Elsevier

Early Human Development

Volume 88, Issue 2, February 2012, Pages 95-98
Early Human Development

Cognitive outcome at 24 months is more predictive than at 18 months for IQ at 8–9 years in extremely low birth weight children

https://doi.org/10.1016/j.earlhumdev.2011.07.013Get rights and content

Abstract

Background

It is unclear whether developmental assessment later or earlier in childhood is the better predictor of intelligence at 8 years of age. This is an important distinction as many clinical trials assess their final outcomes only in early childhood, assuming the results are valid for later childhood cognitive functioning.

Aims

To compare the ability of developmental assessment at 18 months with 24 months in predicting general intellectual functioning at 8–9 years of age in extremely low birth weight (ELBW, birthweight < 1000 g) children.

Study design

Cohort study.

Subjects

58 ELBW survivors born during 1997 at the Royal Women's Hospital, Melbourne, Australia.

Outcome measures

Cognitive assessments at each of 18 months, 24 months (Mental Developmental Index [MDI]) and 8–9 years (Full Scale IQ) of age, corrected for prematurity were compared by regression analysis and by the κ statistic (agreement beyond chance).

Results

Both the 18-month and the 24-month MDI were significantly predictive of Full Scale IQ at 8–9 years, but more so for the 24-month MDI, with 38% of variance explained compared with 34% of variance explained by the 18-month MDI. The 24-month MDI, expressed as categories of severe, moderate, mild or no developmental delay, was more predictive of categories of severe, moderate, mild or no intellectual impairment at 8–9 years (weighted κ = 0.43, P < 0.001) than was the 18-month MDI (weighted κ = 0.35, P = 0.001).

Conclusions

Cognitive assessment at 24 months is superior to cognitive assessment at 18 months in predicting IQ and intellectual impairment at 8–9 years of age in ELBW children.

Introduction

Extremely low birth weight (ELBW, birthweight < 1000 g) and extremely preterm (EPT, gestational age < 28 weeks) infants have higher rates of neurosensory disabilities, mostly caused by cognitive delay, than do normal birthweight or term controls [1], [2], [3]. Many perinatal trials and observational studies assess children only to 18 months of age, when cognitive skills are immature, developing rapidly and difficult to assess reliably; the data thereby obtained are sometimes used in making end-of-life decisions. It is therefore vital to know if cognitive outcomes determined in early childhood are consistent with findings later in childhood.

There is a compromise between assessments earlier and later in childhood. Earlier assessments are useful for the identification of high risk infants, are likely to have higher follow-up rates, and being more timely are likely to appeal to researchers and funding agencies who want quicker answers. On the other hand, earlier assessments are less reliable, sensitive to developmental delay which can be misinterpreted as impairment, and only able to provide a broad assessment of cognitive functioning given that certain cognitive skills have not yet emerged or are in the very early stages of development. Hence, cognitive assessments earlier in development may not be as predictive of later cognitive outcomes.

Two common ages for cognitive assessment are 18 months and 24 months; several large randomised controlled trials have assessed cognitive outcomes at 18 months [4], [5], [6], whereas typically Victorian Infant Collaborative Study (VICS) cohorts have been assessed at 24 months [1], [2], [3]. Cognitive assessments in early childhood are only weakly related to cognitive assessments at school-age [7], [8]. Even though assessments at both ages might be poor predictors of later cognitive outcome, the issue remains as to whether assessment at one age might be more predictive than at another age. The aim of this study was to compare the ability of cognitive development at 18 months with 24 months at predicting general intellectual functioning at 8–9 years of age in extremely low birth weight (ELBW, birthweight < 1000 g) children.

Section snippets

Methods

ELBW survivors cared for at the Royal Women's Hospital, Melbourne, during 1997 were potentially enrolled in two different studies. The first study was a randomised controlled trial of prophylactic indomethacin (TIPP) which recruited infants of 500–999 g birthweight within the first 6 h after birth from 1996 to 1998 [4]. The second study was the 1997 VICS cohort which recruited all infants < 1000 g born in the state of Victoria in the calendar year 1997 [1]. Hence some children were enrolled in both

Results

Of the 78 ELBW infants cared for at the Royal Women's Hospital and enrolled in the TIPP study in 1997, 9 died before 18 months of age. Of the 69 survivors, 59 had cognitive data recorded at all ages (18 months, 24 months and 8–9 years). One child was assessed for both studies within 3 months and hence excluded from the analysis. Consequently there were 58 ELBW children with data at all ages assessed more than 3 months apart that were analysed for this study.

The mean (SD) for the cohort for

Discussion

A dilemma for surveillance programmes of high risk infants such as those born very preterm or small is deciding the most appropriate age to arrange developmental follow-up. Developmental assessment at a very young age will enable earlier identification of children with developmental issues, including those who may benefit from intervention; however if the goal is to predict later cognitive functioning, findings from the current study suggest that it may be more appropriate to delay follow-up.

Conflict of interest statement

No authors have any conflicts of interest to declare.

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