Myasthenia Gravis

Myasthenia Gravis is a topic covered in the 5-Minute Emergency Consult.

To view the entire topic, please or .

Emergency Central is a collection of disease, drug, and test information including 5-Minute Emergency Medicine Consult, Davis’s Drug, McGraw-Hill Medical’s Diagnosaurus®, Pocket Guide to Diagnostic Tests, and MEDLINE Journals created for emergency medicine professionals. Explore these free sample topics:

-- The first section of this topic is shown below --

Basics

Description

  • Autoimmune antibody–mediated condition that results in painless, fatigable skeletal muscle weakness
  • Ocular or generalized:
    • Ocular (eyelids and extraocular) muscle weakness:
      • Most common initial symptom (60%)
      • ∼80% of myasthenia gravis (MG) patients who present with ocular weakness initially will progress to general weakness within 2 yr
    • Generalized:
      • Usually affects proximal limbs, axial muscle groups such as neck, face, bulbar muscles
  • Acute or subacute, with relapses and remissions
  • Associated with thymoma in 10–15% and thymic hyperplasia in 60–70%
  • Myasthenic crisis:
    • Respiratory failure or inability to protect airway due to weakness
    • Triggers:
      • Infection
      • Surgery
      • Trauma
      • Pregnancy
      • Medication changes (e.g., rapid tapering of steroids)
    • Difficult to distinguish from cholinergic crisis resulting from excessive doses of acetylcholinesterase (AChE) inhibitors:
      • Cholinergic crisis may also include muscarinic effects such as sweating, lacrimation, salivation, and GI hyperactivity in addition to weakness
      • Cholinergic crisis due to therapeutic AChE inhibitor use is rare, typically at very high doses

Epidemiology

  • Pediatric MG is rare and distinct:
    • Congenital MG: Genetic defect
    • Juvenile MG: Autoimmune disorder
    • Transient neonatal MG: Postdelivery complication from placental transfer of maternal antibodies
  • Adult MG has bimodal distribution:
    • First peak in second and third decades affecting mostly women
    • Second peak in sixth and seventh decades affecting men
  • Patients with MG may have other autoimmune conditions (e.g., lupus, rheumatoid arthritis, Graves)

Etiology

  • Antibody-mediated attack on nicotinic acetylcholine receptors on the postsynaptic membrane of the neuromuscular junction
  • Up to 20% of patients may be acetylcholine receptor antibody (AChR Ab) negative:
    • Antibodies against other postsynaptic proteins (e.g., muscle-specific receptor tyrosine kinase, MuSK) may be involved
  • Many medications may worsen myasthenic weakness:
    • Aminoglycosides, macrolides, quinolones, antimalarials
    • Local anesthetics
    • Antidysrhythmics (propafenone, quinidine, procainamide)
    • β-blockers, calcium-channel blockers
    • Anticonvulsants (phenytoin, carbamazepine)
    • Antipsychotics (phenothiazine, atypicals)
    • Neuromuscular blocking agents
    • Statins
    • Iodine-containing radiocontrast
  • Penicillamine can induce anti-AChR antibody production

-- To view the remaining sections of this topic, please or --

Basics

Description

  • Autoimmune antibody–mediated condition that results in painless, fatigable skeletal muscle weakness
  • Ocular or generalized:
    • Ocular (eyelids and extraocular) muscle weakness:
      • Most common initial symptom (60%)
      • ∼80% of myasthenia gravis (MG) patients who present with ocular weakness initially will progress to general weakness within 2 yr
    • Generalized:
      • Usually affects proximal limbs, axial muscle groups such as neck, face, bulbar muscles
  • Acute or subacute, with relapses and remissions
  • Associated with thymoma in 10–15% and thymic hyperplasia in 60–70%
  • Myasthenic crisis:
    • Respiratory failure or inability to protect airway due to weakness
    • Triggers:
      • Infection
      • Surgery
      • Trauma
      • Pregnancy
      • Medication changes (e.g., rapid tapering of steroids)
    • Difficult to distinguish from cholinergic crisis resulting from excessive doses of acetylcholinesterase (AChE) inhibitors:
      • Cholinergic crisis may also include muscarinic effects such as sweating, lacrimation, salivation, and GI hyperactivity in addition to weakness
      • Cholinergic crisis due to therapeutic AChE inhibitor use is rare, typically at very high doses

Epidemiology

  • Pediatric MG is rare and distinct:
    • Congenital MG: Genetic defect
    • Juvenile MG: Autoimmune disorder
    • Transient neonatal MG: Postdelivery complication from placental transfer of maternal antibodies
  • Adult MG has bimodal distribution:
    • First peak in second and third decades affecting mostly women
    • Second peak in sixth and seventh decades affecting men
  • Patients with MG may have other autoimmune conditions (e.g., lupus, rheumatoid arthritis, Graves)

Etiology

  • Antibody-mediated attack on nicotinic acetylcholine receptors on the postsynaptic membrane of the neuromuscular junction
  • Up to 20% of patients may be acetylcholine receptor antibody (AChR Ab) negative:
    • Antibodies against other postsynaptic proteins (e.g., muscle-specific receptor tyrosine kinase, MuSK) may be involved
  • Many medications may worsen myasthenic weakness:
    • Aminoglycosides, macrolides, quinolones, antimalarials
    • Local anesthetics
    • Antidysrhythmics (propafenone, quinidine, procainamide)
    • β-blockers, calcium-channel blockers
    • Anticonvulsants (phenytoin, carbamazepine)
    • Antipsychotics (phenothiazine, atypicals)
    • Neuromuscular blocking agents
    • Statins
    • Iodine-containing radiocontrast
  • Penicillamine can induce anti-AChR antibody production

There's more to see -- the rest of this entry is available only to subscribers.