Overview of Duchenne Muscular Dystrophy

Emma Ciafaloni, MD: Duchenne muscular dystrophy is the most common muscular dystrophy with the onset of symptoms in children. It’s a disease that primarily occurs in children. Although nowadays, patients do survive into young adulthood because of better standard of care and new treatments. The cause of this muscular dystrophy is it’s a genetic disease. It’s X-linked recessive disease that affects only men and is due to a genetic mutation in the gene called the dystrophin gene. Those mutations cause a lack of the dystrophin protein. The dystrophin protein is necessary to maintain the stability and the health of the muscle fibers, and without dystrophin, the muscle fibers degenerate. They essentially die off, and they get replaced by connective tissue or scar tissue over time.

Because it is an X-linked genetic disease, the disease manifests only in men. Women carry the disease, and they can transmit it to their male offspring, but it’s an extremely rare circumstance where a woman actually expresses the full disease. For the prevalence of the disease, new cases of Duchenne are about 1 in 6000. Overall, in the United States, there are about 12,000 patients with Duchenne muscular dystrophy. In the big picture, this is a rare disease by all means. The FDA definition of a rare disease is fewer than 200,000 total cases in the United States. In this instance, we’re talking about 12,000 cases. So they’re very rare and yet the most common dystrophy in children.

We have done quite a number of studies on the first symptoms. We know that the symptoms present very early on, and certainly on average around age 2, 2½. Either the parents or the teacher realizes that something is not right with the child. Usually the most common symptoms are trouble running, walking—a waddling type of walking, a run that is not really up to peers, the inability to keep up with other children of the same age, an inability to jump or hop, difficulty going upstairs, frequent falls, and also toe walking. These are all the presenting common symptoms. Essentially there is motor weakness and an issue with motor function. It’s also important to remember that about 30% to 40% of the children will also have some cognitive impairment, maybe some language delay, some behavioral changes, perhaps ADHD [attention deficit hyperactivity disorder]—like symptoms, and speech delay. Frequently they can have delay in ambulation, so they can start walking late.