By: Jenna Klotz, MD, MS, FAAP
Muscular dystrophies are genetic muscle disorders which cause progressive muscle weakness. There are more than 40 different types. Symptoms can be present at birth or may not occur until adolescence or adulthood. Similarly, muscle weakness can progress at different rates. Here’s what families need to know.
What causes muscular dystrophy?
Muscular dystrophies are caused by abnormalities in proteins that are important for the structure and function of muscle. These abnormalities are caused by genetic changes, which can be inherited or can be a new change found only in the affected person.
When you move, your muscle fibers contract and stretch, which puts stress on the muscle. In healthy muscles, the muscle can withstand this physical stress and heal itself when injured. However, with muscular dystrophy, the muscle fibers have abnormal structure or function; even routine physical activity can injure the muscles and the muscles do not heal as they should. Over time, this causes progressive muscle loss and weakness.
What are the different types of muscular dystrophy?
The most common types of muscular dystrophies in children are Duchenne muscular dystrophy (DMD) and Becker muscular dystrophy (BMD). DMD and BMD are caused by mutations in dystrophin which is located on the X chromosome. DMD affects 1 in about 5000 males. Although primarily males are affected, females can have symptoms, particularly related to the heart muscle.
Duchenne muscular dystrophy
Males with DMD may have borderline delays in gross motor skills in the first 1-2 years of life. Symptoms typically become more apparent between 3-5 years old, when they may have more falls than typical for their age. They may also have difficulty walking up and down stairs and standing up from the floor. In addition, they may appear to “waddle” when they walk and can have enlarged calf muscles. Children with DMD typically lose the ability to walk in their early teenage years and pass away in their 20s to 40s. This is because of weakness in their breathing muscles (respiratory failure) or abnormalities in their heart muscle (cardiomyopathy).
Becker muscular dystrophy
Becker muscular dystrophy (BMD) generally has less severe muscle weakness, later onset and slower, less predictable progression. However, the heart muscle can be affected like it is with DMD. In some cases of BMD, cardiomyopathy causing heart failure may be the first sign with little to no apparent limb weakness.
Other types of muscular dystrophy
- Limb girdle muscular dystrophy (LGMD): In LGMD, muscle weakness starts in the muscles closest to the center of the body, including the hips, pelvis, shoulders, upper arms and legs. Over time, weakness can spread to other muscles. There are over 25 types of LGMD.
- Facioscapulohumeral muscular dystrophy (FSHD): In FSHD, the earliest muscles with weakness include those in the face, shoulder and upper back, lower abdomen, and lower leg. Shoulder blades may appear to stick out (“winged scapula”). Weakness can be asymmetric (different on each side of the body) and usually progresses slowly. Most individuals with FSHD develop symptoms in adolescence or early adulthood. However, 5% to 10% have symptoms before age 10. (Parents: if your child is affected by FSHD, share with them “Strength Isn’t Just Physical,” below.)
- Myotonic Dystrophy: Myotonic dystrophy is the most common muscular dystrophy in adults but can also affect newborns and children. Newborns with myotonic dystrophy have low muscle tone and weakness at birth and typically need support for breathing and feeding. This often improves over time before they develop increased muscle weakness in later teenage years and difficulty relaxing muscles (myotonia). Intellectual disability is common. People with childhood-onset myotonic dystrophy may have learning or cognitive issues as their primary symptom and later develop muscle weakness and myotonia.
- Congenital muscular dystrophy: Children with congenital muscular dystrophy have low muscle tone and weakness at birth. (See “Parent to Parent,” below.) Kids with some types of congenital muscular dystrophy may have abnormal brain structure on MRI, abnormal eye structure and sometimes seizures.
- Emery-Dreifuss muscular dystrophy (EDMD): Individuals with EDMD typically develop symptoms between 5-10 years old. They develop contractures (stiff or fixed joints), followed by slowly progressive weakness in their upper arms and lower legs. Later, they develop weakness in their shoulders, pelvis, and hips. Heart failure and abnormal heart rhythms are common and usually start after age 20.
- Collagen-related muscular dystrophy: Collagen is a major component of connective tissue, which provides support for joints, skin and other structures in the body. So, children with collagen-related muscular dystrophy may have contractures (fixed joints) or overly flexible joints (hyperlaxity) early on. They can also have hyperkeratosis (bumpy, dry skin), very soft skin on their palms and soles, and poor wound healing. Ullrich congenital muscular dystrophy is the most severe form, with signs shortly after birth. Bethlem myopathy is the mildest form with slowly progressive weakness starting in childhood or early adulthood. There are also intermediate forms.