Supplementary Material for: Muscle MRI of the Upper Extremity in the Myotonic Dystrophy Type 1

<i>Background:</i> The purpose of this study was to explore the relation between muscle MRI findings and weakness of the upper extremity muscles in patients with myotonic dystrophy type 1 (DM1). <i>Methods:</i> Nineteen DM1 patients from 15 families were enrolled in this study. Muscle weakness was evaluated using the modified Medical Research Council scale. Subjects also underwent a genetic study and muscle MRI of the upper extremities. <i>Results:</i> In patients with DM1, the flexor digitorum profundus (FDP), flexor pollicis longus, flexor digitorum superficialis (FDS), extensor pollicis, abductor pollicis longus (APL), lateral head of triceps brachii and infraspinatus (INF) muscles were frequently and severely affected. Muscle strength was significantly correlated with the severity of muscle MRI findings in the FDP, short head of biceps brachii (SBB), and medial head of triceps brachii muscles. Disease duration was correlated significantly with MRI findings in the FDP, FDS, long head of biceps brachii, INF, APL, and SBB muscles. Unexpectedly, the degree of trinucleotide expansion of myotonin protein kinase was not correlated with muscle MRI findings. <i>Conclusion:</i>Muscle MRI of the upper extremity is useful to detect affected muscles in DM1 patients.