Myasthenia Gravis

Question 1

What is MG?

Answer 1

Autoimmune condition causing disorder of neuromuscular junctions–> muscle weakness

Question 2

Classic feature?

Answer 2

Muscle weakness that gets progressively worse with movement and better with rest

Question 3

Typical patient?

Answer 3

Man over 60 or woman under 40

Question 4

Strong link with?

Answer 4

Thymoma (tumour of thymus gland)10-20% of patients with myasthenia gravis have a thymoma
20-40% of patients with a thymoma develop myasthenia gravis

Question 5

Pathophysiology?

Answer 5

Presynaptic neuron releases ACh, ACh travels accross synapse and attaches to ACh receptors on postsynaptic neurone–> induces muscle contraction. In MG, the IS produces ACh receptor antibodies that attach to ACh receptors and block ACh binding.

ACh receptor antibodies also activate the compliment system in the NMJ so causes cell damage at postsynaptic membrane.

Question 6

Why are symptoms worse during activity?

Answer 6

During activity, more receptors are used so more become blocked up. On rest, the receptors are freed up for use again.

Question 7

Presentation?
Which muscles are most affected?

Answer 7

severity varies drastically
weakness on repetitive movements
Mostly affects the head, neck, eyes

Question 8

Main symptoms?

Answer 8

Extraocular muscle weakness-double vision (diplopia)
Eyelid weakness- drooping of the eyelids (ptosis)
Weakness in facial movements
Difficulty with swallowing
Fatigue in the jaw when chewing
Slurred speech
Progressive weakness with repetitive movements

Question 9

Is there sensory loss?

Answer 9

No

Question 10

Examinations?

Answer 10

Excessive blinking
Move arm up and down

Question 11

How is it diagnosed?

Answer 11

Antibody test
Endrophonium test

Question 12

What is the endrophonium test?

Answer 12

cholinesterase enzymes break down ACh. Endrophonium blocks cholinesterase so means more ACh in the NMJ–> therefore a temporary improvement in symptoms. So diagnoses MG

Question 13

Treatment?

Answer 13

Reversible acetylcholinesterase inhibitors (usually pyridostigmine or neostigmine)

Immunosuppressants: azathioprine or prednisolone

Monoclonal ABs: Rituximab (B cells) and Eculizumab (complement system)

Thymectomy- can improve sx even in patients without a thymoma

Question 14

What is a myasthenic crisis?

Answer 14

acute worsening of sx
often triggered by a resp infection
can cause resp failure
treated with IV IG and plasma exchange