Background: Autism spectrum disorder (ASD) is a developmental disorder with a wide variety of clinical phenotypes and co-occurrences with other neurodevelopmental conditions. Symptoms may change over time.

Aims: The aim of the present study was to prospectively follow 96 children, initially assessed for suspected ASD at an average age of 2.9 years.

Methods and procedures: All children had been identified with autistic symptoms in a general population child health screening program, and had been referred to the Child Neuropsychiatry Clinic in Gothenburg, Sweden for further assessment by a multi-professional team at Time 1 (T1). This assessment included a broad neurodevelopmental examination, structured interviews, a cognitive test and evaluations of the childis adaptive and global functioning. Two years later, at Time 2 (T2), the children and their parents were invited for a follow-up assessment by the same team using the same methods.

Outcomes and results: Of the 96 children, 76 had met and 20 had not met full criteria for ASD at T1. Of the same 96 children, 79 met full ASD criteria at T2. The vast majority of children with ASD also had other neurodevelopmental symptoms or diagnoses. Hyperactivity was observed in 42% of children with ASD at T2, and Intellectual Developmental Disorder in 30%. Borderline Intellectual Functioning was found in 25%, and severe speech and language disorder in 20%. The children who did not meet criteria for ASD at T2 had symptoms of or met criteria for other neurodevelopmental/neuropsychiatric disorders in combination with marked autistic traits. Changes in developmental profiles between T1 and T2 were common in this group of young children with ASD. The main effect of Cognitive level at T1 explained more than twice as much of the variance in Vineland scores as did the ASD subtype; children with IDD had significantly lower scores than children in the BIF and AIF group. Co-existence with other conditions was the rule.

Conclusions and implications: Reassessments covering the whole range of these conditions are necessary for an optimized intervention adapted to the individual child's needs.