Transient neonatal myasthenia is a rare complication of maternal myasthenia gravis.
Around 10-15% of infants born to mothers with antibodies against acetylcholine receptor (AChR), and less frequently against muscle-specific kinase (MuSK).
Symptoms are usually apparent from the 3rd day of life, and in almost all cases there is bilateral facial paresis and feeding difficulties. Symptoms usually resolve within the first month of life.
However, in a small group of patients, a much more severe condition can occur, with fetal arthrogryposis and bulbofacial paresis persisting into adulthood, known as fetal AChR inactivation syndrome, caused by antibodies against the fetal gamma subunit, which is replaced in adults by the epsilon subunit.
Receptor inactivation occurs due to exposure to specific antibodies during a critical period of development.