Dr. Kevin Yip

Dr Kevin Yip
Orthopaedic Surgeon
MBBS(UK), FRCS(EDIN), FAM(SING), FHKCOS(ORTHO)

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Muscular Dystrophies

Basics
Description
  • Muscular dystrophies are a group of inherited disorders characterized by progressive degeneration and weakness of skeletal muscle without apparent cause in the nervous system.
  • Skeletal and cardiac muscles are affected, and secondary effects occur in the lungs, skeleton, and many other systems.
  • These conditions have been categorized by clinical distribution, severity of muscle weakness, and pattern of genetic inheritance.
  • Because of limited space, only Duchenne muscular dystrophy is described in detail.
  • Classification:
    • Sex-linked muscular dystrophy: Duchenne muscular dystrophy, Becker, Emery-Dreifuss
    • Autosomal-recessive muscular dystrophy: Limb-girdle, infantile fascioscapulohumeral
    • Autosomal-dominant muscular dystrophy: Fascioscapulohumeral, distal, ocular, oculopharyngeal
Epidemiology
Incidence
  • Duchenne muscular dystrophy occurs in young boys.
  • Duchenne muscular dystrophy occurs in 1 in 3,500 live male births.
  • Becker dystrophy occurs in ~1 in 30,000 live male births.
Risk Factors
Male gender
Genetics
  • Duchenne muscular dystrophy is sex-linked, as is Becker-type tardive dystrophy.
  • Other dystrophies are autosomal recessive and autosomal dominant.
Etiology
  • A single gene defect in the short arm of the X chromosome has been identified as being responsible for Duchenne muscular dystrophy and Becker muscular dystrophy.
    • The gene encodes the protein dystrophin, which is a component of the cell membrane cytoskeleton.
Diagnosis
Signs and Symptoms
  • Duchenne muscular dystrophy:
    • The disease occurs only in males, and it usually becomes evident at 3-6 years of age.
    • Common presentations include:
      • Delayed walking
      • Waddling, Trendelenburg gait, or lordotic gait
      • Frequent tripping and falling
      • Inability to hop and jump
    • Progressive weakness occurs in the proximal muscle groups, including the gluteus, quadriceps, abdominal muscles, and shoulder girdle muscles
    • Pseudohypertrophy and contracture of calf muscles is common.
    • Most patients have cardiac involvement, most commonly tachycardia and right ventricular hypertrophy.
    • Many also have static encephalopathy with mental retardation.
    • Death from pulmonary and cardiac failure occurs during the 2nd or 3rd decade of life.
    • Because of hip muscle weakness, patients compensate by carrying the head and shoulders behind the pelvis during gait, thus producing an anterior pelvic tilt and increased lumbar lordosis.
    • Weakness in the shoulder girdle occurs 3-5 years after presentation.
      • It is difficult to lift the patient under the arms because of the weakness.
      • This weakness has been termed the Meryon sign.
    • No sensory deficits are detected.
    • Children usually are unable to ambulate effectively beyond 10 years of age.
  • Becker muscular dystrophy:
    • Similar to Duchenne muscular dystrophy in clinical appearance and distribution of weakness, but less severe
    • The onset usually occurs after the age of 7 years.
    • The rate of progression is slower than in Duchenne muscular dystrophy
  • Many more types of muscular dystrophy exist (not described here).
Physical Exam
  • History, physical examination, measurement of creatine phosphokinase and dystrophin, and electromyography help in making the diagnosis.
  • Electromyography shows a myopathic pattern, with reduced amplitude, short duration, and polyphasic muscle action potentials.
  • Muscle biopsy also may be performed.
  • Evaluate muscle bulk to assess for pseudohypertrophy of the calves.
  • Observe the patient’s gait and look for Trendelenburg gait.
  • Starting proximally, look for muscle weakness.
  • Evaluate the patient’s ability to stabilize the shoulder; test for Meryon sign.
  • Note contracture, developing later, followed by scoliosis.
Tests
Lab
  • Serum creatine phosphokinase markedly is elevated in the early stages of Duchenne muscular dystrophy.
    • It may be 200 times normal, but it later declines as muscle degeneration becomes complete.
  • Dystrophin levels are completely absent in Duchenne muscular dystrophy; they are less than normal in Becker dystrophy.
Pathological Findings
  • Muscle degeneration, with subsequent loss of fibers
  • Variation in fiber size
  • Proliferation of connective tissue
Differential Diagnosis
  • Peripheral neuropathy
  • Anterior horn cell disease
  • Poliomyelitis
Treatment
General Measures
Most patients with Duchenne muscular dystrophy die in their 2nd or 3rd decade of life; therefore, orthopaedic treatment should be designed to improve or maintain the functional capacity of the involved adolescent.
 
Activity
  • No restrictions on activity.
  • Activity is to be encouraged as much as possible.
Special Therapy
Physical Therapy
  • Test muscle strength to assess the rate of deterioration.
  • Use ankle-foot orthoses for correctable deformities.
  • The best treatment for fractures is closed reduction and immobilization.
  • Fractures of the lower extremities occur frequently in children with Duchenne muscular dystrophy, especially in children who are wheelchair bound.
  • Contractures of both lower and upper extremities may occur.
    • Surgical release of contractures sometimes is indicated to improve function.
  • ~95% of patients with Duchenne muscular dystrophy develop progressive scoliosis.
    • Surgical correction of scoliosis improves sitting balance and minimizes pelvic obliquity.
    • Posterior spinal fusion is recommended for curves of >20-30°.
  • Programs of vigorous respiratory therapy and the use of home negative-pressure and positive-pressure ventilators may promote life extension.
  • Proper diagnosis and early genetic counseling may help parents to be aware of the risk of additional male infants with Duchenne muscular dystrophy.
Medication
  • No drugs have been proved effective.
  • Steroids have some benefit (delaying scoliosis and prolonging function), but they also are associated with long-term problems, including weight gain and osteoporosis.
Surgery
  • Contracture release (Achilles, fascia lata) may be indicated.
  • Correction of scoliosis involves fusion of nearly the entire thoracic and lumbar spine (T2-L5 or sacrum).
    • Rods are used to straighten and hold the spine.
    • This intervention should be performed for curves of >30°.
Follow-up
Prognosis
  • Duchenne muscular dystrophy is fatal in the 2nd or 3rd decade of life.
  • Becker dystrophy is more slowly progressive, and life expectancy is greater.
Complications
  • Respiratory failure
  • Cardiac failure
  • Fracture
  • Scoliosis
Patient Monitoring
Patients must be followed frequently (every 4-6 months) by a neurologist to assess their progression.
 
Miscellaneous
Codes
ICD9-CM
359.1 Duchenne muscular dystrophy
 
Patient Teaching
Genetic counseling is important, to warn of the risk of additional affected infants.
FAQ
Q: What is the benefit of scoliosis surgery in patients with Duchenne muscular dystrophy?
A: It improves sitting balance and prevents discomfort that develops as the spine collapses.

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