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Key Features

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Essentials of Diagnosis
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  • Asymmetric, variable weakness, usually coming on or increasing with use (fatigue)

  • Involves extraocular, bulbar, and respiratory muscles

  • Positive response to neostigmine and edrophonium

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General Considerations
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  • Characterized by easy fatigability of muscles, particularly the extraocular, bulbar, and respiratory muscles

  • In the neonatal period or in early infancy, the weakness may be so constant and general that an affected infant may present nonspecifically as a "floppy infant"

  • Three general categories are recognized

    • Neonatal (transient) myasthenia

    • Autoimmune myasthenia gravis

    • Congenital myasthenia

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Clinical Findings

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Symptoms and Signs
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  • Neonatal (transient) myasthenia

    • Bulbar weakness, difficulty feeding, weak cry, and hypotonia present before the third day of life

    • Occurs in 12–19% of infants born to myasthenic mothers as a result of passive transfer of maternal acetylcholine receptor antibody across the placenta

  • Autoimmune myasthenia gravis

    • Characterized by fatigable and asymmetric weakness

  • Tends to recur and remit

  • Can be precipitated by illness or medications such as aminoglycoside antibiotics

    • Typical signs include difficulty chewing foods like meat, dysphagia, nasal voice, ptosis, ophthalmoplegia, and proximal limb weakness.

    • Other autoimmune disorders such as rheumatoid arthritis and thyroid disease may be associated findings

    • Systemic or bulbar symptoms develop within 2 years in 50% of children and in 75% within 4 years

  • Congenital myasthenic syndromes

    • These syndromes are a heterogenous group of hereditary, nonimmune disorders of presynaptic, synaptic, or postsynaptic neuromuscular transmission

    • Symptom onset is before the age of 2 years and can vary from mild motor delay to dramatic episodic apnea

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Diagnosis

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Laboratory Findings
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  • Neostigmine test

    • May be preferable to the edrophonium (Tensilon) test in newborns and very young infants because the longer duration of its response permits better observation, especially of sucking and swallowing movements

    • There is a delay of about 10 minutes before the effect may be manifest

    • Clinician should be prepared to suction secretions, and administer atropine if necessary

  • Edrophonium testing

    • Used in older children who are capable of cooperating in certain tasks and who exhibit easily observable clinical signs, such as ptosis, ophthalmoplegia, or dysarthria

    • Maximum improvement occurs within 2 minutes

  • Serum acetylcholine receptor binding, blocking, and modulating antibodies typically, though not always, are found in autoimmune myasthenia gravis

  • Commercially available genetic testing is limited for patients with congenital myasthenic syndromes

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Imaging
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  • Chest radiograph and CT scanning in older children may show thymic hyperplasia

  • Thymomas are rare

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Diagnostic Procedures
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  • Electrophysiologic studies

    • May be helpful when myasthenic syndromes are considered

    • Repetitive stimulation of a motor nerve at slow rates of 2–3 Hz with recording over an appropriately chosen muscle reveals a progressive fall in compound muscle action potentials by the fourth to fifth repetition in myasthenic patients

    • At higher rates of stimulation of 50 Hz, there may be a transient repair ...

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