Why should we take a dimensional approach to studying developmental disorders?

Developmental disorders like attention deficit disorder, attention deficit hyperactivity disorder (ADD/ADHD), autism spectrum disorder (ASD), language, learning and movement disorders are relatively common, more common then we might think. Furthermore, these disorders have a considerable impact upon the daily lives of those who struggle with them. Because some of these disorders are more apparent in some contexts than others, and their severity is highly variable, they may go unrecognised for some time. Indeed, in many cases these disorders are only formally recognised when a child has already progressed through years of formal schooling. This means that they may already have had a largely negative experience of learning, may have lost motivation, and may already have fallen far behind their peers.

When trying to study these disorders, researchers normally use a case-control approach. It’s an observational study in which two groups differing in outcome are identified and compared on the basis of some presumed causal attribute. Researchers use this method to identify factors that may contribute to a medical condition with the help of comparing subjects who have that condition (“cases”) with patients who do not have the condition but are otherwise similar (“controls”).

Case-control studies are relatively cheap and they are a frequently used type of epidemiological study that can be carried out by small teams or individual researchers in single facilities in a way that more complex experimental studies often cannot be. This design is often used in the study of rare diseases or as an exploratory study where little is known about the connection between the risk factor and the disease. In several cases they have bigger statistical power than cohort studies. This approach has largely been translated from the clinical sphere to study developmental disorders.

Well-designed observational studies, like case-control designs, can provide valuable evidence. It is however worth noting that they are quasi-experimental in nature and thus do not bring the same level of evidence as randomized controlled experiments. That said, case-control designs can sit well alongside complementary randomized controlled experiments. There are however other problematic features of case-control designs, which are particularly highlighted when studying developmental disorders. Selecting an outcome of choice, indeed the basis for choosing one particular group, may produce unintended biases that can have a strong effect in overall findings.

One such example is the exclusion of children with any comorbid symptoms – this is routine practice when using a case-control design to study developmental disorders. The most important disadvantage in case-control studies relates to the problem of acquiring reliable information about an individual’s status over time, and then using this as a basis for choosing some children whilst excluding others. The children actually included may be atypical of those with a particular disorder. This exemplifies why this design does not always translate well into the study of developmental disorders; in developmental disorder comorbidity is more the rule than the exception. This approach can also give a false impression of the nature of these disorders, which can be graded rather than discrete.

However, it also is possible to use a dimensional approach instead of the case-control approach. A dimensional approach puts focus on the kind of problem a person is experiencing and on the extent to which that aspect of cognition is impaired. It doesn’t place people into diagnostic categories but along dimensions. Diagnosis then becomes not a process of deciding the presence or absence of a symptom or disorder, but the degree to which particular characteristic is present. This is entirely the approach taken by the Centre for Attention Learning & Memory (CALM; http://calm.mrc-cbu.cam.ac.uk/). Children are referred not on the basis of any discrete disorder or diagnosis, but because they are experiencing problems in the areas of attention, learning and memory. The researchers are taking a dimensional approach to exploring the nature of these impairments.

Instead of making judgements of “present or not?” the dimensional approach asks the question “how much?”. It ranks disorder on a continuum based upon multiple domains of cognition, assessed using standardised materials. A dimension is viewed as a cluster of related psychological/behavioural characteristics that occur together. This approach generates profiles, rather than discrete diagnostic categories. Of course, one could argue that this approach is far ‘messier’ than a simple case-control approach. However, one might also argue that this unique profiling is far more informative about the nature and extent of the impairments themselves, and provides a far clearer picture about the pattern of deficits actually present in a population of children with problems of learning.

Whilst working at CALM I have been trying to understand how children attend, listen and remember and how these skills impact on learning. These include difficulties in language, literacy and maths. By improving our understanding of the cognitive and brain processes involved in learning, we hope to develop ways of identifying and overcoming problems that might appear during childhood. We also hope to provide an information hub for researchers and professionals in children’s services, and to run regular workshops.

A child visiting the CALM clinic is profiled by grading the severity of symptoms from a number of dimensions using standardised tests. For example these dimensions include working memory, attentional control, short-term memory, phonological skills, the ability to inhibit and control responses, to initiate, plan, organise and set goals, inattention, hyperactivity, aggression, conduct problems, emotional symptoms, peer relations, prosocial behaviour, as well as aspects of communication (speech, syntax, semantics, coherence, initiation, use of context and non-verbal communication). This information can be fed back to the referrer and can then be used to help guide the support that the child receives. In parallel to this we are building a large and rich dataset. A dimensional approach is better able to capture the complexities that a categorical approach may miss. Of course, this approach is not without its challenges also. How does one define a dimension? What statistical approaches ought we to use? And what kind of scores would warrant some form of intervention?

Despite these challenges, we think that the dimensional approach will provide a way of capturing the rich complexities of these data. Whilst other disciplines may strongly favour an approach with rigid categorical boundaries, this approach is not always appropriate for studying developmental disorders. Whilst strict case-control studies can be valuable, reliance on these designs alone can provide a biased and unrealistic view of the children with problems of learning.

References

Gelder M, Harrison P, Cowen P. Classification and diagnosis. In: Shorter oxford textbook of psychiatry. 5 th ed. Oxford: Oxford University Press; 2006. p. 21-34.

Helzer, J. E., Kraemer, H. C., & Krueger, R. F. (2006). The feasibility and need for dimensional psychiatric diagnoses. Psychological Medicine(36), 1671–1680. http://psych.colorado.edu/~willcutt/pdfs/Helzer_2006.pdf

Kendell RE. Five criteria for an improved taxonomy of mental. In: Helzer JE, Hudziak J, editors. Defining psychopathology in the 21 st century: DSM-V and beyond. Washington DC: American Psychiatric Publishing; 2002. p. 3-18.

Lewallen, S., & Courtright, P. (1998). Epidemiology in Practice: Case-Control Studies. Community Eye Health(11(28)), 57–58. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1706071/

http://www.healthknowledge.org.uk/elearning/epidemiology/practitioners/introduction-study-design-ccs

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