5.6 Myasthenia Gravis

Question 1

What is the “key part” of a Myasthenia Gravis Hx?

Answer 1

Fatigability: with repeated movements, weakness gets worse.

Question 2

Where are some common sites of weakness of Myasthenia Gravis?

Answer 2

• Eyes
• Limbs

Question 3

Outline the general pathophysiology of Myasthenia Gravis

Answer 3

• Autoimmune
• Autoantibodies target post-synaptic membrane proteins at NMJ

Question 4

What sensory issues do we see in Myasthenia Gravis?

Answer 4

• None
• This is a neuromuscular junction problem; no sensory involvement

Question 5

What is the classical history of Myasthenia Gravis?

Answer 5

• FATIGABLE:
• Extra-ocular: Diplopia, due to extra-ocular weakness, not justone cranial nerve = “complex opthalmoplegia”
• Limb/axial proximal limb weakness, symmetrical
  -Oropharyngeal/resp: SOB, dysphagia

Question 6

Physical exam findings of Myasthenia Gravis

Answer 6

• FATIGABLE:
• Extra-ocular: ptosis, opthalmoplegia
• Limb/axial: Decreased power. Recovery of strength occurs when ice is applied, and heat worsens. Reflexes/sensation normal.
• Oropharyngeal/resp: Count to 50 (voice gets quieter as fatigue sets in)

Question 7

Myasthenia Gravis Ix

Answer 7

• Spirometry/single breath test (resp muscles)
• Nerve conduction studies/EMG; decrement on repetitive nerve stimulation
• Icepack test (usually for ptosis)
• Anti-Ach receptor antibodies test
• Chest CT (Thymoma)

Question 8

Do you being Myasthenia Gravis treatment before diagnosis is confirmed?

Answer 8

• It depends:
• If resp compromised, consider intubation/ventilation
• If it’s not an emergency, then usually wait until diagnosis

Question 9

Outline the short-term management of Myasthenia Gravis

Answer 9

• 1st line: Pyridostigmine
• If myasthenic crisis: plasmapheresis/IVIg

Question 10

Pyridostigmine mech

Answer 10

• Cholinesterase inhibitor
• Prevents breakdown of ACh in synapse
• Improves transmission

Question 11

Describe long-term management of Myasthenia Gravis

Answer 11

• If thymoma: thymectomy
• If pyridostigmine is insufficient: consider prednisolone and steroid preserving agent. If long-term steroids, we need vaccines, bone-health management.

Question 12

What medications can trigger Myasthenia Gravis?

Answer 12

• Aminoglycosides (which are antibiotics e.g. gentamicin)
• Beta blockers