What does a low Bayley Score Mean? Predicting Long term outcomes: part 1.

The title of the commentary listed below might put off some neonatologists, it looks like it has to do with studies in full term babies of their nutrition, when it really has to do with the meaning of the Bayley Scores. Colombo J, Carlson SE. Is the Measure the Message: The BSID and Nutritional Interventions. Pediatrics 2012. http://pediatrics.aappublications.org/content/early/2012/05/23/peds.2012-0934 For many years now the Bayley Scales of Infant Development have been the standard for analyzing short to medium term outcomes of babies surviving intensive care. As a screen for developmental delay they can be helpful. What has not been helpful is the tendency to refer to a low Bayley score on the Mental Development Index (MDI) scale as being cognitive impairment. But as Colombo and Carlson point out:

The BSID is a global test designed to identify developmental delay. Its role and place within the field of developmental science is relatively well established. The BSID is, to be charitable, only modestly related to school-age cognitive development (ie, the outcome that is most meaningful to investigators in this field). The BSID is a global measure of developmental status in infancy that assesses and aggregates the timely attainment of relatively crude milestones in infancy and early childhood.

The lack of predictive ability of the Bayley has been pointed out by Maureen Hack, Laura Ment, and by the long term follow up of the CAP trial (references below) among many other examples. Many children with Bayley MDI more than 2 SD below the mean have intellectual abilities in the normal range at long term. The CAP trial showed that only 18% of those babies who had an MDI at 18 months which was more than 2SD below the mean had an IQ more than 2 SD below the mean at 5 years. The authors of the commentary go on to say :

Simply, the BSID is not an adequate indicant of specific cognitive skills that may be differentially affected by interventions or exposures, nutritional or otherwise, and so its use to evaluate the construct of infant cognition is seriously deficient in the context of recent advances in developmental science.

Apart from being unsure about the use of the word ‘indicant’ I agree absolutely with this. A low score on the Bayley does not equal cognitive impairment.

I think this is extremely important for our patients. Predictions of so-called ‘cognitive impairment’ sound terrifying for parents, and persuade physicians that there will be major effects on intelligence.

In fact, many infants in the past have had consideration of withdrawal of intensive care based on expected ‘Cognitive Impairment’. In other words, a baby has a finding on head ultrasound, or a medical complication (this is especially among preterm infants) and the doctors go to the medial literature and find that ‘Oh No, the baby has a very high risk of Cognitive Impairment, so we should counsel the parents about this, they might want to consider limiting intensive care’.

But there are very few studies of quality, with high retention rates, that go beyond an 18 to 24 month Bayley score and really investigate the impacts of a particular finding on the functioning of a baby. So few that the significance of much of what we see clinically, and then discuss with parents, is highly questionable if we really are considering the long term functional outcomes of our patients.

On the other hand, to hide behind prognostic uncertainty is also not serving our patients, and their parents, best interest. I don’t mean to suggest that we can never say anything about long term outcomes, and that we should never therefore consider limiting intensive care. We should be basing our decision making on reliable data.

We should consider which things actually have impacts on long term functional outcomes, and how severe those impacts really are.

Hack M, Taylor HG, Drotar D, Schluchter M, Cartar L, Wilson-Costello D, Klein N, Friedman H, Mercuri-Minich N, Morrow M: Poor Predictive Validity of the Bayley Scales of Infant Development for Cognitive Function of Extremely Low Birth Weight Children at School Age. Pediatrics 2005, 116(2):333-341.

Ment LR, Vohr B, Allan, Walter, Katz KH, Schneider KC, Westerveld M, Duncan CC, Makuch RW: Change in Cognitive Function Over Time In Very Low Birth Weight Infants. JAMA : the journal of the American Medical Association 2003, 289(6):705-711.

Schmidt B, Anderson PJ, Doyle LW, Dewey D, Grunau RE, Asztalos EV, Davis PG, Tin W, Moddemann D, Solimano A, Ohlsson A, Barringotn KJ, Roberts RS: Survival Without Disability to Age 5 Years After Neonatal Caffeine Therapy for Apnea of Prematurity. JAMA: The Journal of the American Medical Association 2012, 307(3):275-282.

About Keith Barrington

I am a neonatologist and clinical researcher at Sainte Justine University Health Center in Montréal
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