NEUROMUSCULAR DISORDERS

Question 1

The diagnosis of myasthenia gravis test for antibodies to what?

Answer 1

ACh receptors

Question 2

Why does muscle fatigue ability happen in myasthenia gravis?

Answer 2

With repeated nerve stimulation there is a decrease in ACh release at neuromuscular junction. Since there are also antibodies competing for ACh receptor this leads to muscle weakness with more use

Question 3

50% of patients with myasthenia gravis present with what type of complaint?

Answer 3

Eye complaints: diplopia, ptosis

Question 4

Mainstay of MG treatment

Answer 4

ACh esterase inhibitors +/- Immunosuppression

Question 5

What are the two main causes of exacerbation of MG? How can you differentiate between the two?

Answer 5

• Insufficient dose of AChE inhibitor
• Too much AChE inhibitor to point where muscle becomes refractory to ACh
• Tensilon test: give edrophonium (short acting AChE inhibitor):
  
  • muscle function improves ⇒ increase dose of AChE inhibitor
  • fails to improve ⇒ cholinergic crisis ⇒ decrease dose

Question 6

Why might the Tensilon test exacerbate asthma?

Answer 6

Activates parasympathetic activity thus causing symptoms such as bronchoconstriction, abdominal cramping, bradycardia, salivation

Question 7

What is the key symptom of MG crisis syndrome?

Answer 7

Difficult or laboured breathing

Question 8

What is enlarged in around 85% of patients with MG? What do remaining 15% usually have?

Answer 8

• Thymus: develops germinal centres where B-cells proliferate and produce antibodies to ACh receptors
• Thymoma

Question 9

MG symptoms often resolve or improve with which surgery?

Answer 9

Thymectomy

Question 10

What is pathophysiology of Lambert-Eaton Myasthenic syndrome (LEMS)? What is classic presenting complaint? How is this different from MG?

Answer 10

Antibodies attack pre-synaptic calcium channels at neuromuscular junction. This prevents ACh being released and binding to nicotinic receptors.

→ Slow onset symmetrical proximal muscle weakness

MG presents with eye symptoms first and antibodies attack nicotinic receptors

Question 11

What is a differential diagnosis for symmetrical proximal muscle weakness other than Lambert Eaton syndrome? How will you differentiate the two?

Answer 11

Myositis

• ​Normal CK levels in LEMS
• No muscle pain or myalgia in LEMS

Question 12

When considering systemic symptoms, how can you differentiate MG from LEMS?

Answer 12

• MG: acts on nicotinic receptors and therefore symptoms are confined to muscles
• LEMS: prevents release of ACh and therefore is anti-cholinergic systemically e.g. dry mouth, Erectile dysfunction, constipation

Question 13

How does muscle use affect LEMS and MG differently?

Answer 13

• MG: muscle fatigability
• LEMS: muscle use improves symptoms as more depolarisation increases ACh release

Question 14

What is the main treatment of Lambert Eaton syndrome?

Answer 14

Locating and treating underlying malignancy: up to 60% of people with LEMS also have a small cell lung cancer. Calcium channels expressed on these cells can trigger the formation of antibodies that cross react at neuromuscular junction