Myasthenia gravis

Background

  • Autoantibody degradation, dysfunction, and blockade of acetylcholine receptor at the NMJ[1]
  • Thymus is abnormal in 75% of pts
    • Thymectomy resolves or improves symptoms in most pts, especially those with a thymoma
  • No sensory, reflex, pupillary, or cerebellar deficits

Clinical Features

  • Muscle weakness
    • Proximal extremities
    • Neck extensors
    • Facial/bulbar muscles (dysphagia, dysarthria, dysphonia)
  • Ocular weakness
    • Ptosis
    • Diplopia
    • CN III, IV, or VI weakness

Differential Diagnosis

Drug-induced myasthenia

Weakness

Diagnosis

  • Symptoms worsen with repetitive use / as the day progresses[3]
    • Ice Pack Test- should improve symptoms temporarily (usually ptosis; high specificity)
      • Place ice-pack on eyes for 2 mins, if ptosis decreases by ≥2mm the test is positive

Myasthenic Crisis versus Cholinergic Crisis

  • Myasthenic Crisis
    • Respiratory failure is feared complication
    • Much more common
    • D/t med non-compliance, infection, surgery, tapering of immunosuppressants, meds
  • Cholinergic Crisis
    • Excessive anticholinesterase medication may cause weakness and cholinergic symptoms
    • Rarely if ever seen w/ dose limitation of pyridostigmine to less than 120mg q3hr
    • If on usual dose of meds assume exacerbation due to MG even w/ cholinergic side effects
  • Edrophonium (Tensilon) test to distinguish the two is controversial
    • Give 1-2 mg IV slow push. If any fasciculations, resp depression, or cholinergic symptoms within a few minutes, problem is likely cholinergic crisis (no more edrophonium). If no evidence of cholinergic excess, give total of 10 mg and observe improvement in case of myasthenic crisis.
    • Side effects of Edrophonium: Arrhythmias, Hypotension, Bronchospasm
    • Treatment: Atropine

Treatment

  1. Always evaluate tidal volume, FEV, negative inspiratory force, ability to handle secretions
  2. Meds
    • Pyridostigmine
      • If pt's usual dose has been missed the next dose is usually doubled
      • PO route: 60-90mg q4hr
      • IV route: 1/30th of the PO dose (2-3mg) by slow IV infusion
    • Neostigmine
      • 0.5mg IV
  3. Intubation
    • If possible avoid depolarizing AND non-depolarizing agents
      • If pt requires paralysis use non-depolarizing agent at smaller dose
      • If must use depolarizing agents, will need higher doses
  4. Plasmapherisis
  5. IVIG

See Also

Source

  1. Medications and Myasthenia Gravis (A Reference for Health Care Professionals) PDF
  2. Sanders DB, Guptill JT. Myasthenia Gravis and Lambert-Eaton Myasthenic Syndrome. Continuum. 2014 Oct;20(5)
  3. Tintinalli's Emergency Medicine: A Comprehensive Study Guide, 7e (2010), Chapter 167. Chronic Neurologic Disorders