Myasthenia Gravis

Question 1

What is MG?

Answer 1

MG is an autoimmune disease, in which autoantibodies mediate damage and destruction of acetylcholine receptors (AChR) in striated muscle 🡪 impairment of neuromuscular conduction 🡪 weakness and fatigability of skeletal musculature, but not of cardiac and involuntary muscles

Question 2

what are the symptoms of systemic MG?

Answer 2

• Painless fatigue, often brought on by exercise, commonly w/ ptosis and diplopia
• Fatigability affects musculature of the limbs, facial expression, ocular movements, chewing, and speech
• Bulbar symptoms: dysphagia, dysarthria; difficulty with breathing is rare

Question 3

what are the signs of MG?

Answer 3

Peripheral weakness (most important feature), particularly of proximal muscles
Wasting (in long-standing cases)
Lack of facial expression (myopathic facies)

Question 4

what is the cogan lid twitch sign?

Answer 4

brief upshoot of the eyelid as the eye saccade from depression to the primary position

Question 5

what is the lash peak sign?

Answer 5

30 seconds of sustained forced tight eyelid closure 🡪 induce fatigue of the orbicularis oculi muscles 🡪 patient is unable to completely close his eyelids and appears to ‘peek’/scleral show

Question 6

what is the investigations for MG?

Answer 6

Ice pack test (simple clinical test): Tests for improvement after an ice pack is placed on the ptotic eyelid (or other affected muscle) for 3-5 mins, as cold inhibits breakdown of ACh by AChE

Antibody testing

• Supports diagnosis of MG and predicts the likelihood of thymoma
• Anti-AChR (acetylcholine receptor): Present in 90% of systemic cases, but only 50-70% of ocular MG
• Anti-MuSK (muscle specific kinase) (expensive, not done as first line)
• Anti-voltage-gated Ca2+ channels: Characteristic of Lambert Eaton (LEMS)

IV edrophonium test (tensilon test) (not commonly done)

• Edrophonium is a short-acting AChE inhibitor that confers a transient improvement of weakness in MG (rapid onset of action (30-45s), short duration of action (30 mins)) – look for improvement of ptosis or diplopia
• Potential (but uncommon complications) include: bradycardia, death

**TFT for thyroid autoimmune disease

**CT thorax
To detect thymoma (present in 10% of MG)

To r/o lung tumour if Lambert Eaton (LEMS) is suspected (LEMS is often a paraneoplastic syndrome of lung carcinomas)

Question 7

what is the management of M?

Answer 7

Long-acting cholinesterase inhibitor, e.g. pyridostigmine, AND steroids/other systemic immunosuppressive treatment (e.g. azathioprine) –> Refer neurology for medication titration

Plasmapheresis, intravenous immunoglobulin (IVIG)
- Short term measures to address acute illness

Thymectomy
- Performed if thymoma is present