A 2016 review concludes that enteric nervous system abnormalities might play a role in neurological disorders such as autism. Neural connections and the immune system are a pathway that may allow diseases originated in the intestine spread to the brain.Several lines of evidence point to synaptic dysfunction as a cause of autism. Some rare mutations may lead to autism by disrupting some synaptic pathways, such as those involved with cell adhesion. All known teratogens (agents that cause birth defects) related to the risk of autism appear to act during the first eight weeks from conception, and though this does not exclude the possibility that autism can be initiated or affected later, there is strong evidence that autism arises very early in development.In general, neuroanatomical studies support the concept that autism may involve a combination of brain enlargement in some areas and reduction in others. 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.
ICD
The World Health Organizations International Classification of Diseases (11th Revision) ICD-11, regarded as the global standard, was released in June 2018 and came into full effect as of January 2022. It describes ASD as follows:
Autism spectrum disorder is characterised by persistent deficits in the ability to initiate and to sustain reciprocal social interaction and social communication, and by a range of restricted, repetitive, and inflexible patterns of behaviour, interests or activities that are clearly atypical or excessive for the individuals age and sociocultural context. The onset of the disorder occurs during the developmental period, typically in early childhood, but symptoms may not become fully manifest until later, when social demands exceed limited capacities. Deficits are sufficiently severe to cause impairment in personal, family, social, educational, occupational or other important areas of functioning and are usually a pervasive feature of the individuals functioning observable in all settings, although they may vary according to social, educational, or other context. Individuals along the spectrum exhibit a full range of intellectual functioning and language abilities. ICD-11 was produced by professionals from 55 countries out of the 90 countries involved and is the most widely used reference worldwide. Clinicians use the ICD as a reference for diagnosis and reporting but researchers, particularly in the US, continue to use the Diagnostic and Statistical Manual of Mental Disorders (DSM-5-TR from 2022, DSM-5 from 2013, or their predecessors) as some material is not included in the ICD (the ICD is broader in scope, covering general as well as mental health).