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Aim  To investigate the psychiatric and cognitive phenotype in young individuals with the childhood form of myotonic dystrophy type 1 (DM1).

Method  Twenty-eight individuals (15 females, 13 males) with childhood DM1 (mean age 17y, SD 4.6, range 7–24y) were assessed using standardized instruments and cognitive testing of general intelligence, visual attention, and visual–spatial construction abilities.

Results  Nineteen patients had repeated a school grade. The mean (SD) Full-scale IQ was 73.6 (17.5) and mean Verbal IQ was significantly higher than the mean Performance IQ: 80.2 (19.22) versus 72.95 (15.58), p=0.01. Fifteen patients had one or more diagnoses on the DSM-IV axis 1, including internalizing disorders (phobia, n=7; mood disorder, n=6; other anxiety disorders, n=5) and attention-deficit–hyperactivity disorder, inattentive subtype (n=8). Twelve out of 22 patients had alexithymia (inability to express feelings with words and to recognize and share emotional states). Cognitive testing found severe impairments in visual attention and visual–spatial construction abilities in four out of 18, and 14 out of 24 patients respectively. No diagnosis was correlated with the transmitting parent’s sex or with cytosine–thymine–guanine (CTG) repeat numbers. Patients with severe visual–spatial construction disabilities had a significantly longer CTG expansion size than those with normal visual–spatial abilities (p=0.04).

Interpretation  Children and adolescents with childhood DM1 have frequent diagnoses on DSM-IV axis 1, with internalizing disorders being the most common type of disorder. They also have borderline low intelligence and frequent impairments in attention and visual–spatial construction abilities.