The clinical reality of neurodevelopmental disorders

Post written for the BPS Research Digest by guest host Dorothy Bishop, Professor of Developmental Neuropsychology and a Wellcome Principal Research Fellow at the Department of Experimental Psychology in Oxford, Adjunct Professor at The University of Western Australia, Perth, and a runner up in the 2012 UK Science Blogging Prize for BishopBlog.

Some years after I had completed my clinical training, I remember meeting a ten-year-old boy, Peter. Peter's parents wanted my opinion on his language abilities, and I was able to assess him and discover he did have poor language comprehension, as well as some difficulties formulating sentences. It seemed to me that he would fit the picture of having a developmental language disorder.

However, when I discussed his history with his parents, it turned out that Peter had all sorts of other problems. A paediatrician had noted that he was overactive and inattentive, and had diagnosed attention deficit hyperactivity disorder (ADHD). He'd later seen an educational psychologist who thought he was dyslexic. An occupational therapist who visited the school had noticed he was clumsy and suggested he had developmental dyspraxia, and more recently, as his rather obsessive interest in dinosaurs had become more pronounced, his teacher had wondered if he should be assessed for autistic spectrum disorder. His parents, needless to say, were utterly baffled and were continuing to seek out specialists in the hope that they might find someone who would give them the right diagnosis.

In fact, it is likely that the experts were all right in some regards and wrong in others. The so-called neurodevelopmental disorders tend to co-occur, so that a child who has symptoms of one condition often also has symptoms of another. Thus an expert looking at the child through the lens of their own specialism may well find evidence of problems in that area. However, the experts were all wrong insofar as they assumed the child's difficulties could be accounted for by a single diagnostic label. It is easy to be misled by the diagnostic framework that is exemplified in systems such as the International Classification of Diseases (ICD) of the World Health Organisation, or the Diagnostic and Statistical Manual (DSM) of the American Psychiatric Association into thinking that children's neurodevelopmental problems divide neatly into discrete and clearcut disorders.

The Swedish psychiatrist, Christopher Gillberg, has for many years been interested in the overlaps between neurodevelopmental disorders, and in 2010 he proposed the term ESSENCE (Early Symptomatic Syndromes Eliciting Neurodevelopmental Clinical Examinations) to cover the whole group of conditions affecting attention, inhibition, social behaviour, language and learning, motor skills and perception. In a recent paper from his group, Pettersson et al have gone a step further and attempted to find out more about the causes of the symptom overlap.

They were particularly interested in the idea that there might be a general genetic factor that put the child at risk for the entire range of neurodevelopmental disorders. They were able to take advantage of a giant nationwide study of Swedish twins, from which they could use data from around 6000 same-sex twin pairs. If genes play a part in determining who gets a neurodevelopmental disorder, then monozygotic twins – formed by the splitting of a single fertilised egg – should be more similar to one another than dizygotic twins, formed when two eggs are fertilised at the same time. And, of particular interest for this study, it is possible to extend the logic to see if there is evidence for genetic influence that operates across different symptoms. For instance, if one twin has poor attention, is their co-twin at increased risk of tics, and if so, does the twin similarity depend on their genetic relationship?

This study used fiendishly complex analytic methods that combine factor analysis and structural equation modelling to address this issue for 53 questionnaire items that spanned the range of ESSENCE symptoms. The results, however, seemed extremely clearcut. The pattern of results agreed with a model in which there was a single genetic factor that increased the risk for the whole gamut of symptoms. There were other genetic factors that conferred more specific risks relating to symptoms clusters of impulsivity, learning problems, and tics/autism, but these had smaller effects. Remarkably, there was no evidence of factors that made twins more similar regardless of their zygosity, as would be expected if neurodevelopmental disorders were affected by aspects of the home environment.

The implications of this work for clinicians are spelled out by the authors thus: "When children display problems in one area, it might be more important to, as early as possible, set up a strategy for helping with all related symptoms rather than trying to help only with a specific diagnosis (which often will change over time)." We may feel comfortable with our domain-specific labels for neurodevelopmental disorders, but they do not capture the clinical reality.

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Pettersson E, Anckarsäter H, Gillberg C, & Lichtenstein P (2013). Different neurodevelopmental symptoms have a common genetic etiology. Journal of child psychology and psychiatry, and allied disciplines, 54 (12), 1356-65 PMID: 24127638