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Epidemiology
Reviews tend to estimate a prevalence of 6 per 1,000 for autism spectrum disorders as a whole,

Most recent reviews tend to estimate a prevalence of 1–2 per 1,000 for autism and close to 6 per 1,000 for ASD; because of inadequate data, these numbers may underestimate ASD's true prevalence. PDD-NOS's prevalence has been estimated at 3.7 per 1,000, Asperger syndrome at roughly 0.6 p        er 1,000, and childhood disintegrative disorder at 0.02 per 1,000. The number of reported cases of autism increased dramatically in the 1990s and early 2000s. This increase is largely attributable to changes in diagnostic practices, referral patterns, availability of services, age at diagnosis, and public awareness, though as-yet-unidentified environmental risk factors cannot be ruled out, and the available evidence does not rule out the possibility that autism's true prevalence has increased.

Reviews tend to estimate a prevalence of 6 per 1,000 for autism spectrum disorders as a whole, [13] 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.[15] These rates are consistent across cultures and ethnic groups, as autism is considered a universal disorder (Klinger, Dawson, & Renner, 2003). While rates of autism spectrum disorders are consistent across cultures, they vary greatly by gender, with boys being affected fa r more frequently than girls. The average male-to-female ratio for ASD’s is 4.2:1 (Fombonne, 2009), affecting 1 in 70 males, but only 1 in 315 females (ADDM, 2009). 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 (Volkmar, Lord, Bailey, Scultz, & Klin, 2004).

Comorbidity with Autism Spectrum Disorders
Autism spectrum disorders tend to be highly comorbid with other disorders. Comorbidity may increase with age and may worsen the course of youth with ASD's and make intervention/treatment more difficult. Distinguishing between ASD’s and other diagnoses can be challenging because the traits of ASD’s often overlap with symptoms of other disorders and the characteristics of ASD's make traditional diagnostic procedures difficult. In spite of these difficulties, comorbid disorders are readily identified and tend to fall into six categories, medical conditions, intellectual disabilities, anxiety disorders, mood disorders, behavior-related disorders, and sensory processing disorders.

Medical Conditions
A variety of medical conditions commonly occur in individuals with ASD’s. The most common is seizure disorder or epilepsy, which occurs in 11-39% of individuals with ASD. Typically, onset of epilepsy occurs before age five or during puberty, and is more common in females and individuals who also have comorbid mental retardation. Tic disorder is another common medical condition seen in individuals with ASD’s. While only about 6.5% of individuals with an ASD have full blown Tourette syndrome, nearly 30% show some form of tics. Tuberous sclerosis, a medical condition in which non-malignant tumors grow in the brain and on other vital organs, occurs in 1-4% of individuals with ASD’s. Sleep disorders are also commonly reported by parents of children with ASD’s, including late sleep onset, early morning awakening, and poor sleep maintenance.

Intellectual Disabilities
Intellectual disabilities are some of the most common comorbid disorders with ASD’s. Recent estimates suggest that 40-69% of individuals with ASD have some degree of mental retardation, with females more likely to be in severe range of mental retardation. Learning disabilities are also highly comorbid in individuals with an ASD. Approximately 25-75% of individuals with an ASD also have some degree of learning disability, although the types of learning disability vary depending on the specific strengths and weaknesses of the individual.

Anxiety Disorders
A variety of anxiety disorders tend to co-occur with autism spectrum disorders, with overall comorbidity rates of 7-84%. Specific phobia is the most common comorbid condition over the lifetime for those with ASD, with comorbidity rates of 38-63%. Common phobias for children with ASD include the fear of certain places or situations, the fear of medically related people, places, or things, and the fear of loud noises. Obsessive-compulsive disorder (OCD) occurs in 11-35% of individuals with ASD, with nearly 16-81% showing features of the disorder without a full diagnosis. While individuals with ASD exhibit rigid thinking and compulsions, similar to OCD, the repetitive behaviors displayed in ASD (i.e., flapping, spinning, repeating phrases) are distinct and serve alternate functions than the repetitive behaviors displayed in OCD (i.e., checking, cleaning, counting). Social Phobia or Social Anxiety Disorder is seen in approximately 7.4% of individuals with ASD, but is more common in higher-functioning individuals who have a desire for social interactions, but are also aware of their social deficits.

Mood Disorders
Rates of comorbid depression in individuals with an ASD range from 4-58%. The presentation of depression in ASD’s can depend on level of cognitive functioning, with lower functioning children displaying more behavior issues and higher functioning children displaying more traditional depressive symptoms. Depression is thought to develop and occur more in high-functioning individuals during adolescence, when they develop greater insight into their differences from others. Bipolar Disorder may also be comorbid with an ASD, although it is far less common than many other disorders. Rates of comorbidity vary greatly, but tend to be around 2-8%.

Behavior-related Disorders
Deficits in ASD are often linked to behavior problems, such as difficulties following directions, being cooperative, and doing things on other people’s terms. These behavior problems are also characteristic of Oppositional Defiant Disorder (ODD) and Conduct Disorder (CD), but the reasons for the behaviors often differ. For this reason, comorbidity rates of ODD and CD range from 7% to 73%. Attention Deficit Hyperactivity Disorder (ADHD) has ASD comorbidity rates of 30-80%; however, current diagnostic guidelines prevent diagnosing ADHD along with ASD. Rather, ADHD-like symptoms are seen to be part of the ASD diagnosis.

Sensory Processing Disorders
Sensory processing disorder is also comorbid with ASD, with comorbidity rates of 42-88%. Three patterns of sensory processing difficulties are commonly seen, hyperresponsiveness (behavioral over-reactivity to sensory stimuli), hyporesponsiveness (behavioral under-reactivity to sensory stimuli), and sensory-seeking (craving or fascination with certain stimuli). These patterns of processing difficulties may be present with auditory, visual, or tactile stimuli.

Developmental Course
Although autism spectrum disorders are thought to follow two possible developmental courses, most parents report that symptom onset occurred within the first year of life. One course of development follows a gradual course of onset in which parents tend to report concerns in development over the first two years of life and diagnosis is made around 3-4 years of age. Some of the early signs of ASD’s in this course include decreased looking at faces, failure to turn when name is called, failure to show interests by showing or pointing, and delayed pretend play (see Table 1). A second course of development is characterized by normal or near-normal development followed by loss of skills or regression in the first 2-3 years. Regression may occur in a variety of domains, including communication, social, cognitive, and self-help skills; however, the most common regression is loss of language. There continues to be a debate over the differential outcomes based on these two developmental courses. Some studies suggest that regression is associated with poorer outcomes and others report no differences between those with early gradual onset and those who experience a regression period. Overall, the prognosis for individuals with autism is poor with respect to academic achievement and independent living abilities, particularly for those who have not received early intervention. However, many individuals show improvements as they grow older. The two best predictors of favorable outcome in autism are non-retarded intellectual ability and the development of some communicative speech prior to 5 years of age. Overall, the literature stresses the importance of early intervention in achieving positive longitudinal outcomes.

Table 1: Early Symptoms of Autism

Etiology
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.

Neuroanatomical Findings
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.

Frontal lobe
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.

Limbic system
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.

Temporal lobe
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.

Corpus Callosum
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.

Controversial Theories
There has been a great deal of controversy over the years surrounding various theories of the etiology of autism spectrum disorders. In the 1950’s 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-1960’s. 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 1970’s 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.