Condition- Myasthenia Gravis

Question 1

What is myasthenia gravis?

Answer 1

An autoimmune disease affecting the NMJ producing weakness in muscle weakness that increases with exercise (fatiguability) and improves on rest

Question 2

What are the auto-antibodies in myasthenia gravis against?

Answer 2

Against the nAChR (nicotinic acetycholine receptor) on the post-synaptic membrane on the NMJ

Anti-AChR and Anti-MuSK antibodies

Question 3

Why do you get fatiguability of skeletal muscle from myasthenia gravis?

Answer 3

• Antibodies block or destroy AChR –> decreased neuronal excitation–> inconsistent generation of muscle fibre APs –> skeletal muscle weakness

Question 4

What is the order of onset of muscle weakness in a patient with myasthenia gravis?

Answer 4

FATIGUABILITY OF MUSCLE

​Specific order of muscle weakness: extraocular –> bulbar –> face –> neck –> limb girdle –> trunk

Question 5

List some of the presenting symptoms of myseathenia gravis….

Answer 5

OCULAR:

• Drooping eyelids
• Diplopia

BULBAR:

• Facial weakness (myasthenic snarl)
• Dysarthria- Disturbed hypernasal speech
• Dysphagia - Difficulty chewing or swallowing + smiling
• Limb weakness
• SOB

Question 6

How can you test the ocular signs of myasthenia gravis?

Answer 6

• Can see ptosis
• Check for ocular fatigue: Ask patient to sustain an upwards gaze for 1 min. Watch the progressive ptosis that develops
• Place ice on closed eyelide for 2mins => improved NMJ transmission –> reduced ptosis

Question 7

What are the bulbar signs of myasthenia gravis? (+what does bulbar mean)

Answer 7

• Bulbar: relating to medulla oblongata (CN 9,10,11 and 12 which have their nuclei in the medulla)
  
  • Reading aloud: causes dysarthria (= nasal speech)- ask them to count to 50 and their voice will fade

Question 8

How can you test the fatiguability of the limbs in someone with myasthenia gravis? What do you expect the tendon reflexes to be like?

Answer 8

• Test power of limbs before and after repeated use
• Tendon reflexes should be normal

Question 9

What could you look for in the bloods of someone with suspected myasthenia gravis?

Answer 9

• Bloods:
  
  • anti-AChR (positive in 90%)
  • If negative look for anti-MuSK (multiple specific tyrosine kinase)
  • TFTs (assocaited with hyperthyroidism)
  • Anti-VGCC antibodies (lambert eaton syndrome)

Question 10

Which investigations would you order for someone with suspected Myasthenia gravis?

Answer 10

• Bloods
• EMG/ Nerve conduction study: repetitive nerve stimulation will see >10% decline in compound muscle action potential (CMAP)
• CT Thorax/CXR:
  
  • to detect thymoma- thymic enlargement which is often associated with MG
  • To detect Small Cell lung cancer in LES

Question 11

Lambert Eaton Syndrome- what is it? and how does it present

Answer 11

Muscle weakness which improves on use

Question 12

State a malignancy that Lambort Eaton Syndrome is often associated with

Answer 12

Small Cell Lung Cancer