmetropisation, to be complete in 82% of full term infants by 12 months of age." Emmetropisation has also been described for preterm infants who tend to be more myopic and astigmatic at birth than fullterm infants. (1 2 14-19) Among preterm infants with retinopathy of prematurity (ROP) in the neonatal period, a proportion demonstrate increasing myopia from 6 months corrected age onwards. The proportion of infants affected and the degree of myopia is higher when neonatal ROP is severe. (18 20 21) This myopia is retained beyond infancy into childhood. (4 19 22-24) Although these findings suggest a link between ROP and myopia, prematurity (or low birth weight) may also impact on refractive development when ROP is absent or clinically un detectable. (23 25) Some authors have reported that infants who do not develop ROP demonstrate a more normal pattern of refractive development. (14 23 26-29) Others find these infants have an increased risk for developing significant refractive errors, in particular myopia. (19-21 23-25 30 31)

Fielder and Quinn's editorial eloquently summarises the associations between prematurity, ROE and myopia. (32) While the three are clearly linked, the relation between them is not well defined. They highlight the need to more fully understand the processes involved in refractive development of the visual system following preterm birth. Certainly, ROP increases the risk of myopia but even in its absence prematurity signals an increased risk for abnormal refractive development. Few reports on refractive outcome contain detailed information of the neonatal period. This study aimed to address this issue by excluding ROP as a factor and examining early refractive development in preterm infants without clinically detected eye disease and for whom extensive perinatal data are available. Refractive data are related to a fullterm group and to neonatal findings including cranial ultrasound.

MATERIALS AND METHODS

Full, informed consent was gained before testing and the protocol adhered to the tenets of the Helsinki Declaration of 1975, as revised in 1983 and was approved by the ethics committee of the Yorkhil NHS Trust. All refractive testing was performed by authors KJS and DLM. Brain imaging data were collected and analysed by AGW.

Subjects were 59 preterm infants without clinically diagnosed ROP (mean gestation 31.6 weeks, range 28-35, average weight 1.72 kg) and 39 fullterm infants (mean gestation 40.3 weeks, average weight 3.47 kg).

The authors tested all preterm infants born less than 35 weeks' gestational age who were well enough to be handled within 5 days of birth. Preterm infants in the unit were screened fortnightly for ROP from 6 weeks after birth until around term age. Screening was performed by an experienced ophthalmologist using indirect ophthalmoscopy through dilated pupils with a speculum and scleral indentation. Weekly screening between 34-37 weeks' gestational age was instigated for those babies considered at risk for developing severe ROE Only babies for whom no abnormal findings were recorded at any stage or to any degree were enrolled into the follow up study and included in the current analysis.

Refraction was performed by retinoscopy without the use of a speculum 30 minutes after instillation of two drops of 0.5% cyclopentolate HCl in each …