Résumé :
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Myotonic dystrophy type 1 (DM1), the most common form of muscular dystrophy in adults (1/8,000 individuals), is an inherited, autosomal dominant disease characterizedmainly by myotonia, progressive muscle weakness (especially of distal limbs, neck, and face), muscle wasting, and variable multisystemic features. The MuscularImpairment Rating Scale (MIRS) is an ordinal five-point rating scale, established in accordance with the clinically recognized distal to proximal progression of the muscularinvolvement in DM1.In the present study we analyzed the clinical characteristics and muscle strength in a sample of 38 Mexican individuals genetically diagnosed as DM1(30 subjects with clinical characteristics for DM1 and 8 asymptomatic patients) and 5 subjects without clinical significance for DM1 but with the presence of intermediatealleles (pre-mutation). Molecular diagnosis of DM1 patients was performed by applying PCR and TP-PCR with capillary electrophoresis analysis of the amplified products.Moreover, we evaluated the correlation between MIRS and age of onset (AO) according its time evolution.In symptomatic DM1 patients (more than 100CTG repeats),the classic clinical characteristics for DM1 were present at different percentage: myotonia, muscular atrophy, facial muscular weakness and ptosis (96.7%), predominantdistal limb weakness (83.4%), finger flexors weakness (36.7%), neck weakness (63.4%), sternocleidomastoid weakness (56.7%) and cuadriceps weakness (40%).With respect to proximal weakness, DM1 patients presented severe weakness (13%), moderate weakness (16.7), isolated weakness (16.7%), mild weakness (43.4%).Asymptomatic patients (lower than 100 CTG repeats) and subjects carrying pre-mutated alleles showed no alteration in muscle strength. MIRS evaluation and AO wereanalyzed using Spearman's rank correlation coefficient; patients were divided into three groups depending on the disease time evolution (<10 years of evolution, between11 to 20 years, and >20 years). In the first group, a significant inverse correlation was found between MIRS and AO (rho=0.813), in contrast the other two groups showedno correlation (rho=0.293 and rho=0.242 respectively). In the first years of disease evolution, we observe values for rho that indicate a highly significant correlation;however in the posterior years of disease evolution, the variance in MIRS has no correlation with AO. Therefore, another genetic and environmental factors might beimplicated
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