Duchenne muscular dystrophy

An X-linked recessive condition caused by the loss of one protein. Here's how that one loss adds up to whole body disease.

If you haven't covered the six inheritance patterns yet, read our genetics basics article first. The rest of this page assumes you know what autosomal dominant, autosomal recessive, and X-linked recessive mean.

Inheritance: a quick recap

Duchenne muscular dystrophy, usually shortened to DMD, is X-linked recessive. The gene sits on the X chromosome, at locus Xp21. A female needs two altered copies to be affected, written XᵃXᵃ. A male needs only one, written XᵃY, since he only has one X chromosome to begin with.

This is why DMD is almost entirely a disease of boys. A carrier mother, genotype XᴬXᵃ, shows no symptoms or only mild ones, but each son has a 50 percent chance of inheriting her Xᵃ and being affected. Each daughter has a 50 percent chance of inheriting Xᵃ and becoming a carrier herself, usually without symptoms.

About two thirds of DMD cases are inherited from a carrier mother. The remaining third arise from a new mutation in the dystrophin gene, with no family history at all. This matters clinically: a negative family history does not rule out DMD, and it does not mean the mother isn't a carrier going forward for any future children.

The same father to son rule that applies to every X-linked recessive condition applies here. A father cannot pass DMD to his son, since he gives his son a Y chromosome, not an X. If you ever see a pedigree with an affected father and an affected son, and the mother shows no sign of the condition, you are not looking at DMD. That single relationship rules out X-linked recessive inheritance outright.

What dystrophin actually does

DMD is caused by mutations in the gene for dystrophin, a structural protein. Dystrophin sits just inside the membrane of every muscle fiber, where it connects the internal cytoskeleton to a group of proteins called the dystrophin associated glycoprotein complex, which in turn anchors to the structure surrounding the cell on the outside. The whole assembly works like a shock absorber. Every time a muscle fiber contracts, dystrophin spreads the mechanical force evenly across the membrane instead of letting it concentrate at any one point.

Without dystrophin, that shock absorber is gone. The membrane is left to take the full force of every contraction directly, and it begins to tear.

From a torn membrane to a failing body

This is the chain worth understanding, because it explains everything else in this article.

A torn cell membrane is no longer a sealed barrier. Calcium, which sits at much higher concentration outside the cell than inside it, floods in through the tears. Once inside, that excess calcium activates enzymes that break down muscle proteins from within. The cell dies. This isn't a slow chemical imbalance. It's a structural failure with a direct biochemical consequence, repeated with every contraction, in every fiber, for years.

Muscle tissue can normally repair this kind of damage. Specialized stem cells called satellite cells sit dormant in healthy muscle and activate to rebuild damaged fibers. In DMD, the damage never stops, so the satellite cells never stop being called on, and after years of constant repair their capacity runs out. Once that happens, dead muscle fibers stop being replaced by new muscle. They get replaced by fat and fibrous scar tissue instead, which cannot contract and cannot be converted back.

This same process happens everywhere dystrophin is normally active, not just in the muscles you can see working. It happens in the diaphragm, which is a muscle, and it happens in the heart, which is also a muscle. That single fact, that this is fundamentally a problem of muscle tissue everywhere in the body and not just the limbs, is the reason DMD eventually becomes a respiratory and cardiac disease as much as it is a mobility disease.

Symptoms and how the disease progresses

Symptoms typically begin between ages 3 and 5. Early signs include difficulty climbing stairs, frequent falls, a waddling gait, and enlarged calf muscles, caused by fat and fibrous tissue replacing muscle in that area, not by extra strength. A classic clinical sign is Gowers' sign, where a child has to push off the floor with their hands and walk their hands up their own legs to stand up, because their hip and thigh muscles are too weak to lift their body weight directly.

Muscle weakness is progressive and moves from the body's center outward. Hip and thigh muscles go first, followed by shoulder muscles, then the muscles further down the limbs. Most boys with DMD lose the ability to walk independently between ages 7 and 13, depending on access to treatment.

As the disease progresses, the same membrane damage and fibrotic replacement reach the respiratory muscles and the heart. Respiratory muscle weakness reduces the ability to cough and clear the lungs, increasing the risk of infection, and eventually requires ventilatory support. The heart muscle develops cardiomyopathy, a weakening of the heart's pumping ability, which becomes a leading cause of death.

Prevalence

DMD is the most common form of childhood muscular dystrophy. Global estimates put birth prevalence at roughly 1 in 5,000 male births, with most epidemiological studies reporting figures between 1 in 3,500 and 1 in 9,000. It affects males almost exclusively, since it requires only one altered X chromosome to cause full disease.

Why life expectancy is shortened, and how that's changing

Historically, before modern supportive care, most people with DMD died in their late teens to early twenties, almost always from respiratory failure or cardiomyopathy. Both of those causes trace directly back to the same mechanism described above: years of membrane damage and fibrotic replacement in the diaphragm and the heart, the two muscles you cannot survive without.

That figure has changed substantially. With coordinated care, including corticosteroids, non invasive ventilation, cough assist devices, and active cardiac monitoring and treatment, average life expectancy is now reported at over 30 years, and a growing number of people with DMD live past 40. The disease has not changed. The ability to manage its respiratory and cardiac consequences has.

Current treatment

There is no treatment that restores normal dystrophin function throughout the body for a lifetime. Current care is built around slowing the disease and protecting the organs most at risk.

Corticosteroids remain the backbone of standard care. They slow the decline in muscle strength and extend the time a child can walk independently, although the exact mechanism for why they help is not fully settled. Physical therapy and orthopedic management address joint contractures and scoliosis, which develop as muscle imbalances pull the skeleton out of alignment. Regular cardiac monitoring catches cardiomyopathy early enough to treat it with standard heart failure medications, and non invasive ventilation supports breathing as respiratory muscles weaken.

Newer approaches target the genetic cause more directly. Exon skipping drugs work for a subset of patients whose specific mutation makes them candidates, allowing cells to produce a shortened but partially functional dystrophin protein instead of none at all. Microdystrophin gene therapy delivers a compact, lab built version of the dystrophin gene using a viral vector. Both approaches aim at restoring some dystrophin function rather than only treating the downstream consequences of having none, but neither one reverses damage that has already occurred, and long term outcomes are still being studied.

For a comparison with how autosomal recessive conditions behave on a pedigree, including why they can skip generations in a completely different way, read our article on cystic fibrosis.