In DMD, the first signs of muscle weakness typically occur before the age of four years. Rising from the floor, walking, running, jumping and climbing stairs are difficult. If untreated, most boys lose the ability to walk by ten years of age and need to use a wheelchair. This is due to a combination of muscle weakness and contractures (tightness of muscle affecting joint movement) in the ankles, knees and hips. Boys in a wheelchair are at high risk of developing spinal curvature, which can eventually develop into scoliosis. Some boys with DMD may have learning or behavioural difficulties (see entry Learning Disability).
Over time, the heart and respiratory muscles are also affected and this becomes a clinical problem usually in the teenage years. Sometimes there is respiratory failure, which is often demonstrated by difficulty breathing at night.
If untreated, limbs, breathing and heart muscle weakness become severe and this limits life expectancy.
Cardiomyopathy can develop in individuals with DMD, usually after ten years of age. It is recommended that an echocardiogram (heart ultrasound scan) be done every two years till age ten years, and then yearly after so any abnormality can be detected and treated.