Myasthenia Gravis

Question 1

Define myasthenia gravis

Answer 1

An AUTOIMMUNE condition affecting the NEUROMUSCULAR JUNCTION causing weakness in skeletal muscle

Question 2

Explain the aetiology/risk factors of myasthenia gravis

Answer 2

• Impairment of neuromuscular junction transmission
• Most commonly due to autoantibodies against the nicotinic acetylcholine receptor

• Myasthenia gravis is associated with other autoimmune conditions (e.g. pernicious
anaemia)

Question 3

What is the paraneoplastic subtype of MG and what is its aetiology?

How does it differ from MG?

Answer 3

• Lambert-Eaton Syndrome - paraneoplastic subtype of myasthenia gravis caused by autoantibodies against pre-synaptic CALCIUM CHANNELS, leading to impairment of acetylcholine release

Question 4

Summarise the epidemiology of myasthenia gravis

Answer 4

• Prevalence: 8-9/100,000
• More common in FEMALES at younger ages
• Equal gender distribution in middle age

Question 5

What are the presenting symptoms of MG?

Answer 5

Muscle weakness that worsens with repetitive use or towards the end of the day.

○ NOTE: in Lambert-Eaton syndrome, muscle weakness improves after repeated use

Ocular symptoms
○ Drooping eyelids
○ Diplopia

Bulbar symptoms
○ Facial weakness (myasthenic snarl)
○ Disturbed hypernasal speech
○ Difficulty smiling, chewing or swallowing

proximal myopathy - difficulty with stairs

Question 6

What are the signs of myasthenia gravis on physical examination

Answer 6

• May be generalised (affecting many muscle groups)
• May be bulbar (affecting the bulbar muscles i.e. those associated with cranial nerves 9, 10, 11 and 12)

○ NOTE: bulbar = relating to the medulla oblongata (cranial nerves 9, 10, 11 and 12 have their nuclei in the medulla)

• May be ocular
• Eye Signs
○ Ptosis
○ Complex ophthalmoplegia
○ Check for ocular fatigue by asking the patient to sustain and upward gaze for 1 min and watch the progressive ptosis that develops

• Ice on Eyes Test
○ Placing ice packs on closed eyelids for 2 mins can improve neuromuscular transmission and reduce ptosis

• Bulbar Signs
○ Reading aloud may cause dysarthria or nasal speech
• Limbs
○ Test the power of a muscle before and after repeated use of the muscle

Question 7

Identify appropriate investigations for myasthenia gravis

Answer 7

Lecture:
Mainly: EMG and antibody tests (2)

CXR - thymoma exclude
Ice test - should relieve eyelid sx

• Bloods
○ CK - exclude myopathies
○ Serum acetylcholine receptor antibody (positive in 80%)
○ muscle-specific tyrosine kinase (MuSK)

○ TFTs (it is associated with hyperthyroidism)
○ Anti-voltage gated calcium channel antibody (in Lambert-Eaton syndrome)

• Tensilon Test
○ Short-acting anti-cholinesterase (edrophonium bromide) increases ACh levels and causes a rapid and transient improvement in clinical features

○ Risk of bradycardia - so is generally avoided

• Nerve Conduction Study
○ Repetitive stimulation shows decrements of muscle action potential

• EMG

• Serial Pulmonary function test; if SOB. Low FVC

• CT Thorax - visualise thymoma in the mediastinum (MG can cause this in 15%)

Question 8

Mx mg?

Answer 8

Mild disease:

1. Pyridostigmine (Acetylcholinesterase inhibitor)
   1b. Steroid; + immune modifying therapy (azathioprine)
   - consider thymectomy

Question 9

what is a myasthenic crisis?

Answer 9

acute exacerbation of symptoms
development of SOB
often SOB severe enough to require mechanical ventilation

Question 10

presentation of a myasthenic crisis?

Answer 10

SOB

Respiratory failure

Question 11

mx of myathenic crisis?

Answer 11

Breathing support - stepwise:
NIV
Bipap
Intubation + ventilation

Meds:
ivig
plasma exchange