A comprehensive review of the clivus: anatomy, embryology, variants, pathology, and surgical approaches
The clivus is a bony structure formed by the fusion of the basioccipital and basispheniod bone at the sphenooccipital synchondrosis. This downward sloping structure from the dorsum sellae to the foramen magnum is derived from mesoderm and ectoderm properties.
This comprehensive review of the clivus will discuss its basic anatomy, embryology, pathological findings, and surgical implications. The clivus is an endochondral bone, formed under two processes; first, a cartilaginous base is developed, and it is secondly reabsorbed and replaced with bone. Knowledge of its embryological structure and growth of development will clarify the pathogenesis of anatomical variants and pathological findings of the clivus.
Understanding the anatomy including proximity to anatomical structures, adjacent neurovasculature properties, and anatomical variants will aid neurosurgeons in their surgical management when treating pathological findings around the clivus.
KeywordsClivus Skull base Notochord Anatomy Embryology Surgery
The clivus takes on a concave shape when viewed posterosuperiorly and is more prominent inferiorly at the location of the jugular tubercles, which project on the lateral inferior margins of the clivus [3, 10]. The fibrous raphe of the pharynx (superior pharyngeal constrictor muscle), longus capitus, rectus capitus anterior, and the anterior atlanto-occipital membrane are found in connection to the inferior surface, exocranial part, of the clivus . The posterior surface, endocranial part, of the clivus is smooth, composed of cortical bone, and is covered with dura mater [7, 10]. On a midsagittal section, the clivus is a wedged shaped bone, which is thicker anteriorly and becomes thinner posteriorly . Close examination of the central portion of the clivus demonstrates cancellous bone containing bone marrow, which may be variably pneumatized [7, 10, 13].
Understanding the embryology of the clivus may explain pathologies that occur in the region secondary to developmental remnants. The mesoderm and ectoderm are both involved in the development of the skull base; mesoderm develops into the posterior cranial fossa base, and neural crest cells are involved in the growth of parts anterior to the notochord . The skull base is composed of vertebral parts; this includes the notochord (chorda dorsalis), which ends at the junction below the dorsum sellae . The clivus is an endochondral bone; its progression involves cartilaginous formation followed by reabsorption preceding bone deposition .
As mentioned earlier, the clivus is developed from the fusion of the basiocciput and basisphenoid across the spheno-occipital synchondrosis. This portion of the central skull base has variations to its structure based on its development. A study conducted by DiChiro and Anderson (1965) observed 94 cadaver skulls in normal adults and found the majority (77.66%) had a slight concavity of the clivus surface of mostly 1 to 2 mm with a few outliers of 3 mm. From the remainder of the population, 19% had a flat clivus, and 0.03% had a convex clivus. The study also noted occasionally findings of more than 4-mm concavity in the posterior portion of the clivus in patients with posterior fossa tumors and conditions causing longstanding increased intracranial pressure. Other changes noted in the clivus include grooves from the overlying basilar artery with an incidence of 1% [13, 16].
Basilar impression is softening of surrounding bones such as the clivus and is due to, for example, osteoporosis, rickets, and von Recklinghausen disease . Clival scalloping is sometimes seen in the Chiari II malformation . Platybasia is another variation where the clivus is positioned more horizontally, causing the sphenoidal angle to widen. This angle enlargement can be due to elevation of the dorsal or frontal side of the angle .
A shortened clivus is a typical finding in all patients with achondroplasia and Chiari II malformation. In patients with idiopathic, or without achondroplasia, findings of a shortened clivus can be attributed toward the premature merging or due to reduced developmental growth of the spheno-occipital synchondrosis .
Rare anatomical variants of the clivus include Sauser’s fissure, also known as the basilar transverse fissure. Its development may be present on one or both sides and can be formed either completely or partially. The basilar fissure may distinguish the margin of the basioticum and basioccipital bones contingent upon the development of the basilar portion of the occipital bone [7, 23].
The basilar processes are variants seen in 4% of the population; they consist of unilateral or bilateral small bony protuberance at the anterior boundary of the foramen magnum. It is believed that their pathogenesis is derived from remnants of the lateral hypochordal blastema of the proatlas. In rare occurrences, the larger basilar processes communicate with the anterior arch of the atlas resulting in instability of the atlantooccipital joint . If the hypochordal blastema of the proatlas persisted in the medial aspect, it would give rise to a third condyle at the anterior margin of the foramen magnum . A much scarcer variant is the arcus praebasioccipitalis occurring in 0.03% of the population. It is an osseous mass in the shape of a horseshoe positioned at the anterior margin of the foramen magnum formed by the complete persistence of the hypochordal blastemas. It may affect the functioning of the craniovertebral joints.
The apical ligament (Fig. 5) ascends from the apex of the dens to anterior margin (basion) of the foramen magnum. If ossified, it can present as a bony spur on the clivus [6, 7]. Another adaptation includes the ossification of the enclosed material while the core remains ligamentous, resulting in a tube-like development instead of spur .
Arterial supply and venous relationships
Fibrous dysplasia is a benign skeletal anomaly that may affect one or multiple bones, with a greater tendency to affect long bones, ribs, and craniofacial bones . The mature bone becomes replaced with a weaker substitute of woven bone and fibrous tissue. More commonly, the disorder affects a single bone in 70% of cases and multiple bones in 30% . McCune-Albright syndrome involves multiple bone subtype of fibrous dysplasia, café-au-lait spots, and endocrine disorders, in which the clivus and sphenoid bone may be involved . Imaging with a ground glass appearance is pathognomonic for fibrous dysplasia.
If CT images display an integral cortex with lytic lesions, it should raise concerns of a possible neurenteric cyst . A location at the clivus is unusual but not surprising given that the embryological nature involves dysgenesis of the notochord during early development .
Ecchordosis physaliphora is a congenital hamartomatous lesion derived from notochord remnants and is seen in approximately 2% of the population. It is located in the retroclival region and projects toward the intradural prepontine cistern. The tissue appears hypotense on T1-weighted MRI and hypertense on T2-weighted MRI with a lack of contrast enhancement compared to other tumors such as chordomas. The clinical repercussion of this tumor-like mass is bleeding or CSF leakage .
The dorsal dural surface of the clivus increases its propensity of being affected by meningiomas. Due to the lack of arachnoid granulations at the central aspect of the clivus, meningiomas arise more commonly at the petroclival line. As a result, meningiomas are often located medial to the trigeminal nerve [7, 20]. When excising the tumor, neurosurgeons must be attentive toward the vasculature surrounding the mass.
Chordomas are similar to ecchordosis physaliphora in that they are both relics of notochordal tissue. The direction of travel of the notochord from the odontoid process moving anteriorly toward the basiocciput can lead to susceptibility of notochord remnants within the area and lead to the development of chordomas . However, compared to ecchordosis physaliphora, chordomas are malignant tumors arising within the bone in a slow growing manner with a potential to metastasize via lymphatics or hematogenously . Chordomas are uncommon and account for less than 1% of all intracranial tumors . They have a greater prevalence in males by the third and fourth decades of life and commonly occur is the sacrum, clivus, and cervical vertebrae. Approximately one-third occur at the clivus originating within the bone as a well circumcised, midline extradural soft-tissue mass and often affecting the abducens nerve at initial presentation . On imaging, clival chondromas appear as soft tissue masses with lytic bone destruction. Due to high fluid content of the chordomas, on T1-weighted MRI, they appear hyperintense, while on T2-weighted MRI, they appear hypotense (as opposed to ecchordosis physaliphora) with moderate degree of contrast enhancement . Treating chordomas are difficult as they often recur and occupy a difficult location in terms of neighboring neurovascular structures. Clival chordomas sit in close proximity to the brain stem, thus compressive growth and inadequately managed tumors often lead to high morbidity and mortality .
The location of masses on the clivus presents a challenge to surgeons who must maintain vital structures adjacent to it including the pituitary gland, internal carotid artery, cavernous sinus, basilar artery, and brain stem. With slow growing tumors such as chordomas, the extension of the tumor to surrounding structures is already compromised during initiation of symptoms and creates another barrier for surgeons to achieve clear margins.
When accessing a clival mass, craniotomies usually require excessive brain retraction, which can lead to complications including cerebral infarction, hematoma, and regional brain atrophy. Additionally, these approaches do not give access to extradural portions of the tumor. The microscopic transsphenoidal approach allows for better visualization of the sella turcica and upper to middle clivus . The lower clivus and craniocervical junction (CCV) are not adequately viewed, as they would be with a transoral approach. This approach is limited, preventing access to the upper clivus, pharyngeal space, and jugular foramen. However, a maxillary osteotomy allows for access to the upper, middle, and lower clival regions and the CCV. However, the maxillary osteotomy approach requires a facial incision, which may cause dysfunction and deformities to adjacent regions. Lastly, the endoscopic approach to the clivus provides a complete 360-degree view of the clivus and its adjacent structures allowing for complete resection of the tumor. This approach is not without its limitations as it is more likely to cause intracranial infection. The endoscopic technique provides an off-center line of sight of the clivus, although this can be conquered by the surgeon’s knowledge of clival anatomy and practical skills using the endoscope .
The clivus is adjacent to important anatomical structures, which makes surgical approaches to this region complicated. The embryology of the clivus highlights the intricate course of the notochord, and how remnants of the notochord may lead to the development of pathological findings. Understanding the anatomy and adjacent neurovasculature structures and anatomical variants will aid skull base surgeons in their management when treating pathological findings around the clivus.
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Conflict of interest
On behalf of all authors, the corresponding author states that there is no conflict of interest.
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