Neuroanatomic examination of spina bifida aperta and the Arnold-Chiari malformation in a 130-day human fetus.

A 130-day human female fetus with the Arnold-Chiari malformation and thoracolumbar myeloschisis revealed evidence of neuroectodermal-mesodermal spatial dyssynchrony. The rhombencephalon and the cervico-medullary junction appear most affected. The phylogenetic and ontogenetic development of the transition zone between brain and spinal cord is reviewed. It is hypothesized that the etiologic event responsible for the Arnold-Chiari malformation is the caudal "displacement" of the site of initial fusion of the neural folds. This is believed to result in the posterior displacement of the cervico-medullary junction and myeloschisis (the Arnold-Chiari malformation, type II).