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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
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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)
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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
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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
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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
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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
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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
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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
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mitiochondrial myopathies
co-occurance of a myopathy with a peripheral neuropathy. serum lactate and pyruvate often increased
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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
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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
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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
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steroid myopathy
due to inreased glucocorticoids
weakness proximal, wparing of ocular, bulbar, and facial muscles, ck and emg normal
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necrotizing myopathy
drug/toxin -HMG-CoA reductase inhibitors, cyclosporin, propofol, alcohol
clinical - proximal muscle weakness, ck elevated
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mitochondrial myopathy (drug induced)
drug/toxin - zidovudine
clinical - proximal muscle weakness, ck normal or mildly elevated
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drug induced inflammatory myopathy
drug/toxin - cimetidine, procainamide, l-dopa, phenytoin, lamotrigine, d-penicillamine
clinical - proximal muscle weakness, CK increased
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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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