Regression in Autism (1):

Complex developmental paths

Regression in autism refers to the loss of previously acquired developmental skills—such as spoken language, social interaction, play behaviour, motor coordination, or toileting skills. For example, a toddler who once used several words regularly might suddenly stop speaking, or a child who previously engaged in play with others may withdraw socially. What makes regression distinct from general developmental delay is that the child initially reaches certain milestones and then appears to reverse – returning to an earlier, less advanced stage of development.

Historical Context

Although now a well-recognised concept in autism research, regression was largely overlooked in early discussions of the condition. The phenomenon came to wider attention in the 1990s, with the inclusion of Childhood Disintegrative Disorder (CDD) – also known as Heller’s Syndrome – within the Pervasive Developmental Disorders (PDDs) category, alongside autistic disorder, Asperger’s disorder, Rett’s disorder, and PDD-NOS (Not Otherwise Specified), in earlier diagnostic manuals such as DSM-IV.

CDD was defined by significant regression in language, motor, toileting, and social skills, often accompanied by restricted and repetitive behaviours after at least two years of typical development. Despite its overlap with autism, CDD was considered a rare condition.

With the release of DSM-5 in 2013, CDD and other PDD subtypes (except Rett syndrome, which was removed from this category[1]) were subsumed under the single umbrella of Autism Spectrum Disorder (ASD). This shift not only recognised the spectrum nature of these conditions but also acknowledged regression as one possible pattern of onset.

Regression in ASDs

There is an increasing consensus that for some autistic children, development is not uniformly atypical from birth. Instead, they experience a period of typical development followed by a loss of skills, which may be temporary, partial, or permanent. This idea of "acquired autism"[2] or ”regressive autism” – where autistic features emerge after regression—has attracted a lot of attention, especially in light of findings from animal models showing that neurotypical animals can be induced to display autism-like behaviours under certain conditions (Kim et al. 2023).

Research highlights the long-standing division of autism onset into two patterns: early onset (symptoms are noticeable from infancy) and regressive (Boterberg et al. 2019). Approximately one-third of autistic children fall into the latter category, experiencing a loss of previously acquired skills—typically around 18-24 months (Tan et al. 2021), though it can begin as early as 12 months or as late as 3 years. In many cases, the most apparent losses involve language and social engagement. A child might stop speaking, avoid eye contact, or show less interest in playing with others. Less commonly, regression may affect motor skills.

Recent studies have begun to refine our understanding of this phenomenon. For example, Manelis et al. (2019) found that autistic children who lose words begin to crawl, talk, and walk at the same age as typically developing children—and significantly earlier than autistic children who do not regress. Other physical characteristics (head circumference, height, weight) are similar except that autistic children without regression are significantly more likely to have low muscle tone.

Not All Regression Is the Same

In autism, regression varies widely between individuals in both the type and severity of skills lost. Some lose skills suddenly, while others show a more gradual decline. Some children regress after reaching typical early milestones. Others may have delays from the start but still lose additional skills around age 2.

In some cases, development appears to stall entirely—a phenomenon sometimes referred to as a developmental plateau, where new skills fail to emerge, but previously acquired ones remain (Shumway et al. 2011). However, children with early language regression and average non-verbal intelligence tend to follow a characteristic pattern: early first words, followed by a regression, a plateau, and then a catch-up phase in language development (Tan et al. 2021).

The trajectory after regression varies greatly and depends on factors such as the severity of symptoms, the domains involved, and the presence of co-occurring conditions.

Clinical Features of Regressive Autism

Children with regression tend to have distinct neurological profiles and clinical outcomes compared to non-regressive autistic peers (Martin-Borreguero et al., 2021).

Children who experience regression often show:

• More severe autism symptoms, especially in social communication and repetitive behaviours than autistic children without regression (Zachor & Ben-Itzchak 2016; Thompson et al. 2019).

• Neurodevelopmental regression associated with specific neurological phenotypes (Martin-Borreguero et al. 2021)

• Lower cognitive and adaptive skills

• A higher likelihood of intellectual disability, epilepsy (Gadow et al., 2017) and gut microbiota differences

• Poorer emotional and language functioning, especially if regression occurs across multiple domains (Reyes et al., 2024)

However, not all regression leads to poor outcomes. 76% of children who regressed regained lost skills (Reyes et al., 2024), and children without intellectual disability often achieve typical or near-typical language skills over time.

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Evidence suggests regressive autism may represent a distinct subtype / subgroup within the ASDs whose developmental trajectory differs from that of autistic children without regression (Barbeau 2017; Whiteley et al. 2021), and possibly distinct genetic or biological underpinnings (Kaba & Arıcı Canlı 2024).

Prevalence of regression in ASD

It is broadly accepted that between 20% to 40% of autistic children experience some form of regression, though figures span more widely in the literature, e.g., the average rate of regression was about 30%, but it ranged from 10% to over 50% depending on the study.[3]

Estimates vary significantly depending on definitions, methodologies, and whether data are collected retrospectively (relying on parents’ memories) or prospectively (tracking children over time). In fact, some prospective studies have reported rates much higher than earlier estimates – in some studies well over 80%. Ozonoff & Iosif (2019) proposed that regression in autism may be more widespread than believed, perhaps it is even “the rule, not the exception”, suggesting that most children with ASD experience some form of developmental decline – though not always in a way that’s easily observed.

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Regression in autism reflects a critical developmental phenomenon. It affects a significant proportion of children and often signals a more complex clinical profile. Its presence has led to proposals for distinct autism subtypes, with ongoing research aimed at improving early detection and understanding underlying mechanisms. It also contributes to a broader understanding of the heterogeneity of autism, challenging the idea that all autistic children follow a similar developmental path.

Whether regression defines a distinct subtype or simply reflects a variation in autistic development, one thing is clear: understanding it better can improve how we diagnose, support, and advocate especially for the most vulnerable autistic individuals whose early development seems to change course unexpectedly.

Regression can be distressing for families, particularly when it appears suddenly or affects key skills like language. However, with proper support and intervention, many children can recover some or even most of the lost abilities over time.  

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[1] Some researchers argued that Rett syndrome belonged in a category with autism (Rutter 1994), but others disagreed (Gillberg 1994).

[2] The concept of "acquired autism" refers to the hypothesis that amongst the massive heterogeneity that encompasses ASD there may be several phenotypes that are neither syndromic nor innate (Whiteley et al. 2021).

[3] The highest reported rates (over 50%) were reported by Ekinci et al. (2012) and Kern et al. (2014).