Introduction: Cardiac myopathy manifesting with conduction disturbances and arrhythmias is common in the neurologic disease myotonic dystrophy. We studied whether the severity of cardiac involvement in myotonic dystrophy correlates with the severity of the genetic abnormality cytosine-thymine-guanine (CTG) repeat expansion.
Methods and results: History, physical examination, ECG evaluation, and genetic testing were performed in patients with a clinical diagnosis of myotonic dystrophy. In 342 of 385 patients, the diagnosis was confirmed by CTG repeat expansion. In these patients, the muscular disability severity correlated with age and CTG repeat length (r = 0.44, P < 0.001). An arrhythmia diagnosis was present in 19 (5.6%) patients with a likelihood of diagnosis correlating with age (relative risk [RR] 2.2 per decade, 95% confidence intervals [CI] 1.4 to 3.4, P = 0.001) and CTG repeat length (RR 2.9 per 500 repeats, 95% CI 1.5 to 5.4, P = 0.001). ECGs were abnormal in 222 (64.9%) of the patients. Age, CTG repeat length, and male gender were factors found to correlate with ECG conduction abnormalities quantitated by the PR interval (r = 0.43, P < 0.001) and QRS duration (r = 0.32, P < 0.001). A 24-hour ambulatory ECG was abnormal in 95 (29.6%) of 321 recordings. The presence of an abnormality correlated with age (RR 1.5 per decade, 95% CI 1.2 to 1.9, P < 0.001) and CTG repeat length (RR 1.6 per 500 repeats, 95% confidence intervals 1.1 to 2.2, P = 0.01).
Conclusion: The severity of skeletal and cardiac myopathy in myotonic dystrophy correlates with age and CTG repeat length, suggesting a similar mechanism causing a time-dependent degenerative process.