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Myasthenia Gravis

Myasthenia Gravis is an autoimmune disease characterised by fatiguable muscle weakness. It has a strong association with Thymic hyperplasia (more common) and Thymoma.


Pathophysiology

  • Anti-AChR binds to the postsynaptic NMJ receptors, therefore preventing muscle stimulation. As the muscle is used more during activity, the receptors become more blocked up by these antibodies, therefore leading to the characteristic fatigable muscle weakness.

  • Anti-MuSK is also produced and has the same effect as anti-AChR, but is much less common

    • MuSK is an important protein for the production of AChR


N.B. Anti-MuSK is also known as anti muscle-specific tyrosine kinase.


Presentation

Fatiguable weakness – Typically minimal in the morning and worst at end of the day. Usually affects the proximal muscles first, and small muscles of the 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


Symptoms tend to be exacerbated by Infection, Pregnancy, and Hypokalaemia.


Signs seen O/E include:

  • Repeated blinking exacerbates ptosis

  • Prolonged upward gaze exacerbates diplopia

  • Repeated arm abduction results in unilateral weakness

  • Normal reflexes and sensation

Patient with Myasthenia Gravis - Posey & Spiller

Investigations

  • Anti-AChR – Raised in 90%

    • If -ve, do Anti-MuSK

  • EMG – Decrement of amplitude on repetitive stimulation

  • Imaging – CT/MRI of Thymus


Management

Acute relapse - Prednisolone

  • Dose decreased on remission, and bone protection is given

Long-term - 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 (can't see, can't pee, can't spit, can't s*it).


Consider Thymectomy. It's beneficial even in those w/o thymoma, i.e. younger patients.


Myasthenic Crisis

This is an acute, life-threatening worsening of symptoms that's usually triggered by infection. It often presents with respiratory involvement, therefore leading to respiratory failure and requiring NIV w/BiPAP, or Intubation. The severity is assessed by monitoring FVC.


Management - IVIG or PLEX (aim is to remove the auto-antibodies)



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