Rare diseases affect a relatively small number of people, but with nearly 7,000 known rare diseases, they impact up to 30 million people in the United States. Many of these conditions are caused by genetic mutations (changes in DNA that can lead to disease). Duchenne muscular dystrophy (DMD) is considered rare, but it is also one of the most common genetic diseases affecting muscle function. Other types include Becker muscular dystrophy (BMD) and limb-girdle muscular dystrophy.

In this article, we’ll learn more about what makes DMD a rare disease and how it affects research and treatment.

1. DMD Is a Rare Disease

DMD is considered a rare disease by health experts and health organizations.

A rare disease is defined as a medical condition that affects a small percentage of the population. However, the exact definition varies by country and health agency:

• In the U.S., a disease is considered rare if it affects fewer than 200,000 people.
• In the United Kingdom, a condition is rare if it affects fewer than 1 in 2,000 people.
• The World Health Organization (WHO) defines a rare medical condition as one that affects less than 65 in 100,000 people.

To understand how many people have DMD, researchers look at prevalence — the proportion of people in a given population who have the condition. According to the Muscular Dystrophy Association, DMD affects about 6 in 100,00 people in Europe and North America. However, prevalence estimates vary worldwide, ranging from 0.9 to 16.8 per 100,000 people. A 2020 study published in the Orphanet Journal of Rare Diseases estimated the global prevalence of DMD to be 7.1 in every 100,000 males.

2. DMD Is Even Rarer in Females

While DMD is rare in males, it’s even rarer in females. It’s estimated that only 1 in 50 million females are born with DMD, according to research cited in Sleep Science.

DMD is much more common in males because of how it’s inherited (passed down from parent to child). The condition is caused by a genetic mutation in the dystrophin gene (or DMD gene). This gene provides instructions for making dystrophin protein, which helps protect and support muscle cells.

How DMD Is Inherited

The dystrophin gene is located on the X chromosome, one of the two sex chromosomes that determine a person’s biological sex:

• Females typically have two X chromosomes (one from each parent).
• Males typically have one X chromosome from their mother and one Y chromosome from their father.

Since nearly all females have two X chromosomes, they usually have a backup copy of the dystrophin gene. If one X chromosome carries the DMD mutation, the other can often compensate, preventing symptoms. Females with one mutated dystrophin gene are called carriers. Carriers can pass the mutation to their children but usually don’t experience symptoms.

Males, however, typically do not have a second X chromosome to provide a backup copy of the dystrophin gene. If they inherit an X chromosome with a DMD mutation, they will develop symptoms of DMD, such as muscle weakness.

Because DMD primarily affects males, many studies only report its prevalence in males.

3. The Number of People With DMD Is Rising

It may seem concerning that more people are living with Duchenne muscular dystrophy than ever before. However, this increase is largely due to advances in treatment that have helped people with DMD live longer. As life expectancy improves, more people with DMD are alive at the same time.

This trend becomes clearer when looking at incidence — the number of new DMD cases diagnosed each year. Research shows that DMD incidence has remained stable over time, meaning the number of new cases hasn’t significantly increased.

A 2019 study estimated that about 362 new cases of DMD occurred in the United States that year. The study also calculated the prevalence of DMD to be 6.09 per 100,000 males, translating to more than 10,000 people living with DMD in the U.S.

4. Most People With DMD Are Under 20

A 2019 U.S. study found that about 65 percent of people with DMD were younger than 20 years of age. Although treatment advances have improved survival rates, DMD remains a life-limiting condition, and many individuals do not live past their mid-20s.

DMD leads to early death because it weakens the muscles of the heart and respiratory (breathing) muscles. As the disease progresses, it can cause cardiomyopathy (a disease of the heart muscle that makes it harder for the heart to pump blood). Weakness in the respiratory muscles can lead to respiratory failure, making it difficult to breathe.

Although life expectancy has increased over the years, most people with DMD are still diagnosed in childhood and do not reach older adulthood:

• About 30 percent of people with DMD live past the age of 30.
• Only 2 percent of people in this study were between the ages of 40 and 45.

5. Rare Diseases Can Be Difficult To Diagnose

Diagnosing rare diseases can be challenging because there are often no standard guidelines for healthcare providers to follow. Additionally, many rare diseases have nonspecific symptoms, meaning doctors must rule out other conditions first. Without clear guidelines, some healthcare providers may not recognize the signs of a rare disease, leading to delayed diagnoses.

Because DMD is one of the most common rare diseases, health experts in several countries have developed diagnostic and treatment guidelines. However, it can still take years to receive a diagnosis, especially for the 30 percent of people who develop DMD with no family history.

A 2022 study found that in the United States, the average time to diagnose DMD after symptoms first appear is 2.2 years. Despite advances in DMD research, this delay in diagnosis has remained unchanged for more than 40 years.

Learn more about the diagnosis of DMD.

6. Treatments for Rare Diseases Like DMD Are Often Called Orphan Drugs

An orphan drug is a medication developed to treat a rare disease. Rare diseases are sometimes called orphan diseases because they have been abandoned by the drug development process — because they were historically overlooked by the drug development process — often because developing treatments was not considered profitable. According to StatPearls, pharmaceutical companies may not develop drugs for rare diseases due to lack of funding or interest.

The Orphan Drug Act, passed in the U.S. in 1983, created incentives for pharmaceutical companies to research and develop treatments for rare diseases. Before this law, only 10 drugs were available to people with rare diseases. Since then, the number of orphan drugs has grown significantly, including several new treatments for DMD.

New DMD Treatments

At first, the only medications available to treat DMD were corticosteroids (steroids), like prednisone and deflazacort. Steroids can slow DMD progression, improve muscle strength, and improve lung function, but they do not cure the condition. Additionally, steroids can cause significant long-term side effects, such as weight gain, osteoporosis (bone thinning), and hypertension (high blood pressure).

In 2016, the U.S. Food and Drug Administration (FDA) approved (Exondys 51) — the DMD treatment with an orphan drug designation. Since then, additional FDA-approved antisense oligonucleotide therapies have followed, including:

• Casimersen (Amondys 45)
• Golodirsen (Vyondys 53)
• Viltolarsen (Viltepso)

In 2023, the FDA approved the first gene therapy for DMD in 2023: elandistrogene moxeparvovec (Elevidys).

In 2024, the FDA approved givinostat (Duvyzat), a histone deacetylase inhibitor that targets disease-causing processes to reduce inflammation and slow muscle atrophy (loss).

7. Research for Rare Disease May Be Eligible for Special Funding

Additional funding for research and clinical trials may also be available to study DMD. Clinical trials are research studies that test new treatments to determine if they are safe and effective for people.

One research program is called Therapeutics for Rare and Neglected Diseases, launched by the National Institutes of Health. This program funds research aimed at developing safe and effective treatments for rare diseases, including gene therapy for DMD.

With increased funding, researchers may continue to develop new and improved DMD treatments in the future.

Talk With Others Who Understand

On myMDteam, the site for people with muscular dystrophy (MD) and their loved ones, members come together to gain a new understanding of MD and share their stories with others who understand life with the condition.

Have you or a loved one been diagnosed with DMD or another rare form of muscular dystrophy? What challenges have you and your family faced due to DMD being a rare disease? Share your experiences in the comments below or on your Activities page.