Learn The BasicsDuchenne muscular dystrophy (DMD) is a rare genetic neuromuscular disease that causes progressive muscle weakness and loss of muscle function. It mainly affects males and is caused by mutations in the DMD gene, which provides instructions for making dystrophin, a protein essential for muscle health.
The disease is inherited in an X-linked recessive pattern, meaning the faulty gene is passed down from carrier mothers to their sons. However, it can also be the result of a so-called de novo mutation, or a genetic alteration that happens for the first time in a family member.
The disease can be diagnosed based on a medical history and physical examination, blood tests, electromyography, muscle biopsy and genetic testing. An electrocardiogram or echocardiogram may also be conducted to monitor cardiac function.
Medical history and physical exam
A male affected by DMD has inherited the faulty gene from their mother who is a carrier. This means that other female relatives, such as the affected child’s grandmother or aunts, may also be carriers and male relatives may have the disease. A medical history is the starting point for a DMD diagnosis.
During a physical examination features like delayed growth, mild hypotonia (low muscle tone), difficulty walking, toe walking, waddling gait and frequent falls may be observed. Enlarged calf muscles are also common as are contractures of the ankles, knees, hips and elbows. Children with DMD often use the “Gower’s maneuver,” a technique where they push off their thighs to stand up due to weak leg muscles.
In later stages of the disease scoliosis (a sideways curvature of the spine) is common. Learning difficulties are also common.
Blood tests
A blood test measuring the levels of creatine kinase supports a diagnosis of DMD. Creatine kinase is an enzyme that is secreted from damaged muscle into the bloodstream and can be 10 to 20 times higher than normal in patients with DMD in the early stages of the disease.
As the disease progresses and muscle tissue is gradually replaced by scar tissue, the levels of creatine kinase in the blood also start to fall.
However, levels of this enzyme are not specific to DMD and cannot be the only diagnostic sign. The levels of other enzymes such as aldolase and aspartate transferase may also be elevated in the blood of patients with DMD.
Electromyography
Electromyography is a test that measures the electrical activity of muscles in response to electrical stimulation. The test involves the placement of electrodes on the skin or their insertion into a muscle to take a recording.
This test can show myopathic features but is also not specific for DMD.
Muscle biopsy
A muscle biopsy is a procedure where a small piece of muscle tissue is taken, usually using a needle, for examination.
For the diagnosis of DMD, a muscle biopsy from the thigh or calf may be taken and shows characteristic features of the disease. However, this approach is rarely used nowadays as most patients are diagnosed based on genetic testing.
Genetic testing
Genetic testing can reveal the presence of a mutation in the DMD gene. It can also be used to tell exactly which kind of mutation is present. This is important for treatment as some available therapies are only beneficial in patients with certain types of mutations.
It involves taking a blood sample and analyzing the DNA it contains.
A genetic test can also identify female carriers of the disease.
Echocardiogram and electrocardiogram
Patients with DMD show characteristic changes in an electrocardiogram (ECG), a test that records the heart’s electrical activity.
An echocardiogram, an imaging test that uses ultrasound to visualize the heart and its blood vessels, is used to evaluate cardiac function and structure. It usually shows evidence of dilated cardiomyopathy or heart muscle disease in almost all patients with the disease by the end of their teens.