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Corresponding author at: The University of Tokyo, Department of Medical Education Studies, International Research Center for Medical Education, Graduate School of Medicine, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033 Japan
Department of Medical Education Studies, International Research Center for Medical Education, Graduate School of Medicine, The University of Tokyo, Bunkyo-ku, Tokyo, JapanDepartment of Internal Medicine, Suwa Central Hospital, Chino, Nagano, Japan
A 39-year-old male presented to our hospital for an annual complete medical checkup
(so-called human dock in Japanese). He had a 10-year history of malaise. Physical examination revealed
frontal balding (Figure 1) and grip myotonia (sustained muscle contraction; Figure 2 and Supplementary (Video). Spirometry suggested restrictive ventilatory impairment
(vital capacity percentage, 65.6%; forced expiratory volume in 1 second as forced
vital capacity percentage, 86.6%). Further workup investigation was performed because
myotonic dystrophy was suspected. Laboratory studies revealed a mildly elevated creatine
phosphokinase level of 224 U/L (non-disease associated range, 24–195 U/L). Electromyography
showed myotonic discharges in the right biceps muscle. Although the patient's family
history was unremarkable, dystrophia myotonica protein kinase (DMPK) genetic testing revealed a CTG repeat expansion of 400 (non-disease associated range
<35), confirming the diagnosis of myotonic dystrophy type 1 (DM1). Regular follow-up
showed no signs and symptoms of critical complications (i.e., cardiac arrhythmia,
cataract, and endocrine abnormalities).
Fig. 2Grip myotonia in the hands. Although the patient was asked to release his grasp after
a strong grip, it took him a few seconds to fully open his hands.
