Duchenne Muscular Dystrophy

Topic updated on 03/20/16 6:35pm
  • Disorder of worsening neurologic dysfunction characterized by progressive muscle weakness
    • caused by absent dystrophin protein
  • Epidemiology
    • demographics
      • prevalence is 2-3/10,000
      • affects young males only
      • age of onset is between 2-6 years of age
  • Pathophysiology
    • dystrophin absence leads to 
      • poor muscle fiber regeneration
      • progressive replacement of muscle tissue with fibrous and fatty tissue
    • skeletal and cardiac muscle lose elasticity and strength
  • Genetics
    • X-linked recessive   
    • Xp21.2 dystrophin gene defect due to point deletion and nonsense mutation
    • one third of cases result from spontaneous mutations
  • Associated conditions
    • orthopaedic manifestations
      • calf pseudohypertrophy 
      • scoliosis
      • equinovarus foot deformity
      • joint contractures
    • nonorthopaedic conditions
      • cardiomyopathy
      • static encephalopathy
    • Becker's Muscular Dystrophy  
      • similar to Duchenne's in that
        • it is sex-linked recessive
        • calf pseudohypertrophy is present
        • CPK is elevated
      • differs from Duchenne's in that
        • dystrophin protein is decreased instead of absent
        • later onset with slower progression and longer life expectancy (average diagnosis occurs at age 8 compared to 2 years of age with Duchenne's) 
        • more prone to cardiomyopathy
  • Prognosis
    • most are unable to ambulate independently by age 10
    • most are wheelchair dependent by age 15
    • most die of cardiorespiratory problems by age 20
Physical Exam
  • Symptoms
    • progressive weakness affecting proximal muscles first (begins with gluteal muscle weakness)
    • gait abnormalities
      • delayed walking
      • toe walking
      • clumsy, waddling gait
      • difficulty climbing stairs, hopping, or jumping
    • decreased motor skills
  • Physical exam
    • calf pseudohypertrophy (infiltration of normal muscle with connective tissue)
    • deep tendon reflexes present (unlike spinal muscular atrophy) 
    • lumbar lordosis
      • compensates for gluteal weakness
    • Gower's sign  
      • rises by walking hands up legs to compensate for gluteus maximus and quadriceps weakness
    • Trendelenburg sign
  • Labs
    • markedly elevated CPK levels (10-200x normal) 
      • CPK leaks across defective cell membrane 
  • Muscle biopsy
    • will show connective tissue infiltration and foci of necrosis
    • will show absent dystrophin with staining 
  • DNA testing
    • shows absent dystrophin protein
  • EMG
    • myopathic
      • decreased amplitude, short duration, polyphasic motor
Differential Diagnosis
Similar traits to Duchenne's
Distinguishing traits from Duchenne's
Becker's • calf pseudohypertrophy
• markedly elevated CPK
• x-linked transmission 
• Becker's has slower progression of weakness with diagnosis made later (~8 yrs)
• prone to cardiomyopathy
Spinal muscular atrophy • proximal weakness • onset of weakness is earlier in childhood
• absent deep tendon reflexes and fasciculations 
CPK levels are normal
• pseudohypertrophy is absent
Emery-Dreifuss dystrophy • similar clinical picture • no calf pseudohypertrophy
• CPK levels near normal 
• elbow and ankle contractures develop early
Limb girdle dystrophy • progressive motor weakness • no calf pseudohypertrophy
• CPK levels are only mildly elevated
Guillain-Barre syndrome • acute onset of weakness • absent deep tendon reflexes
• CPK levels are normal
  • Nonoperative
    • corticosteroid therapy (prednisone 0.75 mg/kg/day)
      • indications
        • 5 to 7-year-old child with progressive disease
      • goals
        • to maintain ambulatory capacity as long as possible
      • outcomes
        • significant positive effect on disease progression
          • acutely improves strength, slows progressive weakening, prevents scoliosis formation, and prolongs ambulation
          • delays deterioration of pulmonary function
      • side effects
        • osteonecrosis
        • weight gain
        • cushingoid appearance
        • GI symptoms
        • mood lability
        • headaches
        • short stature
        • cataracts
    • pulmonary care with nightly ventilation
    • rehabilitation
      • techniques
        • physical therapy for range of motion exercises
        • adaptive equipment
        • power wheelchairs
        • KAFO bracing (controversial)
  • Operative
    • soft tissue releases to prolong ambulation
      • indications
        • ambulatory child with Duchenne's
      • techniques
        • hip abductor and hamstring releases
        • Achilles tendon and posterior tibialis lengthenings
      • postoperative care
        • early mobilization and ambulation to prevent deconditioning
    • scoliosis surgery (see below)
  • Introduction
    • considered a neurogenic curve
    • occurs in 95% of patients after becoming wheelchair dependent
    • curve progresses rapidly from age 13 to 14 years
      • begins with mild hyperlordosis
      • progresses with general kyphosis and scoliosis with varying degrees of pelvic obliquity
      • progresses 1° to 2° per month starting at age 8 to 10 years
    • patients may become bedridden by age 16
    • treatment is complicated by restrictive pulmonary disease (significant decrease in forced vital capacity)
    • cardiac and pulmonary function studies should be obtained pre-operatively as significant declines in function of both organ systems may make spinal fusion too high-risk 
  • Treatment
    • nonoperative
      • bracing is contraindicated
        • may interfere with respiration
    • operative  
      • early PSF with instrumentation 
        • indications
          • curve > 20° in nonambulatory patient (treat early, < 30° curve, before pulmonary function declines) 
            • can wait slightly longer (Cobb ~ 40°) if patient is responding well to corticosteroids
          • FVC drops below 35%
          • rapidly progressive curve
        • techniques
          • extension to pelvis is controversial
        • complications
          • malignant hyperthermia is common intraoperatively
            • pretreat with dantrolene
          • intraoperative cardiac events
      • anterior and posterior spinal fusion
        • indications
          • rarely for stiff curves
Equinovarus Foot
  • Introduction
    • common foot deformity seen with Duchenne muscular dystrophy
  • Pathoanatomy
    • muscle imbalance secondary to muscle replacement with fibrofatty tissue
  • Diagnosis
    • made upon clinical exam
  • Treatment
    • nonoperative
      • stretching, physical therapy, and night time AFO use
    • operative
      • Tendinoachilles lengthening with posterior tibialis tendon transfer, toe flexor tenotomies


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Qbank (12 Questions)

(SBQ04.47) An 11-year-old boy is evaluated after reporting progressive difficulty walking. After extensive workup, he is diagnosed with Becker muscular dystrophy. Which of the following best describes the characteristic findings associated with this diagnosis? Topic Review Topic

1. Pseudohypertrophy of the calves, markedly increased creatine kinase levels, and abnormal dystrophin
2. Proximal muscle weakness occuring early in childhood, absent deep tendon reflexes and fasciculations
3. Mildly elevated creatine kinase levels, severe limb contractures present at an early age
4. Acute onset of weakness, hypotonia, and arefelxia resulting from postinfectious demyelination of the peripheral nerves
5. X-linked transmission, absence of dystrophin, and significantly increased creatine kinase levels

(OBQ10.49) A 9-year-old boy with Duchenne muscular dystrophy has increasing difficulty with ambulation. He denies back pain, difficulty sitting in a chair, or shortness of breath. Annual screening spine radiographs demonstrate a 20 degree thoracolumbar curve. Which of the following statements best describes the appropriate treatment plan for his scoliosis? Topic Review Topic

1. Given the poor prognosis, surgical treatment is not appropriate
2. Posterior spinal fusion should be performed before curve progresses beyond 30 degrees
3. Posterior spinal fusion should be performed if the curve progresses to greater than 55 degrees
4. Posterior spinal fusion should be performed if FVC drops below 60%
5. With appropriate bracing, curve progression and surgical treatment will most likely be unnecessary

(OBQ09.131) What is the inheritance pattern for Duchenne's muscular dystrophy? Topic Review Topic

1. autosomal recessive
2. autosomal dominant
3. X-linked recessive
4. X-linked dominant
5. random

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