Duchenne muscular dystrophy (DMD) is a progressive, X-linked recessive neuromuscular disorder characterized by the absence of dystrophin, a protein crucial for muscle fiber stability. While DMD affects all skeletal muscles, the lower limb is particularly impacted, leading to significant functional limitations and mobility challenges.
Pathophysiology and Mechanisms of Lower Limb Involvement
DMD is caused by mutations in the DMD gene, resulting in a lack of functional dystrophin. Without dystrophin, muscle fibers are susceptible to repeated injury during contraction, leading to cycles of degeneration and regeneration. Over time, regenerative capacity is exhausted, and muscle tissue is replaced by fat and connective tissue (fibrosis).
The lower limbs are often the first and most severely affected regions. The proximal muscles (hip and thigh) weaken before distal muscles (lower leg and foot), following a characteristic pattern:
- Gluteal and quadriceps muscles: Early and pronounced weakness, causing difficulty in rising from the floor (Gowers’ sign) and climbing stairs.
- Hamstrings and calf muscles: Progressive involvement leads to altered gait and compensatory postures.
- Tibialis anterior and peroneal muscles: Later in the disease, distal weakness contributes to foot drop and contractures.
Clinical Features and Progression in the Lower Limb
Early Stage (Preschool to Early School Age)
- Delayed motor milestones: Late walking, frequent falls, and difficulty running or jumping.
- Waddling gait: Due to hip abductor weakness.
- Gowers’ maneuver: Using hands to push off the thighs when rising from the floor.
School Age to Early Adolescence
- Progressive weakness: Loss of independent ambulation typically occurs between ages 10-13.
- Calf pseudohypertrophy: Enlargement of calf muscles due to fat and connective tissue infiltration, not true muscle growth.
- Equinus contractures: Tightness of the gastrocnemius-soleus complex leads to toe walking and increased risk of falls.
- Scoliosis and pelvic tilt: Secondary to lower limb and trunk muscle weakness.
Late Stage (Adolescence Onward)
- Loss of ambulation: Wheelchair dependence becomes necessary.
- Joint contractures: Fixed deformities at the hips, knees, and ankles.
- Osteoporosis and fracture risk: Due to immobility and corticosteroid use.
Evidence-Based Management of Lower Limb Complications
Physical Therapy and Stretching
Regular stretching and range-of-motion exercises are essential to delay contracture development. Evidence supports:
- Daily passive stretching: Especially for the Achilles tendon, hamstrings, and hip flexors.
- Night splints and orthoses: To maintain ankle dorsiflexion and prevent equinus deformity.
Orthopedic Interventions
- Serial casting and orthotic devices: Used to manage early contractures and support ambulation.
- Surgical interventions: Tendon lengthening or release procedures may be indicated for severe contractures, though timing and selection are critical to optimize function and minimize complications.
Pharmacologic Approaches
- Corticosteroids (prednisone, deflazacort): Shown to slow muscle degeneration, prolong ambulation, and delay scoliosis. However, side effects include weight gain, osteoporosis, and behavioral changes.
Mobility and Assistive Technology
- Wheelchairs and mobility aids: Early introduction of powered mobility enhances independence and quality of life as ambulation declines.
- Standing frames and walkers: Help maintain bone health and joint range of motion.
Multidisciplinary Care
- Podiatry: Routine assessment for pressure areas, skin breakdown, and orthotic needs.
- Physiotherapy and occupational therapy: Tailored programs for stretching, strengthening, and functional adaptation.
- Orthopedics: Monitoring for scoliosis and lower limb alignment.
Practical Considerations in Lower Limb Management
- Family and patient education: Emphasize the importance of daily stretching and early intervention for contractures.
- Regular assessment: Monitoring muscle strength, joint range, and functional status to guide interventions.
- Prevention of secondary complications: Skin care, fracture prevention, and pressure relief are priorities as mobility decreases.
The lower limb in individuals with Duchenne muscular dystrophy undergoes a predictable progression of weakness, contracture, and loss of function. Early, evidence-based interventions—delivered by a multidisciplinary team—are essential for optimizing mobility, minimizing complications, and enhancing quality of life. Ongoing research into gene therapies and new pharmacologic treatments holds promise for altering the natural history of DMD in the future.
