Myasthenia

Question 1

Test of choice to differentiate between myopathy and NMJ disorder

Answer 1

EMG/NCS

(electromyography / nerve conduction study)

Question 2

Classic physical exam findings of myasthenia gravis

Answer 2

• Ptosis which becomes more pronounced with prolonged upward gaze
• Muscle fatigability with return to baseline after a few minutes of rest

Question 3

Anti-MuSK antibodies

Answer 3

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

Question 4

Bulbar weakness

Answer 4

Weakness in muscle groups innervated by cranial nerves

Most often results in dysphagia, dysphonia, dysarthria, slurring of speech

Question 5

Anti-Lrp4 antibodies

Answer 5

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

Question 6

Acquired ptosis

Answer 6

A sign of underlying neurologic problem which requires urgent medical evaluation

Question 7

Ptosis + proximal muscle weakness

Answer 7

Suggests myopathy vs myasthenia gravis

Question 8

Ptosis + contralateral hemiparesis or hemitremor

Answer 8

Suggests ischemia in the midbrain affecting CN III

Question 9

Ptosis, occulomotor abnormalities, and upper facial anesthesia

Answer 9

Suggests cavernous sinus syndrome:

• CN III
• CN IV
• CN V (V1 and V2, V3 is relatively spared)
• CN VI

Question 10

Respiratory metrics affected in myasthenia gravis before oxygenation

Answer 10

FVC and peak inspiratory pressure

Question 11

Concomitant __ disease is often seen in myasthenia gravis

Answer 11

Concomitant thyroid disease is often seen in myasthenia gravis

Question 12

Tensilon test

Answer 12

aka edrophonium bromide test

Administer edrophonium bromide (a short acting acetylcholinesterase inhibitor). If this improves symptoms, the diagnosis of myasthenia gravis is made.

Question 13

Ice test

Answer 13

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

Question 14

Uthoff phenomenon in. . . myasthenia?

Answer 14

Heat worsens NMJ conduction, so myasthenics tend to first present in summer!

Question 15

EMG/NCS findings in MG

Answer 15

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

Question 16

Anyone with myasthenia gravis deserves a ___ as part of their diagnostic workup

Answer 16

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.

Question 17

Treating myasthenia gravis

Answer 17

• 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

Question 18

Ptosis in brain herniation

Answer 18

Associated with uncal herniation, which can compress CN III

Question 19

Why do we prefer neostigmine and pyrimidostigmine for the treatment of myasthenia gravis?

Answer 19

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

Question 20

Etiologies of Lambert-Eaton myasthenia

Answer 20

Often paraneoplastic, typically from small cell lung cancer