Myasthenia gravis

Question 1

What is myasthenia gravis?

Answer 1

An autoimmune condition that causes muscle weakness which progressively worsens with activity and improves with rest.

Question 2

What is the pathophysiology of myasthenia gravis?

Answer 2

The immune system abnormally produces ACh receptor antibodies which bind to the postsynaptic neuromuscular junction, stopping ACh from binding and being able to trigger muscle contraction.
When the receptors are being used more during exercise, more will become blocked hence there is less effective stimulation of the muscles and therefore it becomes weak.

Question 3

How does myasthenia gravis present?

Answer 3

Muscle weakness that gets worse with use and improves with rest.
- extraocular muscles = diplopia
- eyelid weakness = ptosis
- weakness in facial movements
- difficulty swallowing
- jaw fatigue
- slurred speech

Question 4

What other condition is associated with myasthenia gravis?

Answer 4

thymoma (tumours of the thymus gland)

Question 5

How is myasthenia gravis diagnosed?

Answer 5

Antibody testing
- ACh-R ( acetylchlorine receptor) antibodies - in 85%
- Muscle specific kinase antibodies - in 10%
Can also do a CT or MRI scan of the thymus to look for a thymoma.
Or can use a Tensilon (edrophonium) test???

Question 6

How is myasthenia gravis managed?

Answer 6

• Reversible acetylcholinesterase inhibitors - e.g. pyridostigmine
• immunosupression - e.g. prednisolone
• may need a thymectomy
• monoclonal antibodies such as rituximab

Question 7

What is a myasthenic crisis?

Answer 7

A life threatening complication of myasthenia gravis that causes acute worsening of symptoms. It is often triggered by a respiratory tract infection. It can then lead to respiratory failure due to respiratory muscle weakness.

Question 8

How is a myasthenic crisis managed?

Answer 8

• non invasive ventilation with BiPAP or full intubation and ventilation
• IV immunoglobulins and plasma exchange