Myotonic dystrophy is an autosomal dominant disease affecting many organ systems, including the heart. Abnormalities of the cardiac conduction system are a frequent and well-documented finding in this neuromuscular disease, whereas overt signs of heart failure are rarely reported. However, controversy exists about the prevalence of preclinical left ventricular (LV) dysfunction in patients with myotonic dystrophy who have no symptoms of heart failure. To address this issue, load-independent LV function indexes were compared in patients with myotonic dystrophy and in normal subjects. LV measurements were obtained with M-mode echocardiography in 43 consecutive patients with myotonic dystrophy (mean age 35 +/- 14 years) who had no clinical evidence of heart failure, and in 35 sex- and age-matched controls. A cuff sphygmomanometer was used to determine blood pressure. No difference was found between patients and control subjects in the percentage of LV ejection fraction predicted for end-systolic stress (103 +/- 7 vs 103 +/- 7%; p = 0.97) or in the end-systolic stress/volume index ratio (2.9 +/- 0.5 vs 2.7 +/- 0.4 dyne7/cm3; p = 0.09). Similarly, no significant difference in LV myocardial function was detected between the 19 patients with a mild degree of the neuromuscular disease and in the 24 with a moderate or severe degree of the disease. Finally, LV myocardial function was compared between patients with myotonic dystrophy and atrioventricular or intraventricular conduction disturbances (n = 8) and those without these disturbances (n = 35), but no significant difference in the percentage of LV ejection fraction predicted for end-systolic stress or in the end-systolic stress/volume index ratio could be detected.(ABSTRACT TRUNCATED AT 250 WORDS)