Causes of autism
Many causes of autism, including environmental and genetic factors, have been recognized or proposed, but understanding of the theory of causation of autism is incomplete.[1] Attempts have been made to incorporate the known genetic and environmental causes into a comprehensive causative framework.[2] ASD (autism spectrum disorder) is a neurodevelopmental disorder marked by impairments in communicative ability and social interaction, as well as restricted and repetitive behaviors, interests, or activities not suitable for the individual's developmental stage. The severity of symptoms and functional impairment vary between individuals.[3]
There are many known environmental, genetic, and biological causes of autism. Research indicates that genetic factors are predominant in the appearance of autism; however, the heritability of autism is complex, and many of the genetic interactions involved are unknown.[1] In rare cases, autism has been associated with agents that cause birth defects.[4] Many other causes have been proposed.
Different underlying brain dysfunctions have been hypothesized to result in the common symptoms of autism, just as completely different brain types result in intellectual disability.[1][5] In recent years, the prevalence and number of people diagnosed with the disorder have increased dramatically. There are many potential reasons for this occurrence, particularly the changes in the diagnostic criteria for autism.[6]
Environmental factors that have been claimed to contribute to autism or exacerbate its symptoms, or that may be important to consider in future research, include certain foods,[7] infectious disease, heavy metals, solvents, diesel exhaust, PCBs, phthalates and phenols used in plastic products, pesticides, brominated flame retardants, alcohol, smoking, and illicit drugs.[6] Among these factors, vaccines have attracted much attention, as parents may first become aware of autistic symptoms in their child around the time of a routine vaccination, and parental concern about vaccines has led to a decreasing uptake of childhood immunizations and an increasing likelihood of measles outbreaks.[8][9] However, there is overwhelming scientific evidence showing that there is no causal association between the measles-mumps-rubella (MMR) vaccine and autism. Although there is no definitive evidence that the vaccine preservative thimerosal causes autism, studies have indicated a possible link between thimerosal and autism in individuals with a hereditary predisposition for autoimmune disorders.[10][11] In 2007, the Center for Disease Control stated there was no support for a link between thimerosal and autism, citing evidence from several studies, as well as a continued increase in autism cases following the removal of thimerosal from childhood vaccines.[12]
Perinatal environment[edit]
Autism is associated with some perinatal and obstetric conditions. Infants that are born pre-term often have various neurodevelopmental impairments related to motor skills, cognition, receptive and expressive language, and socio-emotional capabilities.[91] Pre-term infants are also at a higher risk of having various neurodevelopmental disorders such as cerebral palsy and autism, as well as psychiatric disorders related to attention, anxiety, and impaired social communication.[91] It has also been proposed that the functions of the hypothalamic-pituitary-adrenal axis and brain connectivity in pre-term infants may be affected by NICU-related stress resulting in deficits in emotional regulation and socio-emotional capabilities.[91] A 2019 analysis of perinatal and neonatal risk factors found that autism was associated with abnormal fetal positioning, umbilical cord complications, low 5-minute Apgar score, low birth weight and gestation duration, fetal distress, meconium aspiration syndrome, trauma or injury during birth, maternal hemorrhaging, multiple birth, feeding disorders, neonatal anemia, birth defects/malformation, incompatibility with maternal blood type, and jaundice/hyperbilirubinemia. These associations do not denote a causal relationship for any individual factor.[92] There is growing evidence that perinatal exposure to air pollution may be a risk factor for autism, although this evidence has methodological limitations, including a small number of studies and failure to control for potential confounding factors.[93][94] A few studies have found an association between autism and frequent use of acetaminophen (e.g. Tylenol, Paracetamol) by the mother during pregnancy.[95][96] This association does not necessarily demonstrate a causal relationship.