Updated: 1/17/2019

Duchenne Muscular Dystrophy

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Introduction
  • 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
Evaluation
  • 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
 
Treatment
  • 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 abduction contractures treated by release of iliotibial band 
        • Hip flexion contractures treated by release of sartorius, rectus femoris, and tensor fascia lata
        • hamstring releases
        • Achilles tendon and posterior tibialis lengthenings
      • postoperative care
        • early mobilization and ambulation to prevent deconditioning
    • scoliosis surgery (see below)
Scoliosis
  • 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 20-30° in nonambulatory patient 
            • treat early before pulmonary function declines
            • can wait longer ~ 40° if responding well to corticosteroids
          • FVC drops ≤ 35%
          • rapidly progressive curve
      • PSF with instrumentation to pelvis
        • indications
          • curves ≥ 40°
          • pelvic obliquity ≥ 10°
          • lumbar curve where apex is lower than L1
        • 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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Questions (19)
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(SBQ13PE.67) A 4-year-old boy presents to your clinic for evaluation of 'clumsiness.' His parents state that he began walking at 30 months of age. They note that he is not able to keep up with his peers in athletic activities. He is unable to climb steps without the aid of a handrail. On exam, he stands with increased lumbar lordosis and a wide based stance. When asked to rise from a seated position on the floor, the child first rolls into a crawling position on all four limbs, stands up by extending his knees and uses his upper extremities bracing against his thighs to help elevate his trunk. You confirm the diagnosis with serum enzyme assay followed by DNA analysis. The parents ask you what the chances are of their child passing on this genetic disease to his male offspring. Review Topic

QID: 5175
1

0%

62%

(1097/1781)

2

25%

7%

(131/1781)

3

50%

20%

(356/1781)

4

75%

1%

(14/1781)

5

100%

10%

(173/1781)

L 4

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(SBQ13PE.36) Which of the following is true regarding scoliosis in patients with untreated Duchenne muscular dystrophy (DMD)? Review Topic

QID: 5051
1

Small curves are likely to remain stable over time

2%

(44/2190)

2

50% of all patients with DMD will develop scoliosis

18%

(393/2190)

3

Spinal fusion should be offered when the Cobb angle reaches 20 to 30 degrees

69%

(1505/2190)

4

Scoliosis in DMD is responsive to bracing

2%

(53/2190)

5

Scoliosis surgery is ideally performed at skeletal maturity patients with DMD

9%

(187/2190)

L 3

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(OBQ13.199) Which of the following is most commonly inherited as a X-linked recessive disorder? Review Topic

QID: 4834
1

Charcot-Marie-Tooth disease

6%

(199/3494)

2

Marfan's syndrome

2%

(77/3494)

3

Larsen's syndrome

2%

(73/3494)

4

Duchenne's muscular dystrophy

83%

(2913/3494)

5

Turner's syndrome

6%

(217/3494)

L 2

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(SAE13BS.10) A 5-year-old boy has a history of delayed walking and difficulty climbing stairs. Physical examination shows a positive Gower sign and scoliosis. Laboratory tests show elevated serum creatine phosphokinase. A mutation of what protein results in this disorder? Review Topic

QID: 8242
1

Dystrophin

97%

(148/153)

2

Collagen type I

1%

(1/153)

3

Neurofibromin

1%

(1/153)

4

Runt-related transcription factor 2 (Runx2)

1%

(2/153)

L 1

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(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? Review Topic

QID: 3137
1

Given the poor prognosis, surgical treatment is not appropriate

2%

(49/2533)

2

Posterior spinal fusion should be performed before curve progresses beyond 30 degrees

70%

(1764/2533)

3

Posterior spinal fusion should be performed if the curve progresses to greater than 55 degrees

12%

(310/2533)

4

Posterior spinal fusion should be performed if FVC drops below 60%

10%

(242/2533)

5

With appropriate bracing, curve progression and surgical treatment will most likely be unnecessary

6%

(158/2533)

L 2

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

QID: 2944
1

autosomal recessive

4%

(51/1179)

2

autosomal dominant

6%

(74/1179)

3

X-linked recessive

77%

(903/1179)

4

X-linked dominant

13%

(149/1179)

5

random

0%

(2/1179)

L 2

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(SAE07PE.60) Figure 24 shows the sitting AP and lateral spinal radiographs of a nonambulatory 12½-year-old boy with Duchenne muscular dystrophy who is being evaluated for scoliosis. The lumbar curve from T12 to L5 measures 36 degrees, and the thoracic curve from T3 to T12 measures 24 degrees on the AP radiograph. He has 5 degrees of pelvic obliquity. His forced vital capacity is 45% of predicted for height and weight. What is the most appropriate treatment for the spinal deformity? Review Topic

QID: 6120
FIGURES:
1

Posterior spinal fusion from T2 to L5 with segmental instrumentation

45%

(61/137)

2

Anterior spinal fusion from L1 to L4, followed by posterior spinal fusion from T2 to the sacrum with segmental instrumentation including iliac fixation

18%

(24/137)

3

Custom-molded spinal orthosis worn 23 hours per day until skeletal maturity

18%

(25/137)

4

A spinal orthosis until age 14 years, followed by posterior spinal fusion with segmental instrumentation

7%

(10/137)

5

Adapted wheelchair seating with a custom-molded back support to correct scoliosis and kyphosis

12%

(16/137)

L 4

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(SAE07PE.43) Duchenne’s muscular dystrophy is a genetic disorder that is transmitted by which of the following modes of inheritance? Review Topic

QID: 6103
1

X-linked

90%

(115/128)

2

Autosomal-dominant

5%

(7/128)

3

Autosomal-recessive

3%

(4/128)

4

Chromosomal duplication

1%

(1/128)

5

Chromosomal deletion

1%

(1/128)

L 1

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(SBQ04PE.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? Review Topic

QID: 2232
1

Pseudohypertrophy of the calves, markedly increased creatine kinase levels, and abnormal dystrophin

78%

(1613/2061)

2

Proximal muscle weakness occuring early in childhood, absent deep tendon reflexes and fasciculations

8%

(159/2061)

3

Mildly elevated creatine kinase levels, severe limb contractures present at an early age

3%

(67/2061)

4

Acute onset of weakness, hypotonia, and arefelxia resulting from postinfectious demyelination of the peripheral nerves

2%

(31/2061)

5

X-linked transmission, absence of dystrophin, and significantly increased creatine kinase levels

9%

(176/2061)

L 2

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