The electroretinogram and vitamin A in preterm infants

MacTier, Helen (1987) The electroretinogram and vitamin A in preterm infants. MD thesis, University of Glasgow.

Abstract

The electroretinogram (ERG) records an electrical potential arising from cells within the retina in response to stimulation by light. As an objective measure of retinal function, electroretinography has potential clinical application in neonates. However, technical difficulties inherent in recording low amplitude responses from unco-operative subjects have limited work in this field, and to date little is known about the normal development of the ERG during the early weeks of life. Plasma concentrations of retinol in the majority of preterm infants are marginal or deficient by adult standards, and there is indirect evidence that this reflects depleted liver stores of vitamin A. At present the functional significance of these observations is unclear. The earliest sign of vitamin A deficiency in adults and older children is impaired dark adaptation, and this is associated with changes in the ERG prior to the onset of subjective nightblindness. Thus, it was anticipated that the development of a reliable method for recording the ERG in the neonatal period might help to determine vitamin A requirements in preterm infants. A technique is described by which the ERG could be studied soon after birth in extremely low birthweight infants. The success rate in recording an interpretable averaged ERG was 96% in term infants and 92% in preterm infants. Results with this method in adult controls were comparable to results obtained by standard techniques. Continuous observation of the infant and assessment of the degree of eye opening was achieved by video monitoring. Minimal restraint was necessary, and no major incidents or complications were encountered. Electroretinograms were recorded successfully in 50 term infants aged between 7 and 148 hours. The amplitude of the ERG was less and the a- and b-wave latencies were longer than in adult controls. Eyelid closure during recording of the ERG significantly increased the a-wave latency and diminished the amplitude of the response. Both the a- and b-wave latencies shortened with advancing postnatal age. Fifty-nine preterm infants were studied at ages ranging from 7 hours to 87 days. The amplitude of the ERG was less and the a-wave latency was longer than in term infants of comparable postnatal age. The ERG was absent initially in two of the most immature infants although each subsequently demonstrated a clearly defined response. Eyelid closure did not have a significant effect upon any of the ERG parameters in preterm infants. The b-wave latency decreased after 48 hours' exposure to light (p < 0. 01) and subsequently was related inversely to postconceptional age. Longitudinal and cross-sectional observations both showed a reduction in the a-and b-wave latencies and an increase in the amplitude of the ERG over time. Maturation of the ERG in preterm infants appears to be mediated, at least in part, by exposure to light. The preterm infants had significantly lower plasma levels of retinol, retinol-binding protein, prealbumin and a-tocopherol than term infants of comparable postnatal age. Plasma concentrations of retinol were below accepted normal levels for older children in all but three preterm infants, and did not change over time. The rise in plasma retinol concentration following an oral dose of 5000 IU retinol (the retinol dose response) suggested that low circulating levels of retinol in preterm infants reflect reduced hepatic reserves of vitamin A. Tocopherol levels were adequate in the majority of preterm infants after the fifth day of life and were higher in infants of all gestational ages fed with own mother's milk. Standard oral vitamin supplementation did not affect plasma levels of either retinol or alpha-tocopherol. A relationship was not demonstrated between any of the averaged ERG parameters and either the plasma concentration of retinol or the retinol dose response. The electroretinographic threshold was measured in 12 preterm infants. Allowing for eye opening, there was a significant reduction in the threshold over time. The logarithm of the electroretinographic threshold correlated with the retinol dose response, but was not related significantly to the predose plasma concentration of retinol. This work suggests that retinal stores of vitamin A in apparently healthy preterm infants are reduced in conjunction with depletion of hepatic reserves. Current recommendations for vitamin A supplementation of preterm infants may be inadequate and require review subsequent to further studies of vitamin A tissue function.

• Item Type: Thesis (MD)
• Qualification Level: Doctoral
• Keywords: Medicine, Ophthalmology
• Date of Award: 1987
• Depositing User: Enlighten Team
• Unique ID: glathesis:1987-76676
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 19 Nov 2019 13:55
• Last Modified: 19 Nov 2019 13:55
• URI: https://theses.gla.ac.uk/id/eprint/76676