Atlanto-occipital assimilation with arcuate foramen, vertebral artery hypoplasia, and elongated styloid process: a case report.

We encountered a partial type of atlanto-occipital assimilation (AOA), characterized by the fusion of the atlas (C1) with the base of the occiput, in a male cadaver during a student dissection course. The lateral mass and lateral part of the anterior arch of C1 were fused with the corresponding basilar part of the occipital bone on both of the right and left sides. The transverse process of C1 was involved in the fusion on the right side but not on the left side. The atlanto-occipital joint was obliterated on the right side, while it was present on the left side. Middle part of the anterior arch and the entire posterior arch were free of the occipital bone. The foramen magnum was stenosed and deformed due to the protrusion of the hypertrophied fused bone mass. Complete form of arcuate foramen (AF) was unilaterally present on the right side. A marked difference was observed in the diameters between the right and left vertebral arteries (VAs) at the C2-C6 levels, indicating right VA hypoplasia (VAH). In addition to these anomalies, the specimen had bilateral elongated styloid processes (ESPs). The coexistence of multiple anatomical variations, including AOA, AF, VAH, and ESP, in the same cadaver is rare. This report describes the observed anatomical characteristics and discusses the significance of these variations from morphological and clinical perspectives, thereby contributing to a deeper understanding of craniocervical anatomical variations.