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.