The Arnold-Chiari type II malformation at midgestation.
The Arnold-Chiari malformation type II (ACMII) is reported to be reversible after closure of a myelomeningocele at midgestation. To elucidate the developmental state of the ACMII malformation at the approximate time fetal surgery is performed, the ACMII of a 20-week human fetus was investigated in vitro using high-field magnetic resonance microscopy at 9.4 T and compared with the hindbrain of a neurologically intact fetus of the same gestational age. Up to 20 weeks of gestation, the developmental failures caused by the early embryonic herniation of the posterior fossa contents are the dominant feature of fetal ACMII, but after 20 weeks, the accelerated and disproportionate growth of the cerebellum dominates. As midgestational surgery stops the leakage of cerebrospinal fluid, the posterior fossa will expand in time to allow further normal growth of both the cerebellum and brain stem. Some early developmental anomalies already present in the primitive rhombencephalon due to early embryonic hindbrain herniation as well as some intra-axial anomalies are probably not reversible.