The proatlas: a comprehensive review with clinical implications.

BACKGROUND: The proatlas is derived from the fourth occipital sclerotome in human embryos. It usually fuses with the three upper occipital sclerotomes to form the occipital bone. However, this does not always occur. Manifestations of a partial proatlas structure may persist due to failure of fusion. CLINICAL CONSIDERATIONS: These embryological remnants can induce several symptoms in humans, ranging from mild to severe. On occasion, this structure can go unnoticed until a precipitating traumatic event results in symptoms. Proatlas segmentation abnormalities form bony masses at C1 and the foramen magnum. A number of surgical procedures have been devised to rectify the resulting neural compression and vascular compromise. DISCUSSION: This paper will discuss the development of the proatlas and the resultant anomalies associated with its failure to merge with the occipital sclerotomes to form the occipital bone. In addition, some consideration of comparative anatomy and surgical techniques will be presented.