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A 60-year-old woman came to the neuromuscular clinic because of a progressive gait difficulty that had started insidiously 2 years before presentation. She could walk and climb stairs unassisted but with some difficulty. The physical examination revealed slight bilateral eyelid ptosis, temporalis muscle atrophy, and a waddling gait. Muscle strength was 4/5 in the scapular-girdle musculature and 3/5 in the hip flexors and extensors. Percussion of the finger extensor muscles and thenar eminence muscles evoked a myotonic phenomenon (figure and video). There was no grip myotonia. Sensation and coordination were preserved. Electromyography showed myotonic discharges and low-amplitude, short-duration motor-unit potentials, indicating myopathy. The creatine kinase level was 107 U per liter, and serum electrolytes were within the normal range. The patient had a history of hypercholesterolemia and cataracts and was receiving levothyroxine-replacement therapy after partial thyroidectomy for nodular disease; she had a normal thyroid level at the time of examination. There was no family history of any similar disorder. A molecular test detected a CTG triplet expansion in the dystrophia myotonica–protein kinase (DMPK) gene, confirming the diagnosis of type 1 myotonic dystrophy. Since the myotonia did not substantially reduce the patient's functioning, antimyotonic drugs were not recommended. At the last follow-up, 2 years after diagnosis, her muscle strength had deteriorated but she remained ambulatory.
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