Myotonic muscular dystrophy is the most common form of muscular dystrophy that affects adults and is characterized by myotonia, a symptom involving prolonged muscle stiffening or spasms that worsen in cold temperatures, explains WebMD. Other symptoms include daytime sleepiness, cataracts and heart problems, notes the Muscular Dystrophy Association.
There are two forms of adult-onset myotonic muscular dystrophy: MMD1 and MMD2, states the Muscular Dystrophy Association. Both have the potential to impact multiple bodily systems, although their presentations differ in some aspects. Daytime sleepiness occurs mostly in individuals with MMD1, often making it a challenge for them to work or attend school full-time, but can also affect people with MMD2 to a lesser extent. Adults with MMD1 commonly develop a heart condition called a conduction block, which blocks an important electrical signal in the body responsible for maintaining a healthy heart rhythm. This complication can also impact patients with MMD2, but there has not been as much research into its occurrence with this form of the disease, as of 2015.
Muscle weakness is common in both forms of myotonic muscular dystrophy, but different muscle groups tend to be affected depending on the type of the disease, notes the Muscular Dystrophy Association. The muscles farthest from the body's core, such as feet muscles, are commonly weakened in MMD1. Weakness in muscles nearer to the middle of the body, such as the thigh muscles, is more common with MMD2.