Aphasia Leading to the Diagnosis of Myotonic Dystrophy Type 1: A Case Report

Myotonic dystrophy type 1 (DM1) is a multisystem autosomal dominant disorder primarily characterized by myotonia and distal muscle weakness. Central nervous system (CNS) involvement, including cognitive, executive, and emotional dysfunctions, is increasingly being recognized; however, language impairment as an initial presentation is rare. A 50-year-old right-handed woman with a six-month history of progressive word-finding difficulty, vague speech, and social withdrawal was referred for evaluation. Neurological examination revealed distal muscle atrophy (grip strength: 5 kg right, 8 kg left) without overt dysarthria or dysphagia, and intact reflexes and coordination. Neuropsychological testing revealed fluent spontaneous speech with anomia, semantic paraphasia, impaired comprehension of longer sentences, and surface dyslexia/agraphia (Mini-Mental State Examination-Japanese: 22/30, Frontal Assessment Battery: 8/18, Raven's Colored Progressive Matrices: 28/36, Montreal Cognitive Assessment-Japanese: 21/30). Brain magnetic resonance imaging revealed left-sided frontotemporal and limbic atrophy, and ^99mTc-ethyl cysteinate dimer single-photon emission computed tomography showed a corresponding left-dominant hypoperfusion. Amyloid positron emission tomography (PET) scan results were negative. Two weeks later, percussion and grip myotonia emerged. Genetic analysis revealed a cytosine-thymine-guanine repeat expansion (~1500 repeats) in the myotonic protein kinase 1 gene, confirming the diagnosis of DM1. The patient's semantic-variant primary progressive aphasia-like profile (impaired semantic processing with preserved fluency) and frontotemporal imaging findings were consistent with rare CNS phenotypes reported in DM1. Previous studies have described frontotemporal atrophy, hypoperfusion, and cognitive/emotional changes in DM1. Negative amyloid PET excluded Alzheimer-related primary progressive aphasia. The subsequent detection of myotonia and a positive family history were key to diagnosis. We conclude that this case expands the clinical spectrum of DM1 to include progressive aphasia as an initial manifestation. Clinicians should maintain a high suspicion of neuromuscular disorders and actively pursue targeted genetic testing when atypical aphasia symptoms are accompanied by distal muscle atrophy.