As with other types of muscular dystrophy, myotonic dystrophy involves progressive muscle weakness and muscle wasting. However, it's often the smaller muscles that are affected first, such as those in the face, jaw and neck.
Myotonic dystrophy can appear at any time between birth and old age. It affects the same number of men and women.
As well as muscle weakness and wasting, symptoms can include:
Myotonic dystrophy is highly variable and often gets worse very slowly, with little change over long periods of time. However, it can become more severe as it's passed down through the generations.
Some people with myotonic dystrophy may never have a significant disability, although their heart rate will need to be monitored for abnormalities. This is because there's a risk of the electrical impulses that control the heartbeat travelling too slowly through the heart. In some people, the condition can also cause cataracts to develop at a younger age than usual.
Life expectancy for people with myotonic dystrophy can vary considerably. Many people have a normal life expectancy, but people with the more severe congenital form (present from birth) may die while still a newborn baby, or only survive for a few years.
Some people who first develop symptoms as a child or teenager may also have a shortened life expectancy. Most deaths related to myotonic dystrophy are related to pneumonia, breathing problems or heart problems.
If you have myotonic dystrophy, it's important that you're well informed about your condition, and that you tell any healthcare professionals you see that you have it. Myotonic dystrophy can cause difficulties with general anaesthetics and childbirth.