Retroperitoneal ectopic neural mass: "abdominal brain"--presentation of two cases and proposal of classification of paraneuraxial neural ectopia.

An encapsulated mass of brain tissue was found in the retroperitoneum of a fetus of gestational week 15 and a boy of age 3 years. The masses possessed fibrous tissue that bound them to the spine and intraspinal connective tissue, respectively, but there was no evidence of direct continuity of the ectopic brain tissue with the normal central nervous system. There was no dysraphism. In our fetal case, possible Foix-Alajouanine anomaly was additionally found. The ectopic neural tissue in the retroperitoneal region may be termed "abdominal brain." In the literature, an identical state has been described in the head (paracranial region) but there are no other records of the paraspinal region. Despite the different locations of the masses (head/paracranial or retroperitoneum/paraspinal), these ectopic brain masses should belong to the same disorder spectrum of the paraneuraxial neural ectopia, a new concept.