Research Issues Involving the Biology of Autism

1419

Abstract

This article provides an overview and discussion on many of the major topics currently discussed in the autism research community. These topics include: prevalence, male-to-female ratio, diagnosis, genetics, environmental factors, neurology, medical comorbidities, sensory processing, and behavior. The article concludes with a discussion of future research in the field of autism.

General Information

Keywords: autism spectrum disorder, prevalence, male female ratio, diagnosis, medical comorbidities, sensory processing, behavior

Journal rubric: Research & Diagnosis of ASD

Article type: scientific article

DOI: https://doi.org/10.17759/autdd.2019170102

For citation: Edelson S.M. Research Issues Involving the Biology of Autism. Autizm i narusheniya razvitiya = Autism and Developmental Disorders, 2019. Vol. 17, no. 1, pp. 4–14. DOI: 10.17759/autdd.2019170102.

Full text

Introduction

 

The term ”autism” was first used in 1943 to describe a small group of children whose behaviors were noticeably different from the behaviors of those who were mentally retarded or schizophrenic [29]. A turning point in the field came in 1964, when Dr. Bernard Rimland argued convincingly that autism was not caused by emotionally neglectful parents—the prevailing theory at that time—but rather was a result of one or more underlying biological impairments involving genetics and neurology [43]. In this article, I will discuss numerous research issues involving the biology of autism.

 

Prevalence

 

The prevalence rate of autism began to increase in the late 1980s [39]. Prior to this time, the reported rate was approximately 1 in 2,000 children (or 4.5 in 10,000). More recently, the Centers for Disease Control and Prevention, a U.S. health agency, reported a rate of 1 in 59 children [2]. The prevalence rate varies throughout the United States, with a high of 1 in 34 in the state of New Jersey and a low of 1 in 77 in the state of Arkansas.

A question often posed is: “Is there really an increase in autism, or is there a better awareness, resulting in more children diagnosed with autism?” Research is mixed regarding this important issue [24; 51]; however, one can argue that autism today is prevalent worldwide.

 

Males and females

 

Another important issue is the relatively high number of males with autism spectrum disorder (ASD). Historically, males outnumbered females at a rate of 3 to 1. Starting in the 1990s, the reported ratio increased to 4 or 5 males to 1 female [18]. However, a recent meta analysis indicates a 3 to 1 male to female ratio [37].

The male-to-female ratio is now under scrutiny after a survey conducted by The National Autistic Society in the United Kingdom. Their findings indicated that females with autism are under-diagnosed. Frequently, females with ASD are more sociable than males with the disorder, and their characteristic autistic symptoms are subtler [45]. In addition, some of the females’ behaviors may be interpreted using more neurotypical labels, such as “anorexia” rather than “picky eater” and “obsessive-compulsive behavior” rather than “insistence on sameness.” Eventually, a diagnostic criterion for autism will need to be established for females with ASD.

 

Diagnosis

 

Researchers are focusing much of their effort on identifying biomarkers to objectively diagnose autism [1; 42], but to date, there are no agreed-upon valid biomarkers. Currently, autism is diagnosed based on observable behaviors, and several popular diagnostic tools include the Autism Diagnostic Observation Schedule (ADOS), the Autism Diagnostic Interview (ADI-R), and the Childhood Autism Rating Scale (CARS). However, recent research has questioned the validity of the ADI-R in diagnosing children and adults [21; 23].

 

Genetics

 

Numerous studies have investigated the genetics of autism. For many years, researchers have examined specific genes with the goal of uncovering the autism gene or set of genes. Currently, 18% to 20% of those individuals diagnosed with autism have syndromic autism, i.e., known inheritable disorders. These include Angelman syndrome, Cornelia de Lange syndrome, Fragile X syndrome, Prader-Willi syndrome, Rett syndrome, Smith-Lemli-Opitz syndrome, and tuberous sclerosis complex [14]. In a highly-publicized study, approximately 100 genes, all of which were thought to contribute to autism, were examined in 85 families with two or more children with ASD [56]. Interestingly, shared genetic mutations were found in only 31% of the siblings, whereas 69% carried different autism-associated mutations. This relatively low overlap in autism-associated genes indicates that the underlying genetics of autism are much more complex than initially believed.

More recent research has employed whole genome sequencing in order to identify specific genes associated with an increase in autism [47; 55]. Regarding implications for treatment, researchers using a nanoparticle technique were able to successfully disable a gene responsible for repetitive behaviors in a mouse model of Fragile X [36]. Such an approach to understanding and treating autism will likely lead to major breakthroughs in the field.

There has also been limited research on the therapeutic effects of stem cells. In an open clinical trial, Dawson and colleagues [12] evaluated the effects of giving infusions of umbilical cord blood to a group of 25 children with ASD between two and six years of age. They reported the therapy to be relatively safe. In addition, parent reports indicated improvements in social communication and common autistic symptoms.

 

Environmental Factors

 

Over the years, researchers have suggested a relationship between viral exposure during pregnancy and an increased risk of autism. This relationship has been documented for congenital rubella, herpes simplex virus, measles, mumps, varicella, and cytomegalovirus [33].

Some researchers have speculated that certain insults or toxins play a role in causing autism in a subset of individuals with the disorder [35; 52; 54]. For example, studies have documented proximity relationships between autism and major highways [49; 50] and open fields spread with pesticides [44].

 

Neurology

 

The neurology of autism has been of interest in the research community for over 50 years [4; 43].  In general, the results have been mixed regarding specific structural abnormalities and biochemical imbalances [8; 9; 11]. Such inconsistencies are thought to be due to the use of different measurement techniques and/or the heterogeneity of the autism population [8]. However, there are several reliable findings including relatively large brain size early in life [8; 48], fewer long-range neurons that connect different regions of the brain [10], and an abundance of short-range neurons in specific areas of the brain [10]. Furthermore, impairment in structure and function of column-shaped neural bundles located in the cerebral cortex, called minicolumns, have been well-established and may lead to excessive stimulation in specific regions of the brain [6; 26].

 

Medical Comorbidities

 

Widespread interest in medical comorbidities began in the mid-1990s, and over the past 20 years, studies have indicated that many if not most individuals on the autism spectrum suffer from one or more medical problems [41].

Maternal immune activation, or MIA, is a very interesting area of study, with research revealing associations between maternal anti-brain autoantibodies and autism [19]. The reason for MIA is not known at this time, but some researchers have suggested a genetic susceptibility to environmental factors [5]. Another immune-related issue involves allergies, such as airborne (pollen), skin (eczema), and food allergies [27; 31; 53]. In addition, Jyonouchi and colleagues have documented an inflammatory autism subtype and observed irritability, lethargy, and hyperactivity in the “flare” state [28].

Gastrointestinal (GI) problems are common among individuals with ASD and include constipation, diarrhea, gastroesophageal reflex (or GERD), bloating, and excessive gas [25]. Researchers studying the microbiome have reported a lack of diversity in bacteria strains [32]; an increase in harmful bacteria, such as Clostridium [17]; and a deficiency in certain digestive enzymes, such as lactase [34]. It is interesting to note that clinicians often suspect that GI problems are due to non-nutritious, self-restricted diets [46]. Insufficient water consumption has also been suggested as a reason for the high rate of constipation in individuals with ASD [3].

Other medical comorbidities often associated with autism include seizures [20], sleep problems [30] and severe headaches or migraines [7]. 

 

Sensory Processing

 

Many if not most individuals on the autism spectrum suffer from sensory processing problems. These may involve vision, audition, touch, taste, smell, vestibular input, and proprioception. Some individuals are very sensitive to sensory sensations, or hyper-responsive, whereas others are insensitive to sensations, or hypo-responsive [40]. Unfortunately, published research on sensory processing in autism has been sparse.

Recently, researchers have studied the role of interoception in autism [22; 38]. Interoception refers to the ability to attend to internal sensations, such as hunger, thirst, bladder fullness, discomfort, and pain. The anterior insula, which is responsible for interoception, has been shown to be dysfunctional in autism [13]. This finding is consistent with self-reports and controlled experiments on interoceptive processing in autism. This may account for the exaggerated reactions of many individuals with ASD to discomfort or pain due to medical problems, and the inability of others to detect such internal sensations [15]. In the case of the latter, we have often heard reports of individuals with autism showing no outward signs of pain from a ruptured appendix or a ruptured eardrum.

 

Behavioral

 

Research on behavior modification, often referred to as Applied Behavior Analysis, has received a great deal of empirical support since the early 1960s. Thousands of studies have supported its validity and effectiveness, and research over the past 20 years has focused primarily on optimizing this behavior-based intervention. More recently, researchers in other areas, especially those studying medical comorbidities, have been interested in understanding the relationship between specific medical problems and challenging behavior [16]. When medical issues underlie or contribute to behavior problems, the treatment of choice would involve a medical-behavioral intervention rather than treating only the behavior.

 

Future Research

 

The majority of studies in the field of autism have focused on one aspect of autism—for instance, genetics, metabolism, immunology, GI function, or sensory processing—and researchers typically do not examine how these systems interact with one another. Funding agencies are urged to support multidisciplinary studies in order to obtain a clear picture of autism rather than relying on an incomplete mosaic.

Research is also needed to categorize autism into specific subgroups. Once this is accomplished, researchers can focus on examining homogeneous population samples. This will undoubtedly expedite the discovery of the underlying cause(s) of each subtype as well as help determine the most effective interventions for each group.

Although progress has been made over the past 75 years, there is still much to be learned about the underlying biology of autism. The more we discover about the roots of this disorder, the better we will be able to address the needs of individuals with ASD and the families who care for them.

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Information About the Authors

Stephen M. Edelson, Doctor of Psychology, Executive Director, Autism Research Institute, San Diego, USA, ORCID: https://orcid.org/0000-0002-9856-5936, e-mail: director@autism.com

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