27th October 2023

Overview of Neurodevelopmental Disorders (NDDs)

Important classifications 

The ICD-11 states that NDDs encompass a spectrum of difficulties in the acquisition and execution of intellectual, motor, language or social functions, with onset in the developmental period before the age of 18 (World Health Organisation, 2019).

Three of the most clinically important diagnoses include intellectual disability (ID), autism spectrum disorders (ASD), and attention-deficit hyperactivity disorder (ADHD), which will be outlined in more detail in the subsequent sections. Other disorders include things like:

• Developmental learning disorders, e.g., reading, written expression, and mathematics impairments.
• Developmental speech or language disorders, e.g. speech sound, fluency (stuttering), language disorders.
• Developmental motor disorders, e.g. impairments acquisition and execution of coordinated motor skills, also includes some tic disorders.
• Stereotyped movement disorder, e.g. repetitive, apparently purposeless motor behaviour, e.g. hand flapping, rocking, head banging etc.

Shared components

Aetiology

There is significant co-occurrence of multiple NDDs across the levels of diagnosis, symptoms/problems, and underlying biology, including genetics, to the point where comorbidity is the ‘rule rather than the exception’.

For example, the overlap between ASD and ADHD is high:

• 22-83% of children with ASD have symptoms of ADHD, and 30-65% of children with ADHD have symptoms of ASD (Morris-Rosendahl & Crocq, 2022).
• Having a twin or sibling with ADHD or ASD puts other twins or siblings at a significantly greater likelihood of developing ASD, ADHD, and a host of other NDDs (Dewey, 2018).

However, if symptoms do not meet a diagnostic threshold (leading to a dual diagnosis), then vital clinical and research information risks being lost. Given the trait- or symptom-level overlap and subsequent blurring of boundaries between formal diagnoses, it might be appropriate to conceptualise NDDs as existing on a continuum of their own (Morris-Rosendahl & Crocq, 2022). Further study of the shared genetics, epigenetics and neurobiology, as called for by the research domain criteria (RDoC) project, is likely to make significant contributions to advancing this understanding and the subsequent classification systems.

Aside from biological predisposition, there are also thought to be some shared environmental factors for the development of NNDs, such as – prenatal exposure to alcohol or drugs, premature birth or low birth weight, and exposure to lead, mercury or certain components of plastics. The developmental origins of health and disease (DOHaD) hypothesis argues that NNDs are likely to be related to exposure to such environmental factors during critical periods of development, which subsequently affect genetics and body systems lifelong. Further research is required to elucidate the specifics of these relationships (Dewey, 2018).

Time course

For the vast majority of people, NNDs begin the developmental periods of early life and persist into adulthood; e.g. two thirds of children diagnosed with ADHD will have ongoing impairments into adulthood (RCPsych, 2023). Over time, those with NNDs are also at greater risk of exploitation and further psychiatric and physical health concerns. With early, specific intervention and ongoing support where necessary, there can be a significant reduction of disability for many people, highlighting the importance of timely and accurate assessment of needs.

Sex, gender and cultural factors

Perhaps unsurprisingly, NDDs are more commonly diagnosed in males than females, e.g. 4:1 in ASD, 2:1 in ADHD (Morris-Rosendahl & Crocq, 2022), although there is widespread variation in this. Presentation in females often varies significantly; e.g. females are more likely to present with inattentive symptoms of ADHD and may be better at ‘masking / camouflaging’ their symptoms of ASD (especially as people get older) and so go undiagnosed (Bölte et al., 2023). It is vital to consider the appropriateness of any formal testing (e.g. neuropsychological testing in ID) and the norms used for comparison, as performance might be affected by cultural biases and limitations of translation.

Neurodiversity

Although initially born from activists with ASD, the neurodiversity paradigm remains applicable to all NDDs, given their high heritability and frequent lifelong time courses. It is a relatively recent approach, centred on the fact that humans all vary in their neurological make up and subsequently how they process information: people with the most common sets of capacities are referred to as ‘neurotypical’, whilst those with broadly different ways of processing would come under the heading of ‘neurodiverse’.

In the context of disorders, it employs a social model for disability, i.e. disability arises not within the individual as the medical model would suggest but within the interaction between an individual’s capacities and their broader social context. One example of this relating to ASD might include failures for certain workplaces to sufficiently specify task requirements in ways that the autistic person might need. In this way, NNDs are seen as both natural variations (that, in a few instances, might also provide advantages) as well as disabilities. Consequently, there have been many useful insights, e.g. emphasising the importance of meaningful co-production in research, calling attention to support required by those with NNDs whilst striving for societal acceptance and emphasising the value of what people can bring. These perspectives also offer an important counterpoint to many interventions for things like ASD that continue to place a reduction of core symptoms as their goal, something which many of the community feel risks undermining efforts towards greater social provision (Den Houting, 2019).

References

• Bölte, S., Neufeld, J., Marschik, P. B., Williams, Z. J., Gallagher, L., & Lai, M. C. (2023). Sex and gender in neurodevelopmental conditions. Nature Reviews Neurology, 1-24.
• Den Houting, J. (2019). Neurodiversity: An insider’s perspective. Autism, 23(2), 271-273.
• Dewey, D. (2018). What is comorbidity, and why does it matter in neurodevelopmental disorders? Current Developmental Disorders Reports, 5, 235-242.
• Morris-Rosendahl, D. J., & Crocq, M. A. (2022). Neurodevelopmental disorders—the history and future of a diagnostic concept. Dialogues in clinical neuroscience.
• Royal College of Psychiatrists (2023) College Report CR235 ADHD in adults: Good practice guidance.
• World Health Organisation. (2019). Neurodevelopmental disorders. International statistical classification of diseases and related health problems (11th ed.).