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  • Autoimmune disease where antibodies are produced to the (postsynaptic) acetylcholine (ACh) receptor of neuromuscular junction (NMJ), possibly originating in thymus
  • Ten percent of patients have congenital myasthenia with abnormal ACh receptors; cholinesterase inhibitors will be ineffective in these patients
  • Results in functionally decreased (70–80%) number of postsynaptic receptors
  • Incidence of about 1:10,000
  • Before age of 40 years, 2F/1M, 40% thymus enlargement
  • After age of 40 years, 1F/1M, 20% thymoma
  • Most commonly affects ocular muscles, but can affect any skeletal muscle, including muscles of respiration
  • Characterized by skeletal muscle weakness that improves with rest
  • Clinical course often marked by periods of exacerbations and remissions
  • Treatment:
    • Cholinesterase inhibitors to increase amount of ACh at NMJ
    • Corticosteroids
    • Immunosuppression (mycophenolate mofetil, azathioprine, cyclosporine, tacrolimus, and occasionally cyclophosphamide)
    • Plasmapheresis to remove antibodies, IVIg
    • Thymectomy
  • Pregnancy:
    • Worsening in first 4 months, at delivery, and postpartum (up to 3 weeks)
    • Optimize therapy; immunosuppressants are contraindicated
    • Epidural for delivery or c/s; avoid high level; consider ropivacaine (less motor block)
    • Breastfeeding OK if well controlled
    • Neonatal myasthenia (antibodies cross placenta) in 20–30%: weak cry, difficulty feeding, occasional respiratory distress; treat with cholinesterase inhibitors, plasmapheresis

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Differences between Eaton–Lambert Syndrome and Myasthenia Gravis
SyndromeMyasthenia gravisEaton–Lambert syndrome
Association with cancerNot associated with cancer
  • Usually small cell lung cancer (SCLC), but also non-SCLC, lymphosarcoma, malignant thymoma, or carcinoma of the breast, stomach, colon, prostate, bladder, kidney, or gallbladder
  • Clinical manifestations often precede cancer diagnosis by 2–4 years
PathophysiologyAutoimmune attack directed against the postsynaptic acetylcholine receptor, results in impaired ACh effect at the NMJ
  • Autoimmune attack directed against the voltage-gated calcium channels (VGCCs) on the presynaptic motor nerve terminal results in impaired ACh release
  • Parasympathetic, sympathetic, and enteric neurons are all affected
Clinical diagnosis
  • Increasing weakness with repeated effort
  • Improvement with rest
  • Increasing strength with repeated effort
  • Reappearance of tendon reflexes after a period of muscle contraction by the patient
Epidemiology
  • Relatively common
  • Bimodal distribution (see above)
  • Rare (˜400 cases in the United States at any given time)
  • Typically in >60 years old, but has been reported in children
Autonomic findingsNoneDysautonomia: dryness of the mouth, impotence, difficulty in starting the urinary stream, and constipation
Treatment (besides supportive)See below
  • Treatment of underlying neoplasm
  • Cholinesterase inhibitors
  • 3,4-Diaminopyridine
  • Immunosuppression with corticosteroids, IVIg, guanidine, aminopyridines, and azathioprine
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Main Cholinesterase Inhibitors Used to Treat MG
MedicationOnset/durationTypical dosageSide effects
Pyridostigmine (Mestinon)PO: 15–30 min/3–4 hPO: 60 mg qidMuscarinic side effects (diarrhea, abdominal cramps, salivation, nausea), treated with atropine/diphenoxylate or loperamide
Neostigmine (Prostigmin)
  • PO: 1 h/90 min
  • IM: 30 min/1 h
  • IV: immediate/20 min
  • PO: 15 mg
  • IM: 1.5 mg
  • IV: 0.5 mg
Same
Ambenonium (Mytelase)PO: 20–30 min/3–8 hPO: 5–25 mg qidSame
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Severity: Osserman–Genkins Classification...
Stage IOcular symptoms only (diplopia, ptosis)
Stage IIA

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