Myasthenia Gravis

Question 1

Define myasthenia gravis

Answer 1

Myasthenia gravis is an autoimmune condition that causes muscle weakness that gets progressively worse with activity and improves with rest typically women under 40 and men over 60

Question 2

What type of hypersensitivity reaction is MG?

Answer 2

Type 2

Question 3

What is the epidemiology for MG?

Answer 3

Females = 40 yrs old related to autoimmune disease
Males = 60 yrs old related to thymoma (thymus tumour)

Question 4

Describe the pathophysiology of MG

Answer 4

2 types of autoantibodies
- 85% anti Ach-R: bind to post synaptic receptor + inhibit, competitively Ach binding
- more binding w/ EXERTION (worse symptoms later)
- activate complement –> NMJ destruction
- 15% anti MuSK: MuSK helps synthesise Ach-R –> decreased Ach-R expression on post synaptic membrane

Question 5

What are causes of MG?

Answer 5

Can be inherited

Question 6

What are signs symptoms of MG?

Answer 6

→ muscle weakness that worsens during physical activity and improves after rest
→ Nearly all patients with the condition will experience some form of eye-related symptoms at some point e.g ptosis, diplopia
→ Proximal muscles are more affected
→ Weakness may be observed in the small muscles of the hands, as well as the deltoid, triceps, and bulbar muscles, which are responsible for chewing and speaking –> myasthenic snarl, jaw fatigability, swallowing difficulty
→ does not typically cause muscle wasting or affect sensation
→ Seizures may be a possible complication

Question 7

What is the differential diagnosis for MG?

Answer 7

Lambert Eaton syndrome
- similar symptoms

Question 8

What investigations do you do for MG?

Answer 8

→ Clinical diagnosis
→ Serology: Detection of Ach receptor antibodies (Tensilon/ Edrotropium test)
→ TFT’s
→ CT/MRI of the thymus to look for thymoma

Question 9

Describe the Tensilon/ Edrophonium test

Answer 9

• Administer edrophonium (rapid acting Achase inhibitor)
• +ve = increased muscle power in a few seconds for a few seconds

Question 10

How do you treat MG?

Answer 10

→ 1st line: Symptomatic treatment with reversible acetylcholinesterase inhibitors - pyridostigmine/neostigmine
→ 2nd line: immunosuppression - prednisolone/azathioprine suppresses the production of antibodies
→ Monoclonal antibodies - Rituximab/eculizumab - targets B cells and reduces production of antibodies
→ Thymectomy

Question 11

What are the complications of MG?

Answer 11

• Myasthenic crisis= acute symptoms worsening w/ severe resp weakness
• Do plasma exchange + IV Ig + BiPAP for resp failure