Flashcards in Myasthenia Gravis Deck (22)
What is myasthenia gravis?
autoimmune disorder of neuromuscular transmission
What are the hallmarks of myasthenia gravis?
fluctuating degree and variable combination of weakness in ocular, bulbar, limb, and respiratory muscles
What is the cause of myasthenia gravis?
antibody-mediated T-cell dependent immunologic attack directed at proteins in the postsynaptic membrane of the neuromuscular junction (acetylcholine receptors)
What are common bedside tests for myasthenia gravis?
ice pack test and edrophonium test
What is the ice pack test?
used for patients with ptosis - bag of ice is place on closed eyelid for 2 minutes and then removed - improvement in ptosis is a positive response
What is the edrophonium test?
Tensilon test - used for patients with ptosis or ophthalmoparesis - 1 mL of edrophonium (10 mg/mL) is drawn up and administered IV - improvement in symptoms after administration of 4-6 mg is a positive response
What is edrophonium chloride (Tensilon)?
acetylcholinesterase inhibitor with rapid onset (30-45 seconds) and short duration of action (5-10 minutes) that prolongs presence of acetylcholine in the neuromuscular junction and results in immediate increase in muscle strength (AEs: cardiac decrease and bronchial asthma)
What are the serologic indicators for myasthenia gravis?
autoantibodies against the acetylcholine receptor (AChR-Ab) or receptor-associated protein, muscle specific tyrosine kinase (MuSK-Ab)
What is the relationship between AChR-Ab and MuSK-Ab in myasthenia gravis?
those positive for AChR-Ab do not have antibodies to MuSK-AB, but nearly half of those who are negative for AChR-Ab are positive for MuSK-Ab (latter less likely to have thymic pathology)
What is seronegative myasthenia?
patients who have negative standard assays for both AChR and MuSK antibodies - more likely to have purely occular disease - respond to pyridostigmine, plasma exchange, glucocorticoids, immunosuppressive therapy, thymectomy
What types of electrodiagnostic studies are used to confirm diagnosis of myasthenia gravis?
repetitive nerve stimulation and single-fiber electromyography
What is repetitive nerve stimulation?
placement of recording electrode over endplate of a muscle and stimulating the motor nerve to that muscle (6-10 times at 2-3 Hz) - progressive decline (> 10%) in compound muscle action potential is consistent with myasthenia gravis
What is single fiber electromyography?
most sensitive diagnostic test for myasthenia - allows simultaneous recording of action potentials of two muscle fibers innervated by same motor axon (must be a limb and a facial muscle) - variability of second action potential relative to the first is called "jitter" - increased jitter consistent with myasthenia gravis
What are the most common thymic disorders associated with myasthenia gravis?
thymic hyperplasia and thymic tumors (thyoma)
What other autoimmune disorders are commonly seen in patients with myasthenia gravis?
rheumatoid arthritis and systemic lupus erythematous
What are the clinical forms of myasthenia gravis?
ocular (weakness limited to eyelids and extraocular muscles) and generalized (weakness affects ocular, bulbar [responsible for speech and swallowing], limb, and respiratory muscles)
What are the clinical features of myasthenia gravis?
muscle weakness (not fatigue) worse in the evening - ptosis (pupils are always spared), diplopia, dysarthria, dysphagia, fatigable chewing, proximal limb weakness
What is the curtain sign?
ptosis that increases with sustained upward gaze or by holding up the opposite eyelid with the examiner's finger
What is the myasthenic sneer?
weakness of facial muscles causes mid-lip to rise but outer corners of mouth fail to move
What is the dropped head syndrome?
weight of head overcomes weakness in neck muscles
What is myasthenic crisis?
respiratory muscle weakness that leads to respiratory insufficiency and pending respiratory failure