Myotonic Dystrophy Type 1
Myotonic dystrophy type 1 is the most common form of muscular dystrophy and affects distal muscles. Myotonic dystrophy may occur at birth, in children between the ages of 1 and 10 years, or in adults. Congenital myotonic dystrophy is rare but should be suspected in neonates with hypotonia, clubfoot, feeding difficulties, and respiratory distress. Childhood symptoms include cognitive and behavioral impairments and gastrointestinal disorders such as diarrhea, constipation, or fecal incontinence. Most cases present in adults. Symptoms include distal muscle weakness, myotonia, early cataracts and cardiac conduction abnormalities. Myotonia refers to impaired muscle relaxation after contraction.
The diagnosis is based on a patient’s medical history, family history, physical examination, and genetic studies. Muscle biopsies are nonspecific and are not needed for diagnosis. Electromyography findings of myotonic discharges support the diagnosis.
The criterion standard diagnostic test is molecular genetic testing with identification of CTG repeat expansions in the DMPK gene. The presence of more than 34 CTG repeats is categorized as a premutation. These individuals are asymptomatic but the number of repeats can expand in future generations to pathogenic levels. Individuals with 50 or more CTG repeats in the DMPK gene have full-penetrance alleles and typically develop clinical manifestations of myotonic dystrophy type 1. Longer CTG repeat lengths are associated with more severe symptoms and an earlier age of onset.
Since myotonic dystrophy type 1 is an autosomal dominant disorder, it is important to offer genetic counseling to all first-degree relatives. They may have sudden cardiac death without other preceding symptoms.
Reference
Johnson NE. Diagnosis and Management of Myotonic Dystrophy Type 1. JN Learning. Published Online: March 11, 2024. doi:10.1001/jama.2024.2511
