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Editor,—Dr Corrigan and colleagues ask important questions about the value of diagnostic screening for children with mild to moderate developmental delay.1 Unfortunately, their study has many of the methodological weaknesses highlighted in a recent review on this field.2
The vital issue of clear case definitions is not adequately addressed mainly because no standardised cognitive or developmental assessment tools were employed. Furthermore to meaningfully evaluate the performance of the diagnostic tests examined in this study requires not only accurate prevalence data, but also a knowledge of the individual tests’ sensitivities and specificities.
Most of the known organic causes of mild to moderate developmental delay are of a low prevalence, limiting the usefulness of small studies. In the specific case of fragile X screening it is worrying that the authors conclude ‘The failure of this study to detect any cases of the fragile X gene would suggest that until larger studies are completed on similar populations fragile X screening of children with mild to moderate learning difficulties should not be recommended’. The prevalence of fragile X syndrome in the general population is currently estimated at around about 0.2/1000 and in boys with learning difficulties nearer 5/1000.3 It is not therefore surprising that no cases of fragile X were detected in this study’s sample of 95 children. Perhaps a more appropriate conclusion would be that although they have not yet found any affected children, they should keep looking. DNA testing for fragile X has a high sensitivity and specificity, and testing is not only for the child’s benefit but also for the mother and extended family.
Choosing medical investigations in developmentally delayed children remains a difficult area, but there are well described protocols for critically appraising medical investigations which should be followed before making recommendations on the performance of diagnostic investigations.4
Drs Corrigan and Stewart comment:
As has been correctly pointed out research into children with mild to moderate learning difficulties is plagued by difficulties in case definition. In our study we used operational criteria based on school placement and provision of remedial assistance to define learning difficulties. This reflects the clinical reality and outlines the extent of the problem.
The majority of these children were maintained in mainstream education and did not have formal educational assessment. They represent a large number of children (16% of population in our study) that are becoming an increasing focus for paediatricians with a remit in educational medicine. At present there are no formal recommendations regarding medical assessment or investigation in this group.
Any decision to institute a regular screening programme for these children would have significant implications for both the child and health resources. We feel therefore that the benefits of a programme such as screening for fragile X would first have to be proved. Studies should show positive results in an identifiable group of children using easily translatable operational criteria before recommending their inclusion.