Pediatric emergency medicine trisk 2643 2643
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In myasthenia gravis, antibodies directed against the acetylcholine receptor
protein of the postsynaptic neuromuscular junction cause intermittent failure of
neuromuscular transmission. Myasthenia manifests as fluctuating weakness of
cranial and skeletal musculature, exacerbated by exertion. The onset of symptoms
may be insidious or acute. Most cases affect the cranial nerves, and any cranial
nerve can be involved in combination or isolation. Bilateral ptosis is the most
common cranial nerve deficit, followed in incidence by oculomotor impairment.
Generalized truncal and limb weakness is present at onset in up to half of cases
and eventually develops in most children with myasthenia. The diagnosis should
be suspected if there is a history of worsening weakness during continual activity
or if fatigability of muscle strength is demonstrable.
More commonly a disease seen in adults, myasthenia gravis occurs in children
in three major forms: transient neonatal, infantile (congenital), and juvenile (most
common). The juvenile form of myasthenia clinically mimics the adult disease.
The mean age of onset is 8 years, with a female predominance of approximately
4:1. Illnesses confused with myasthenia include the muscular dystrophies,
congenital myopathies, inflammatory myopathies, acute and chronic
polyneuropathies, and in the infant, botulism.
EMG can be used to provide electrophysiologic evidence for myasthenia
gravis, and the Tensilon (edrophonium) test may be used, in consultation with a
neurologist, to confirm the diagnosis.
Although myasthenia gravis is potentially life threatening, specific
management can usually be delayed until after diagnosis is made. Ventilatory
support may be required if there is respiratory compromise. If severe weakness is
present, the child should be hospitalized. Treatment is begun with the use of
cholinesterase inhibitors to prolong the availability of acetylcholine at the
neuromuscular junction. At present, the anticholinesterase of choice is
pyridostigmine (Mestinon).
Myasthenia has a fluctuating, unpredictable course that can be exacerbated by
intercurrent illness and by certain drugs, particularly the aminoglycoside
antibiotics. In a known myasthenic, rapid worsening and respiratory compromise
(myasthenic crises) may be difficult to differentiate from deterioration secondary
to overdose of anticholinesterases (cholinergic crises) because the muscarinic side
effects of the anticholinesterases, such as nausea, vomiting, cramps, and muscle
fasciculations, may be absent.
Differentiation can be made by giving 1 to 2 mg of IV edrophonium after
ensuring respiratory sufficiency. This should result in rapid improvement in the
patient with a myasthenic crisis. This procedure may be falsely positive, however,
