Condition- Myasthenia Gravis Flashcards
What is myasthenia gravis?
An autoimmune disease affecting the NMJ producing weakness in muscle weakness that increases with exercise (fatiguability) and improves on rest
What are the auto-antibodies in myasthenia gravis against?
Against the nAChR (nicotinic acetycholine receptor) on the post-synaptic membrane on the NMJ
Anti-AChR and Anti-MuSK antibodies
Why do you get fatiguability of skeletal muscle from myasthenia gravis?
- Antibodies block or destroy AChR –> decreased neuronal excitation–> inconsistent generation of muscle fibre APs –> skeletal muscle weakness
What is the order of onset of muscle weakness in a patient with myasthenia gravis?
FATIGUABILITY OF MUSCLE
Specific order of muscle weakness: extraocular –> bulbar –> face –> neck –> limb girdle –> trunk
List some of the presenting symptoms of myseathenia gravis….
OCULAR:
- Drooping eyelids
- Diplopia
BULBAR:
- Facial weakness (myasthenic snarl)
- Dysarthria- Disturbed hypernasal speech
- Dysphagia - Difficulty chewing or swallowing + smiling
- Limb weakness
- SOB
How can you test the ocular signs of myasthenia gravis?
- 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
What are the bulbar signs of myasthenia gravis? (+what does bulbar mean)
- 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
How can you test the fatiguability of the limbs in someone with myasthenia gravis? What do you expect the tendon reflexes to be like?
- Test power of limbs before and after repeated use
- Tendon reflexes should be normal
What could you look for in the bloods of someone with suspected myasthenia gravis?
- 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)
Which investigations would you order for someone with suspected Myasthenia gravis?
- 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
Lambert Eaton Syndrome- what is it? and how does it present
Muscle weakness which improves on use
State a malignancy that Lambort Eaton Syndrome is often associated with
Small Cell Lung Cancer