An abbreviated Wechsler Adult Intelligence Scale Revised (WAIS-R) was used to assess verbal and arithmetical cognitive performance in 55 subjects with myotonic dystrophy (DM), covering all grades of disease severity, and 31 controls at 50% risk of inheriting DM. Scaled scores from the assessment were converted into an intelligence quotient (IQ) estimation on each person. Significant IQ differences were found between: (1) all 55 DM subjects (mean 90.2, SD 16.1) and 31 controls (102.6, SD 9.4), with no sex differences in either group; (2) 15 affected parents (99.3, SD 12.2) and their affected children (88.1, SD 17.2), where significance was dependent on parental sex being female; and (3) 15 pairs of affected sibs (89.6, SD 13.2) and their normal sibs (100.2, SD 7.6). IQ steadily declined as (1) the age of onset of signs and symptoms decreased, and (2) the CTG expansion size increased. The correlation appeared to be more linear with age of onset. The correlation of IQ difference and CTG expansion difference in both the DM parent-child pairs and normal sib-affected sib pairs was poor, indicating that CTG expansion is not a reliable predictor of IQ either in individual persons or families. Further analysis of cognitive function in DM is required to clarify specific deficits characteristic of this patient group.
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