Craniorachipagus symmetric conjoined twinning: Identification of a fourth case and delineation of the phenotype

Background: Symmetric craniorachipagus is an exceedingly rare type of bi-umbilical conjoined twinning, known from only three scantily described cases. Case: We identified a fourth, previously described case that was misdiagnosed as janiceps and as pygopagus. It concerned dorsally conjoined twins that were part of a triplet pregnancy, spontaneously born at 22 weeks of gestation. Radiography confirmed union at the occipital craniums and the thoracolumbar vertebral columns. Both twins had their own separate umbilical cords. To delineate the phenotype of craniorachipagus and to differentiate it from rachipagus without cranial involvement, we compared the present case with the previous three reported cases and with the historical literature concerning comparable conditions. Furthermore, we discuss why exceedingly rare conditions such as these are presently underreported in the literature. Conclusion: Symmetric craniorachipagus is a type of bi-umbilical conjoined twinning, to date represented by four confirmed cases that share a similar phenotype. This includes dorsal conjunction at the sides of the occipital craniums and the vertebral columns, in the absence of any visceral connections. Details on its etiopathogenesis and apparent lethality await additional case investigations. No unequivocally confirmed cases of symmetric rachipagus without cranial involvement have been reported and its existence in humans has yet to be proven.