THE occurrence of myasthenia in the newborn infant is relatively rare and during the past 25 years about 40 cases have been reported (Osserman, 1958; Stern et al., 1964). Apart from its potential gravity neonatal myasthenia has aroused much interest from the point of view of its aetiology. The transient nature of the disorder has understandably focused attention on the possible transfer from the myasthenic mother during intrauterine life of some factor, whose adverse influence on the well-being of the infant has seldom persisted beyond about six weeks. The passive transfer of maternal antibodies such as have been detected in the serum of myasthenic adult patients (Strauss et al., 1960; Beutner e ta l . , 1962 ; van der Geld et al., 1963) or of antinuclear factor (White and Marshall, 1962) are attractive possibilities which so far have not been cleaIly shown to be involved in neonatal myasthenia. It has also been suggested that neonatal myasthenia is a result of placental transmission of a neuromuscular blocking substance, the origin of which may be the thymus gland (Keynes, 1949; Wilson et al., 1953; Goldstein, 1968). This explanation has been countered by reports of neonatal myasthenia in cases in which the mother had previously undergone thymectomy (Schlezinger,
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