Life expectancy of type 1 myotonic dystrophy patients ultimately depends on the phenotype, but is often at least 45 years, explains the U.S. National Library of Medicine. The less common type 2 form is not as severe and has little impact on life expectancy, states the Myotonic Dystrophy Foundation.
The onset of congenital type 1 myotonic dystrophy typically occurs from infancy to the age of 10, and brings a life expectancy of roughly 45 years, according to the U.S. National Library of Medicine. Type 1 classic myotonic dystrophy may appear between the ages of 10 and 30, with an average life expectancy of 48 to 55 years. Mild myotonic dystrophy affects patients as young as 20 or as old as 70, and may limit life expectancy to 60 years of age or have no affect at all.