myasthenia%20gravis
MYASTHENIA GRAVIS
Myasthenia gravis is an autoimmune neurological disorder caused by autoantibodies against the acetylcholine receptor or against a receptor-associated protein, muscle-specific tyrosine kinase (MuSK-Ab).
The autoimmune attack at the muscle endplate leads to failure of neuromuscular transmission and eventually muscle weakness.
In the active phase of the disease, symptoms typically fluctuates and then become severe; myasthenic crisis occur in this phase.
In the stable/inactive phase, symptoms are stable but still persist; it usually worsen attributable to infection, fatigue, tapering of medications or other identifiable factors.
In the burnt-out phase, remission may occur wherein patients are on immunotherapy and symptom-free, or may even be off medications.
In 15-20 years, if the symptoms left untreated, patient's weakness becomes fixed wherein the most severely affected muscles become atrophic.

Definition

  • Myasthenia gravis is an uncommon neurological disorder caused by autoantibodies against the acetylcholine receptor or against a receptor-associated protein, muscle-specific tyrosine kinase (MuSK-Ab)
    • The autoimmune attack at the muscle endplate leads to failure of neuromuscular transmission & eventually muscle weakness

Signs and Symptoms

  • Fatigability & weakness of voluntary muscles
    • Weakness tends to increase during repeated use
    • May improve with rest or sleep
  • Diplopia & ptosis
    • Common initial complaints
    • Generally asymmetrical
    • Cogan’s lid twitch or excessive lid elevation
  • Facial weakness which results in snarl-like expression when smiling
  • Nasal or slurred speech
  • Difficulty in swallowing
  • If generalized, there will be weakness in the limb muscles
    • Often asymmetric & proximal
  • Deep tendon reflexes & sensation are preserved
  • If severe, there will be shortness of breath

Clinical Features

  • Acetylcholine Receptor Myasthenia Gravis
    • Muscle weakness predominant in the limb than bulbar, neck extensor than neck flexor, prominent ptosis & external ocular muscle weakness
    • Wasting of proximal limb & ocular muscles in long-standing disease (myasthenic myopathy)
    • Thymus pathology: 65% hyperplasia, 15% thymoma
    • Risk of recurrent crisis is low
  • Muscle-Specific Tyrosine Kinase Myasthenia Gravis
    • Muscle weakness predominant in the bulbar than the limb, neck flexor than neck extensor, mild ptosis & external ocular muscle weakness
    • Early wasting of facial & tongue muscles
    • Thymus pathology: 10% hyperplasia
    • High-risk of recurrent crisis
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