Hypotonia due to postsynaptic myoneural junction disorders may result from destructive, metabolic, or dysgenetic problems (Figure 140.1).

Figure 140.1.— Salient 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.

Neonatal Transient Myasthenia Gravis
Neonatal 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.