Anencephaly, dorsal hypermelia, and duplication of the vertebral column: a rare type of rachipagus conjoined twins.

The rare combination of hypermelia with craniorachischisis and duplication of the vertebral column has been found in three cases, two published and one previously unreported. Even though the previous authors had come to other conclusions, careful analysis of the details in these twins leads to the inescapable conclusion that they were all dorsally united parasitic rachipagus twins. The nature of the original union was clearly indicated by (1) the articulation of autositic ribs to two different vertebral columns but to a single ventral sternum, (2) the articulation of parasitic ribs also to the two vertebral columns but fusion of the ribs and clavicles dorsally without an interposed sternum, and (3) the dorsal location of the rudimentary viscera of the parasite. The orientation of notomelic arms was confirmatory but not diagnostic. The proposed embryologic origin of these twins is presented.