Surgical Management of Lesions of the Clivus

Anatomy of the Clivus

The clivus a midline osseous anatomic structure in the skull base formed by the body of the sphenoid bone and the clival part of the occipital bone. Both bones join in a flat surface, the spheno-occipital synchondrosis. The upper part of the clivus is formed by the sphenoid bone, whereas the middle and inferior part of the clivus are mainly formed by the occipital bone ( Fig. 29.1 ).

The sphenoid bone consists of a body, lesser and greater wings, and pterygoid processes. The clivus forms part of the body of the sphenoid, which also contains the sphenoid sinus. The sphenoid bone forms the upper part of the clivus, formed by the dorsum sellae and the posterior clinoids.

The occipital bone has three parts: clival, squamous, and condylar. The clival part of the occipital bone is the osseous portion in midline from the spheno-occipital synchondrosis to the foramen magnum, and forms the anterior part of the foramen magnum. The clival part forms the lower two-thirds of the clivus. The paired condylar parts are situated along the anterolateral margin of the foramen magnum and connect the clival and squamous parts of the occipital bone. The squamous part of the occipital bone is lateral and posterior to the clival part and forms the remaining two-thirds of the foramen magnum. The petrous part of the temporal bone has a triangular shape seen from above. It articulates with the sphenoid bone anteriorly and with the occipital bone posteriorly. The vertex of the triangle, the petrous apex, is located at the junction of the sphenoid bone, occipital bone, and petrous part of the temporal bone. The petroclival junction forms the posterior side of this triangle and is the articulation between the occipital portion of the clivus and the petrous part of the temporal bone. The petrous ridge, in the middle of the triangle, serves as attachment of the tentorium, which separates the supra and infratentorial compartments. The tentorial hiatus is formed by the two edges of the tentorium posteriorly and laterally and the clival dura mater anteriorly. The tentorial hiatus surrounds the midbrain, and leaves between the midbrain and the tentorial edge a space called the tentorial incisura. The clivus is mostly an infratentorial structure. When the dura mater of the tentorium joins the clival dura mater, it forms a dural fold called the posterior petroclinoidal fold. This fold, sometimes with a true ligament beneath it, is the continuation of the dura mater covering the petrous apex, which joins the posterior clinoid process.

It is of paramount importance to know the neurovascular relationships of the clivus. The brainstem (midbrain, pons, and medulla) in its anterior part is facing the clivus. Regarding cranial nerves (CNs), the nerve most closely related to the clivus is CN VI. CN VI exits the brainstem in the pontomedullary junction and pierces the inner layer of the dura mater of the clivus to be interdural in the Dorello canal until it passes medial to the petrous apex under the petrosphenoidal ligament (Gruber ligament) and enters the cavernous sinus (CS). The porus of the VI CN is 12 mm lateral to midline. The V1 division of the V CN is immediately lateral to CN VI in the CS. CN III and IV are in close relationship with the lateral aspect of the dorsum sellae superior to CN VI. A small bony groove for the CN III can be seen often in the lateral aspect of the dorsum sella just inferior to the posterior clinoids.

The vascular relationships of the clivus can be divided in intradural and interdural/extradural. The carotid arteries are related to the clivus extradurally or interdurally (in the CS), whereas the vertebral arteries, basilar artery (BA), and their branches are intradural. The petrous segment of the internal carotid artery (ICA) in its medial or anterior genu is related to the petrous apex and thus to the spheno-occipital synchondrosis and medial petroclival region. The cavernous segment of the ICA has a posterior bend and an anterior bend, which are closely related to the body of the sphenoid bone. The posterior bend is lateral to the upper clivus, whereas the anterior bend (parasellar carotid) is lateral to the anterior aspect of the sella turcica. The vertebral arteries and BA are closely related to the clivus as they travel anterior to the brainstem (see Fig. 29.1E ).

The clivus can be divided in three parts per two surgical classifications. The first one is the classical surgical classification based on the open approaches to the clivus; the second is a newer classification that has emerged after the endoscopic endonasal approaches based on the endonasal anatomy. Both are based on key anatomic landmarks of each approach. The classical surgical classification considers: upper clivus from the upper part of dorsum and posterior clinoids to the dural porus of the CN VI; middle clivus from the porus of the CN VI to the glossopharyngeal meatus; and lower clivus from there to the foramen magnum. The porus of the CN VI cannot be estimated in preoperative studies, but it is on average 3.4 mm inferior to the petrous apex. The glossopharyngeal meatus is located at the upper part of the jugular foramen and can be easily distinguished in preoperative studies.

Regarding the endoscopic endonasal approaches, the classification is based on the endonasal view and localization of the different segments of the ICA. Three segments can be distinguished: upper clivus, defined by the parasellar portion of the carotid artery (anterior bend of the cavernous carotid), middle clivus defined by the paraclival carotid or vertical segment of the cavernous carotid up to foramen lacerum inferiorly (basically corresponds to the clival recess when it is present), and lower clivus from the inferior aspect of foramen lacerum to the foramen magnum.

Philosophy of the Approaches

Approaches to the clival area can be distinguished in those directed to the pathologies arising from the clivus as an osseous structure, and intradural pathologies that affect the interpeduncular, prepontine, and premedullary cisterns with possible lateral extensions. In general, when considering approach, we like to stay extradural for extradural lesions; it rarely makes sense to do an intradural approach for an extradural lesion in the clivus unless the approach offers less morbidity to the nerves of vessels. The most direct route to the clivus is the endoscopic endonasal route because it does not require any brain retraction or crossing neurovascular structures to access the bone. The equivalent in transcranial approaches is the transbasal approach, which requires a bifrontal craniotomy, sacrifice of olfaction, and frontal lobe retraction as part of the approach. For intradural pathologies in midline, the endoscopic endonasal approach does not require crossing nerves to reach the pathology. Lateral transcranial approaches require working in between the nerves to reach pathologies in midline. They are required for pathologies with lateral extension and, in some occasions, superior extension. Both approaches are complementary and are essential in the surgical armamentarium of the cranial base surgeon.

The other essential aspect to consider for planning is the objective of the approach, complete resection versus biopsy if diagnosis is uncertain. Depending on the pathology, and given the potential morbidity of complete resection and sometimes irresectability of the pathologies, a multidisciplinary team approach is essential for management of these patients ( Fig. 29.2 ).

Pathologies of the Clivus

Derived from the Notochord Remnants

Chordoma

Chordoma is a malignant tumor arising from the remnants of the notochord. Approximately 32% of all the chordomas are localized in the clivus, 32.8% in the mobile spine, and 29.2% in the sacrum. These lesions are typically osteolytic and invasive and have hypointense in T1 and hyperintense in T2 with variable contrast enhancement ( Fig. 29.3 ). These tumors are usually extradural, can have an intradural component, and are rarely primarily intradural. There are three histologic types of chordomas: classical, chondroid with a partially chondroid matrix, and dedifferentiated (or sarcomatous). The dedifferenciated type has a worse prognosis. Chordomas are mainly confused with chondrosarcomas on radiologic diagnosis; however, chondrosarcomas typically have more true calcifications, whereas chordomas can have calcifications within them often representing eroded bone. Inmunohistologically these two lesions are clearly distinct entities separated by histologic sensitive markers cytokeratins and brachyury. Brachyury is a protein found in any tissue derived from the notochord.

Chordomas tend to recur locally. Surgery is the most efficient treatment, and gross total resection is the most important prognostic factor. Endoscopic endonasal approaches are gaining acceptance for chordomas due to their location and direct access to this mostly extradural lesion. Considering their midline location, the endonasal approach avoids crossing CNs, and in some cases, intradural access can be avoided. Often chordomas require complementary transcranial approaches for resection of the involved bone, especially if the tumor has a lateral extension. In a recent meta-analysis, endoscopic approaches showed less incidence of CN deficits and infections but higher rates of a postoperative cerebrospinal fluid (CSF) leak. Although en bloc resection may be curative in the spine, rarely in the skull base can such a strategy be used. However, an aggressive margin-negative resection if able, or alternatively a radiographically negative postoperative image, is the preferred upfront treatment. Proton beam is gaining popularity as the most common adjuvant radiation; however, some centers use IMRT and stereotactic radiosurgery postoperatively. Proton beam therapy after surgery improved survival in some series, and it is proposed after surgical resection. Theoretically proton beam provides the highest treatment dose in close proximity to the brainstem. The average length of survival is 6.3 years, and overall survival is 67.6% at 5 years, 39.9% at 10 years, and 13.1% at 20 years, being better in children older than 5 years and worse in younger children. Metastases may occur but do not affect length of survival. ^,

Ecchordosis Physaliphora

Ecchordosis physaliphora is a benign, congenital lesion representing a notochordal remnant, which occurs particularly at the level of the clivus and sacrum. The magnetic resonance imaging (MRI) characteristics are T1 hypointensity, T2 hyperintensity, and lack of enhancement with gadolinium administration ( Fig. 29.4 ). It is usually asymptomatic and discovered incidentally. If it is asymptomatic, it is managed with serial imaging, and few symptomatic cases may need resection. Histologically they can be distinguished from chordomas for lower Ki-67 index, a very low MIB count, and absence of mitosis; however, this distinction is still difficult.

Benign Notochordal Cell Tumor

Benign notochordal cell tumors are usually incidental microscopic findings found on autopsy in 20% of autopsies in patients ranging in age from 7 to 82 years, with a distribution of 11.5% of the clival region, 5% of cervical vertebrae, 2% of lumbar vertebrae, 12% of sacrococcygeal vertebrae, and 0% in the thoracic vertebrae. Larger ones are seen in a variety of ages and in both sexes. Radiologically they are hypointense on T1, are hyperintense on T2, and do not show bone destruction or soft tissue involvement.

Sarcomas

Chondrosarcoma

Chondrosarcomas account for 6% of skull base tumors, develop from primitive mesenchymal cells, and develop in the skull base with a high predilection for the petroclival synchondrosis. The chondrosarcomas arise more commonly in the appendicular skeleton, and their presentation in the skull base is rare. They are paramedian lesions unlike chordomas that appear in midline. The majority of cranial chondrosarcomas are low-grade, slow-growing indolent tumors that have the capacity to locally engulf critical neurovascular structures. They can cause mass effect on the surrounding brainstem and can extend superiorly, laterally, and inferiorly. The low-grade tumors tend to have more extensive calcification. Bone destruction and calcification occur in approximately 50% of chondrosarcomas. These tumors are generally low-to-intermediate signal on T1-weighted images and high signal on fluid-attenuated inversion recovery, and T2-weighted images. Stippled foci of calcification, a hallmark of this tumor, may be demonstrated as heterogeneous areas of low signal within the tumor ( Fig. 29.5 ). Histologically, the myxoid type of chondrosarcoma can appear similar to chordomas; however, immunocytochemical staining is usually conclusive. Chondrosarcomas do not stain for cytokeratin or other epithelial markers, in contrast to chordomas. The treatment includes maximal safe surgical resection, followed by adjuvant fractionated radiotherapy (preferentially proton therapy). The 5-year progression-free survival is greater than 90% for chondrosarcoma, and it is also influenced by the histologic subtype. ^,

Enchondromas are rare skull base lesions that occur mostly in syndromic cases such as Ollier syndrome. These have the potential to malignantly degenerate to chondrosarcomas; however, the frequency is unknown.

Osteosarcoma

Osteosarcoma is the second most common primary bone malignancy of the skeletal system, behind multiple myeloma. Head and neck osteosarcomas are rare, comprising only 6% to 10% of osteosarcomas; skull base osteosarcomas are a rare subgroup, and the data are often extrapolated from the head and neck group. Osteosarcoma of the craniofacial region tends to present in the third and fourth decades of life. Craniofacial osteosarcomas are frequently the result of previous radiation therapy to the region or an underlying bone condition such as Paget disease or fibrous dysplasia (secondary osteosarcoma), whereas primary osteosarcomas occur de novo and are less frequently encountered. Regarding head and neck osteosarcomas, resection with negative margins is the preferred approach and improves survival, which is not often possible in the skull base. Thus the prognosis of the skull base osteosarcoma is worse. The 5-year survival rate of patients with head and neck osteosarcoma with negative margins was 75%, compared with only 32% for patients with positive resection margins. Although an important treatment in long bone osteosarcoma, chemotherapy remains controversial in skull base osteosarcoma; however, chemotherapy and radiotherapy in conjunction with safe maximal resection are often suggested to treat these tumors. Based on the review of a recent study, the 5-year overall survival rate of patients with osteosarcoma of the skull base was 37.8%, with a 42-month median survival duration.

Fibrous Dysplasia

Fibrous dysplasia is a developmental disorder caused by abnormal proliferation of fibroblasts resulting in replacement of normal cancellous bone by immature osseous tissue. This process of unknown etiology, which mainly affects the younger population, may be monostotic, polyostotic, or associated with McCune-Albright syndrome. It may affect the cranial base, including the clivus. Low signal on both T1- and T2-weighted images improves the confidence of making a correct imaging diagnosis. However, the lesion may have intermediate signal intensity on T1-weighted imaging and the T2-weighted signal intensity varies from low to high. The enhancement patterns include patchy central, rim, homogeneous, or a combination. Vivid contrast enhancement is possible and may lead to confusion with other diseases. At computed tomography (CT), the ground-glass pattern is the most common (56%); however, homogeneously dense (23%) and cystic varieties (21%) account for a significant proportion ( Fig. 29.6 ). The majority of patients are asymptomatic and need observation and conservative treatment. Symptomatic patients may have headache, neck pain, craniocervical instability, or cranial neuropathies. Extensive cases with intractable pain, compressive cranial neuropathy, or malignant degeneration are treated surgically. Cases involving the clivus can be treated endonasally. Less than 1% of cases can have malignant transformation to osteosarcoma.

Metastasis

Metastasis in the clivus are rare, but in patients with known cancer, the probability of a mass lesion in the clivus being a metastasis is high. For an isolated mass lesion of the clivus without definite disease elsewhere, a metastasis appears to be less likely than chordoma or chondrosarcoma. The most common primary sites are prostate, thyroid, and hepatocellular carcinomas. They can be dark in T2, which could aid in differentiating them from chordomas. The surgical approach is usually biopsy/debulking and chemotherapy or radiotherapy ( Fig. 29.7 ). ^,

Plasmacytoma and Multiple Myeloma

Plasmacytoma is a B cell lymphoproliferative disease that can rarely affect the clivus primarily, and treatment consists of biopsy and radiotherapy. On MRI, it is usually an isointense lesion on T1- and moderately hyperintense on T2-weighted imaging. They generally show intense homogeneous uptake with gadolinium ( Fig. 29.8 ) . Surgical resection does not improve prognosis and can increase morbidity unless used for resectable solitary lesions that persist after radiation treatment. Multiple myeloma is the systemic version of the disease that can affect the clivus as any other bone of the body.