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The impact of low birth weight on the visual pathway
  1. ALISTAIR R FIELDER
  1. Academic Unit of Ophthalmology, Western Eye Hospital
  2. Imperial College School of Medicine, London NW1 5YE

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    It is well known that the survival of babies born prematurely has increased greatly in the recent past and that low birth weight carries a significant morbidity for ophthalmic problems. However, the impact of low birth weight on childhood vision impairment in epidemiological terms is largely unknown.

    The article in this issue of the BJO (p 9) by Crofts and colleagues is therefore particularly welcome. Using the Oxford Register of Early Childhood Impairment, these authors gathered information on all babies with severe vision impairment born in the 4 years commencing January 1984. We are told that this population is reasonably representative of the UK as a whole. Vision impairment was defined as an acuity of 6/18 or less in the better eye identified by 5 years of age. From this database the birthweight specific rate of severe vision loss was calculated. The overall rate of severe vision impairment was 1.25/1000 births, but those of birth weight less than 1500 g contributed to 17.5% of the cohort and for this group the rate was 26 times that for babies of 2500–3499 g birth weight. The prevalence of associated impairments was high for all birth weights (60%), but highest for those under 1500 g (72%), decreasing with increasing birth weight to 44% for those over 3500 g.

    The time of origin of the disorder was estimated, as best as possible, according to three major categories—prenatal, perinatal, and postnatal, and each of these contained 60%, 24%, and 10% respectively (5% were unclassifiable). Thus, 84% of childhood vision impairment has its origins at birth or before, and once the perinatal period has passed, from the vision impairment point of view, the visual pathway is more likely than not to remain unscathed. The aetiological front runner was cortical vision impairment which accounted for 36% of all childhood vision impairment, and was followed by optic atrophy (13%) and cataract (10%), with nystagmus, retinal dystrophy, and retinopathy of prematurity (ROP) all accounting for 6% each. Mindful that most childhood vision impairment affects parts of the visual pathway other than the eye, significant reduction of childhood vision impairment in the future has to come from preventing neurological prenatal and perinatal maldevelopment and/or damage.

    It is of interest that ROP accounts for such a small proportion of severe vision impairment in this low birthweight population, particularly as this study predated the introduction, in 1988, of treatment for severe disease. Here, Oxford does not accurately reflect the worldwide situation for there is a rising incidence of severe disease associated with increased survival of preterm babies in middle income countries such as Latin America and eastern Europe, all of which have less than ideal standards of care.1 Thus, 54% of babies requiring treatment in Lithuania2 had birth weights over 1500 g and the birth weight of some of the treated babies in Hungary was over 2000 g.3 In the UK and USA, severe ROP is virtually confined to babies under 1500 g and less than 32 weeks gestational age, hence the use of these criteria for the UK national screening guidelines.4 Clearly, however, these are not internationally appropriate and it is ironic that those countries with sparse resources have the greatest screening load. Both ROP and cataract are important for they are two of the few conditions about which the clinician can influence outcome.

    So far we have concentrated on severe vision impairment, but babies born prematurely are also at risk of developing a range of less severe sequelae of the visual pathway. These include low acuity, colour and contrast sensitivity deficits, field defects, as well as the better known refractive errors and strabismus.5-16 A number of factors might play a role, such as an abnormal prenatal period, the neonatal environment, and illnesses suffered during the perinatal period.17 18 Some of these can be directly attributed to ROP and/or neurological insults, although in many instances it is not possible precisely to tease out their individual contributions.8 Over the past decade or so we have learned much about the visual pathway complications of prematurity, but as yet we know little about the mechanisms by which they occur, or about their functional implications. Fascinating glimpses of future research directions come from the finding that advanced ROP can be associated with a missense mutation of the Norrie gene,19 20 or at a neurological level that intrauterine stress and preterm birth could impair wiring and impede neuronal migration.21 22

    Epidemiological data are essential if we are to learn about the types of vision impairment and their frequency in the community, but they give no insight into the functional impact of visual disability, mindful that most children have more than one handicap. Here the term “disability adjusted life years” (DALYs) is helpful, for it reminds the clinician of the lifelong challenges that lie ahead for the child and family, with education and employment, social interaction, and leisure. We have now entered a difficult area. For all clinicians, telling parents that their baby has a severe vision impairment is a difficult and daunting experience. But, it is nothing compared with what the parents have to face thereafter, and giving the appropriate information sensitively is an essential prerequisite to ensure that the baby and family get the best possible support. A recent survey of parents of impaired children sought information about the support they received at the point of diagnosis—it does not make comfortable reading for the medical profession. However this report, Taking the Time,23 makes several important and positive recommendations of which the following are but a few. If possible, giving the diagnosis of severe vision impairment should be done in private. A review appointment should be offered within a couple of weeks to provide another opportunity for discussion, recognising that this is not strictly medically necessary. Try to give parents written information which can be updated according to progress, this may be in the form of a personal letter. The care of the visually disabled child requires multiprofessional input and links with the community support services, such as the peripatetic teaching service, should be established as soon as possible. Do not forget the value of client based parents’ support organisations. Finally, while the ophthalmologist has to be the bearer of bad news, he or she also has the first opportunity to take positive supportive steps for the family, one which must not be missed. Hopefully by being open with the family and other professionals, the parents will come to regard the clinician as an advocate and ally to work with.

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