Superior View of the Base of the Skull
This article will describe the anatomical structures which can be seen from a superior view of the skull base. This will include the various foramina, the nerves and arteries that pass through them, but also the structures of the brain and cerebellum. This view may expose some daunting anatomy, but it is not difficult to learn once you associate the various structures with a function. The foramina and landmarks all have a precise function and location, all of which is described in detail.
Anterior Cranial fossa
The sella turcica is a saddle shaped cavity which is within the body of the sphenoid and contains the pituitary gland. Its name is from the ‘Turkish saddle’ which was used by soldiers during the times of the Ottoman Empire and was famous for having four corners. Crucial structures that form the boundaries of the sella turcica are the anterior and posterior clinoid processes. They are small projections from the sphenoid bone, and forms the anterior and posterior posts of the ‘four poster bed’ like structure in which the pituitary gland sits. The sella turcica lies within the body of the sphenoid bone, which has a wedgelike shape and looks like a key that slots into the floor of the skull. The sphenoid bone is found as part of the posterior part of the anterior cranial fossa, as well as the middle cranial fossa. It has numerous foramina and harbours the pituitary gland.
Moving anteriorly we have the frontal bone which forms a large part of the anterior cranial fossa with its orbital plates, as well as the wall of the skull itself. In the anterior midline is the ethmoid bone, specifically the cribriform plate section. In between the plates we have a bony ridge known as the Crista Galli which is the latin word for ‘rooster comb.’ The perpendicular plate of the bone also forms the posterosuperior part of the bony nasal septum.
In the anterior cranial fossa between the two lesser wings of the sphenoid is the jugum sphenoidale. It is a flat plane surface anterior to the sella turcica. It also articulates with the orbital surfaces of the frontal bone as well as the cribriform plate anteriorly. Anterior to this we have the superior surface of the orbital plate, which is the region on which the frontal lobe rests.
In the anterior most part of the anterior cranial fossa in the midline, is a small triangular bump known as the frontal crest, and it is structurally continuous with the superior sagittal sinus. Just posterior to the frontal crest is the foramen caecum which is a small opening just anterior to the crista galli. It allows the emissary vein from the nose to communicate with the superior sagittal sinus.
The sphenoid bone is a complex structure and it has many articulations. As a result, it has been given its own subsection which will address its various regions and functions.
Body of Sphenoid bone
There are a number of features of this bone which can be seen from above. The features include anteriorly, a plate of bone that articulates with the orbital plates laterally and the cribriform plate anteriorly. The anterior and posterior clinoid processes can also be seen forming the ‘four poster bed’ like structure that forms the sella turcica (Turkish saddle) in which the pituitary gland rests.
Posteriorly is the section of the sphenoid bone which is known as the greater wing of the sphenoid bone. This forms the anterior part of the middle cranial fossa. The structure contains numerous foramina which allow important cranial structures to leave the skull cavity. Within the greater wing of the sphenoid is the foramen rotundum. This allows the second branch of the trigeminal nerve (the maxillary branch of the trigeminal nerve) to leave the skull, and supply sensation to the skin over the upper jaw. The second foramen of the greater wing is the foramen ovale, which is lateral and slightly posterior to the rotunda. It allows the mandibular branch of the trigeminal nerve to leave the skull. The nerve goes on to supply sensation to the skin over the lower jaw. The third is the foramen spinosum, which forms the dot of the exclamation point formed by the ovale. It transmits the middle meningeal artery, middle meningeal vein and meningeal branch of mandibular nerve to enter the skull, and supply the meninges with blood. The wing also forms the anterior margin of the foramen lacerum, which is filled with cartilage during life.
Anterior to the greater wing of the sphenoid is the lesser wing of the sphenoid bone. This shelf of bone forms the posterior part of the anterior cranial fossa and extends laterally, and articulates with the orbital plate anteriorly.
Just posterior and medial to the lesser wing of the sphenoid in the midline is the prechiasmatic groove which can be found in the sphenoid bone medial to the optic canals. Slightly posterior to this structure is the Tuberculum Sella which is a small elevation. Within the floor of the middle cranial fossa is found the foramen Lacerum which is filled with cartilage in life and has a superior relationship the carotid canal. The Internal Carotid Artery enters the skull in the most medial portion of the opening.
Middle Cranial Fossa
This cavity is formed by the body and greater wings of the sphenoid mainly, as well as the petrous portion of the temporal bone posteriorly and the squamous portions laterally. An important part of the sphenoid bone is the dorsum sellae that is defined as the two posterior clinoid processes laterally and a small intervening section of bone. It forms the posterior part of the sella turcica, and leads into the clivus. The hypophyseal fossa is also known as the sella turcica, and is the socket of bone in which the pituitary gland sits.
In the depths of the middle cranial fossa we have the foramen ovale which is the oval shaped foramen which allows the mandibular branch of the trigeminal nerve to leave the skull. It lies lateral to the foramen lacerum, and anteromedial to the foramen spinosum. Anterior to this foramen we have the foramen rotundum which is a round foramen which lies inferior to the superior orbital fissure and anteromedial to the foramen ovale. Superomedial to the foramen rotundum is the optic canal which lies within the sphenoid bone, and lies medial to the two anterior clinoid processes. The optic chiasm lies posterior to these canals, and lies superior to the pituitary gland. The pituitary is known as the master gland as the hormones it secretes stimulates the formation of the vast majority of hormones within the body.
Posteromedial to the foramen lacerum is the carotid canal which can be seen extending further laterally. It allows the internal carotid artery to enter the skull and supply the anterior component of the circle of Willis.
In the side walls of the foramen magnum, the hypoglossal canal can be found. It allows the hypoglossal nerve to leave the skull. The nerve goes on to supply motor function to all of the muscles of the tongue (aside from palatoglossus which is supplied by the vagus nerve). Closely associated with the basilar part of the occipital bone as well as the foramen magnum is the Clivus. It can be described as the interior of the occipital bone, leading down from the posterior clinoid processes like a slide. The lower border of the clivus forms the anterior border of the foramen magnum. The basilar artery, which is formed by the anastomosis of the two vertebral arteries, lies in close association with the clivus.
Posterior to the foramen magnum is the internal occipital Crest. This is a small fold of bone within the occipital region of the cranial vault in the midline. It leads down into the foramen magnum and demarcates the separation of the two sides of the cerebellum. Moving to the anterior border of the middle cranial fossa is the arcuate eminence. This is a feature of the petrous (stone like) part of the temporal bone. Inferior to the arcuate eminence is the internal acoustic meatus which enables both the facial and vestibulocochlear nerve to leave the skull.
Foramen Magnum- This is latin for ‘large hole,’ and simply, this is the opening for the brainstem to become continuous with the spinal cord.
Within the floor can be found the carotid sulcus which is a small indentation which demarcates the internal carotid artery’s path. The arteries emerge through the carotid canal and travel forwards to pass through the cavernous sinus. They then undertake a complete 180’ degrees turn back and goes onto emerge medial to the anterior clinoid processes.
Posterior Cranial Fossa
Bilateral bowl like indentations are found in the floor of the posterior cranial fossa and are referred to as the cerebellar Fossa. They lie on either side of the internal occipital crest and contain the two lobes of the cerebellum in life.
The posterior cranial fossa is formed by the clivus in its anterior part, and its anterolateral border is the posterior face of the petrous portion of the temporal bone. The jugular foramen and foramen magnum are other obvious features as well as the cerebellar fossa posterolaterally.
Within the floor of the fossa and anterior to the foramen lacerum is found the trigeminal Impression. This is a bony impression of the trigeminal ganglion, which sits within the dura mater towards the apex of the petrous section of the temporal bone. The three major branches of the trigeminal nerve then travel forward to pass through their respective foramina.
Superior to the jugular foramen is the internal acoustic meatus which is located in the anterior medial section of the petrous portion of the temporal bone, and allows both the facial and vestibulocochlear nerve to leave the skull. Both these nerves arise directly next to one another from the pontomedullary angle.
At the posterior end of the posterior fossa is the occipital bone. This bone has bony features including the clivus anteriorly and the internal occipital crest in the posterior midline as well as the cerebellar fossa laterally. Lateral and superior to the occipital bone is the parietal bone. This bone has a very small role in the floor of the cranial vault. It forms the lateral wall of the middle cranial fossa and a small part of the posterior cranial fossa. Another feature is the internal occipital protuberance which is a small projection of the internal surface of the occipital bone in the midline. Running lateral from this point on either side are the transverse sinuses.
The floor of the middle cranial fossa and part of the posterior cranial fossa is formed by the temporal bone. This bone articulates with the sphenoid anteriorly and the occipital bone posteriorly. The various portions contribute to the floor of the middle cranial fossa and a part of the posterior cranial fossa, and has an important relationship with numerous cranial structures.
The dural venous sinuses of the brain converge and eventually drain into the sigmoid sinus which leaves a groove. These will then drain into the internal jugular veins.
- Trauma to the frontal lobe underlying the frontal bone, especially the orbitofrontal cortex will result in personality changes, namely disinhibition.
- Nasal infections may pass directly up into the brain if left untreated and have severe neurological consequences. They spread through to the cavernous sinus and may cause cavernous sinus thrombosis. The triangle of the face (which includes the nose and upper lips and philtrum) is an area which can transmit infection directly to the brain (as there are no venous valves to inhibit their flow).
- If there is raised intracranial pressure, it will cause ‘coning’ which is essentially the external protrusion of the brain stem, which may compress the vital respiratory centres and cause sudden death. Signs of intracranial pressure include dizziness, confusion, headaches and malaise. A late stage symptom is a ‘blown pupil’. This occurs as the parasympathetic fibres of the oculomotor nerve (which lie on the outside of the nerve) are responsible for constricting the pupil, and become compressed first, and therefore unable to perform their function. This is why in an emergency scenario, the pupils are checked to confirm a rise in intracranial pressure secondary to a bleed.
- Surgery to remove pituitary tumours is usually undertaken transphenoidaly, where the surgeon operates through the nose, drills through the thin plate of bone that surround the sphenoid sinus (being careful to avoid the ICA), and removes the tumour.
- Trauma to the cerebellum which lies deep to the inferior occipital bone will cause problems with coordination and proprioception from the lower limbs.
- If the patient has a pituitary tumour such as a prolactinoma, it may cause the gland to enlarge, and compress the optic chiasm superior to it. This will result in a bitemporal hemianopia, as the fibres that cross over (for the peripheral visual fields) will be compromised.