Neurodevelopmental disorders

Autistic disorders lie on a spectrum, which differ in terms of severity and characteristic behaviours. The term ‘autistic spectrum disorder’ includes those with autism (both childhood and atypical), and Asperger Syndrome or other autistic spectrum disorders. ‘Autism’ is defined by characteristics of abnormal functioning in all three areas of social interaction, communication, and restricted, repetitive behaviour. Asperger syndrome is classified as part of the spectrum of autistic disorders but differs from autism primarily in that there is no general delay or retardation in language or in cognitive development.

The charts present global level data on the prevalence of autistic disorders, as well as autism and Asperger syndrome specifically.

The prevalence of autistic spectrum disorders across most countries is roughly 1 percent of the total population; this ranges from 0.8 to 1.1 percent across countries.

Males are approximately 4-5 times as likely to have autistic spectrum disorder versus females. This is shown across countries in the chart.

Globally, around 62 million people were estimated to have autistic spectrum disorder in 2016. Just under 50 million of these cases were in males. Of the 62 million with autistic spectrum disorder, around 18 million had Autism, and 44 million had Asperger syndrome and other spectrum disorders. These breakdowns are shown in the charts.

The chart found here shows the health burden of autistic spectrum disorders as measured in Disability Adjusted Life Years (DALYs) per 100,000. A breakdown of DALYs from autism and Asperger syndrome can be found here.

Autism, which is a sub-category of autistic spectrum disorders, is defined by the ICD as:

Childhood autism

“A pervasive developmental disorder defined by the presence of abnormal and/or impaired development that is manifest before the age of 3 years, and by the characteristic type of abnormal functioning in all three areas of social interaction, communication, and restricted, repetitive behaviour.”

“Usually there is no prior period of unequivocally normal development but, if there is, abnormalities become apparent before the age of 3 years. There are always qualitative impairments in reciprocal social interaction. These take the form of an inadequate appreciation of socio-emotional cues, as shown by a lack of responses to other people’s emotions and/or a lack of modulation of behaviour according to social context; poor use of social signals and a weak integration of social, emotional, and communicative behaviours; and, especially, a lack of socio-emotional reciprocity.

“The condition is also characterized by restricted, repetitive, and stereotyped patterns of behaviour, interests, and activities. These take the form of a tendency to impose rigidity and routine on a wide range of aspects of day-to day functioning; this usually applies to novel activities as well as to familiar habits and play patterns.”

Atypical autism

‘Atypical autism’ differs from childhood autism either based on the age of onset, or the number of diagnostic criteria which are fulfilled. Those with atypical autism develop the condition after the age of 3 and/or they do not demonstrate behaviours in one or two of the three areas which typically diagnose autism (i.e. reciprocal social interactions, communication, and restrictive, stereotyped, repetitive behaviour).

Like the full spectrum of autistic spectrum disorders, males are much more likely to have autism than females. Males are around 2 to 3 times more likely to have autism than females.

Globally, an estimated 18 million people had autism in 2016; 13 million males and 5 million females.

Asperger Syndrome is classified as part of the spectrum of autistic disorders, but is differentiated from childhood or atypical autism. The ICD provide the following definition and diagnostic criteria for Asperger Syndrome:

“A disorder of uncertain nosological validity, characterized by the same kind of qualitative abnormalities of reciprocal social interaction that typify autism, together with a restricted, stereotyped, repetitive repertoire of interests and activities. The disorder differs from autism primarily in that there is no general delay or retardation in language or in cognitive development.”

“Diagnosis is based on the combination of a lack of any clinically significant general delay in language or cognitive development plus, as with autism, the presence of qualitative deficiencies in reciprocal social interaction and restricted, repetitive, stereotyped patterns of behaviour, interests, and activities. There may or may not be problems in communication similar to those associated with autism, but significant language retardation would rule out the diagnosis.”

The prevalence of Asperger Syndrome and other autistic spectrum disorders (not including autism) typically ranges from 0.5 to 0.8 percent across countries. This is higher than the typical prevalence of Autism.

There is a very strong gender difference in the prevalence of Asperger Syndrome and other autistic spectrum disorders; this gender gap is wider than for Autism. Across all countries, males are between 4 to 6 times more likely to have Asperger Syndrome and other autistic spectrum disorders than females.

Globally, an estimated 44 million had Asperger Syndrome and other autistic spectrum disorders (not including Autism) in 2016. 4.5-times as many males had Asperger Syndrome than (36 million) versus females (8 million).

Attention deficit/hyperactivity disorder (ADHD) is termed ‘hyperkinetic disorders’ by the ICD. It uses the following definition and diagnostic criteria:
“this group of disorders is characterized by: early onset; a combination of overactive, poorly modulated behaviour with marked inattention and lack of persistent task involvement; and pervasiveness over situations and persistence over time of these behavioural characteristics.

“Hyperkinetic disorders always arise early in development (usually in the first 5 years of life). Their chief characteristics are lack of persistence in activities that require cognitive involvement, and a tendency to move from one activity to another without completing any one, together with disorganized, ill-regulated, and excessive activity. These problems usually persist through school years and even into adult life, but many affected individuals show a gradual improvement in activity and attention.”

The prevalence of attention deficit/hyperactivity disorder (ADHD) ranges from 0.5 to 2 percent of the population across countries.

Males are typically 2 to 3 times more likely to have ADHD versus females. This is true across all countries.

Globally, it’s estimated that around 63 million had ADHD in 2016. 70 percent of ADHD cases were in males (44 million) versus females (19 million).

The Institute of Health Metrics (IHME) and the WHO International Classification of Diseases (ICD) define a broad category of ‘idiopathic developmental intellectual disability’. This category is broad and incorporates a number id disorders which are defined by delayed or impaired speech, language, motor condition, and visuo-spatial skills.

The ICD categorise this group of disorders as having the following common traits:
“(a) an onset that is invariably during infancy or childhood;
(b) an impairment or delay in the development of functions that are strongly related to biological maturation of the central nervous system; and
(c) a steady course that does not involve the remissions and relapses that tend to be characteristic of many mental disorders.”

“In most cases, the functions affected include language, visuo-spatial skills and/or motor coordination. It is characteristic for the impairments to lessen progressively as children grow older (although milder deficits often remain in adult life).”

The prevalence of intellectual disability disorders typically ranges from 0.4 to 3 percent of the population across countries.

In all countries the prevalence of intellectual disability disorders are higher in males than in females. The gender gap appears to be smallest at low prevalence levels (where males and females are almost equally as likely), with a gradual widening of the gender gap as the prevalence increases.

Globally, an estimated 115 million had an intellectual disability disorder in 2016. 56 percent of this total were male (65 million) and 44 percent female (50 million).