Duchenne Muscular Dystrophy

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Definition: Duchenne Muscular Dystrophy (DMD) is a severe X-linked recessive disorder caused by mutations in the dystrophin gene (Xp21). It results in absent dystrophin protein, leading to progressive muscle degeneration.

Epidemiology:

• Incidence: ~1 in 3500-5000 male births.
• Inheritance: X-linked recessive. Males affected. Females are carriers.
• New Mutations: ~1/3 are sporadic (new mutation).

Natural History:

• Age 3-5: Symptom onset (delayed walking, difficulty climbing stairs).
• Age 5-10: Progressive weakness. Gowers' sign. Toe walking.
• Age 10-12: Loss of ambulation. Wheelchair-bound.
• Age 15+: Scoliosis progression. Respiratory decline. Cardiomyopathy.
• Age 20-30: Death (usually respiratory or cardiac failure).

Genetics:

• Gene: Dystrophin (DMD gene) on Xp21.
• Mutation Types: Deletions (60%), Duplications (5-10%), Point mutations (30%).
• Result: Complete absence of dystrophin protein.

Pathophysiology:

• Dystrophin: Links cytoskeleton to ECM. Stabilizes sarcolemma during contraction.
• Absence: Membrane fragility. Increased calcium influx. Muscle fiber necrosis.
• Progression: Repeated cycles of necrosis and regeneration. Eventually, fibrosis and fatty replacement.
• CK Elevation: Leakage from damaged muscle fibers.

Becker Muscular Dystrophy (BMD):

• Same gene, but in-frame mutations that allow partial dystrophin function.
• Milder phenotype. Later onset. Longer survival.

Key Anatomy: Understanding the relevant anatomy is crucial for diagnosis and management. The structures involved include the osseous architecture and surrounding soft tissues.

Pathomechanics: The injury mechanism often involves specific loading patterns that disrupt the structural integrity.

History:

• Developmental: Delayed walking (18+ months). Difficulty running, climbing, jumping.
• Family History: X-linked pattern. Carrier mother. Affected uncles/brothers.

Physical Examination:

• Gowers' Sign: Uses hands on thighs to stand from floor (proximal weakness).
• Pseudohypertrophy: Calves appear large (fibrofatty replacement).
• Lordosis: Lumbar hyperlordosis (weak hip extensors).
• Toe Walking: Equinus contracture.
• Shoulder Weakness: Weak scapular fixators ('winging').
• Contractures: Equinus, Hip flexion, Iliotibial band.
• Scoliosis: After loss of ambulation.

Orthopaedic Manifestations:

1. Contractures: Equinus (most common), Hip flexion, ITB tightness.
2. Scoliosis: Progressive neuromuscular scoliosis after wheelchair.
3. Fractures: Fragility fractures (osteoporosis from steroids + immobility).

Diagnosis:

1. Serum CK: Massively elevated (10-100x normal). Screening test.
2. Genetic Testing: Confirms mutation. Multiplex PCR, MLPA, Sequencing.
3. Muscle Biopsy: Absent dystrophin on immunohistochemistry. Now less used (genetic testing is gold standard).

Orthopaedic Assessment:

1. Spine X-ray: AP and Lateral for scoliosis. Measure Cobb angle.
2. Whole Spine MRI: If planning fusion.
3. Pulmonary Function (FVC): Essential before scoliosis surgery.
4. Cardiac Assessment: Echo, ECG. Cardiomyopathy is universal.
5. DEXA: Bone density assessment.

Natural History:

• Scoliosis develops in nearly 100% of DMD patients after loss of ambulation.
• Progression is rapid (~15-20 degrees/year).

Indications for Spinal Fusion:

• Cobb angle greater than 20-30 degrees AND progressing.
• FVC greater than 35% (some say greater than 30%) - Below this, perioperative risk is very high.

Surgical Technique:

• Posterior Spinal Fusion (PSF): T2/T3 to Pelvis (Iliac or S1 Alar-Iliac screws).
• Instrumentation: Segmental pedicle screws.
• No Anterior Release: Usually not needed.

Perioperative Considerations:

• Anesthesia: TIVA (Total Intravenous Anesthesia). Avoid succinylcholine and volatiles (MH risk).
• Blood Loss: Expect significant. Cell salvage, Tranexamic acid.
• ICU: Often require post-op ICU. May need prolonged NIV.

Outcomes:

• Halts scoliosis progression.
• Maintains sitting balance.
• May improve respiratory function or slow decline.

After Spinal Fusion:

• ICU admission (often 24-48 hours).
• NIV if respiratory compromise.
• Early mobilization to wheelchair (Day 2-3).
• Wound care.
• No brace required (rigid instrumentation).

After Achilles Lengthening:

• Cast 4-6 weeks.
• AFOs after cast removal.
• Intensive physiotherapy.

• With Modern Care: Life expectancy now 20-30 years (previously teens).
• Steroids: Prolong ambulation ~2 years. Delay scoliosis.
• Spinal Fusion: Halts scoliosis progression. Maintains sitting posture. May improve QOL.

• Neuromuscular Clinics: MDT clinics at major pediatric hospitals (CHW, RCH, LCCH).
• PBS: Steroids (Deflazacort/Prednisolone) are PBS-subsidized for DMD.
• NDIS: Supports equipment, wheelchair, home modifications, therapy.
• Clinical Trials: Australia participates in gene therapy and exon-skipping trials.