Objective. - To evaluate and quantitate cardiac involvement in myotonic dystrophy and assess whether the size of the trinucleotide (cytosine-thymine- guanine [CTG]) repeat expansion is a significant predictor of cardiac abnormalities. Design. - Case-control study of a large kindred with myotonic dystrophy. Patients. - Ninety-one bloodline members of the kindred underwent clinical and cardiac evaluation with electrocardiograms, echocardiography (with Doppler in the majority of cases), and genetic and neurologic evaluations. Affected individuals were age-matched to normal family members. Main Outcome Measures. - Electrocardiographic conduction abnormalities, wall motion abnormalities, mitral valve prolapse, and global parameters of systolic and diastolic function were determined by an observer blinded to all clinical data and genetic analysis. Results. - Compared with age-matched normals, patients with myotonic dystrophy (n=25) were more likely to have conduction abnormality (52% vs 9%), mitral valve prolapse (32% vs 9%), and wall motion abnormality (28% vs 0%) (all P<.05). Left ventricular ejection fraction and stroke volume were reduced compared with normals matched for age and heart rate (P<.05), whereas Doppler indexes of diastolic function were only marginally altered. Using multivariate analysis, the number of CTG repeats (range, 69 to 1367; normal, ≤37) was the strongest predictor of abnormalities in wall motion and electrocardiographic conduction (odds ratio of 16.5 and 5.07 per 500 repeats, respectively). The relation of mitral valve prolapse to the size of the CTG repeat was of borderline significance. Patients with more extensive neurologic findings (n=12) had a higher incidence of wall motion and/or electrocardiographic conduction abnormalities (83% vs 43%; P=.04). Conclusions. - Cardiac involvement in myotonic dystrophy affects predominantly the conduction system and myocardial function. Alterations in myocardial relaxation and diastolic properties, in contrast to skeletal muscle myotonia, are minor. In this kindred, the number of CTG repeats was a significant predictor of cardiac dysfunction in myotonic dystrophy.
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