title: Comparisons of intellectual capacities between mild and classic adult-onset phenotypes of myotonic dystrophy type 1 (DM1).
abstract: BACKGROUND
  Myotonic dystrophy type 1 (DM1) is an autosomal dominant genetic multisystem disorder and the commonest adult-onset form of muscular dystrophy. DM1 results from the expansion of an unstable trinucleotide cytosine-thymine-guanine (CTG) repeat mutation. CTG repeats in DM1 patients can range from 50 to several thousands, with a tendency toward increased repeats with successive generations (anticipation). Associated findings can include involvements in almost every systems, including the brain, and cognitive abnormalities occur in the large majority of patients. The objectives are to describe and compare the intellectual abilities of a large sample of DM1 patients with mild and classic adult-onset phenotypes, to estimate the validity of the Wechsler Adult Intelligence Scale-Revised (WAIS-R) in DM1 patients with muscular weakness, and to appraise the relationship of intelligence quotient (IQ) to CTG repeat length, age at onset of symptoms, and disease duration.
  
  
  METHODS
  A seven-subtest WAIS-R was administered to 37 mild and 151 classic adult-onset DM1 patients to measure their Full-Scale (FSIQ), Verbal (VIQ) and Performance IQ (PIQ). To control for potential bias due to muscular weakness, Standard Progressive Matrices (SPM), a motor-independent test of intelligence, were also completed.
  
  
  RESULTS
  Total mean FSIQ was 82.6 corresponding to low average IQ, and 82% were below an average intelligence. Mild DM1 patients had a higher mean FSIQ (U=88.7 vs 81.1, p<0.001), VIQ (U=87.8 vs 82.3, p=0.001), and PIQ (U=94.8 vs 83.6, p<0.001) than classic adult-onset DM1 patients. In both mild and classic adult-onset patients, all subtests mean scaled scores were below the normative sample mean. FSIQ also strongly correlate with SPM (r s =0.67, p<0.001), indicating that low intelligence scores are not a consequence of motor impairment. FSIQ scores decreased with both the increase of (CTG)n (r s =-0.41, p<0.001) and disease duration (r s =-0.26, p=0.003).
  
  
  CONCLUSIONS
  Results show that intellectual impairment is an extremely common and important feature in DM1, not only among the classic adult-onset patients but also among the least severe forms of DM1, with low IQ scores compared to general reference population. Health care providers involved in the follow-up of these patients should be aware of their intellectual capacities and should adapt their interventions accordingly.
authors.affiliation: Clinique des maladies neuromusculaires, Centre de réadaptation en déficience physique Le Parcours, Centre de santé et de services sociaux de Jonquière, 2230, rue de l'Hôpital, C.P. 1200, Jonquière, Québec, G7X 7X2, Canada. stephane.jean.csssc@ssss.gouv.qc.ca.
authors.forename: Stéphane
authors.initials: S
authors.lastname: Jean
authors.affiliation: Département des sciences de la santé, Université du Québec à Chicoutimi, 555, boul. de l'Université, Chicoutimi, Québec, G7H 2B1, Canada. louis_richer@uqac.ca.
authors.forename: Louis
authors.initials: L
authors.lastname: Richer
authors.affiliation: Département des sciences de la santé, Université du Québec à Chicoutimi, 555, boul. de l'Université, Chicoutimi, Québec, G7H 2B1, Canada. luc_laberge@uqac.ca.|ÉCOBES - Recherche et transfert, Cégep de Jonquière, Québec, Canada. luc_laberge@uqac.ca.|Faculté de médecine et des sciences de la santé, Université de Sherbrooke, Québec, Canada. luc_laberge@uqac.ca.
authors.forename: Luc
authors.initials: L
authors.lastname: Laberge
authors.affiliation: Clinique des maladies neuromusculaires, Centre de réadaptation en déficience physique Le Parcours, Centre de santé et de services sociaux de Jonquière, 2230, rue de l'Hôpital, C.P. 1200, Jonquière, Québec, G7X 7X2, Canada. j.mathieu@usherbrooke.ca.|Faculté de médecine et des sciences de la santé, Université de Sherbrooke, Québec, Canada. j.mathieu@usherbrooke.ca.
authors.forename: Jean
authors.initials: J
authors.lastname: Mathieu
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D015331:Cohort Studies; D005260:Female; D006801:Humans; D008607:Intellectual Disability / Q000175:diagnosis* / Q000453:epidemiology / Q000523:psychology*; D007360:Intelligence; D008297:Male; D008875:Middle Aged; D009223:Myotonic Dystrophy / Q000175:diagnosis* / Q000453:epidemiology / Q000523:psychology*; D010641:Phenotype; D014888:Wechsler Scales / Q000592:standards; D055815:Young Adult