Motor stereotypies in toddlers with and without autism: A transdiagnostic dimension

Motor stereotypies (MS) represent one of the transdiagnostic symptom dimensions identified by the NIMH Research Domain Criteria work group as relevant to psychopathology. MS are common in neurodevelopmental conditions, but they remain poorly understood, particularly in early childhood. The present study examined MS in 648 toddlers with autism spectrum disorder (autism, n = 455) and other neurodevelopmental conditions (non-autism, n = 193) and their concurrent and prospective links with other phenotypic characteristics.

Toddlers were recruited between February 2000 and October 2018 and evaluated at 24 +/- 5 months (Time 1, N = 648) and 41 +/- 6 months (Time 2, N = 455). The presence of MS was determined based on the Autism Diagnostic Observation Schedule assessment. The phenotypic measures included adaptive socialization skills, severity of social symptoms of autism, and verbal, nonverbal, and motor skills. The analysis was conducted using the general linear models while controlling for age, sex, visit year, group, and other relevant covariates.

MS were present in both groups, but were more common and intense in autism, and their prevalence did not vary by sex in either group. Regardless of group, MS was associated concurrently with higher social-affective symptoms [p < .001, ${\eta }_p^2$  = .034], lower socialization [p = .001, ${\eta }_p^2$  = .017], expressive language [p = .008, ${\eta }_p^2$  = .011], visual reception [p = .002, ${\eta }_p^2$  = .015], fine motor [p < .001, ${\eta }_p^2$  = .020] and gross motor [p < .001, ${\eta }_p^2$  = .032] scores, and prospectively with higher social-affective symptoms [p = .011, ${\eta }_p^2$  = .014], lower socialization [p = .039, ${\eta }_p^2$  = .009], and lower verbal scores [p < .014, ${\eta }_p^2$  = .013].

Motor stereotypies are present in toddlers with and without autism and may represent a distinct transdiagnostic dimension expressed early in development, associated with core developmental skills and, putatively, characterized by shared pathophysiology across neurodevelopmental conditions.