Autism Spectrum Disorder (ASD) is a neurodevelopmental disorder defined by persistent deficits in social functioning and the presence of restricted and repetitive behaviours (RRBs)(1). RRBs refer to four subtypes of behaviour including repetitive movements, speech or use of objects; insistence on sameness; restricted interests; and sensory processing abnormalities (1). Additionally, many individuals with ASD experience elevated levels of anxiety compared to the general population with approximately 40% of children with ASD meeting the criteria for an anxiety disorder (2) {van Steensel, 2011 #36}and others displaying subclinical symptoms (3). The presence of anxiety negatively impacts one’s quality of life by inhibiting daily functioning and increasing the risk of comorbid medical and psychiatric conditions (4). These deficits are further compounded in individuals with ASD as anxiety can be triggered by or exacerbate the social difficulties inherent in the disorder resulting in significant distress, problematic behaviours, and compounding relationship difficulties with family members, teachers, and peers (5, 6).
Despite these adverse effects, diagnosing and treating anxiety in individuals with ASD is challenging. This is due, in part, to the overlap that exists between anxiety symptoms and autistic traits with the core features of ASD - social communication deficits and RRBs - closely resembling the symptoms of social anxiety and Obsessive Compulsive Disorder (OCD) respectively (7). The diagnosis and treatment of anxiety is further complicated by its atypical presentation in individuals with ASD. Anticipatory worry related to changes in routine; sensory issues; unusual fears; and social aversion that is not linked to fears of rejection are often experienced by individuals with ASD alongside traditional anxiety symptoms (8). This atypical presentation, paired with overlapping symptomology, may result in anxiety being incorrectly diagnosed or remaining undetected and subsequently untreated (9). Either is potentially detrimental, as anxiety symptoms in youth significantly predict later anxiety disorders (10) and a diagnosis of anxiety directs intervention.
RRBs and Anxiety
There is a body of evidence highlighting the association between RRBs and anxiety symptoms in individuals with ASD (11). Specifically, cross-sectional studies have found moderate to large correlations between the two constructs (r = .41 – .69) with individuals experiencing higher anxiety engaging in more RRBs (12-14). Several longitudinal studies exploring the nature of this relationship have found that the severity of RRBs predicts later anxiety, suggesting that a unidirectional relationship exists between the two constructs (p < .001 – p = 0.026) (11, 15, 16). The strong and consistent associations found across studies suggests the RRBs and anxiety are interrelated; that this relationship is specific to children with ASD (14); and that the presence of RRB’s proceeds the onset of anxiety (11, 15, 16). Subsequently, RRBs have been conceptualised as an early manifestation of anxiety (11, 15) or a maladaptive response to negative affect which subsequently increases one’s risk of anxiety over time (10, 11, 17). Although engaging in RRBs may temporarily reduce anxiety symptoms, their continued use may impede one’s ability to adopt more adaptative coping mechanisms, reduce their ability to engage in their environment, and therefore ultimately perpetuate feelings of anxiety (10, 11, 17). This parallels the trajectory observed in OCD whereby individuals engage in compulsions to neutralise anxiety associated with obsessions (4, 17). However, in reality, these compulsions add considerable distress to individuals with OCD as they do not address the source of anxiety and interfere with daily functioning (4, 17). As the hypothesised relationship between anxiety and RRBs in ASD is thought to mirror the relationship between obsessions and compulsions in the OCD literature, it is plausible to suggest that RRBs could be an observable indicator of future anxiety. This could help clinicians identify and treat anxiety in young children, preventing the manifestation of significant distress in later years.
RRB Subtypes and Anxiety
Although the literature consistently reports that RRBs are associated with anxiety, it is important to clarify if this relationship exists for all or specific RRB subtypes. There is a consistent body of evidence to suggest that insistence on sameness and restricted interests are linked to anxiety (3). However, the relationship between the remaining subtypes and anxiety remains unclear, with conflicting evidence emerging from the literature. This lack of clarity may be associated with the varying definitions of RRBs. Whilst some studies have explored subtypes individually (3, 18), others have combined the subtypes into two broad domains known as low-order and high-order RRBs (10, 19). Low-order RRBs refer to repetitive movements that may be motivated by sensory feedback, whereas high-order RRBs include restricted interests and insistence on sameness behaviours (20). These domains have been criticised for being too broad and therefore disregarding important differences in the functions served by RRB subtypes (3). The grouping of low-order RRBs is problematic as they are thought to have contradictory functions. For instance, sensory processing abnormalities may increase one’s risk of experiencing anxiety as having a low threshold for sensory input is thought to exacerbate environmental stressors (16). This is consistent with the finding that hyperactivity to sensory input in toddlers predicts later anxiety (16).
Contrastingly, repetitive movements are thought to enable one to manage anxiety that stems from sensory processing abnormalities (21). A discrepancy is therefore highlighted between the two low-order RRBs, with one thought to contribute to anxiety, whilst the other a response.
Subtypes of high-order RRBs are also understood to uniquely moderate anxiety symptoms. For instance, restricted interests are thought to be a maladaptive coping response to anxiety (17), whilst insistence on sameness behaviours may enable one to control their environment and thus buffer the effect of anxiety (21). These distinctions are consistent with results of factor analyses which have found restricted interests and insistence on sameness behaviours to be distinct subtypes (22-24). Thus, it is critical to examine these subtypes individually in order to clarify their differential relationship with anxiety. This knowledge could help clinicians tailor diagnostic tools and treatment interventions as specific RRBs may be utilised as a marker for anxiety, whilst others could be targeted for interventions.
The Relationship between RRB Subtypes
To effectively conceptualise the associations between RRBs and anxiety, it is important to also understand the inter-relationships amongst RRB subtypes. There is emerging evidence to suggest that RRBs subtypes are inter-related (21, 25). Specifically, sensory processing abnormalities have been shown to negatively correlate with repetitive movements and high-order RRBs, whilst a positive association has been observed between repetitive movements and high-order RRBs (21, 25). However, not all studies yield consistent results with Lidstone (12) finding that low registration (a measure of hyporeactivity) and sensory sensitivity (a measure of hyperreactivity) were not associated with repetitive movements.
Factors influencing the RRB-Anxiety relationship
The relationship between specific RRB subtypes and anxiety may be influenced by the presence of additional RRBs. Wigham (21) reported that anxiety, in combination with intolerance of uncertainty, mediated the relationship between sensory processing and high-order RRBs. Further study has reported a path from hyporeactivity, but not hyper-reactivity, to high-order RRBs through anxiety alone (25). These findings support the hypothesis that restricted interests and insistence on sameness behaviours enable individuals to reduce or avoid anxiety associated with sensory input (12, 21). However, in contrast, anxiety did not mediate the relationship between sensory processing abnormalities and repetitive movements (21, 25). Given that different studies provide conflicting evidence about the relationship between RRB subtypes, it is critical to explore this in the context of the anxiety-RRB relationship. Collating the associations between RRB subtypes may help clinicians understand if these behaviours are interrelated and whether clusters or specific RRBs are associated with anxiety.
Other individual traits or characteristics may also influence the relationship between RRBs and anxiety. For instance, intolerance of uncertainty, a trait associated with high levels of worry and avoidance when faced with uncertainty, has been linked to both anxiety and RRBs (20, 21, 26). Hwang (26) found that intolerance of uncertainty mediated the relationship between anxiety and all RRB subtypes except for repetitive movements. Age and intelligence quotient (IQ) have also been identified as factors relating to both anxiety and RRB subtypes (3, 27), however the role they play within the RRB-anxiety relationship remains unclear. Findings to date are mixed with some studies finding that anxiety increases with age but not with RRBs (16, 28), whilst others have found the reverse (26). As all individuals with ASD present with at least two RRBs as per the Diagnostic and Statistical Manual of Mental Disorders (DSM: 1), identifying and collating the factors associated with the RRB-anxiety relationships could help clinicians identify who are most vulnerable to developing anxiety symptoms.
Current Study
Our systematic review aims to: 1) meta-analytically assess the association between RRB subtypes and anxiety symptoms in individuals with ASD; 2) meta-analytically assess the association between the RRB subtypes in the context of the anxiety-RRB relationship; and 3) collate factors associated with both RRBs and anxiety symptoms. These aims will be addressed via the following research questions:
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What is the association between RRB subtypes and anxiety symptoms in people with ASD?
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In studies exploring RRBs and anxiety in people with ASD, what is the association between the RRBs subtypes?
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For individuals with ASD, what are the factors associated with both RRBs and anxiety?
This review is registered on the PROSPERO database (CRD42020185434).