Are neurodevelopmental disorders discrete conditions?

Emilia Misheva considers the evidence.

There is abundant evidence to indicate that neurodevelopmental disorders tend to co-occur, rather than existing as individual conditions. Could this ‘comorbidity’ reflect an overall atypical course of neural development, which manifests in various ways?

As psychologists, many of us are familiar with the idea that diagnostic labels could stifle the thinking of both those they’re attached to and the practitioners supposedly helping them. Do these labels describe discrete, objectively defined conditions, with different established causal origins and presentations? The evidence paints a more complex picture.

Instead, some researchers in the field have argued that comorbidity shows neurodevelopmental disorders are not completely distinct conditions. Improved awareness in this area is important: the evidence can help practitioners develop more sophisticated formulations and interventions that incorporate a dynamic, modern account of neurodevelopment as a complex process where similar end results could have been caused by different underlying causal mechanisms. Consider special educational needs provision in schools. Here, a lack of awareness around the possible common origins of many neurodevelopmental disorders may result in an over-emphasis on just one pronounced aspect of the child’s difficulties – for example, reading. This may lead to a diagnosis that does not capture the full profile of the young person… more subtle difficulties in other areas of learning and cognition may go unnoticed and unaddressed.

What are neurodevelopmental disorders?
Neurodevelopmental disorders are commonly defined as disorders of multivariate origin where the cause is unknown or complex, with typical onset in childhood, that may affect areas of cognitive function such as memory, inhibition control, coordination and motor skills, as well as emotion regulation. There is some debate as to which conditions should be formally classified as neurodevelopmental disorders. Some definitions include both neurological disorders of known origin (such as Down’s syndrome and Williams syndrome) and disorders of unclear or multivariate origin (such as autistic spectrum conditions, ADHD, dyslexia, dyspraxia and dyscalculia). Some scholars argue that while understanding the causal mechanisms behind disorders of known origins can be very illuminating for our overall picture of neurodevelopment, there are fundamental differences: the latter are categorised mainly on the basis of behavioural presentation, rather than aetiology. This brings different challenges in terms of diagnostic validity, the assessment process itself, and intervention.

Similarly, some commentators have broadened the definition even further to argue that conditions such as schizophrenia, obsessive-compulsive disorders and even depression can be classified as neurodevelopmental conditions. That’s beyond the scope of this article, but I will incorporate conditions with unclear multivariate origin.

Comorbidity – just a coincidence?
Bonnie Kaplan argues that comorbidity is the rule in developmental disorders, rather than the exception. She goes on: ‘When comorbidity is the rule in physical health, a single underlying disorder is usually assumed. Yet in the area of childhood behavioural/emotional/coordination disorders, we continue to use independent labels such as Reading Difficulties and Developmental Coordination Disorder, as if the child has demonstrated two distinct, unique conditions.’

There is indeed strong evidence that different developmental disorders co-occur and overlap at rates much higher than those expected by random chance. For example, dyslexia and dyspraxia have a comorbidity rate of between 60 and 70 per cent; approximately 40–45 per cent of children diagnosed with dyspraxia would also meet the diagnostic criteria for ADHD, autism spectrum disorder (ASD) or dyslexia; and some studies have discovered comorbidity rates of up to 60 per cent between dyscalculia and dyslexia. Many researchers argue that it is highly unlikely that such comorbidity rates are a coincidence.

Developmental neuropsychologist Dorothy Bishop, for instance, uses ASD and developmental language disorder (DLD) as an example. ASD affects around 1 per cent of the population and DLD occurs in roughly 7 per cent of the population. It would therefore be extremely rare for these two conditions to co-occur, with a probability rate of 7 in 10,000. Yet children diagnosed with ASD often experience language difficulties consistent with a DLD diagnosis, with studies identifying comorbidity rates of up to 76 per cent.

While there is still considerable debate surrounding the possible shared aetiology of neurodevelopmental disorders, explanatory models have emerged in an attempt to address some of the complexity surrounding the origins of these conditions.

Neuroconstructivism, for example, offers a useful framework for examining the complex nature of neurodevelopment and its implications for neurodevelopmental disorders. Neuroconstructivists argue that the brain gradually specialises over developmental time through experience, and while some parts of the brain may have a bias towards processing certain types of input, different parts of the brain are not innately specialised to specific cognitive domains such as memory, language or reading. While neuroconstructivists do not argue that the human brain is a ‘blank slate’ at birth and they acknowledge the role of biological and genetic constraints, they do argue that, initially, all areas of the brain are able to process different types of information, with some areas more relevant to the processing of specific types of input. Specialisation occurs gradually throughout developmental time, where different areas compete against each other. The area that is most relevant to that specific type of input ‘wins’ and becomes specialised in processing this specific type of input.

The neuroconstructivist view of the brain is therefore one of an interacting system where disturbance in one local area in the early stages of development can have a cascading effect on a range of cognitive domains. A particular example used by Annette Karmiloff-Smith and Emily Farran to illustrate this point is the ‘uneven cognitive profile’ observed in children with Williams syndrome. Williams syndrome is a genetic condition with a profile characterised by relatively strong language and social skills and more significant difficulties with visuospatial skills. This example can be used by nativists – scholars who believe that the human brain is innately modularised – as evidence that an impairment in one module does not affect other modules. Upon closer examination, however, it emerges that even in areas of relative strength, children with Williams syndrome use different mechanisms to typically developing children to achieve the same outcome. This suggests that areas of perceived relative strength are not necessarily ‘spared’; rather, the individual has found another way of compensating for their difficulties in other domains that have also been affected in more or less subtle ways.

Therefore, from a neuroconstructivist perspective, it is unlikely that a person will present with difficulties in just one isolated area (i.e. reading) without any other difficulties. Rather, the reading difficulties are likely to originate from a deficit on a more specific local level that has ‘cascaded’ to other areas throughout development. It is therefore important that developmental trajectories are taken into account in order to try to locate an underlying origin. Grouping children in categories based on behavioural presentation alone risks missing the fact that those similar behavioural presentations might be the end result of different trajectories of atypical development that may also have had an effect on other areas of development, and in less obvious ways.

This also has significant implications for intervention. Farran and Karmiloff-Smith have summarised as follows:
Take a patient who presents with a serious deficit in, say, number but scores in the normal range for all other domains. It would be tempting in such a case to tailor remediation solely to the domain of number. However, that misses the very point of the neuroconstructivist framework. First, the scientist would need to trace back to infancy the origins of the number deficit, which might not be in the number domain directly; it could be a deficit in the visual system in scanning arrays of objects. A scanning deficit might affect other domains but to a lesser degree, meaning that these other domains could look normal in subsequent development but may camouflage subtle deficits. Once one explores multiple, low-level interacting processes that underpin early development, this leads to a more dynamic neuroconstructivist view of remediation also.

Moving forward
We have seen that diagnostic labelling may never accurately account for the complex, multivariate origin of developmental disorders, or for the possibility that neurodevelopmental disorders may not have completely distinct aetiologies. So would a broader conceptual category be more useful? It would need to encompass the possibility that developmental disorders may be a manifestation of an atypical neural development rather than completely distinct conditions, in a way that does not discount the objective existence of the difficulties that individual experience regardless of the label attached to them. In 2001 Jeffrey Gilger and Bonnie Kaplan proposed the term ‘atypical brain development’ as an alternative to the distinct diagnostic labels. Their view was that this conceptualisation could ‘serve as an integrative concept of etiology, the expression of which is variable within and across individuals’ and should not be seen as a separate disorder.

Kaplan stated that this approach needn’t be seen as critical of other approaches; rather, it aims to provoke debate and raise awareness of the very complex aetiology of neurodevelopmental disorders and of the limitations of the existing diagnostic terms. More recently, Dorothy Bishop has proposed the term ‘neurodevelopmental disability’ as a possible alternative to separate diagnoses. She argues that this can be used as a generic term, and the specific type of difficulties can then be described in more detail. Bishop explains that she chose the word ‘neurodevelopmental’ in order to point to the broader structural origin of the condition, and ‘disability’, rather than ‘disorder’, as the word disability can be seen as describing the often significant impact of the condition on the daily functioning of the individual, without implying abnormality. Bishop suggests that the term would also prevent the current compartmentalised view of developmental disorders and would not restrict practitioners to putting individuals into constrained diagnostic boxes; rather, under this term, they will be able to take into account the full range of difficulties the individual is experiencing regardless of whether they meet the seemingly arbitrary cut-off points for diagnosis required for separate conditions.

While Bishop acknowledges that the adoption of this model on a broader level would warrant significant changes in the ethos and structure of health, education and social care services, her approach could be used to improve awareness among those working with individuals with neurodevelopmental disorders. It could help to think more broadly about atypical development, to see beyond the diagnostic label. More specifically, in the context of special educational needs provision in schools, this approach would ensure that the children’s difficulties are not conceptualised in an overly simplistic, fragmented way; instead, it points the way to a multilayered account of the child’s profile of strengths and difficulties.

About the author

‘My interest in the field of developmental conditions and the co-occurrence between them was first sparked prior to my training as an educational psychologist. In my previous roles, I was working with children who presented with difficulties in more than one area; however, this was often not reflected in the diagnosis they had been given. Their families and schools often felt that the child’s diagnosis did not fully or in some cases, accurately, reflect his or her needs, difficulties or overall presentation. Those anecdotal examples sparked my academic interest in this topic and I hope that the article will provoke the readers’ curiosity and encourage them to find out more about this important debate.’

- Emilia Misheva a Trainee Educational Psychologist at the Institute of Education, University College London

Key sources
Bishop, D. (2010, 18 December). What’s in a name? [Blog post]. BishopBlog. Retrieved 10 April 2018 from
Farran, E.K. & Karmiloff-Smith, A. (Eds.) (2012). Neurodevelopmental disorders across the lifespan: A neuroconstructivist approach. Oxford: Oxford University Press.
Gilger, J.W. & Kaplan, B.J. (2001). Atypical brain development: A conceptual framework for understanding developmental learning disabilities. Developmental Neuropsychology, 20(2), 465–481.
Kaplan, B.J., Dewey, D.M., Crawford, S.G. & Wilson, B.N. (2001). The term comorbidity is of questionable value in reference to developmental disorders: data and theory. Journal of Learning Disabilities, 34(6), 555–565.
Karmiloff-Smith, A. (2009). Nativism versus neuroconstructivism: Rethinking the study of developmental disorders. Developmental Psychology, 45(1), 56.
Pennington, B.F. & Bishop, D.V. (2009). Relations among speech, language, and reading disorders. Annual Review of Psychology, 60, 283–306.

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