Myasthenia gravis

Background

• Autoantibody degradation, dysfunction, and blockade of acetylcholine receptor at the NMJ
• 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

Myasthenic Crisis versus Cholinergic Crisis

1. Myasthenic Crisis
   1. Respiratory failure is feared complication
   2. Much more common
   3. D/t med non-compliance, infection, surgery, tapering of immunosuppressants, meds
2. Cholinergic Crisis
   1. Excessive anticholinesterase medication may cause weakness and cholinergic symptoms
   2. Rarely if ever seen w/ dose limitation of pyridostigmine to less than 120mg q3hr
   3. If on usual dose of meds assume exacerbation due to MG even w/ cholinergic side effects
3. Edrophonium (Tensilon) test to distinguish the two is controversial
   1. Side effects of Edrophonium: Arrhythmias, Hypotension, Bronchospasm
   2. Treatment: Atropine

Clinical Features

1. Symptoms worsen with repetitive use / as the day progresses
2. Muscle weakness
   1. Proximal extremities
   2. Neck extensors
   3. Facial/bulbar muscles (dysphagia, dysarthria, dysphonia)
3. Ocular weakness
   1. Ptosis
   2. Diplopia
   3. CN III, IV, or VI weakness

DDX

1. Cholinergic crisis
2. Lambert-Eaton Syndrome
3. Botulism
4. Thyroid disorders
5. Drug-induced myasthenia
   1. Abx (aminoglycosides, flouroquinolones, clindamycin, metronidazole, macrolides)
   2. Steroids
   3. Anticonvulsants (phenytoin, barbiturates, lithium)
   4. Psychotropics (haloperidol)
   5. Beta-blockers / calcium-channel blockers
   6. Local anesthetics
   7. Narcotics
   8. Anticholinergics (diphenhydramine)
   9. NMJ blocking agents (roc, sux)

Treatment

• Always evaluate tidal volume, FEV, ability to handle secretions

1. Meds
   1. Pyridostigmine
      1. If pt's usual dose has been missed the next dose is usually doubled
      2. PO route: 60-90mg q4hr
      3. IV route: 1/30th of the PO dose (2-3mg) by slow IV infusion
   2. Neostigmine
      1. 0.5mg IV
2. Intubation
   1. If possible avoid depolarizing AND non-depolarizing agents
      1. If pt requires paralysis use non-depolarizing agent at smaller dose
3. Plasmapherisis
4. IVIG

See Also

• Lambert-Eaton Myasthenic Syndrome

Source

• Tintinalli