A case of myotonic dystrophy showing proximal dominant muscle involvement but not myotonia

A 45-year-old female had progressive difficulty in climbing stairs and standing from a chair for 10 years. She had binocular cataracts which were operated at the age of 42 years. On examination, she had marked muscle wasting in the proximal limbs, scapular and sternomastoid muscles. She presented as marked muscle weakness in the proximal portion of the lower extremities and moderate in the upper extremities and the legs. Deep tendon reflexes were absent in all limbs. There was no grip myotonia, or percussion myotonia of the thenar muscle and tongue. Myotonia was not elucidated even after the hands were exposed to cold water. Moreover, none of the examined muscles revealed insertion myotonic discharge on electromyography. Serum CK level was normal and IgG value decreased to 546 mg/dl. Muscle biopsy of the left biceps muscle showed the variation in fiber size, increased central nuclei and many fiber with pyknotic nuclear clumps on HE staining. Sarcoplasmic mass and ring fibers were also found on HE staining. There were a few percents of ragged-red fibers on Gomori-trichrome staining, and type 1 fiber atrophy was found on pH 4.5 ATP-ase staining. The expansion of lymphocyte CTG trinucleotide repeats in the myotonin protein kinase gene was about 733, so that she was diagnosed as having myotonic dystrophy (MD). MRI of skeletal muscles exhibited marked atrophy especially in the femoral region and the biceps muscle. This patient had the proximal dominant muscle weakness, and absent myotonia even on electromyographic examination, which are unusual clinical features of adult onset MD.