Myotonic dystrophy type 1 (DM1) is an autosomal dominant disorder, caused by an expansion of a CTG triplet repeat in the DMPK gene. The aims of the present study were to classify a cohort of children with DM1, to describe their neuropsychiatric problems and cognitive level, to estimate the size of the CTG expansion, and to correlate the molecular findings with the neuropsychiatric problems. Fifty-seven children and adolescents (26 females; 31 males) with DM1 (CTG repeats > 40) were included in the study. The following instruments were used: Autism Diagnostic Interview-Revised (ADI-R), 5-15, Griffiths Mental Development Scales, and the Wechsler Scales. Based on age at onset and presenting symptoms, the children were divided into four DM1 groups; severe congenital (n = 19), mild congenital (n = 18), childhood (n = 18), and classical DM1 (n = 2). Forty-nine percent had an autism spectrum disorder (ASD) and autistic disorder was the most common diagnosis present in 35% of the subjects. Eighty-six percent of the individuals with DM1 had mental retardation (MR), most of them moderate or severe MR. ASD was significantly correlated with the DM1 form; the more severe the form of DM1, the higher the frequency of ASD. The frequency of ASD increased with increasing CTG repeat expansions. ASD and/or other neuropsychiatric disorders such as attention deficit hyperactivity disorder, and Tourette's disorder were found in 54% of the total DM1 group. In conclusion, awareness of ASD comorbidity in DM1 is essential. Further studies are warranted to elucidate the molecular etiology causing neurodevelopmental symptoms such as ASD and MR in DM1.

Aims  To investigate cognitive abilities and adaptive skills in children and adolescents with myotonic dystrophy type 1 (DM1) and correlate the findings to the cytosine‐thymine‐guanine (CTG) repeat expansion size. Method  Cognitive level was assessed in 55 children and adolescents with DM1 (31 males, 24 females; mean age 12y 1mo, SD 5y 1mo; range 2y 7mo–21y 5mo) divided into the following categories: severe congenital DM1 (n=19), mild congenital DM1 (n=18), and childhood DM1 (n=18). The Griffiths Mental Developmental Scale, the Wechsler Scales, and the Vineland Adaptive Behavior Scales (VABS) for adaptive skills were used for this purpose. Results  Learning disability was found in 95% of the severe congenital group, 83% of the mild congenital group, and 89% of the childhood DM1 group. The more severe the form of DM1, the lower the full‐scale IQ (FSIQ; rs=0.28, p=0.044). The individuals with severe congenital and childhood DM1 had a significantly higher verbal IQ than performance IQ (severe congenital: mean difference 5.7, SD 5.7, p=0.008; childhood DM1: mean difference 9.8, SD 18.0, p=0.038). CTG repeat expansion correlated negatively with FSIQ (rs=−0.63, p<0.006). Almost all participants showed poor results on the VABS. There was a positive relationship between cognitive level and adaptive skills in the mild congenital (rs=0.95, p<0.01) and childhood DM1 groups (rs=0.92, p<0.01). Interpretation  Children and adolescents with DM1 exhibit significant cognitive and adaptive problems.