The onset of clinical symptoms is between 20 and 30 years. Histology Specimens show early selective atrophy of type 1 fibers with type 2 hypertrophy and increased central nuclei and targetoid fibers. Ring fibers are common. There are also sarcoplasmic masses, and sarcolemmal blebs of homgenous eosinophilic cytoplasm devoid of myofibrils. Staining is PAS positive, with or without oxidative activity.
Necrosis is unusual.
Due to unstable CTG repeat in the gene for myotonin, a protein kinase. These are a group of diseases that are often familial and present in early childhood. Main features include delayed motor development, hypotonia, proximal muscle weakness, and often, facial weakness. Frequently, the disease is static or slowly progressive. Congenital myopathies are, overall, considered disturbances in myogenesis that adversely affect maturation, but without necrosis.