Jugular foramen

The jugular foramen courses anteriorly, laterally, and inferiorly as it insinuates itself between the petrous temporal bone and the occipital bone.

The foramen is divided into 3 parts:

1. Petrous part through which the petrosal sinus passes
2. Intrajugular part through which the nerves passpasses
3. Sigmoid part through which the sigmoid sinus passes

The intrajugular part is positioned between the sigmoid and petrosal parts, and is the site of bony prominences called the intrajugular processes on the opposing surfaces of the temporal and occipital bones.

Gross anatomy

The jugular foramen is divided by a fibrous or bony septum, the jugular spine,  into:

• the pars nervosa: smaller and anteromedial
• the pars vascularis: larger and posterolateral

Pars nervosa

The pars nervosa is the anteromedial portion of the jugular foramen and is smaller than the larger, posterolateral pars vascularis. It contains:

• inferior petrosal sinus
• glossopharyngeal cranial nerve (CN IX)

The inferior petrosal sinus drains the cavernous sinus and courses in the petro-occipital fissure adjacent to the clivus prior to its exit through the pars nervosa and subsequent drainage into the jugular vein beneath the foramen.

The glossopharyngeal nerve yields a tympanic branch (Jacobson`s nerve) which reaches and supplies the middle ear along with the inferior tympanic artery via the inferior tympanic canaliculus which is occasionally seen at CT in cross section at the level of the carotico-jugular spine.

Pars vascularis

The pars vascularis is the posterolateral portion of the jugular foramen and is larger than the smaller anteromedial portion termed the pars nervosa. It contains:

• jugular bulb: a venous expansion between the endocranial sigmoid sinus and the exocranial jugular vein
• vagus nerve (CN X)
• spinal accessory nerve (CN XI)

The vagus nerve yields an auricular branch (Arnold nerve) via the mastoid canaliculus on the lateral wall of the foramen adjacent to the mastoid segment of the facial nerve.

Radiographic features

The size of the normal jugular foramen is remarkably variable and asymetrical. However, the normal foramen will always be well-corticated. Therefore, the bony margin (and not the size) of the foramen should guide diagnostic evaluation.

CT

CT appearance of the foramen depends upon the level and angulation of the scan. The foramen has been described as having the shape of a sitting duck on the highest axial cuts.