MYONEURAL JUNCTION DISORDERS
due to postsynaptic myoneural junction disorders may result from destructive,
metabolic, or dysgenetic problems (Figure 140.1).
features of generalized hypotonia due to presynaptic myoneural junction
dysfunction. Arrow indicates the anatomical location of the injury (postsynaptic
myoneural junction); SNST: slow nerve stimulation test; RNST: rapid nerve
stimulation test; DES: destructive; MET: metabolism; DYS: dysgenesis.
Transient Myasthenia Gravis
transient myasthenia gravis occurs in 10% to 20% of neonates born to symptomatic
or asymptomatic myasthenic mothers.
Neonatal transient myasthenia gravis is due to a decreased number of available
nicotine acetylcholine receptors at the postsynaptic striated muscle membrane.
This occurs because maternal antibodies cross the placenta and bind to
the receptor. It is still unclear why all neonates of myasthenic mothers
do not develop transient myasthenia gravis since the antibodies always
cross the placenta. Neonatal transient myasthenia gravis presents in the
first week of life, usually immediately after birth. Limb and axial hypotonia
and weakness is overshadowed by the signs of pontine and medullar cranial
nerve musculature dysfunction (Figure 140.1). Fatigability is the hallmark
of myasthenia gravis. Muscle stretch reflexes are normal. Neonatal transient
myasthenia gravis is diagnosed by using neostigmine 0.15 mg/kg administered
intramuscularly or edrophonium at a dose of 0.15 mg/kg intramuscularly
or subcutaneously or 0.1 mg/kg intravenously.