myasthenia gravis

Question 1

what is myasthenia gravis?

Answer 1

an autoimmune condition that causes muscle weakness that gets progressively worse with activity and improves with rest

Question 2

what causes myasthenia gravis?

Answer 2

• autoimmune condition

- not a clear cause but there is a link between thymomas and myasthenia gravis

Question 3

who is more commonly affected by myasthenia gravis?

Answer 3

females <40 years

males >60 years

Question 4

how does myasthenia gravis present?

Answer 4

muscle weakness that gets worse the more the muscles are used and improves on rest.

• difficulty swallowing
• eyelid weakness leading to drooping eyelids (ptosis)
• extraocular muscle weakness causing double vision (diplopia)
• slurred speech
• fatigue in jaw when chewing

Question 5

what auto antibodies are found in a patient with myasthenia gravis?

Answer 5

• serum acetyl choline receptor antibodies (AChR) in 85%
• muscle specific tyrosine kinase antibodies (MuSK) in 10%
• anti-LRP4 antibodies in <5%

Question 6

what is used to increase neurotransmission in the treatment for myasthenia gravis?

Answer 6

anti-cholinesterase agents (pyridostigmine)

Question 7

what investigations are done for someone with suspected myasthenia gravis?

Answer 7

• repetitive nerve stimulation (could get a patient to attempt to lift up arm 20x)
• scan thymus gland (to look for thymoma)
• lab test to look for antibodies (Anti AChR antibodies, Anti-MuSK antibodies, Anti-LRP4 antibodies)
• edrophonium (or neurostigmine test)= patient gets given an IV dose of edrophonium chloride or neostigmine, if muscle weakness temporarily releived then MG is diagnosed

Question 8

what is the pathophysiology behind how acetylcholine receptor antibodies cause myasthenia gravis?

Answer 8

• acetylcholine receptor antibodies are produced and bind to the acetylcholine receptors in the post synaptic membrane, blocking them and stopping the signals from getting passed through the synapse. As the muscle gets used more, more of the receptors get blocked up, causing more muscle weakness. And as the muscles get used less, less acetyl receptor antibodies bind to the receptor and the muscles become stronger again.
• the acetylcholine receptor antibodies trigger the complement system which leads to damage of the cells at the post synaptic membrane

Question 9

what is the pathophysiology of how anti-MusK and anti-LRP4 cause myasthenia gravis?

Answer 9

-MusK and LRP-4 are important proteins in the formation and organisation of the acetylcholine receptor. Destruction of MusK and LRP-4 leads to a problem with the formation and organisation of the acetylcholine receptors

Question 10

what is an edrophonium or neostigme test?

Answer 10

Patients get given an IV dose of edrophonium chloride or neostigme, if muscle weakness is temporarily releived theb MG is diagnosed

Question 11

what is first line treatment for mild/moderate myasthenia gravis?

Answer 11

reversible anti cholinesterase agents (e.g. pyridostigmine pill)