Adult-onset myotonic muscular dystrophy, also called Steinert’s disease, is caused by a genetic mutation, explains Mayo Clinic. Symptoms begin with weakness and degeneration of the voluntary muscles, usually starting in the face and the neck. Another symptom is myotonia, meaning that the muscles cannot be relaxed at will. The disease progresses to weaken involuntary muscles, including the heart and the muscles that control breathing and digestion.
Type 1 myotonic muscular dystrophy can begin in adolescence or early adulthood, says the Muscular Dystrophy Association. Type 2 only appears during adulthood. Type 2 is less common and less severe than Type 1 myotonic muscular dystrophy.
In addition to muscle weakness, adult-onset myotonic muscular dystrophy can cause other symptoms. These symptoms include muscle pain, early cataracts in the eyes, early balding in males, excessive daytime sleepiness, cognitive impairment and insulin resistance, according to the Muscular Dystrophy Association.
In both types of myotonic muscular dystrophy, the genetic mutations that cause the disease have been identified by researchers. Type 1 originates on chromosome 19, while Type 2 has its origins on chromosome 3. These genetic mutations have an autosomal dominant inheritance pattern, meaning that a child who has one parent with myotonic muscular dystrophy has a 50 percent chance of inheriting the disease, reports the Muscular Dystrophy Association.