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Neonatal myasthenia gravis

Weakness and hypotonia in newborns can be caused by disorders of the neuromuscular junction. Transient acquired neonatal myasthenia gravis is characterized by abnormal neuromuscular transmission leading to muscle fatigability and weakness. Transient neonatal myasthenia gravis occurs in 10 to 20 percent of infants born to mothers with myasthenia gravis. Most mothers of affected infants have active disease, although some may have no clinical symptoms or may be in remission.

Myasthenia gravis is an autoimmune disorder caused by antibodies directed against the acetylcholine receptor (AChR), resulting in postsynaptic inhibition of neuromuscular transmission. Maternal AChR antibodies transferred to the fetus are responsible for transient neonatal myasthenia gravis. Newborns with myasthenia have generalized weakness and hypotonia. Bilateral facial weakness often occurs while ptosis and ophthalmoplegia occur less frequently. Bulbar weakness is frequent, leading to poor sucking, swallowing and weak cry. Pooling of secretions and respiratory muscle weakness may lead to respiratory failure and the need for assisted ventilation.

Management of neonatal myasthenia is supportive with small frequent feedings via nasogastric/orogastric tube and assisted ventilation as needed. Neostigmine is given IM or SC prior to feedings until swallowing and respiratory symptoms improve at which time neostigmine can be given PO. As symptoms continue to improve clinically, neostigmine doses can gradually be decreased until ultimately discontinued. Upwards of 90% of patients recover fully by two months; tube feeding and assisted ventilation are generally only necessary for one to two weeks while the average duration of neostigmine treatment is four weeks.