OBJECTIVE: Cardiac disease accounts for a large burden of premature mortality and morbidity in patients with type 1 myotonic dystrophy (MD). However, little is known about structural cardiac abnormalities particularly in asymptomatic patients with MD. We sought to describe the prevalence and extent of structural cardiac abnormalities in patients with MD and to assess their association with functional, electrical, biochemical and genetic disturbances. METHODS: In this case-control study, 40 adults with MD who had no contraindications to cardiac MRI (CMR) were identified from the Grampian region genetic database. Forty-one age-and-gender-matched healthy volunteers were also recruited. All subjects underwent detailed assessment including CMR, echocardiography, electrocardiography, signal-averaged electrocardiography, Holter monitoring and quantification of serum B-type natriuretic peptide (BNP). Genetic testing of patients with MD was performed with quantification of CTG trinucleotide repeat sequences. Results of clinical, electrical, genetic and biochemical investigations were correlated with cardiac structural and functional abnormalities detected on CMR. RESULTS: Electrical disturbances including prolongation of PR (187±29 vs 156±23 ms, p<0.001) and QRS intervals (99±11 vs 89±9 ms, p<0.001) were the most prevalent abnormality. Patients with MD had a significantly lower left ventricular (LV) mass (142±44 vs 172±73 g, p=0.03) and lower right ventricular (RV) ejection fraction (46±9 vs 50±7%, p=0.02) compared with controls, although LV ejection fraction was similar between the groups (58±8 vs 59±6%, p=0.34). LV non-compaction was also significantly more prevalent in the MD cohort (35% vs 12%, p=0.019). Late gadolinium enhancement was present in 13% of patients with MD. Muscular disability scores correlated with electrical changes (r=0.529, p<0.001); however, the number of CTG repeat sequences did not correlate with either electrical or structural abnormalities. CONCLUSIONS: Patients with MD have a high prevalence of both electrical and structural abnormalities. These include reduced LV mass, impaired RV contractility, a high prevalence of LV non-compaction and myocardial fibrosis. These findings illustrate the potential utility of CMR detecting subclinical disease in otherwise asymptomatic patients with MD.