Elongated Clivus with Deficient Anterior Atlantal Arch and Congenital Posterior Atlantooccipital Dislocation: Pathoembryology and Management Nuances of a Rare Form of Proatlas Segmentation Anomaly

Background: Proatlas segmentation anomalies represent a rare subset of congenital craniovertebral junction anomalies. In this condition, the structures originating from the proatlas, such as the clivus, occipital condyles, foramen magnum rim, and odontoid tip, may demonstrate congenital anomalies, usually without any spinal instability. Elongated clivus, as a result of nonsegmentation of the odontoid tip from the rest of the proatlas, has been reported before to cause ventral spinal cord compression. We report such a case with certain unreported other associations and explore the pathoembryology and management options of such complex anomalies. Case Description: An 8-year old girl presented with a 2-year history of progressive spastic quadriparesis. On neuroimaging, the anterior arch of the atlas was deficient, the odontoid process was foreshortened, and the clivus was elongated, encroaching into the spinal canal leading to ventral spinal cord compression. Additionally, there was rotatory posterior dislocation of the occipital condyles onto the posterior atlantal arch and vertebral artery anomaly. This patient underwent transoral decompression followed by occipitocervical fusion using rods and screws with satisfactory results. Conclusions: Proatlas anomalies are rare, varied, and often subtle enough to go unrecognized. Knowledge of the embryology and its aberrations is necessary to understand these anomalies. Our case describes a rare form of bony anomalies pertaining to the fate of the proatlas with accompanying atlanto-occipital dislocation.