Muscular dystrophy

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Muscular dystrophy (MD) – differs from myositis (e.g. polymyositis or dermatomyositis) in that myositis does not have any cardiac effects, CK levels are much higher in MD, IQ is rarely affected in myositis and MD has progressive worsening of symptoms while myositis patients typically get better with steroids

  • i. Duchenne MD
    • 1. X-linked recessive disorder due to a mutation of the gene for dystrophin occurring in 1:3000 male infants; female carriers are generally normal but some may have weakness due to Lyonization; patients have absence of dystrophin
      • a. For a girl to have Duchenne MD she must have Turner’s syndrome
    • 2. symptoms: broad based waddling gait with exaggerated lumbar lordosis (Gower’s sign); delayed developmental motor milestones; very prone to contractures
    • 3. muscles innervated by the cranial nerves are normal
    • 4. 90% have EKG abnormalities with tall precordial R waves in V1 and deep narrow Q waves in V6 due to intratrial conduction system defects
    • 5. average IQ is one standard deviation below the mean
    • 6. respiratory failure occurs in second or third decade
    • 7. CK levels often 50-100 times normal during the first 3 years of life
    • 8. microscopically see type I fiber predominance with connective tissue proliferation, fat infiltration, endomysial fibrosis
  • ii. Becker’s MD
    • 1. also caused by abnormalities in the dystrophin gene but patients have some dystrophin as opposed to Duchenne where the patients have none; X-linked
    • 2. clinical features similar to Duchenne but less severe with proximal muscle weakness occurring after leg weakness
    • 3. typically present before age 10 but may survive into the fourth to fifth decade; congestive heart failure or mental retardation is rare but does occur
    • 4. CK levels 20-100 times normal
  • iii. Limb-Girdle MD
    • 1. may be autosomal recessive, autosomal dominant or X-linked
    • 2. onset is late in the fourth or fifth decade
    • 3. clinically see slow progressive weakness in either the shoulder (winging of the scapula) or the pelvic girdle
    • 4. 10% associated with cardiomyopathy
    • 5. CK levels elevated but less than 10 times normal
  • iv. Oculopharyngeal MD
    • 1. autosomal dominant
    • 2. caused by expansion of GCG repeats encoding polyalanines in the N-terminus of polyadenylate-binding protein on chromosome 14q
    • 3. clinical features: ptosis, incomplete extraocular muscle weakness, dysphagia, dysphonia
    • 4. CK levels 3 times normal
  • v. Fascioscapulohumeral MD
    • 1. autosomal dominant in 90% of cases linked to chromosome 4q
    • 2. onset first to fifth decade
    • 3. third most common MD after Duchenne and Becker’s
    • 4. clinical features: weakness of facial muscles with functional changes such as sleeping with eyes open, difficulty using straws, dysarthria; also associated with leg weakness and scapular winging; labile hypertension and sensorineural deafness also noted; may have congential absence of involved muscle
    • 5. CK levels may be normal or mildly elevated
  • vi. Myotonic dystrophy
    • 1. most common adult MD with males and females affected equal incidence
    • 2. autosomal dominant on chromosom 19q; gene codes for myotonin protein kinase with up to 3000 CTG repeats
    • 3. onset in any decade – adults typically present with weakness and cataracts
    • 4. distal>>proximal weakness
    • 5. clinically see cardiac conduction defects, impaired ventilator drive, dysphagia, MR, hydrocephalus, hypogonadism, frontal bossing, high arched palate, cataracts, difficulty relaxing their grip, hypersomnolence
      • a. EMG typically has a ‘dive bomber’ pattern because of the characteristic sound produced with evoked potentials – i.e. repetitive discharges are heard with just mild stimulation
    • 6. CK levels normal to slightly elevated
    • 7. muscle biopsy typically has type I fiber atrophy
  • vii. Emery-Dreifuss dystrophy – a benign X linked dystrophy which includes contractures of the elbow flexors, neck extensor, and posterior calf muscles
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