Duchenne muscular dystrophy (DMD) causes progressive muscle weakness that typically becomes noticeable between ages 2 and 4, though subtle signs can appear earlier. It affects roughly 1 in every 3,500 to 5,000 boys born worldwide, making it the most common form of muscular dystrophy. The symptoms start in the legs and pelvis, then gradually spread to the upper body, heart, and breathing muscles over the course of childhood and adolescence.
Early Motor Delays
The first signs of DMD are usually delays in reaching physical milestones. Most children walk independently by 12 months, but boys with DMD often don’t walk until 18 months or later. Once walking, they may seem clumsy, fall frequently, or struggle to keep up with peers. Running, jumping, and climbing stairs are particularly difficult because DMD weakens the muscles closest to the trunk first, especially in the hips, thighs, and pelvis.
Parents often notice that their child has trouble getting up from the floor. Instead of standing straight up, a boy with DMD will roll onto his stomach, push himself onto his hands and knees, then “walk” his hands up his own legs to pull himself upright. This characteristic movement is called the Gower maneuver, and it compensates for weak hip and thigh muscles. In the earliest stages, it may look like nothing more than mild hand pressure against the thighs. As weakness progresses, the child needs to push off the floor with increasing force. Most children outgrow the need to use this prone-to-standing technique by age 3, so its persistence beyond that age is a red flag.
A waddling gait is also common. Boys with DMD tend to walk on their toes and lean their upper body from side to side with each step, shifting their weight to compensate for pelvic muscles that can’t stabilize the hips properly.
Enlarged Calves and Muscle Changes
One of the most recognizable physical signs of DMD is unusually large calf muscles. This looks like the child has strong, muscular legs, but it’s misleading. The enlargement is called pseudohypertrophy: the muscle tissue is being replaced by fat and scar tissue through repeated cycles of damage and failed repair. Over time, the proportion of non-contractile tissue increases while functional muscle shrinks. MRI studies of boys with DMD show that both real muscle growth and fatty replacement happen simultaneously in the calf muscles, which is why the calves can look impressively large even as they become weaker.
This same replacement process happens throughout the body, but it’s most visible in the calves because those muscles are used heavily in walking and standing.
Loss of Walking Ability
DMD is progressive, meaning symptoms worsen steadily over time. Boys typically reach their peak physical ability somewhere between ages 4 and 8, then gradually lose strength. The median age at which boys with DMD lose the ability to walk independently is about 11 years and 8 months, based on data from 160 patients tracked over time. Corticosteroid treatment can extend this timeline: boys on daily steroid regimens lost ambulation at a median of 12 years and 5 months compared to 11 years and 5 months for those on intermittent dosing. The specific genetic mutation also plays a role, with certain deletions associated with later loss of walking.
Before full-time wheelchair use, there’s usually a transitional period where a boy can walk short distances but needs a wheelchair for longer outings, and where fatigue becomes a major daily factor.
Spinal Curvature After Wheelchair Use
Once a boy with DMD stops walking, the muscles supporting the spine weaken further, and scoliosis (a sideways curve of the spine) frequently develops. The weakened trunk muscles can no longer hold the spine straight during sitting, and the curve tends to progress as growth continues. Hip and knee contractures, where joints stiffen in a bent position, add to the postural imbalance.
Not all scoliosis in DMD causes problems. Moderate curves that don’t cause pain or affect sitting balance may not need treatment. But rapidly worsening curves can compress the lungs and make breathing harder, which sometimes requires surgical correction before respiratory function declines too far for safe surgery.
Heart Problems
The same protein missing in skeletal muscle, dystrophin, is also missing in heart muscle. This leads to a form of heart disease called dilated cardiomyopathy, where the heart gradually weakens and enlarges. Signs of heart involvement can usually be detected through cardiac testing by age 10, though rare cases have been identified as early as age 6.
Heart symptoms are often silent in the early stages because boys with DMD aren’t physically active enough to stress their hearts the way a healthy child would. As the condition progresses, symptoms can include fatigue, shortness of breath, swelling in the legs or feet, and irregular heartbeat. Regular cardiac screening with echocardiograms is standard practice because by the time a boy feels heart symptoms, the damage may already be significant.
Breathing Difficulties
As DMD weakens the diaphragm and the muscles between the ribs, breathing becomes less effective. This happens gradually and often shows up first during sleep, when the body naturally breathes more shallowly. The earliest breathing symptoms are easy to miss because they’re nonspecific: morning headaches, poor sleep, daytime fatigue, nightmares, difficulty concentrating during the day, and poor appetite at breakfast. These all result from the body not getting enough oxygen or clearing enough carbon dioxide overnight.
Over time, breathing difficulty extends to waking hours. Lying flat becomes uncomfortable because the weakened diaphragm can’t work as effectively against gravity in that position. Coughing becomes weak, making it harder to clear mucus from the lungs and increasing the risk of chest infections. Respiratory support, starting with nighttime ventilation, eventually becomes necessary for most people with DMD.
Cognitive and Behavioral Differences
DMD doesn’t only affect muscles. The dystrophin protein is also present in the brain, and its absence can affect cognitive development. Boys with DMD show higher rates of intellectual disabilities, difficulties with verbal working memory, reading challenges, and learning disabilities compared to their peers. ADHD is also more common, though the exact relationship between specific genetic mutations and attention difficulties is still being studied.
These cognitive and behavioral features are present from early childhood, not a result of disease progression. They don’t worsen over time the way the muscle symptoms do. The severity varies widely: some boys have no noticeable learning differences, while others need significant educational support. The specific location of the genetic mutation within the dystrophin gene appears to influence which brain functions are affected, with mutations that disrupt certain shorter forms of the protein linked to greater cognitive impact and behavioral inflexibility.
How DMD Is Diagnosed
When DMD is suspected based on symptoms, the first step is usually a blood test measuring creatine kinase (CK), an enzyme that leaks out of damaged muscle cells. In boys with DMD, CK levels peak around age 2 at 10 to 20 times the upper limit of normal, then gradually decline by about 25% per year as muscle tissue is replaced by fat and scar tissue. Even with this decline, levels remain significantly elevated.
Genetic testing is the gold standard for confirming the diagnosis. It identifies mutations in the dystrophin gene, most commonly large deletions or duplications of sections of the gene. If those aren’t found, more detailed sequencing can detect smaller mutations. When genetic testing clearly confirms the diagnosis, a muscle biopsy is typically unnecessary.