CHAPTER 1
An Introduction to Neurodevelopmental Disorders

Munib Haroon

OVERVIEW

• Neurodevelopmental disorders (NDDs) are relatively common conditions.
• They arise in the developing brain and so have features typically present in childhood.
• They can present with developmental differences and alterations in personal, social, academic or occupational functioning.
• They include autism, ADHD, Tourette's disorder/tic disorders, developmental coordination disorder, intellectual developmental disorders (intellectual disability) and specific learning disorder.
• NDDs often co-occur in the same individual.
• NDDs are associated with mental health difficulties and other physical conditions.
• NDDs can be viewed as diversity/difference/divergence, difficulty, disorders or disabilities, according to context and different models of health.

What is a neurodevelopmental disorder?

Neurodevelopmental disorders (see Box 1.1) arise in the developing brain; their features are typically present from childhood (but may escape attention until later on in adolescence or adulthood). They may include differences in patterns of development or result in alterations in personal, social, academic or occupational functioning.

Depending on the person and their situation, these conditions may be considered as natural (neuro) diversity, difference, neurodivergency and/or as difficulties, disorders or disabilities (see Box 1.1). There is typically a strong genetic component behind them, meaning that they can cluster in families. In addition, they often seem to co-occur in the same individual.

Classification schemes

Two of the main classification schemes in healthcare, the International Classification of Diseases (ICD) and the Diagnostic and Statistical Manual of Mental Disorders (DSM) (see Figure 1.1), define what conditions are classed as neurodevelopmental disorders. Both of these are well-established, internationally recognised systems and at the time of writing are in their 11th and 5th iterations, plus further text revisions for the latter. While they continue to use slightly different terminology for some conditions, they have become more closely aligned over the years. This is helpful in ensuring the use of consistent terms. That is important, because consistent language can be helpful for clinicians who make diagnoses, but also for researchers, patients, carers and other professionals.

Table 1.1 lists some of the conditions classed as neurodevelopmental disorders; as you can see, the lists under each classification are very similar. Because the DSM is probably the most widely used system and the one that most people are familiar with, this book will use the DSM terminology.

Even after taking account of the conditions deliberately left out of the list in Table 1.1 for simplicity, there do seem to be some notable omissions. Tic disorders do not appear primarily under the ICD-11 classification, but instead appear as a secondary class. There are other conditions that could be said to belong in the classification schemes for neurodevelopmental conditions but are not listed. Examples include epilepsy and schizophrenia. Compelling arguments could be made for including these on the lists, although clearly counter-arguments against locating them there have also been made - and for the moment seem to dominate the discussion.

This demonstrates the somewhat arbitrary nature of classification schemes.

There are other conditions that will be familiar to some people as neurodevelopmental disorders but are not on these lists, for example Asperger's syndrome (see Box 1.2). This illustrates the changing nature of classification schemes. For example, conditions like Asperger's or 'pervasive developmental disorder not otherwise specified' have become subsumed within the more overarching term 'autism spectrum disorder'. There are a number of reasons for this; perhaps the most important is that many conditions are increasingly described as existing on a continuum, with variations in the extent to which individuals with these conditions manifest different features. Whereas previously conditions like autism and Asperger's were seen as more distinct, albeit very similar conditions (similar enough to create a diagnostic dilemma over whether someone had autism or Asperger's), modern terminology sees these conditions as points on a broad spectrum.

Box 1.1 Some definitions

A number of different terms are used throughout this text, such as condition, disorder, disability, neurodiversity, neurodivergent and neurotypical, of which brief definitions are given here. The reader should be aware that there may be variations in how these are used outside of this text.

• Condition: A general or specific state of physical/mental health (i.e. 'you're a highly conditioned athlete' vs 'the condition you have is called eczema'). In ABC of Neurodevelopmental Disorders the word is therefore frequently used to imply a variation from the 'usual' and sometimes in relation to a specific condition.
• Disorder: A condition or illness that affects the usual functioning of the body or mind.
• Disability: A physical or mental condition that has a substantial and long-term negative effect on the ability to do normal daily activities.
• Neurodiversity: The thinking, perceiving, learning and behavioural variations that exist across all humans (e.g. 'Neurodiversity is a fact of life' but not 'I am neurodiverse').
• Neurodivergent: The presence of variations from 'the norm' in thinking/perceive/learning/behaving (e.g. 'I am neurodivergent').
• Neurotypical: As distinct from neurodivergent. The type of thinking/perceiving/learning/behaving that conforms to what is understood to be 'usual' by society and does not fit a recognised pattern of thinking that might be identified as neurodivergent.

Figure 1.1 The classification schemes for neurodevelopmental disorders.

Table 1.1 Comparison of classification schemes ICD-11 and DSM-5-TR for neurodevelopmental disorders (not all disorders are included).

Box 1.2 John

John is a 25-year-old ex-solicitor. He was originally referred as an 8-year-old to see a paediatrician for possible dyspraxia before being diagnosed two years later with Asperger's syndrome. This was followed with being diagnosed with attention deficit hyperactivity disorder (ADHD) just after starting university. He recently left his job after developing a severe anxiety disorder and is currently tutoring law students online from his home in Cambridge. He has always had insomnia, but has found that not having to work 'office hours' has worsened his sleep-wake routine and he has just started taking melatonin to help with this. He is wondering if he needs to see the adult autism team to have his Asperger's diagnosis reclassified as autism.

Medical models and social models

This title of this book contains the word 'disorders', but is that the correct term for a person with a neurodevelopmental condition?

When viewed from a clinical point of view, the reasons for the word choice are clear. People do not go to an autism clinic (for example) out of a sense of curiosity. It is generally because they are experiencing difficulties of some sort and they (or their carers/parents/partners) are seeking an explanation for these difficulties and/or some kind of support. But are those difficulties necessarily down to an individual's innate make-up?

As we see in the case of Jane (Box 1.3), while there may have been notable differences in behaviour from an early age and she was subsequently diagnosed as being autistic, Jane was doing perfectly well until there was a lot of upheaval in her life (because of events not within her control), and once things were resolved she settled down again.

Is it therefore fair in Jane's case to see the autism as a disorder/disability or even as a medical condition when she is perfectly fine in the right environment? This argument can be generalised to state that in neurodivergent individuals, if the environment is right (whether through support or leaving them alone to get on with things...