Case of myotonic dystrophy type 1 : NEJM
A 27-year-old woman presented with a 1-year history of progressive weakness and grip myotonia in both hands. There was no family history of any similar condition and no personal history of cataracts. Physical examination revealed weakness of the muscles of the face, neck, shoulder girdle, and hands.

Myotonia was elicited with the percussion of the tongue (see video), finger extensors, and thenar eminence. Sensation, coordination, and cognition were normal. Electromyography showed myotonic discharges and low-amplitude motor-unit potentials of short duration, indicating myopathy. Serum levels of electrolytes and thyrotropin were normal; the level of serum creatine kinase was mildly elevated at 245 U per liter (reference range, 2 to 200 U per liter).

Molecular analysis of peripheral blood revealed more than 167 trinucleotide repeats in the region of the gene encoding dystrophia myotonica protein kinase (DMPK).

The patient received a diagnosis of myotonic dystrophy type 1 and a prescription for carbamazepine for the relief of symptomatic myotonia. At 1 year of follow-up, she remained ambulatory, and there was slight abatement of grip myotonia.

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