Myasthenia Gravis

Question 1

def myasthenia gravis

Answer 1

autoimmune disorder affecting the neuromuscular junction

Question 2

what kind of ab can be involved in myasthenia gravis? (3)

Answer 2

• nicotinic acetylcholine receptors
• muscle-specific kinase
• lipoprotein-related protein 4

Question 3

who gets myasthenia gravis?

Answer 3

in <40Y its more common in females
in >50Y its more common in males

childhood MG (<15Y) is more prevalent in Asian countries

this is the most common disorder affecting the NMJ

Question 4

RF for myasthenia gravis

Answer 4

occurs in genetically susceptible indiv

can be triggered by infections, immunizations, surgeries, or drugs

Question 5

in general what is the patho for myasthenia gravis?

Answer 5

1. antibodies bind to NMJ
2. activate complement
3. MAC destroys

OR
1. binds to proteins in NMJ
2. decreases number of n-acetylcholine receptors
3. unable to generate postsynaptic potential
4. no muscle contraction

Question 6

what are common symptoms of myasthenia gravis?

Answer 6

MC: extraocular muscle weakness (85%): diplopia or ptosis

Other:
- bulbar muscle weakness: diff chewing/choking, dysphagia, hoarseness, dysarthria
- facial: expresionless face
- neck: dropped-head syndrome
- limb muscle weakness: proximal > distal; upper > lower

“chair, hair, stairs” for muscle weakness

spared/normal reflexes ➔ they have fatiguability for muscle weakness

Question 7

T/F: there are autonomic symptoms (palpitations, BM/urinary disturbances) in myasthenia gravis

Answer 7

False!

there aren’t any bc it only involves the n-acetylcholine receptors, not the actual nerves

Question 8

what is a myasthenia crisis?

Answer 8

when there is intercostal muscle and diaphragm muscle weakness ➔ can lead to acute resp failure requiring intubation and mechanical ventilation

Question 9

how do you dx myasthenia gravis?

Answer 9

mostly a clinical dx
refer to neurology for formal
can do antibody tests – anti n-AChR, anti-MuSK, anti-LRP4, anti-striated muscle

can also consider doing nerve condution tests ➔ single finger EMG w/ repetitive nerve stimulation

could consider imaging (CXR) for presence of thymoma cause some pts have that bc of the increase production of ab

Question 10

how to tx myasthenia gravis?

Answer 10

firstline: acetylcholinesterase inhibitors ➔ increase levels of ACh at the NMJ

others
- consider immunosuppressants (glucocorticoids/MXT)
- could consider immunotherapy (rituximab) if the acetylcholinesterase inhibitor + immunosuppressants don’t work

consider thymectomy if thymoma present

Question 11

how to tx myasthenia crisis?

Answer 11

IVIG or plasmapheresis

intubation + ventilation + supportive measures

Question 12

what physical exam tests could you do if you suspect myasthenia gravis?

Answer 12

• ice pack test: cold reduces muscle weakness ➔ ptosis resolves
• rest test: pt rests eyes and ocular symptoms should improve
• curtain’s sign ➔ pulling one eyelid up causes the other to droop down
• cogan lid twitch sign ➔ look downwards for 15s, gaze upwards then primary gaze ➔ + if upper lip makes an upwards twitch or overshoots at primary gaze position

Question 13

what other things can cause NMJ disorders?

Answer 13

• botulism ➔ diffuse, flaccid paralysis (honey)
• lambert-eaton syndrome ➔ proximal muscle weakness, autonomic dysfunction, and impaired DTR (paraneoplastic syndrome from SCLC)
• congenital myasthenia syndrome