Myasthenia Gravis (3)

Question 1

What is this?

What does it have a strong association with?

What is its pathophysiology?

Answer 1

➊ Autoimmune disease, causing fatiguable muscle weakness

➋ Thymic hyperplasia and Thymoma

➌ • Anti-AChR binds to the postsynaptic NMJ receptors, therefore preventing the stimulation of muscle. As the muscle is used more during activity, the receptors become more blocked up by these Ab’s, therefore leading to the characteristic fatigable muscle weakness.
• Anti-MuSK is also produced to have the same effect, but is much less common than Anti-AChR – MuSK is an important protein for the production of AChR

Question 2

How does it present?

What is seen O/E?

What tends to exacerbate it?

Answer 2

➊ • Fatiguable weakness – Typically minimal in the morning and worst at end of the day
• Usually proximal muscles first, and small muscles of head and neck, leading to:
‣ Ptosis, Diplopia – extraocular muscles
‣ Difficulty smiling or chewing – Facial muscles
‣ Slurred speech, Difficulty swallowing and chewing – Bulbar symptoms
‣ Neck flexion weakness

➋ • Repeated blinking will exacerbate ptosis
• Prolonged upward gaze will exacerbate diplopia
• Repeated arm abduction will result in unilateral weakness
• Normal reflexes and sensation

➌ Infection, Pregnancy, Low K+

Question 3

Which investigations need to be done?

Answer 3

• Anti-AChR – Raised in 90%
  ‣ If -ve, do Anti-MuSK (aka. anti muscle-specific tyrosine kinase)
• EMG – Decrement on repetitive stimulation
• Imaging – CT/MRI of Thymus

Question 4

What is given acutely, during relapse?

What is given prophylactically?

What should be considered?

Answer 4

➊ Prednisolone - Dose decreased on remission, with bone protection to be given

➋ Reversible AchE inhibitors e.g. Pyridostigmine or Neostigmine
• SEs – Increased salivation, teary eyes, sweating, vomiting, diarrhoea

N.B. This drug class increases cholinergic activity, therefore its side-effects are the opposite of anti-cholingerics

➌ Thymectomy – Beneficial even in those w/o thymoma, esp. in younger pts

Question 5

Myasthenic Crisis:
What is this?

What usually triggers it?

What does it often present with?
→ How is the severity assessed?

How is it managed?

Answer 5

➊ Acute, life-threatening worsening of symptoms

➋ Infection

➌ Respiratory involvement, leading to respiratory failure – Pt may require NIV w/BiPAP or Intubation
→ FVC monitoring

➍ IVIG or PLEX (remove antibodies)