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Autism should not be viewed as a single condition with a unified underlying cause, according to scientists who found that those diagnosed early in childhood typically have a distinct genetic profile to those diagnosed later.
The international study, based on genetic data from more than 45,000 autistic people in Europe and the US, showed that those diagnosed in early childhood, typically before six years old, were more likely to show behavioural difficulties from early childhood, including problems with social interaction, but remain stable.
Those diagnosed with autism later, typically after the age of 10, were more likely to experience increasing social and behavioural difficulties during adolescence and also had an increased likelihood of mental health conditions such as depression.
“The term ‘autism’ likely describes multiple conditions,” said Dr Varun Warrier, from Cambridge’s department of psychiatry, senior author of the research. “For the first time, we have found that earlier and later diagnosed autism have different underlying biological and developmental profiles.”
The scientists are not advocating for a move towards two diagnostic categories, saying that this could be unhelpful for the many who fall somewhere in the middle.
“It is a gradient,” said Warrier. “There are also many other factors that contribute to age of diagnosis, so the moment you go from averages to anything that is applicable to an individual, it’s false equivalency.”
The findings come at a time when autism diagnosis has risen steeply, with a nearly 800% increase in diagnoses in the UK between 1998 and 2018. Experts say this is due largely to a widening of the diagnostic criteria and greater recognition of the condition.
And, while autism is defined by having challenges with social communication, sensory processing and restrictive behaviours, there is huge variability in how these difficulties present between individuals. Scientists have been looking at whether the population clusters into subgroups, with shared traits or trajectories, that could make studying autism more tractable.
The latest study used behavioural data from four birth cohorts, ranging from 89 to 188 people, and genetic data from two large studies, with more than 45,000 participants.
Previously, it was generally assumed that those diagnosed earlier tended to be those with more marked autistic traits, underpinned by people carrying a higher proportion of autism-linked gene variants. However, the latest study revealed a different pattern.
The analysis, published in Nature, found that the underlying genetic profiles differed between those diagnosed with autism earlier and later in life, with only a modest overlap. The average genetic profile of later-diagnosed autism is closer to that of ADHD, as well as to mental health conditions such as depression and PTSD, than it is to autism diagnosed in early childhood.
Those diagnosed before the age of six years were more likely to be slow to walk and have difficulty interpreting hand gestures and tended to experience social and communication difficulties that appeared early but remained stable. Those diagnosed after the age of 10 years were more likely to experience an increase in difficulties during adolescence and, by late adolescence, presented with more severe challenges.
Prof Uta Frith, emeritus professor of cognitive development at University College London, who was not involved in the research, said: “It makes me hopeful that even more subgroups will come to light, and each will find an appropriate diagnostic label.
“It is time to realise that ‘autism’ has become a ragbag of different conditions.”
