Diagnosis of Myotonic Dystrophy Based on a History of Grip Myotonia in a 21-Year-Old Woman Undiagnosed for Five Years: A Case Report

Myotonic dystrophy (DM) is an inherited neuromuscular disorder characterized by myotonia and progressive muscle weakness. A 21-year-old female patient presented with a one-month history of fever, myalgia, and a threefold increase in creatine kinase (CK) levels following a diagnosis of coronavirus disease 2019. The patient had undergone hemithyroidectomy at 14 years of age and was being followed up with regular blood testing, which revealed that she had slightly elevated CK levels (232-265 U/L) but consistently normal thyroid hormone levels. At the initial visit, detailed history recording revealed that since attending high school, the patient had occasionally experienced difficulty in opening her gripped hand. Physical examination confirmed grip myotonia, and apart from the elevated CK levels (333-488 U/L; CK isozyme MM 98%), blood tests revealed no obvious abnormalities. Genetic analysis revealed CTG trinucleotide repeat expansion, confirming a diagnosis of DM type 1. DM should accordingly be considered as a differential diagnosis in cases of an unexplained elevation in CK levels, particularly in younger patients. Detailed history recording and physical examination of myotonia should be performed if DM is suspected.