Craniopagus twins: surgical anatomy and embryology and their implications.

Craniopagus is of two types, partial and total. In the partial form the union is of limited extent, particularly as regards its depth, and separation can be expected to be followed by the survival of both children to lead normal lives. In the total form, of which three varieties can be recognized, the two brains can be regarded as lying within a single cranium and a series of gross intracranial abnormalities develops. These include deformity of the skull base, deformity and displacement of the cerebrum, and a gross circulatory abnormality. It is considered that these and other abnormalities, unlike the primary defect, which is defined, are secondary ones; explanations for them, based on anatomy and embryology, are put forward. The implications of the various anomalies are discussed and the ethical aspects of attempted separation in these major unions considered.