Symptoms usually appear in male children before age 6 and may be visible in early infancy. Even though symptoms do not appear until early infancy, laboratory testing can identify children who carry the active mutation at birth.^[2] Progressive proximal muscle weakness of the legs and pelvis associated with a loss of muscle mass is observed first. Eventually this weakness spreads to the arms, neck, and other areas. Early signs may include pseudohypertrophy (enlargement of calf and deltoid muscles), low endurance, and difficulties in standing unaided or inability to ascend staircases. As the condition progresses, muscle tissue experiences wasting and is eventually replaced by fat and fibrotic tissue (fibrosis). By age 10, braces may be required to aid in walking but most patients are wheelchair dependent by age 12. Later symptoms may include abnormal bone development that lead to skeletal deformities, including curvature of the spine. Due to progressive deterioration of muscle, loss of movement occurs, eventually leading to paralysis. Intellectual impairment may or may not be present but if present, does not progressively worsen as the child ages. The average life expectancy for patients afflicted with DMD is around 25.^[1]

There is no current cure for DMD, although phase 1-2a trials with exon-skipping treatment for certain mutations have halted decline and produced small clinical improvements in walking.

Treatment is generally aimed at controlling the onset of symptoms to maximize the quality of life, and include the following:

• Corticosteroids such as prednisolone and deflazacort increase energy and strength and defer severity of some symptoms.^[13]
• Randomised control trials have shown that beta2-agonists increase muscle strength but do not modify disease progression. Follow-up time for most RCTs on beta2-agonists is only around 12 months and hence results cannot be extrapolated beyond that time frame.^{[citation needed]}
• Mild, non-jarring physical activity such as swimming is encouraged. Inactivity (such as bed rest) can worsen the muscle disease.
• Physical therapy is helpful to maintain muscle strength, flexibility, and function.
• Orthopedic appliances (such as braces and wheelchairs) may improve mobility and the ability for self-care. Form-fitting removable leg braces that hold the ankle in place during sleep can defer the onset of contractures.
• Appropriate respiratory support as the disease progresses is important.