USMLE Step 1 Web Prep — Skeletal Muscle and Peripheral Nerve
Pathology
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SLIDE 1 of 9
SKELETAL MUSCLE
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SLIDE 2 of 9
INFLAMMATORY MYOPATHIES
Polymyositis
Clinical
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Adults
Bilateral proximal muscle weakness
o Shoulder + pelvic girdle
Micro
o
Lymphocytic inflammation- endomysial
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INFLAMMATORY MYOPATHIES CON’T
Dermatomyositis
Clinical
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Micro
Children or adults
Bilateral proximal muscle weakness
“Heliotrope rash”- periorbital
Periorbital edema
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Perimysial + vascular lymphocytic inflammation
Perifascicular fiber atrophy
Fiber degeneration + regeneration
Increased risk of lung, stomach, + ovarian cancers
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MYASTHENIC SYNDROMES
Myasthenia gravis
Definition
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Autoimmune
Autoantibodies against neuromuscular junction
muscular weakness
Clinical
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Females
Facial muscles
Proptosis + diplopia
Weakness worsens with use
Respiratory involvement
death
Mechanism
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Autoantibodies against ACh receptor
 Treatment
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Anticholinesterase agents, steroids + thymectomy
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SLIDE 5 of 9
MYASTHENIC SYNDROMES CON’T
Eaton- Lambert syndrome
 Paraneoplastic syndrome of small cell lung cancer
 Mechanism- autoantibodies against calcium channel
 Clinical
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Proximal muscle weakness- hip flexers
Weakness improves with use
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SLIDE 6 of 9
MUSCULAR DYSTROPHY
Duchenne’s muscular dystrophy
Genetics
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X-linked
Dystrophin gene on X chromosome
Dystrophin protein
muscle structural protein
Mutation
absence of dystrophin protein
Clinical
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Normal at birth
Onset by age 5
Progressive ascending muscular weakness
Respiratory insufficiency
Calf pseudohypertrophy
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SLIDE 7 of 9
DUCHENNE’S CON’T
Lab
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Elevated creatine kinase
Micro
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Necrosis, degeneration + regeneration of fibers
Fatty infiltration
Diagnosis
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DNA analysis by PCR
Muscle biopsy
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SLIDE 8 of 9
BECKER’S MUSCULAR DYSTROPHY
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Less common + severe than Duchenne’s
Mutation
altered dystrophin protein
Later onset
Rare cardiac involvement
Normal life span
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SLIDE 9 of 9
GUILLAIN-BARRE SYNDROME
Clinical
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Preceded by virus
Muscular weakness
ascending paralysis
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Loss of DTRs
Pathology
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Inflammation + demyelination of peripheral nerves + spinal nerve roots
Diagnosis
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Nerve conduction studies
LP- elevated protein
Prognosis
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5% fatal- respiratory paralysis