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Autoimmune Myasthenia Gravis: Clinical Manifestations²

Epidemiology

• Prevalence
  • 50 to 400 cases per million
  • Higher > 40 years
• Annual incidence: 2.5 to 20 per million
• Lifetime risk: 500 per million
• Onset Age
  • General adult: Bimodal pattern
    • Early peak
      • Age: 2nd & 3rd decade
      • Female predominance
      • Association with HLA B8 & DR3
    • Late peak⁴
      • 6th to 8th decade
      • Male = Female
      • HLA association: B7 & DR2
      • Frequency: May be increasing in Western countries
      • Antibodies
        • AChR
          • Lower titer than in younger patients
          • Sero-negative MG uncommon
        • Titin
          • Frequency in late onset MG: 30%
          • Early onset MG: Rare except with thymomas
          • Associated with higher frequency of DR 7 antigen & lower frequency of DR 3 antigen
        • Ryanodine receptor
  • Childhood: Up to 30% of MG in China & Japan
• Autoimmune MG may rarely be a familial disorder .
• Ocular MG: Associated with HLA-BW46 in Chinese patients
  
  

Onset

• Presenting symptoms
  • Ocular (50%): Ptosis; Diplopia
  • Weakness, systemic (35%)
    • Distribution: Variable; Bulbar, Legs, or Arms
    • Painless
  • Fatigue (10%)
  • Respiratory failure: Rare
• Progression: Generally insidious over weeks to months
• Aggravating factors
  • Systemic disease: Infections; Thyroid
  • Emotional stress
  • Pregnancy
  • Medications

Symptoms & Signs

• Weakness
  • Variable: May increase through the day, or with prolonged physical activity
  • Onset: Often diplopia or ptosis 2° to extraocular muscle or levator palpebrae weakness
    • Most patients develop weakness in other muscles
    • Ocular myasthenia
      • Weakness remains limited to ocular muscles during entire course of the illness

    from Baylor

  • Specific muscle groups
    • Ocular
      • Ptosis & Ophthalmoplegia
      • Usually asymmetric & bilateral
      • Pupils: Normal
      • Rule out focal neural lesions
        • III or VI nerve lesion; Internuclear ophthalmoplegia
        • Especially when unilateral signs
    • Bulbar
      • Symptoms: Dysarthria, Dysphagia, Weak mastication
      • Signs: Poor gag reflex & palate elevation; Weak tongue
      • May result in aspiration pneumonia
      • Considered life-threatening
        • Usually an indication for rapidly-acting therapeutic intervention
        • Plasma exchange most commonly used
    • Facial
      • Frequency: > 95% of MG
      • Reduced facial expression, or snarl with attempt to smile
      • Distribution: Orbicularis oculi most common; Also orbicularis oris
    • Respiratory failure
      • Considered life-threatening
      • Usually due to Diaphragmatic and Intercostal muscle weakness
        • Strong indication for rapidly-acting therapeutic intervention
        • Pyridostigmine & Plasma exchange most commonly used
      • May be due to vocal cord paralysis¹
        • Vocal cords in adductor position: Produces stridor
        • May require intubation
      • Monitor with forced vital capacity
    • Limbs & Trunk
      • Typical
        • Proximal > Distal
        • Arms > Legs
        • Symmetric
      • Weakness in selective areas
        • Posterior neck (head ptosis)
        • Triceps
        • Quadriceps
        • Occasionally: Distal musculature
    • Differences from LEMS³
      • LEMS never begins with ocular weakness
      • LEMS usually has weakness in Legs > Arms
• Fatigue
  • Induced by
    • Repetitive muscle strength testing
    • Prolonged tonic contraction
  • Quantitation
    • Timed upward gaze
    • Forward arm abduction
• Muscle wasting: Uncommon, except when MG is chronic & untreated
• Deep tendon reflexes
  • Usually preserved
  • May be brisk in clinically weak muscles
• Sensory: Normal