Central nervous system in twin reversed arterial perfusion sequence with special reference to examination of the brain in acardius anceps.

The twin-reversed arterial perfusion (TRAP) sequence, or acardia, is the most severe complication in monozygotic twinning. Although more than 400 cases with TRAP sequence were reported since 1533, thorough investigations of the brain in those cases with a rudimentary head remained infrequent. We report a TRAP sequence with microcephaly and a severely rudimentary brain anlage. Neuropathologic examination clearly demonstrated two types of change: (1) developmental arrest of brain at the prosencephalic stage (holoprosencephaly), and (2) hypoxic damage to the holospheric brain mantle with cystic change (hydranencephaly). With reference to previous studies in experimental animals showing that lack of oxygen during early embryogenesis can induce severe disruptions of head-brain and heart formation, it is concluded that oxygen deficiency due to TRAP may be responsible not only for the encephaloclastic changes in the acardius anceps, but for the developmental arrest of the brain cases as well. This would make it unnecessary to postulate additional primary causes such as asymmetric zygote cleavage (Schwalbe, '07) for the maldevelopment.