Occipital vertebra

Occipital vertebrae are rare anatomical variants that result from incomplete or aberrant fusion of occipital bone ossification centers. There is a broad spectrum of occipital vertebrae, the more common variants include:

• third condyle (condylus tertius)

• basilar process

• prebasioccipital arch

• paracondylar process

Clinical presentation

Occipital vertebrae are usually identified incidentally. If they fuse or articulate with the atlas (C1) or axis (C2) they can disrupt the occipitocervical articulation, causing neck pain and dysfunction that may necessitate surgery ^1,2.

Radiographic features

These variants are best detected on CT, where their characteristic location and appearance aids in differentiating them from fractures ^1,3.

Development

The hypochordal arch (or bow) is a part of the embryonic proatlas, derived from the first cervical sclerotome and the caudal part of the fourth occipital sclerotome ⁴. It lies on the ventral surface of the the clivus, extending across the anterior rim of the foramen magnum between the anteromedial borders of the occipital condyles ^1,4. The hypochordal arch gives rise to the anterior arch of the atlas and subsequently undergoes a process of controlled assimilation into the occiput, with the midline part disappearing before the lateral parts ¹.

Incomplete regression of the hypochordal arch leads to a spectrum of anatomical variants:

• third condyle (condylus tertius): midline part persists

• basilar process: lateral part persists (can be unilateral or bilateral)

• prebasioccipital arch: entire hypochordal arch persists

The paracondylar process develops due to a phenomenon known as caudal shifting, whereby the component sclerotomes of the embryonic proatlas fail to separate normally. The broad array of paracondylar process subvariants is said to be a result of variable degrees of separation between the sclerotomes ⁵. Some authors would advocate for an alternate acquired or post-traumatic mechanism secondary to tension on the rectus capitis lateralis muscle or lateral atlanto-occipital ligament, as both attach near the jugular process from which the paracondylar process arises ^5,6.