Neuro Shelf - skeletal muscle

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ashboeri
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102323
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Neuro Shelf - skeletal muscle
Updated:
2011-09-17 21:17:13
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neurology
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  1. How muscles are simulated to contract
    At the presynaptic bouton, secretory vesicles containing Ach concentrated at active zones formed by P/Q type voltage-gated Ca channels, which leads to Ca influx, which triggers release of Ach

    Ach binds at the postsynaptic cleft, which generates an inward Na current through the Ach-receptor ion pore, which leads to depolarization of the muscle endplate
  2. myasthenia gravis
    antibodies directed against the postsynaptic nicotinic Ach receptor

    bimodal peak (20-30, 80-90)

    earlier -onset MG associated with thymic hyperplasia, whereas thymoma associated with pts with later onset

    fatigable muscle weakness, ocular involvment (pstosis, diplopia), bulbar muscle weakness (dysarthria or dysphasia), limb weakness is proximal and symmetric

    be sure to check pulmonary function bc even those who don't look like it could be in crisis

    Dx with edrophonium chloride (anti-choniesterase agent) should help

    Tx with pyridostigmine (antiacetylcholinesterase)
  3. lambert-eaton syndrome
    *associated with small cell carcinoma of the lung*

    antibodies directed against the presynaptic P/Q-type voltage-gated Ca channel, reducing release of Ach

    fatigable proximal muscle weakness, reduced DTRs. Unlike MG, bulbar and ocular symptoms rare, but autonomic complaints (dry eyes, dry mouth, impotence) common. Muscle strenth increases with brief intense exercise
  4. duchenne musclular dystrophy, becker muscular dystrophy
    mutations of dystrophin (allelic), x-linked inheritance (usually in young boys)

    onset in childhood, may use an arm to push down on thighs when arising from floor (Gowers sign), pseudohypertrophy of calf muscles, proximal muscle weakness (including neck flexors), usually sparing of ocular and bulbar muscles

    in DMD, death around age 20 due to respiratory insufficiency and aspiration

    CK very elevated (normal CK strongly against diagnosis), muscle biopsy shows dystrophic features with absent or reduced staining for dystrophin
  5. limb-girdle muscular dystrophies
    proximal muscles of arms and legs affected predominantly, sparing of other structures. Shoulder and hip girdle weakness, may require muscle biopsy to distinguish from muscular dystrophy

    CK usually elevated
  6. myotonic dystrophy
    most common inherited skeletal muscle disorder affecting adults. inheritance is AD - genetic defect either unstable CTG expansion in DMPK gene or CCTG expansion in the ZNF9 gene

    weakness and stiffness of distal muscles present in young adults, action and percussion myotonia, proximal weakness develops later. Cataracts, ptosis, arrhythmias, dysphagia, insulin resistance, testicular atrophy, frontal balding, neurobehavioral changes

    CK normal or mildly elevated, Cardiac evaluation important to prevent arrhythmias

    Phenytoin and carbamazepine may reduce myotonia
  7. emery-dreifuss muscular dystrophy
    mutations in emerin (transnuclear membrane protein) gene on x-chromosome (AD form rare, mutations in lamin A and C genes on chromosome 1)

    early onset of joint contractures (elbows, ankles, c-spine), slowly pregressive humeroperoneal pattern of weakness and atrophy, and cardiomyopathy (conduction abnormalities). Key is appearance of contractures prior to onset of weakness)

    although femal carriers do not develop weakness or atrophy, they are at risk for the cardiac complications

    CK mildly to moderately elevated
  8. Periodic paralyses
    AD conditions with episodic muscle weakness. attacks of weaknessassociated with change in serum K concentration

    hypokalemic pp - mutation in pore-forming alpha IS-subunit of skeletal muscle Ca channel that causes secondary dysfunction of the NA,K ATPase

    hyperkalemic pp - mutation in skeletal muscle voltage-gated Na channel
  9. mitiochondrial myopathies
    co-occurance of a myopathy with a peripheral neuropathy. serum lactate and pyruvate often increased
  10. inflammatory myopathies
    polymyositis - proximal muscle weakness, often associated with other autoimmune diseases (crohns disease, vasculitis, sarcoidosis, MG, etc). mediated by CD8 T lymphocytes

    dermatomyositis - antibodies against intramuscular blood vessels, inflammation due to muscle ischemia. proximal muscle weakness. purplish discoloration of eyelids (heliotrope) and papular erythematous scaly lesions over the knuckles (Gottron patches). Cardiac manifestations, interstitial lung disease (anti-Jo-1 antibodies), increased risk of malignancy

    inclusion body myositis - inflammatory and neurodegenerative. assymmetric, both proximal and distal muscles (early selective involvement of forearm and finger flexors as well as knee extensors and ankle extensors. usually diagnosed when pts thought to have PM fail to respond to steroids . Endomysial inflammation and basophilic rimmed vacuoles

    in all - CK elevated (amount doesn't correlate with degree of weakness), muscle pain and weakness
  11. Thyrotoxic myopathy
    proximal muscle weakness and atrophy with normal or brisk reflexes, ck normal

    tx underlying thyrotoxic state with beta-blockers, will improve muscle strength
  12. hypothyroid myopathy
    proximal muscle weakness, fatigue, myalgias, and cramps. muscle reflexes may show delayed relaxation, associated distal polyneuropathy

    CK elevated, weakness improves following thyroid replacement
  13. steroid myopathy
    due to inreased glucocorticoids

    weakness proximal, wparing of ocular, bulbar, and facial muscles, ck and emg normal
  14. necrotizing myopathy
    drug/toxin -HMG-CoA reductase inhibitors, cyclosporin, propofol, alcohol

    clinical - proximal muscle weakness, ck elevated
  15. mitochondrial myopathy (drug induced)
    drug/toxin - zidovudine

    clinical - proximal muscle weakness, ck normal or mildly elevated
  16. drug induced inflammatory myopathy
    drug/toxin - cimetidine, procainamide, l-dopa, phenytoin, lamotrigine, d-penicillamine

    clinical - proximal muscle weakness, CK increased
  17. critical illness myopathy
    drug/toxin - corticosteroids plus neuromuscular blocking agents in pts with sepsis

    clinical - generalized weakness, ck normal or elevated

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