Normal variants and congenital anomalies in the region of the obelion.

A number of anatomic bony variants and several congenital anomalies are known to occur at the level of the obelion, that is, in the posterior interparietal region of the head, a short distance in front of the lambda. Among them are: (1) remnants of the embryonic parietal incisura, including parietal fissure, small and large parietal fontanelle, obeliac bones, persistant midline parietal foramen, and small and large parietal foramina; (2) encephalomeningoceles and related defects, including true encephaloceles, herniation of midline intracranial cysts (diencephalic and Dandy Walker cyst), epicranial arachnoid cysts, and scalp defects with ectopic glial tissue; and (3) congenital scalp and skull defects (cutis aplasia congenita). A review of these defects is presented with some illustrative examples not previously reported.