Skull Base

The skull base houses lots of valuable real-estate. It is the primary site of many pathologies that a radiologist often encounters-malignant tumors, benign osseous tumors or fibrous dysplasia, infection, congenital anomalies, and traumatic findings among them. It contains key conduits to the orbit, face, neck, spine, and contains the organs of hearing and vestibular function. These conduits include important foramen and fissures containing vital nerves (including the cranial nerves), arteries, and venous structures. Treatment of skull base pathology is complex, a cross-roads of disciplines.

Viewed from above, the skull base has three separate regions, the anterior, middle, and posterior cranial fossae. These are comprised of 5 bones, the: frontal bones, temporal bones, ethmoid bone, sphenoid bone, and occipital bone. The skull base forms from endochondral ossification whereas the remainder of the skull forms from membranous ossification. This is the reason the skull base (and foramen magnum) is small in the setting of achondroplasia whereas the rest of the skull is relatively large.

Traumatic shearing of the basal frontal lobe over the anterior cranial fossa and the basal temporal lobe over the petrous ridge can result in cerebral contusion.

Reference

Raut AA et al. Imaging of the skull base: pictorial essay. Indian J Rad Imaging 2012;22(4):305-316

The anterior cranial fossa is formed predominantly by the frontal bones, with a small contribution by the the ethmoid bone medially and lesser wing of the sphenoid posteriorly. The middle cranial fossa extends from the crest of the petrous ridge to the lesser wing of the sphenoid. It houses many important foramen and fissures that contain critical neurovascular structures and communicate with the suprahyoid neck and orbit via the pterygopalatine fossa. The posterior cranial fossa extends from the petrous ridges to the foramen magnum.

3D volume rendered image of the skull base in a 10 week old, inferior view. The mendosal suture fuses early, is faintly perceptible here, and separates the endochondral portion of the occipital bone (including supraoccipital) from the membranous (interparietal). The Kreckring tubercle fuses in the first month of life. The anterior and posterior intraoccipitial sutures normally close in mid-childhood.

Reference

Idriz S et al. CT of normal developmental and variant anatomy of the pediatric skull: distinguishing trauma from normality. Radiographics 2015;35(5)

The craniocervical junction (CVJ) is an important region at the cross-roads of the brain/skull base and spinal cord/spine. There is a rich body of literature and numerous described lines/angles, and measurements for assessment of a wide range of congenital or acquired pathology of the CVJ, but for some reason this region is often under-emphasized in educational resources. The CVJ consists of related areas of the occipital bone, the atlas (C1) and axis (C2). Beyond significance for the CVJ, C1 and C2 are further detailed in the spine section. Pathology of the CVJ can affect cranial nerves IX-XII and the cervicomedullary junction.

Here, just the key landmarks, lines, angles, and concepts will be introduced. For more in-depth reading, please see the excellent reference below by Dr. Smoker.

Key landmarks include the hard palate, tuberculum sella, posterior border of the clivus, basion, opisthion, odontoid process, and posterior border of the C2 vertebral body.

Key lines and angles include.......

A variety of acquired and congenital pathologies can afflict the CVJ. These can result in os odontoidium, platybasia (flat head), basilar invagination (especially anomalies of the occipital bone and atlanto-occipital segmentation anomalies), and basilar impression.

including involvement with several syndromes that 'soften' the bone. Key Syndromes include trisomy 21, achondroplasia, Klippel-Feil, mucopolysaccharidoses, and osteogenesis imperfecta.