Myasthenia Flashcards
Test of choice to differentiate between myopathy and NMJ disorder
EMG/NCS
(electromyography / nerve conduction study)
Classic physical exam findings of myasthenia gravis
- Ptosis which becomes more pronounced with prolonged upward gaze
- Muscle fatigability with return to baseline after a few minutes of rest
Anti-MuSK antibodies
Antibodies against muscle-specific receptor tyrosine kinase
A surface membrane enzyme that is critical for aggregating ACh receptors during NMJ development
Seen in 40% of myasthenics who are seronegative for anti-AchR antibodies
Bulbar weakness
Weakness in muscle groups innervated by cranial nerves
Most often results in dysphagia, dysphonia, dysarthria, slurring of speech
Anti-Lrp4 antibodies
Low-density lipoprotein receptor-related protein-4 antibodies
Lrp4 is a protein that binds to MuSK and facilitates its actions in aggregating ACh receptors
Seen in some double-seronegative (AchR-, MuSK-) myasthenics
Acquired ptosis
A sign of underlying neurologic problem which requires urgent medical evaluation
Ptosis + proximal muscle weakness
Suggests myopathy vs myasthenia gravis
Ptosis + contralateral hemiparesis or hemitremor
Suggests ischemia in the midbrain affecting CN III
Ptosis, occulomotor abnormalities, and upper facial anesthesia
Suggests cavernous sinus syndrome:
- CN III
- CN IV
- CN V (V1 and V2, V3 is relatively spared)
- CN VI
Respiratory metrics affected in myasthenia gravis before oxygenation
FVC and peak inspiratory pressure
Concomitant __ disease is often seen in myasthenia gravis
Concomitant thyroid disease is often seen in myasthenia gravis
Tensilon test
aka edrophonium bromide test
Administer edrophonium bromide (a short acting acetylcholinesterase inhibitor). If this improves symptoms, the diagnosis of myasthenia gravis is made.
Ice test
In a patient with ptosis who is thought to have myasthenia gravis
Place ice over the ptotic eyelid for 2 minutes, then re-evaluate. If ptosis improves, a diagnosis of NMJ disorder can be made.
Cold improves NMJ transmission, while heat worsens NMJ transmission
Uthoff phenomenon in. . . myasthenia?
Heat worsens NMJ conduction, so myasthenics tend to first present in summer!
EMG/NCS findings in MG
Classically, NCS is normal. EMG can be normal or can show myopathic features.
A more specialized, single-fiber EMG is the most sensitive test for detecting myasthenia gravis, however it is not very specific and can be abnormal in neuropathies, ALS, and muscular dystrophy
Anyone with myasthenia gravis deserves a ___ as part of their diagnostic workup
Anyone with myasthenia gravis deserves a CT scan as part of their diagnostic workup
To screen for a thymoma, which is present in ~60% of cases. Removal of the thymoma (thymectomy) may improve symptoms, lead to remission, or even cure the disease.
Treating myasthenia gravis
- Two forms of therapy done simultaneously
- Immunosuppression (disease modifying): Corticosteroids, cyclosporine, azathioprine, mycophenolate, plasmapheresis
- Pro-cholinergic (symptom improving): Pyrimidostigmine, neostigmine (specifically these – not the other ones!)
- Look for and resect any thymomas
Ptosis in brain herniation
Associated with uncal herniation, which can compress CN III
Why do we prefer neostigmine and pyrimidostigmine for the treatment of myasthenia gravis?
Of the stigmines, it does not cross into the CSF!!!
This prevents all kinds of CNS cholinergic effects that you do not want.
So, if you are given the option to treat myasthenia gravis with physostigmine or one of the others, it is the wrong answer
Etiologies of Lambert-Eaton myasthenia
Often paraneoplastic, typically from small cell lung cancer
