Why do we find it so hard to calculate the burden of Neurodevelopmental Disorders?

Neurodevelopmental Disorders (NDDs) comprise a spectrum of disorders that include Autism Spectrum Disorder (ASD), Attention Deficit/Hyperactivity Disorder (ADHD), Developmental Coordination Disorder (DCD, also called Dyspraxia), Developmental Language Disorder (DLD, also called Dysphasia and previously called Specific Language Impairment), Dyscalculia (also called Mathematical or Arithmetic Disability), Dyslexia (also called Reading Disability), Intellectual Disability (ID, also called cognitive disability, previously called mental retardation) and Tic Disorders (including Tourette Syndrome and chronic tic disorder). Some definitions of NDDs also include foetal alcohol syndrome, although we have not included it. The exact prevalence rates of NDDs vary considerably between countries and are generally higher in the USA than in the UK (Table 1). Although it was historically considered that children ‘grew out of’ most NDDs excepting ASD and ID, there is now good evidence that the impact of NDDs is lifelong for many individuals. In ASD, it has controversially been claimed that ‘optimal outcome’ (i.e. no or sub-clinical ASD symptoms) may occur in 3-20 % of individuals, dependent on symptom severity, age of diagnosis and age of treatment (Helt et al., 2008). In any case, most if not all individuals with have a persist challenges to a greater or lesser degree throughout life. The majority of studies indicate ADHD also predominantly persists into late adolescence and adulthood, with persistence occurring in between 50 % and 80 % of cases (Biederman, Petty, Clarke, Lomedico, & Faraone, 2011; Cadman et al., 2016; Guelzow, Loya, & Hinshaw, 2017; Hechtman et al., 2016; Mick et al., 2011; Tuithof, Ten Have, Van Dorsselaer, & De Graaf, 2014). However, a few studies indicate much lower persistence of childhood ADHD and an independent cohort of young adult-onset ADHD (Agnew-Blais et al., 2016; Moffitt et al., 2015). Approximately 70 % of children with DCD continue to exhibit significant motor difficulties through adolescence and into adulthood (Losse et al., 1991) and around 73 % of 5-year-old children with language impairments continue to have these impairments when aged 18-20 years (Johnson et al., 1999). Dyslexia and Dyscalculia show similarly high rates of persistence. Jacobson found that among dyslexic children aged 8-9 years, 83 % failed to achieve reading standards expected for their age and experience at age 15-16 years despite the majority of these pupils receiving remedial instruction (Jacobson, 1999). Additionally, 85 % of these children had persisting phonological deficits and 60 % had persisting decoding deficits at age 18-19 years (Svensson & Jacobson, 2006). Mussolin et al. found that adults aged 18-50 years who had been diagnosed with Dyscalculia as children were slower and more error-prone when completing a mathematics battery (Mussolin, Martin, & Schiltz, 2011). The persistence of Tic Disorders depends on the diagnosis: Provisional Tic Disorder (formerly Transient Tic Disorder) represents the bulk of childhood cases and is classified as a tic disorder that resolves within a year (American Psychiatric Association, 2013); however, Chronic Tic Disorder and Tourette Syndrome are more persistent. For example, in between 50 % and 90 % of individuals with Tourette Syndrome at least some tic symptoms persist into adulthood (Burd et al., 2001; Leckman et al., 1998; Pappert, Goetz, Louis, & Leurgans, 2003). The impact of NDDs on the individual is not only lifelong but also complex as among NDDs co-occurrence is the rule not the exception. Relatively few individuals exhibit a single ‘pure’ disorder and many exhibit more than two co-occurring disorders (Table 2). Even among those with a ‘pure’ NDD, subclinical co-occurrence is common, for example autism symptoms in children with ADHD (Green et al., 2015) or with DLD (Conti-Ramsden, Simkin, & Botting, 2006; Howlin, Mawhood, & Rutter, 2000). Indeed, it has been argued that the term ‘comorbidity’ is of questionable value in the context of NDDs (Kaplan, Dewey, Crawford, & Wilson, 2001). This is certainly the case for Tourette Syndrome, where 88 % of individuals have at least one co-occurring condition (Freeman et al., 2000), and ASD, where 70 % of individuals have at least one co-occurring NDD or psychiatric disorder (Simonoff et al., 2008). The specific pattern and degree of co-occurrence among NDDs has clinical relevance: for example the relationship with severity of symptoms is unsurprisingly associated with poorer outcomes as there are complex cumulative and interactional impacts between NDDs (Gadke, McKinney, & Oliveros, 2016; J. L. Green et al., 2015; Rasmussen & Gillberg, 2000; Schoemaker, Lingam, Jongmans, van Heuvelen, & Emond, 2013).