Behavioral phenotype in childhood type of dystrophia myotonica

To the Editor:

We report on the emotional, behavioral and cognitive characteristics of 24 children and adolescents with childhood-type dystrophia myotonica (DM). With this study we confirm earlier findings on a smaller sample of subjects [Steyaert et al., 1997]. Childhood-type DM is defined as DM where the first symptoms appear before the age of 12 years, but with uneventful pre- and neonatal history and normal development in the first year of life. Unlike children with congenital DM, children with childhood DM are not known to have important neurological symptoms or motor impairment before adulthood. However, learning disabilities are known to be often the most prominent symptom in this type of DM. Most subjects in the study were nonclinical family members of affected patients with DM, and participated after a call through a patients association for muscular disorders. DM was confirmed by means of DNA analysis of the expansion length in the DM gene and a clinical geneticist (CDD) diagnosed the subjects as having childhood-type DM. All subjects were between 9 and 22 years, 14 boys and 10 girls. In 13 children the DM was transmitted by the mother and in 11 by the father. Twenty-two DM parents had the adult type of DM, one had the mild type, and one was free of symptoms.

The 24 subjects underwent a cognitive evaluation with a standardized intelligence test (Wechsler scales). Possible behavior problems were assessed by means of a standardized behavior checklist (Achenbach's Child Behavior Checklist: CBCL), completed by the parents in all subjects and by the teachers in 11 subjects. The ADIKA, a structured psychiatric interview based on the Diagnostic and Statistical Manual for Mental Disorders, was completed in all subjects.

We found that only 4 of the 24 subjects were in a regular school program, while the other subjects either attended a special education program or worked in a sheltered workshop. Their full-scale intelligence quotient varied between 50–97 (mean = 72; SD = 14). The lower IQ's correlate with longer expansions in the DM gene (Pearson correlation = 0.619, P = 0.002). The expansion lengths correlate with the transmitting parent's sex: inheritance from the mother gives a longer expansion length than from the father (ANOVA: P = 0.001). On the CBCL completed by the parents, we found a score in the clinical range in 10 of the 24 subjects (42%). On the same checklist completed by the teachers, 3 of 11 subjects (27%) scored in the clinical range. The subjects had most problems in the problem areas of the CBCL “withdrawn,” “social interaction,” and “attention problems.” Fifteen of the 24 subjects (63%) had a diagnosis on the structured psychiatric interview. The most frequent diagnoses were Attention Deficit Hyperactivity Disorder (33% of subjects) and anxiety disorders (25% of subjects). Neither the transmitting parent's sex nor the expansion length had a significant effect on the level of behavior problems or on the psychiatric diagnoses. The number of subjects scoring in the clinical range on the CBCL and the number of subjects with a psychiatric diagnosis were significantly increased when compared with the prevalences in the general population.

We conclude that a significantly increased number of children and adolescents with the childhood type of DM, who have no or only minimal neurological symptoms of DM and no muscular impairment, are nonetheless cognitively impaired, are in a special education setting, and have emotional and behavior problems. There is no evidence that these problems are the consequence of muscular impairment or impairment in the parents. This supports the view that DM is not only a muscular disorder, but also a brain disorder [Ashizawa, 1998]. The finding that the most frequent psychiatric diagnosis in these 24 subjects is ADHD supports the findings by van Spaendonck et al. [1995] that in DM the subject's cognitive control of behavior, or so-called ‘executive’ mental functions, can be markedly impaired. This suggest that (pre-)frontal areas of the brain may be involves in the pathophysiology of DM.