What To Know About Duchenne Muscular Dystrophy (DMD)

Duchenne muscular dystrophy (DMD) is a rare neurological condition that causes severe muscle weakness and intellectual disability.

DMD is an inherited (passed down) disorder. The condition is linked to the X-chromosome, which means DMD mostly affects people assigned male at birth. DMD affects approximately one in 3,600 male infants.

DMD is a type of muscular dystrophy, a group of genetic diseases that cause muscles to weaken over time. DMD is one of the most severe forms of muscular dystrophy, with symptoms worsening quickly. Symptoms appear in early childhood.

There is currently no cure for DMD. Treatment involves managing symptoms to improve quality of life.

DMD symptoms usually start by age 6. Symptoms may begin as early as infancy, but most boys don’t have any symptoms in the first few years of their life.

The main symptoms of DMD are muscle weakness, intellectual disability, and cardiomyopathy (disease of the heart muscle).

Muscle Weakness

Muscle weakness is the primary symptom of DMD. The weakness usually starts in the legs and pelvis. There is a decline in muscle strength between the ages of 6-11. Due to the loss of muscle mass and movement difficulties caused by muscle weakness, most people with DMD need a wheelchair, braces, or standing frames by the age of 12.

Muscle weakness causes:

• Problems with motor skills (walking, jumping, running)
• A waddling walking pattern (your upper body moves side to side with each step)
• Frequent falls
• Difficulty going up the stairs
• Fatigue and difficulty breathing due to weak lung muscles
• Increased likelihood of fractures
• Pain in the calves

Intellectual Disability

DMD can cause a mild form of intellectual disability. This means there may be challenges in how someone with DMD can learn and limitations in what they can do for themself to function in their daily life.

Many people with DMD have some level of intellectual disability. However, most are not at the level where they would officially be classified as intellectually disabled.

Intelligence quotient (IQ) is a test that measures someone’s intelligence. Most people score between 85-115. A score below 70 is considered intellectually disabled. About 20-30% of people with DMD have an IQ below 70.

The severity of intellectual disability does not worsen over time.

Cardiomyopathy

Cardiomyopathy is a disease that weakens the heart muscle. In DMD, low levels of dystrophin protein cause the heart to grow bigger and the heart muscle to weaken. These changes make it harder for the heart to pump blood.

When blood doesn’t pump as well, tachycardia (elevated heart rate) and heart failure (when the heart pumps poorly) can occur. These symptoms may start in the early teen years, but nearly everyone with DMD will have cardiomyopathy symptoms by their 20s.

Cardiomyopathy is one of the main causes of death for people with DMD.

An abnormality in the DMD gene causes Duchenne muscular dystrophy. The DMD gene produces dystrophin, a protein that maintains the structure of muscle cells. People with DMD have mutations (changes) in the gene that cause a loss of dystrophin protein in cells.

The lack of dystrophin protein causes muscle cells to lose their structure and function, leading to the symptoms of DMD. The lack of dystrophin also negatively affects the ability of muscle cells to regenerate over time, explaining why some symptoms of DMD worsen as the child ages.

The DMD gene is X-linked, meaning it is located on the X chromosome (a sex chromosome). DMD is known as an X-linked genetic disorder because mutations of the gene occur on the X chromosome and affect mostly men.

Women who have the DMD gene mutation don’t usually show symptoms, but they are carriers for the disorder and can potentially pass it on to their children. A woman with the X-linked mutation has a 1 in 4 chance of passing the mutation to a male offspring.

While most cases of DMD among boys are inherited from the birth mother, there are rare cases of spontaneous mutations of the DMD in children whose birth mothers are not carriers of the condition.

DMD is diagnosed through a physical evaluation and blood tests to identify mutations in the DMD gene. Muscle samples might also be evaluated. These tests are usually performed during the first two or three years of someone’s life.

During the physical evaluation, a healthcare professional will assess the child’s muscle strength, including their ability to walk and hold up their head.

Blood testing can be used to perform molecular genetic testing and examine the DNA to identify the mutated DMD gene. Blood tests might reveal elevated creatine kinase levels, a sign of muscle cell damage.

A muscle biopsy (tissue sample) is usually taken from the thigh muscle. Biopsy imaging examines muscle cells to evaluate whether the tissue has deteriorated.

As symptoms develop, your healthcare provider might do cardiac and pulmonary testing to evaluate heart and breathing function.

There is no cure for DMD. Treatment aims to manage symptoms and improve quality of life.

Prescription steroids, like prednisone, may help slow the loss of muscle mass. Steroids have also been shown to improve breathing function slightly.

Other medications, like angiotensin-converting enzyme inhibitors or angiotensin receptor blockers, are prescribed to manage cardiac symptoms and prevent heart failure.

Genetic therapies may help DMD by increasing levels of dystrophin in the body. Examples include Exondys 51 (eteplirsen), Vyondys 53 (golodirsen), and Viltepso (viltolarsen). A healthcare provider must inject the medication into your veins. Not everyone with DMD responds to them.

Your healthcare provider might also suggest physical therapy, which usually involves passive stretching and movement exercises. Physical therapy can help maintain muscle strength and function in limbs.

The main conditions associated with DMD are related to the heart and lungs. Weak muscles in the heart and lungs can make breathing and pumping blood to the heart difficult, leading to complications like:

• Cardiomyopathy
• Heart failure
• Arrhythmia (irregular heartbeat)
• Respiratory infections, like pneumonia
• Respiratory failure

DMD is a condition with a poor prognosis. It gets worse over time, and its complications cause early death.

However, advances in medications have improved outcomes for people with DMD. More people with DMD are now living to 35 than ever before. The development of rigorous treatment regimens that combine medications, physical therapy, and nutrition plans can help improve the quality of life of those with DMD.

Support groups for people with DMD and their families also provide a space to share stories, gather resources, and interact with others who are going through the same experience.