Global Data on Autism Spectrum Disorders Prevalence: A Review of Facts, Fallacies and Limitations

 
Universal Journal of Clinical Medicine 5(2): 14-23, 2017                                             http://www.hrpub.org
DOI: 10.13189/ujcm.2017.050202

Global Data on Autism Spectrum Disorders Prevalence:
     A Review of Facts, Fallacies and Limitations
                                            Onaolapo A Y1, Onaolapo O J2,*

       1
         Behavioural Neuroscience/Neurobiology Unit, Department of Anatomy, Ladoke Akintola University of Technology,
                                               Ogbomoso, Oyo State, Nigeria
  2
    Behavioural Neuroscience/Neuropharmacology Unit, Department of Pharmacology, Ladoke Akintola University of Technology,
                                                Osogbo, Osun State, Nigeria

Copyright©2017 by authors, all rights reserved. Authors agree that this article remains permanently open access under
the terms of the Creative Commons Attribution License 4.0 International License

Abstract        Autism spectrum disorders (ASDs) are a         in the early to mid-1960s [2, 3], while the first
range of neurodevelopmental disorders whose aetiologies        hospital-based survey in sub-Saharan Africa was
are largely unknown. In the past few decades, studies have     conducted in five countries in the late 1970s [4]. Autism
demonstrated that ASDs occur globally, and that the            surveys have since become global, with different countries
numbers of recorded cases are rising; however,                 conducting surveys to determine the prevalence and risk
determining the true prevalence figures is still a major       factors in their communities [1, 5], and thereby propose
challenge, especially in developing nations. Also, subtle      strategies to improve identification, diagnosis,
differences in cultural norms might impede timely/accurate     management as well as promote policy reforms that bring
diagnosis and categorisation of patients. In this review, we   awareness to the plight of the autistic child.
examine the globally-available data on the prevalence of          The term ‘Autism’ derives from the Greek word "autos",
ASD and discuss some of the challenges of data acquisition,    meaning "self”; and it was in the year 1911 that Eugen
with reference to how these may impact the reliability of      Bleuler first used the word autism in describing a symptom
figures obtained. Some of the facts, fallacies and             of schizophrenia. However, in 1943 and 1944 respectively,
limitations relating to the presently-available figures are    Leo Kanner (USA) and Hans Asperger (Germany)
also highlighted.                                              described individuals with social/emotional limitations and
                                                               withdrawn behaviours. Kanner reported behavioural
Keywords Neurodevelopment, Aetiology, Prevalence, aberrations such as poor social interaction, obsessiveness,
Autism, Data                                                   stereotypic movement, and echolalia in the affected
                                                               children. It was Kanner who first coined the term ‘infantile
                                                               autism’ (earlier called Kanner’s syndrome) to describe his
                                                               observations in this group of children who seemed
1. Introduction                                                socially-isolated and withdrawn [6]; while Asperger named
                                                               the condition he observed Asperger’s syndrome. Decades
   Epidemiology is not only concerned with patterns of of research will eventually reveal that both syndromes are
disease occurrence within human populations, but it is also parts of a highly-variable spectrum of disorders. Autism
concerned with the factors that determine or influence spectrum disorders (ASDs) are an array of
development of disease, or maintenance of health. Over the neurodevelopmental disorders characterised by cognitive
years, different study designs (surveillance, descriptive, and neurobehavioural deficits, whose underlying factors
analytical, cohort, case-control and cross-sectional) have are genetic and/or neurobiological [7]. In the Diagnostic
been employed in examining the relationships that may and Statistical Manual (DSM) III, autism debuted in 1980
exist between disease and disease determinants in defined as "infantile autism", which was replaced by the term
human       populations.    Generally,      epidemiological "Autistic Disorder" in 1987. In the DSM IV, it was referred
investigations of neuropsychiatric diseases begin with to as pervasive developmental disorders (PDDs); and
cross-sectional surveys, which help to determine the consisted of autistic disorder (autism), Asperger disorder,
prevalence of the disease condition, patterns of risk factors, childhood disintegrative disorder, Rett syndrome, and
as well as the possible correlates between disease and risk PDD-not otherwise specified (NOS).
factors among representative samples [1]. The first               Over the years, a lot has changed about our perception of
epidemiological survey of autism was initiated in England ASDs, the perceived risk factors, core diagnostic criteria,
Universal Journal of Clinical Medicine 5(2): 14-23, 2017                                 15

and even approaches to management. Presently, according           the human brain (notably the cerebellum and cingulate
to the DSM V, persistent deficits in social communication/        gyrus) in autistic children) [15].
social interaction and a restricted/repetitive (stereotypical)       The cause(s) of the above-mentioned pathological
patterns of behaviour, interests or activities; which must be     changes are traceable to both genetics and the environment.
present in the early developmental period(but might not           The heterogeneity of ASD is not only reflected in its
become obvious until social demands exceed limited                behavioural manifestations, but also in the genetic basis, as
capacity), cause clinically significant impairment (in social,    many candidate ASD genes have been studied or
occupational or other important areas of current                  implicated [16]. Also, while studies may differ in terms of
functioning) and must not be better explained by                  their conclusions regarding the degree of involvement of
intellectual disability or global developmental delay, are        genetics, it is generally accepted that genetics plays a
the core criteria for the diagnosis of ASD [8]. DSM 5 also        crucial role in ASD. Presently, candidate genes that have
makes provision for differential diagnosis such as ‘social        been implicated in ASD include those that encode for
communication disorder’. Level of severity of symptoms            neural adhesion molecules, ion channels, scaffold proteins,
{3 levels given for both social communication/social              protein kinases, receptor/carrier proteins, signaling
interaction, and restricted/repetitive (stereotypical) patterns   modulator molecules, and circadian relevant proteins [17].
of behaviour} determines the degree of supportive care            Environmental factors that had been linked to
needed, with level 3 severity requiring very substantial          ASD-associated       neuroinflammation        and    immune
support. Apart from the core symptoms, patients often have        dysfunction include early-life exposure to various
co-morbid psychiatric and behavioural manifestations like         xenobiotics including heavy metals such as lead, mercury
self-injury, aggression, impulsivity, hyperactivity, anxiety      and aluminium [18]. The Ethyl mercury compound
and mood symptoms; all of which may worsen the overall            (Thimerosal) is a bacteriostatic agent that was a component
prognosis, stress the caregivers, and further complicate          of a number of childhood vaccines in Western nations. The
management. Recently, results from epidemiological                possibility of its involvement in ASD led to its removal
surveys indicate that the global prevalence of ASDs is on         from many vaccines meant for children; and while it must
the rise; although data from sub-Saharan Africa and a             be noted that this was not accompanied by a sharp decline
number of other developing countries still show numbers           in ASD prevalence, its continued use in vaccines given to
that are generally lower than those from the developed            pregnant women further complicates the picture [18]. The
countries [9-11]. Whether this reflects an absolute low           use of aluminium(Al) as an adjuvant in many vaccines has
prevalence, deficits in diagnostic skills, mal-adaptation of      continued, due to the fact that Al salts help to stimulate the
diagnostic criteria as it relates to cultural differences in      immune system to yield adequate antibody titres [19].
behaviour, or under-sampling is still being studied.              However, there is abundant scientific literature on the
   The aetiology of ASD is still being studied, and despite       neurotoxic effects of Al; and its ability to adversely affect
years of research, a complete understanding of the                both the immune and the nervous systems, hence making it
causative factors is still elusive; and while it is now known     a plausible risk factor for triggering ASD [18]. Al’s ability
that genetics may plays a big role in ASD, a rapidly              to alter neuronal gene expression can also affect the
increasing prevalence suggests a bigger role of                   response of neurons to environmental insults such as toxins
environmental factors.                                            [18]. The presence and abundance of environmental risk
   Generally, abnormalities in neural connectivity                factors may be a major determinant of geographical
involving synapses, tracts and neuronal communication via         variations in the true prevalence figures of ASD; however,
neurotransmitters are key pathological features of the brain      the wide variety of candidate environmental risk factors
in autism [12]. Neurodevelopmental defects in synapse             makes matching environmental factors to prevalence a
formation/elimination and changes in ratios of                    challenging task.
inhibitory/excitatory synapses leads to defective local and          There have also been reports suggesting that some of the
long range connectivity [12]. An active neuroinflammatory         behavioural characteristics of “autistic” children in
process involving the cerebral cortex and the cerebellum;         developing countries differ, when compared to those in
with associated loss of cerebellar Purkinje cells, marked         developed nations [20, 21]. Along this line, how
reactivity of the Bergmann’s astroglia in areas of Purkinje       prevalence figures are influenced by the beliefs and
cell loss, marked astroglial reactions in the granule cell        cultural heritages of the different regions of the world is a
layer and cerebellar white matter, and astroglial reactions       matter of debate. In this review, we discuss the global
in the middle frontal/cingulate gyri had been demonstrated        epidemiology of ASDs by highlighting the incontrovertible
by     neuropathological        examinations      [13]. This      facts relating to its prevalence, fallacies of
neuroinflammation may have a strong link to                       misinterpretation of available figures as they are, and
autoimmunity, based on demonstrable elevated levels of            limitations of data acquisition, diagnosis and categorisation
immunoglobulins (IgG, IgM and IgA) against select                 of patients. We also summarise how these factors impact
neuron-specific antigens in ASD children [14]; and a              strategies to improve identification, diagnosis and
significantly higher presence of serum auto-antibodies to         management; as well as promote policy reforms.
16            Global Data on Autism Spectrum Disorders Prevalence: A Review of Facts, Fallacies and Limitations

1.1. Global Prevalence of ASDs                                  the Study of Autism in Canada (NEDSAC) ranks ASD as
                                                                one of the most common developmental disabilities,
1.1.1. Prevalence in USA, Europe, China, Japan, Middle          affecting 1 in 94 children [38]. A pilot study that was
       East, Ecuador, Brazil and Mexico                         conducted in Brazil reported a prevalence rate of 27/10,000
   Global data on the prevalence of disorders that now fall     [39]; while the prevalence of ASD in regular
under the term ASDs had been available before 2013. In a        schoolchildren in Quito, Ecuador was found to be
2012 review that compared prevalence data from different        11/10,000 persons [40]. The Leon survey in Mexico
parts of the world (excluding sub-Saharan Africa), the          reported an overall prevalence of 87/10, 000; a rate that
researchers estimated the median global prevalence of           was deemed consistent with other studies conducted at a
17/10,000 (approx. 1 in 588) for autistic disorder, and         similar period [41].
62/10,000 (approx. 1 in 161) for all pervasive
developmental disorders; while suggesting that the              1.1.2. Prevalence of Autism in Sub-Saharan Africa, India
increase in estimates over time and the variability between            and the Caribbean Islands
countries and regions may be linked to diagnostic                  According to the World Health Organisation, the global
switching, the broadening diagnostic criteria, service          prevalence of ASD has been estimated as 1 per 160 persons
availability and the increasing global autism awareness         [42]. However, the contribution of sub-Saharan Africa
[22]. Even prior to this, the epidemiology of ASDs had          (SSA) to this burden is largely unknown due to a paucity of
portrayed a shifting dynamics, with prevalence increasing       population-based surveys on autism in this region [10, 11,
from 2-4/10,000 children as reported in the 1940s [23], to      22, 42, 43]. Data (largely from hospital-based studies) are
approximately 4-5/10,000 children, according to the             however available from a few countries in the region. In
studies that were published in the 1960s and 1970s [24, 25],    south-west Nigeria, Lagunju et al [44] reported a
11/1000 children in the USA, according to a survey              prevalence rate of 2.3% (1 in 43.5) amongst 2,320
conducted about a decade ago [26]; while a Centres for          first-time attendees of a neurology or child psychiatry
Disease Control and Prevention surveillance report              clinic. In another clinic-based study in south-eastern
revealed a combined estimated prevalence of 14.6 per            Nigeria, prevalence of ASD was reported to be 0.8% (1 in
1,000, or 1 in 68 children (aged 8 years, whose parents or      125) of the total number of children attending the clinic for
guardians resided in 11 ADDM Network sites) in the              that year [45]; while in Uganda, a survey of 1,169 children
United States [27]. The most recent CDC figures on ASD          aged between 2 and 9 years reported an unadjusted
published in 2015 (which are believed to be more accurate)      prevalence for ASD of 6.8/1000 [46]. All studies reported a
say 1 in 45 children in the USA have ASD; this is however       male predilection, and middle to high socioeconomic status
based on a parent survey which was designed to track the        improved the likelihood of a child being evaluated for ASD.
prevalence of developmental disorders in children aged 3        While these studies provided some data, the absence of
to 17 years [28]. Studies in Europe recorded before 1999        data from large scale epidemiological studies from a
reported prevalence rates ranging from 1.9 to 72.6 per          number of countries in this region have been suggested as a
10,000 with a median of 18.75/10,000; while studies             possible reason for the perceived low incidence and/or
conducted after 1999 reported rates ranging from 30.0/10        prevalence of autism observed in SSA [10, 11].
000 to 116.1/10 000, with a median rate of 61.9/10 000 [22].       Until recently, India (like SSA) had a paucity of
In the United Kingdom, a 2006 study in 9-10 year-olds           community-based population studies for autism; although
reported a prevalence of childhood autism as 38.9/10,000,       there was a number of hospital-based autism clinical
while other ASDs (DSM III) was 77.2/10,000 (52.1-102.3),        assessment studies [47-49]. However, in a 2017
making the total prevalence of all ASDs 116.1/10,000 [29].      population-based study (of 28, 078 children aged between
However, in 2014, the National Autistic Society reported        1 and 10 years, cutting across geographical regions that
that 1 out of every 100 children has ASD [30]. In China, it     represent rural, urban, and tribal populations in India) using
was estimated that 1.1 in every 1,000 children are              the Hindi version of the Indian Scale for Assessment of
diagnosed with autism [31]; while in an analysis of five        Autism; a prevalence of 0.15% (1 in 667) was reported,
studies from mainland China, an estimated pooled mean           with a male sex predilection [50]. Also, a high
prevalence of 24.4 per 10,000 children was observed [32].       socioeconomic status increased the likelihood of an ASD
In a 2012 review of epidemiological studies conducted in        diagnosis [50]. The prevalence of ASD in the study was
the Western Pacific region (including Japan and China)          observed to be higher amongst children aged between 4
since 2000, Elsabbagh et al. [22] reported that prevalence      and 10 years with majority of them diagnosed with mild
rates varied from 2.8/10 000 to 94/10 000 with a median         autism; while amongst children aged between 1 and 7 years,
value of 11.6/10 000. In the Middle East, the prevalence of     the diagnosis of moderate autism was prevalent. This was
ASD was reported to be 1.4/10,000 in Oman, 29/10,000 in         however attributed to families delaying presentations till
the United Arab Emirates, and 4.3/10,000 in Bahrain             the onset of delayed motor and speech development [50].
[33-37].                                                        In an earlier study, the same authors (sampling 11,000
   In Canada, the National Epidemiological Database for         children within the same age range, and from the same
Universal Journal of Clinical Medicine 5(2): 14-23, 2017                               17

region) reported a prevalence of 0.0009 % (1 in 1100), with    underreporting stemmed from studies conducted amongst
low socioeconomic status correlating positively with           children born to African immigrants in Sweden [53-55],
autism [51].                                                   that reported high prevalence rates in this subset of the
  In the Caribbean islands, the Aruba Autism Project           population, compared to the indigenous population. The
determined the prevalence of ASDs in birth years               possibility of late diagnosis or misdiagnosis has also been
1990–1999 in Aruba, and found a figure of 53/10,000 [52].      considered, and studies have shown that African children
                                                               are more likely (than Caucasian children) to have a late
                                                               diagnosis of ASD [56], or a misdiagnosis [57].
2. Autism Spectrum Disorders: The                                 Despite the lower figures (when compared to developed
     Facts and Fallacies                                       nations), it is safe to say that even in SSA, the prevalence
                                                               rates of ASDs are on the rise; and there has been an
    Autism fact sheets from different advocacy groups ‘improvement’ from an era of no prevalence figures [22] to
(Autism Society, National Autism Association, National the            availability      of     hospital-based    [44],    or
Autism Network, Talk About Curing Autism, National community–based [46] prevalence figures. Also, in India,
Autism Network, Autism South Africa, Action In Autism, the prevalence from a 2017 study [38] revealed a rise from
Autism Speaks etc.) irrespective of region or country an earlier 2015 study [51].
generally accept and reiterate that autism is a                   In developed nations, a number of researchers have
bio-neurological developmental disability which generally attributed the rise in ASD prevalence to an increasing
appears before the age of 3; impacting normal brain ability to diagnose, or a possible reflection of the success of
development in the areas of social interaction, public health systems in better-identifying children who
communication skills, and cognitive function. There have were previously undiagnosed [58]; and this is also related
also been suggestions that autism is a disease of developed to the continued redefinition of diagnostic criteria [22]. An
countries, with a male sex predilection, that does not affect increased awareness is also thought to be largely
life expectancy. While currently, there is yet no cure for responsible for changing figures in less developed regions
autism, early diagnosis and treatment helps to improve the of the world, such as SSA [10, 11], and Asia [50, 51].
diverse symptoms associated with autism.                       Comparing the two Indian studies cited earlier [50, 51], it is
                                                               obvious that more children were recruited in the 2017 study
2.1. ASDs Occur Globally                                       than were in the 2015; suggesting that in the 2 year period,
                                                               more families were aware enough to want to seek diagnosis
    In relation to the prevalence of ASDs, data emanating and treatment for their wards. There is also evidence of
from developed nations appear more comprehensive and increasing awareness, cultural reorientation and the
reliable, compared to those from developing nations. availability of well-trained child and adolescent care
Despite this, one fact that can no longer be denied is that as specialists and allied professionals in SSA. In the last
it stands today, ASDs occur globally irrespective of culture, decade, particularly in the last 5 years, the number of ASD
geography or degree of industrialisation; with variable related studies in this region [44, 46, 59-61] has increased
degrees of tangible data from different regions of the world considerably, compared to the preceding decades.
where studies had been conducted. Hence, contrary to what         A review of a recent CDC data published in the National
might have been believed in the past, especially in the Health Statistics Reports [28] also supports the theory that
developing countries, it is not a disorder of ‘the West’ or of changes in awareness, earlier diagnosis, and redefinition of
advanced nations. The realisation of this fact is of diagnostic criteria has a lot of impact on prevalence figures.
particular importance in SSA and other regions of the While the autism rate appeared to rise 79 % over a 3-year
world where awareness levels are still growing.                period, the rate of developmental delay dropped 36 % [28];
                                                               and the combined prevalence of children diagnosed with
2.2. ASD Prevalence Figures are Still Rising                   autism,  developmental delay and intellectual disability did
                                                               not change over the 3 years. The unchanging statistic of the
    Globally, the recorded prevalence figures for ASDs are sum data suggests that a number of children who were once
rising; and from a statistical point of view, this is tagged as having developmental delay or intellectual
undeniable when we examine data from both developed disability are now being identified as having autism [28].
and developing countries. However, in many developing             In a study involving Danish children, Hansen and
countries, the reported rates continue to be significantly colleagues [62] were also of the opinion that the increase in
lower than the developed countries [9]. Whether this truly prevalence of ASD was actually due to changes in how the
reflects an absolute low prevalence, deficits in diagnostic disease is diagnosed [62]. By using data from a large study
skills, mal-adaptation of diagnostic criteria as it relates to involving 677, 915 Danish children (born between 1980
cultural differences in behaviour, or under sampling are and 1991) and tracked until they either had an autism
issues that continue to be discussed. The idea that the low diagnosis or reached the terminal date of the study (Dec. 31,
prevalence rates observed in SSA could be due to 2011); the authors concluded that significantly more
18            Global Data on Autism Spectrum Disorders Prevalence: A Review of Facts, Fallacies and Limitations

children were diagnosed with autism in 1995 and thereafter,     improved ability to diagnosis b) The public response that
than before 1994 (when autism became a spectrum of              followed the Wakefield article attests to the societal and
disorders in Denmark) [62]. However, the authors also           social burden of ASDs, and the dangers of the inability of
noted what appears to be a certain degree of real increase in   science to find a sound and timely answer in relation to the
prevalence.                                                     cause(s) of a disorder that continues to plague more
   Several researchers also share the opinion that there is a   children. It is therefore imperative to continue to research
real global rise in prevalence of ASDs, and the rise may be     and identify environmental and social factors that may
the product of several factors such as exposure to              increase risk of ASDs.
environmental toxins and increasing parental age/genetic
susceptibility [63-65]. Immune dysregulation in pregnancy       2.3. ASD has a Male Sex Predilection
has also been linked to the rising prevalence of ASD; and
autoimmune inflammatory disorders, such as coeliac                 Sexual predilections in disease prevalence are
disease and rheumatoid arthritis in mothers are believed to     well-recognised, and female predilections have been
increase the risk [66]. Inflammation due to common              reported consistently in a number of disorders with
infections and endemic parasitic illnesses are probably not     autoimmune aetiologies [70] while a male preponderance
likely associated with increased risk; and this is consistent   has been observed in some neurodevelopmental disorders
with the observation that in some developing nations such       [71-73] ASDs have also been observed to have a higher
as Nigeria or Cambodia, where parasitic and infectious          prevalence in males, compared to age-matched females; in
diseases are still common, recorded ASD prevalence is still     a 4:1 ratio. This observation has been consistent across
low [45, 67].                                                   populations, regions and time; strongly suggesting the
   The continued increase in prevalence figures of ASDs in      involvement of sex-specific biological factors in ASD
developed countries led to the postulation of the “Biome        aetiology [74, 75]. Genetic studies have demonstrated that
Depletion Theory” by some researchers, who believe that         females are protected from the effects of de novo and
maternal immune response overreaction is a factor               heritable ASD risk variants, with suggestions that sex
underlying the development of ASDs in offsprings.               hormones or sex chromosomal genes may modulate these
According to this theory, our immune system has                 genetic variations [74], for a detailed review on the
co-evolved alongside microbes and parasites, but a lack of      influence of sex in autism (see Halladay et al., [75]).
these pathogens in urban, post-industrial societies leads to
immune system over reactivity [67]. Along this line, it was     2.4. ASD and Life-expectancy
suggested that pro-biotics may be able to control the
inflammatory response in pregnant women, and prevent               It is generally believed that autism per se does not affect
some cases of ASD; however, this notion still seems like a      life expectancy; however, research continues to show
shot in the dark, and studies will have to be conducted to      higher mortality risk among individuals with ASD. In
examine this.                                                   comparison with mortality statistics from the general
   The apparent induced immune system over-reactivity           population or general population controls, the risk of
also formed the basis of an implied link between                premature mortality has been estimated to be 2-fold to
vaccination and ASDs. In 1998, a case series published by       10-fold higher in the ASD population; even autistic adults
Wakefield and colleagues, but later retracted by 10 of the      without a learning disability are 9 times more likely than
12 authors suggested that the measles, mumps, and rubella       controls, to die by suicide [76]. Also, risk of mortality is
(MMR) vaccine could predispose to behavioural symptoms          higher in females, compared to males [77]. Accidents such
that are representative of ASDs in children. Despite certain    as drowning and co-morbid medical conditions such as
obvious shortcomings of this study {including the very          epilepsy are among the classic causes of death; however,
small sample size (n=12)}; the publication generated a          these cannot fully account for the life span gap between
social firestorm that led to some parents rejecting             autistic and non-autistic people, or the difference in
vaccination for their children. However, epidemiological        mortality between autistic people with and without an
studies conducted thereafter failed to establish such a link    intellectual disability [76]. One of the factors that could
[68, 69]. The CDC also continues to reiterate its position      account for this gap is an increase in the prevalence of
that no such links exist. Again, a number of developing         health problems such as diabetes and respiratory disease;
countries who have contributed to the global data on ASD        and the fact that these health problems may go unnoticed
prevalence do not have immunisation regimens like MMR;          until it is too late [76]. Among autistic adults, pressures of
and even if they do, data establishing a link appear            meeting the obligations of ‘normal life’ may lead to
non-existent. From the foregoing, the following are             continued isolation, depression and suicide. The
apparent: a) an undeniable deduction from the study by          relationship between ASD and life-expectancy deserves
Hansen et al [62]is that is not all new cases of ASDs are       further studies, especially, with the realisation that certain
explicable by changes in diagnostic ability; hence, it might    life-threatening medical conditions are more common in
be unscientific and over simplistic to attribute all to         ASD patients. Therefore, it might be erroneous to simply
Universal Journal of Clinical Medicine 5(2): 14-23, 2017                                19

state that ASD does not affect life-expectancy without            the true figures are likely to be gotten from the community,
explaining the ever-increasing complexity of the                  and the present figures should be treated with caution until
relationship.                                                     larger community-based studies are carried out. However,
                                                                  while the value of an epidemiological study for assessment
                                                                  of needs and priorities within a community cannot be over-
3. ASD Global Epidemiology:                                       emphasised; the cost implication of such studies may be
   Limitations of Acquisition,                                    high, with a number of developing countries not finding
                                                                  them easily-affordable, especially in the face of pressures
   Interpretation and Viability of Data                           from other competing priorities.
   An accurate global comparison of data relating to the
prevalence of ASDs is a rather challenging task, due to           3.2. Impact of Culture
factors such as; a) the absence of data from large scale
epidemiological studies from a number of countries,                  Cultural differences play a large role in disease diagnosis,
especially in the sub-Saharan countries like Nigeria (where       especially when considering the diagnosis of disorders that
most of the available rates emanated from hospital-based          tends to attract a lot of social stigma; therefore, despite
studies); even, developed nations like the USA cannot             adequate sensitisation and awareness, parents may still
boast of a truly all-inclusive national survey, probably due      refuse to submit their children for ASD evaluation. For
to impediments of cost effectiveness and logistics b) the         instance, in South Korea, a risk of intense stigmatisation
information gathering tools are not totally adaptable to or       makes many families of children with developmental
may not yield 100 % accuracy in some regions of the world         delays to intentionally avoid diagnosis of ASD; also
because of certain cultural peculiarities that might affect       cultures that believe that speech development in the male
diagnosis or categorisation c) there could be wide regional       child is usually slower than females make seeking early
variations in figures arising from the same country, as is the    help for the child to be impossible [9]. Therefore, societal
case with China [32].                                             or parental behaviours affect ASD prevalence figures.
                                                                     The lack of a culturally-adaptable tool for ASD
                                                                  screening constitutes another impediment to having
3.1. Absence of Population-based Prevalence Data in
                                                                  reliable prevalence data; since culture/language differences
     Developing Countries
                                                                  might affect diagnosis rates. Different cultures have
   The prevalence figures of ASD in sub-Saharan Africa            peculiar behaviours that are at least acceptable, if not even
have been based mainly on a few hospital-based studies,           desirable. Diagnosis of ASD is not based on laboratory
and hardly any community-based study, except that of              tests, but rather, on the exhibition of certain behaviours.
Kakooza-Mwesige et al [46] in Uganda. Under these                 These behaviours include the tendency to establish eye
circumstances, under-sampling or ‘skewed’ sampling have           contact, ability to form relationships with others,
been suggested to negatively impact the reported                  milestones in language development, and the presence of
prevalence [10, 11]. The study by Lagunju et al [44] was          certain repetitive motor and play behaviours. Cultural
conducted in a tertiary hospital setting in south-west            perceptions affect the exhibition of at least some of these
Nigeria, while that of Bakare et al [45] was conducted in a       behaviours; and in different cultures, parental perceptions
similar setting in south-eastern Nigeria. These centres are       of their presence or otherwise affect the timing of seeking
the ultimate referral centres for paediatric patients in their    help for a child with possible ASD. Therefore, behaviours
respective regions and they represent a ‘gathering point’         that are considered normal in some cultures may fall within
for all ‘puzzling’ and ‘unusual’ childhood illnesses. First, it   the inclusion criteria for ASD diagnosis.
must be noted that there are other regions (in the country),         Generally, in Europe and America, children are expected
for which there are no data. Secondly, confidently drawing        and encouraged to make eye contact with others; but in
an inference from such studies is associated with two             other parts of the world such as Asia and Africa, or even
potential pitfalls; that of under-representation, due to an       among native communities in America and Europe, this is
apparent small sample size, compared to the total                 not encouraged, and may even be perceived as rudeness or
population of children within the age range; or of                insubordination, especially, when interacting with adults.
over-representation, because the sampling frame is                This has made certain researchers to be of the opinion that
compelled to fall on an area where such cases are likely to       the matter of use of eye contact as a diagnostic criterion for
cluster. This clustering effect may the apparent from the         autism needs to be handled with caution. There are also
figures; in Nigeria, Lagunju et al [44] sampled from the          cultural variations in the degree of a child’s interaction
paediatric neurology clinic and got a prevalence of 23/1000       with others, especially with adults. There are cultures all
and Bakare et al [45] sampled from a general paediatric           over the world (Africa, Asia, Central and South America)
clinic to return a figure of 8/1000; while in Uganda,             that don’t encourage ‘undue’ interactions between children
Kakooza-Mwesige et al [46] sampled from the community             and adults (especially strangers). How children in these
and got an unadjusted figure of 6.8/1000. It is obvious that      cultures form relationships may not be exactly what is
20             Global Data on Autism Spectrum Disorders Prevalence: A Review of Facts, Fallacies and Limitations

taken to be normal in urban Europe or America; and ASD            Conflict of Interest
diagnosis in these communities may face an uphill task
when the children encounter these ‘strangers’(healthcare             Onaolapo AY declares that she has no conflict of interest.
personnel) that are questioning, examining and observing          Onaolapo OJ also declares that he has no conflict of
them. Along this line also, language development may be           interest.
difficult to assess with all certainty, as the child may simply
be scared or ‘ashamed’ to talk to an unfamiliar adult;
leading to a situation where the parents vouch that the child     Acknowledgements
could speak, but he/she will refuse to communicate during
the assessment.                                                     This research did not receive any specific grant from
    Since cultural differences may impair diagnosis of ASDs,      agencies in the public, commercial, or not-for-profit
it is imperative to develop universally-adaptable diagnostic      sectors.
criteria that are applicable to all cultures. In order to
achieve this, studies must be conducted across cultures to
find common markers that consistently differentiate
children with ASD from typically-developing children.
These markers are to be inserted as probes when gathering
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