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
   1. Ice Pack Test- should improve symptoms temporarily (usually ptosis; high specificity)
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. Antibiotics (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, negative inspiratory force, 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
      2. If must use depolarizing agents, will need higher doses
3. Plasmapherisis
4. IVIG

See Also

• Lambert-Eaton Myasthenic Syndrome

Source

• Tintinalli