The term "autism spectrum" is often used to describe disorders that are currently classified as pervasive developmental disorders. Pervasive developmental disorders include autism, Asperger syndrome, Childhood disintegrative disorder, Rett syndrome and Pervasive Developmental Disorder Not Otherwise Specified. These disorders are typically characterized by social deficits, communication difficulties, stereotyped or repetitive behaviors and interests, and/or cognitive delays. Although these diagnoses share some common features, individuals with these disorders are thought to be "on the spectrum" because of differences in severity across these domains
Pervasive developmental disorders ("autism spectrum" disorders) include  these five:
2. Asperger syndrome
3. Childhood disintegrative disorder
4. Rett syndrome
5. Pervasive Developmental Disorder Not Otherwise Specified
Autism is characterized by delays or abnormal functioning before the age of three years in one or more of the following domains: (1) social interaction; (2) communication; and (3) restricted, repetitive, and stereotyped patterns of behavior, interests, and activities. Social impairments are marked by poor use of nonverbal communication, difficulty in peer relations, lack of social-emotional reciprocity, and lack of shared enjoyment. Communication deficits may include failure to develop speech, use of stereotyped or delayed echolalia, and difficulties maintaining conversations. Social and communication impairments may also cause a lack of symbolic or imaginative play. Restricted and repetitive behaviors may include unusual preoccupations with narrow interests, inflexibility to nonfunctional routines, stereotyped and repetitive mannerisms, and preoccupations with parts of objections.
Asperger syndrome can be distinguished from autism by the lack of delay or deviance in early language development. Additionally, individuals with Asperger syndrome do not have significant cognitive delays. An individual with Asperger syndrome typically demonstrates obsessive interest in a single topic or activity. Other symptoms include repetitive routines or rituals, peculiarities in speech and language, inappropriate affect or social behavior, problems with non-verbal communication, and clumsy or uncoordinated motor movements. Because of these difficulties, individuals with Asperger's Disorder often have trouble interacting with others.
Unlike autism and Asperger syndrome, childhood disintegrative disorder is characterized by significant regression or loss of functioning after at least two years of typical development. A child who is affected with this condition may lose communication skills, nonverbal behaviors, motor functioning, and/or skills that have already been learned (such as toy play).
Rett syndrome appears only in females and is characterized by multiple deficits after a period of normal functioning after birth. At onset, Rett's Disorder is characterized by deceleration of head growth, loss of purposeful hand skills, loss of social engagement and language, and poor physical coordination.
Pervasive Developmental Disorder Not Otherwise Specified (PDD-NOS) is considered "subthreshold autism" and "atypical autism" because it is often characterized by milder symptoms of autism or symptoms in only one domain (such as social difficulties). Persons with PDD-NOS may demonstrate pervasive deficits in the development of reciprocal social interaction or stereotyped behaviors, but do not meet the criteria for a specific pervasive developmental disorder or other psychological disorders (such as schizophrenia or avoidant personality disorder).
 Autism spectrum disorder in the DSM-V
Further information: Autism classification
Autism Spectrum Disorder (ASD) is a proposed revision to the Diagnostic and Statistical Manual of Mental Disorders 5 (DSM-5), which will be released in May 2013. This new diagnosis will encompass current diagnoses of Autistic Disorder, Asperger's Disorder, Childhood Disintegrative Disorder, and Pervasive Developmental Disorder Not Otherwise Specified. Rather than categorizing these diagnoses, the DSM-5 will adopt a dimensional approach to diagnosing disorders that fall underneath the autism spectrum umbrella. It is thought that individuals with ASDs are best represented as a single diagnostic category because they demonstrate similar types of symptoms and are better differentiated by clinical specifiers (i.e., dimensions of severity) and associated features (i.e., known genetic disorders, epilepsy and intellectual disability). An additional change to the DSM includes collapsing social and communication deficits into one domain. Thus, an individual with an ASD diagnosis will be described in terms of severity of social communication symptoms, severity of fixated or restricted behaviors or interests and associated features.
Reviews tend to estimate a prevalence of 6 per 1,000 for autism spectrum disorders as a whole, however prevalence rates vary for each of the developmental disorders in the spectrum. Autism prevalence has been estimated at 1-2 per 1,000, Asperger syndrome at roughly 0.6 per 1,000, childhood disintegrative disorder at 0.02 per 1,000, and PDD-NOS at 3.7 per 1,000. These rates are consistent across cultures and ethnic groups, as autism is considered a universal disorder.
While rates of autism spectrum disorders are consistent across cultures, they vary greatly by gender, with boys being affected far more frequently than girls. The average male-to-female ratio for ASD's is 4.2:1, affecting 1 in 70 males, but only 1 in 315 females. Females, however, are more likely to have associated cognitive impairment. Among those with an ASD and mental retardation, the sex ratio may be closer to 2:1.
While a specific cause or specific causes of autism spectrum disorders has yet to be found, many risk factors have been identified in the research literature that may contribute to the development of an ASD. These risk factors include genetics, prenatal and perinatal factors, neuroanatomical abnormalities, and environmental factors.
Genetic Risk Factors
The results of family and twin studies suggest that genetic factors play a role in the etiology of autism and other pervasive developmental disorders. Studies have consistently found that the prevalence of autism in siblings of autistic children is approximately 15 to 30 times greater than the rate in the general population. In addition, research suggests that there is a much higher concordance rate among monozygotic twins compared to dizygotic twins. These studies suggest a strong genetic component in autism. It is estimated that autism involves 5-10 genes and possibly more. It appears that there is no single gene that can account for autism. Instead, there seems to be multiple genes involved, each of which is a risk factor for part of the autism syndrome through various groups. Possible susceptibility regions include chromosomes 1p, 2q, 7q, 13q, 16p, and 19q.
Prenatal and Perinatal Risk Factors
A number of prenatal and perinatal complications have been reported as possible risk factors for autism. These risk factors include maternal gestational diabetes, maternal and paternal age over 30, bleeding after first trimester, use of prescription medication during pregnancy, and meconium in the amniotic fluid. While research is not conclusive on the relation of these factors to autism, each of these factors has been identified more frequently in children with autism compared to their non-autistic siblings and other normally developing youth.
In general, neuroanatomical studies support the notion that autism is linked to a combination of brain enlargement in some areas and brain reduction in other areas. These studies suggest that autism may be caused by abnormal neuronal growth and pruning during the early stages of prenatal and postnatal brain development, leaving some areas of the brain with too many neurons and other areas with too few neurons. Some research has reported an overall brain enlargement in autism while others suggest abnormalities in several areas of the brain, including the frontal lobe, the mirror neuron system, the limbic system, the temporal lobe, and the corpus callosum.
In neuroanatomical studies, it has been shown that for individuals with Autism spectrum disorder there is reduced activation in the primary and secondary somato-sensory cortices during Theory of Mind and facial emotion response tasks when compared to control. This is consistent with reports of patterns of abnormal cortical thickness and grey matter volume in those regions in individuals with Autism Spectrum Disorder. (Sugranyes) In normal children, there is a bias of the left lateralized network that is essential for language development, as shown by magnetoencephalography. Specifically, there was a left dominance of parieto-temporal coherence in the theta band that was correlted with higher performance on language related tasks. This was not correlated with head circumference or chronological age.
The frontal lobe is central to many functions that are associated with autism, such as language and executive functions. For instance, Broca's area, which is related to language production, is located in the inferior prefrontal lobe. Other important areas of the frontal lobe include: the prefrontal cortex (involved with aspects of executive function such as working memory, inhibition, planning, organizing, set-shifting and cognitive flexibility), the orbitofrontal cortex (involved in social cognition and theory of mind) and the inferior frontal gyrus (part of the mirror neuron system). Current research suggests that dysfunction in the frontal lobe may be associated with some of the deficits observed in individuals with ASD, including social cognition, imitation, face processing, language, attention, working memory, and problem-solving. For example, it has been found that individuals with autism have decreased concentrations of N-acetyl-asparate (NAA) and reduced glutaminergic neurons in the frontal lobe, suggesting some dysfunction in this region. Another study using fMRI found that boys with high-functioning autism had reduced activity in the pars opercularis when observing and imitating emotions. Orbitofrontal cortex deficits have also been implicated with autism, as individuals with high-functioning autism have shown decreased functioning in this area when participating in a task that involved the perception of fearful faces., Finally, individuals with ASD have shown decreased activation in the medial prefrontal cortex relative to a control group during a theory of mind task.
Mirror neuron system
The mirror neuron system (MNS) consists of a network of brain areas that have been associated with empathy processes in both animals and humans. In humans, the MNS has been identified in the inferior frontal gyrus (IFG) and the inferior parietal lobule (IPL) and is thought to be activated during imitation or observation of behaviors. It has been suggested that the MNS generates internal representations of the self and others, which facilitates an understanding of other people. Many researchers have hypothesized that the MNS is related to cognitive processes such as imitative learning, "mind-reading", and empathy; all of which are necessary for social-communication. Several studies using functional brain-imaging have found evidence of mirror neuron dysfunction in autism, suggesting this neural system is associated with social impairments in individuals with ASDs. Specifically, it has been found that reduced mirror neuron activity and MNS cortical thinning are highly correlated with autism severity.
Social skills impairments in autism have been theorized to reflect abnormal functioning in the limbic system. In animal models, it has been found that monkeys with lesions in the medial temporal lobe (e.g., the amygdala and hippocampus) demonstrate autistic-like behaviors, such as a failure to develop normal social relationships, stereotyped movements, and poor eye-contact. Notably, it was found that that the most severe autistic symptoms resulted from lesions in the amygdala and hippocampus whereas less severe forms resulted from lesions to the amygdala alone. Human autopsy studies have also found evidence for limbic system abnormalities in individuals with ASDs. These studies revealed reduced neuronal cell size and increased cell-packing density in the hippocampus and amygdala. However, MRI studies have not found any evidence for abnormalities in the hippocampus.
Functions of the temporal lobe are related to many of the deficits observed in individuals with ASDs, such as receptive language, social cognition, joint attention, action observation and empathy. The temporal lobe also contains the superior temporal sulcus (STS) which may mediate facial processing. It has been argued that dysfunction in the STS underlie the social deficits that characterize autism. Compared to typically developing individuals, one fMRI study found that individuals with high functioning autism had reduced activity in the STS when viewing pictures of faces. Other studies have suggested that the role of the STS may be more complex than simple face processing, as research has found that individuals with ASDs have shown reduced functioning when viewing fear-provoking faces; implying that the STS is involved in understanding the emotions of others. Other areas of the temporal lobe have also been implicated in ASDs. For instance, fMRI research suggests that individuals with ASDs have reduced activity in the right temporoparietal junction and other regions during imitation and observation tasks.
Studies have found evidence of reduced size of the corpus callosum in individuals with autism. These findings suggest that there may be a link between autism and impaired communication between brain hemispheres.
 Environmental Risk Factors
A wide variety of environmental risk factors have been proposed as contributing to autism. These include gastrointestinal or immune system abnormalities, allergies, and exposure of children to drugs, vaccines, infection, certain foods, or heavy metals. The evidence for these risk factors is anecdotal and has not been confirmed by reliable studies. The subject remains controversial and extensive further searches for environmental factors are underway.
There has been a great deal of controversy over the years surrounding various theories of the etiology of autism spectrum disorders. In the 1950s the “refrigerator mother theory” emerged as an explanation for autism. This theory was based on the idea that autistic behaviors stem from the emotional frigidity, lack of warmth, and cold, distant, rejecting demeanor of a child’s mother. Naturally, parents of children with an autism spectrum disorder suffered from blame, guilt, and self-doubt, especially as the theory was embraced by the medical establishment and went largely unchallenged into the mid-1960s. While the “refrigerator mother theory” has been rejected in the research literature, its effects have lingered into the 21st century. Another controversial theory suggests that watching extensive amounts of television may cause autism. This theory is largely based on research suggesting that the increasing rates of autism in the 1970s and 1980’s were due to the growth of cable television at this time. This theory has not been supported in the research literature. Probably the biggest and most widely circulated controversial theory of autism etiology is the “vaccine theory”. This theory suggests that autism results from brain damage caused either by (1) the measles, mumps, rubella (MMR) vaccine itself, or by (2) thimerosal, an MMR vaccine stabilizer that is 50% ethylmercury. The current scientific consensus is that no convincing scientific evidence supports these claims, based on various lines of evidence including the observation that the rate of autism continues to climb despite elimination of thiomersal from routine childhood vaccines. Major scientific and medical bodies such as the Institute of Medicine and World Health Organization as well as governmental agencies such as the Food and Drug Administration and the CDC reject any role for thiomersal in autism or other neurodevelopmental disorders.
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