A review of prevalence studies of Autism Spectrum Disorder by latitude and solar irradiance impact
Introduction
Autism Spectrum Disorder (ASD) is an umbrella term for multiple neurodevelopmental conditions characterized by repetitive or stereotyped behaviors and pervasive deficits in social communications and interactions [1]. ASD is considered a lifelong disability which has an impact on both the individual and the family [2], [3], as well as being a cost to society in general [2], [4]. Among these costs are additional health care, disability support in school and, in some instances, the loss of a productive working life and the provision of social security. In addition, ASD is associated with several comorbidities [5], [6] such as Attention-Deficit/Hyperactivity Disorder [7], [8], [9], [10], Obsessive Compulsive Disorder [7], [8], anxiety disorders [11], [12], [13], [14], [15], sensory over-responsivity [13], [16], [17], [18], [19], sleep disorders [20], [21], [22], and gastrointestinal problems [13], [20], [23], [24].
The prevalence of ASD, or at least reports thereof, have increased substantially from 1 in 500 in 1995, 1 in 250 in 2001 [25], to 1 in 68 in 2010 in the USA in children less than 8 years [26]. However, it is possible that rather than an actual increase in the rate of ASD, these statistics reflect higher prevalence associated with expanded definitions of ASD, increased public awareness and help-seeking. Further changes in the prevalence rates for ASD may also be a product of social de-stigmatization associated with ASD [27].
Despite the efforts of scientists who have investigated a myriad of environmental and genetic factors including air pollution [28], [29], [30], [31], [32]; environmental toxins such as mercury, nickel, selenium, lead, cadmium, aluminum, vinyl chloride and trichloroethylene [33], [34], [35], [36], [37]; genetic heritability [38], [39], [40]; hormonal imbalances such as oxytocin [41], vasopressin [42], and more recently Vitamin D deficiency [43], [44] the etiology of ASD remains uncertain.
Following Cannell’s proposal [43], [44] that Vitamin D deficiency could be a risk factor for ASD several researchers have found low Vitamin D levels in patients with ASD [45], [46], their siblings [47] and also maternal deficiencies [4], [48], [49]. Laboratory research has explored the genes regulated by Vitamin D [50], [51]. Overall, these results have pointed towards an association between Vitamin D deficiency and autism-related traits [44], [52]. Further support for Cannell’s hypothesis comes from reports that some level of improvement in autistic symptoms has been achieved via the administration of Vitamin D supplements [53], [54], [55]. Despite this tentative support for an association between Vitamin D and ASD, no conclusive evidence of this relationship has emerged.
In line with the research into Vitamin D deficiency, recent research has addressed a link between solar irradiance and ASD [56], [57], [58]. Solar irradiance is the measure of the sun’s electromagnetic spectrum over all wavelengths per unit area, usually described in watts per meter2 (W/m2) units [59]. The earth’s atmosphere filters the sun’s electromagnetic radiations leaving Ultraviolet (UV), Visible, and Infrared radiations as the main biogenically relevant components [60] to reach the earth’s surface. UV is further classified into UVA, UVB and UVC [59], [61]. UVA reaches the earth’s surface throughout the year, UVB reaches the surface only when the sun is high in the sky, while UVC is completely blocked by the earth’s atmosphere [59].
When people are exposed to UVB radiation, 7-dehydrocholesterol present in the skin is converted to pre-Vitamin D3 and provides the main source of natural Vitamin D to the body [56] Some research has reported a correlation between low solar UVB and high ASD prevalence [56], [57], [58] with this higher ASD prevalence attributed to lower levels of sunlight based Vitamin D production. These studies were, however, limited in scope. Firstly, they focused only on the UVB based Vitamin D part of the electromagnetic spectrum; secondly, there was a noticeable omission of ASD prevalence rates from countries near the equator, which receive the highest levels of solar irradiance.
The aim of current study was to build on these investigations by examining the incidence of ASD across the globe by latitude and its relationship with solar irradiance which decreases as distance from the equator increases. This study, for the first time, proposes a link between ASD and solar irradiance across all electromagnetic wavelengths. This hypothesis will be investigated by comparing the results of published epidemiological studies from different countries and plotting their ASD prevalence rates by the latitude of the data sources.
Section snippets
Method
Published data were used for this study therefore approval from the University’s Ethics Committee was not required.
Inclusion Criteria. Reports on the prevalence of ASD in children and adolescents were sought via searches of the major databases (PubMed, Google Scholar, ScienceDirect, Medline, Psychinfo) and a non-content database (Web of Knowledge). Search terms of “Autism” and “incidence/prevalence/epidemiology” were used. Subsequent searches included the name of countries/regions where gaps in
Results and discussion
The locations of the 25 studies reviewed are identified on the map in Fig. 1. Although prevalence rates from some regions that is, Russia and southern Africa, are not represented, with respect to street level solar irradiance at latitudes, the entire globe is represented (Fig. 1). An overview of the studies is listed in Table 1 including sample age, ASD prevalence and the diagnostic criteria used. The prevalence rates plotted by their respective latitude and solar irradiance level [62] are
Study limitations and future work
A comprehensive review of the published literature was conducted to establish a global picture of ASD prevalence rates across different latitudes. However, prevalence studies give only a cross-sectional view of the number of the people suffering a disease at a specific point in time. Although attempts were made to use comparable criteria, there is always a challenge in comparing prevalence studies from different regions, with differences in case definitions, case identification (or case
Conclusion
In this review, the aim was to explore the link between solar irradiance and the prevalence of ASD. This study for the first time has shown, theoretically, a relationship between low solar irradiance and ASD, particularly noted by the low rates in countries near the equator and Northern Territory (Australia) and high rates in Europe and Tasmania (Australia). The results of this review provide some support for Cannell’s hypothesis [43], [44] that lack of vitamin D, which in its natural form
Conflict of interest
The authors have no conflicts of interest associated with this paper.
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