The clivus is one of the skull base structures formed by the occipital bone and sphenoid bone at the spheno-occipital synchondrosis. It is wedge-shaped and it extends from the foramen magnum to the dorsum sella.
It can show different anatomical variants such as arrested pneumatization
Fossa Navicularis (FN)
FN is a wider-than-deeper notch-like or rounded defect located on the ventral and inferior surface of the basioccipital portion of the clivus. It is postulated that it originates from a notochordal remnant or the expansion of remnant emissary veins.
Canalis Basilaris Medianus (CBM)
CBM is a rare channel communicating the nasopharynx with the intracranial surface of the basioccipital portion of the clivus. Six types are described, three complete forms and three incomplete forms.
Craniopharyngeal Canal (CPC)
CPC is a defect of more than 1.5 mm wideness that results in communication between the sella turcica and the nasopharynx. It is supposed to be a remnant of the Rathke´s pouch. There are three types described depending on whether they are solitary or associated with cephaloceles or ectopic adenohypophysis. Wide CPCs are related to transsphenoidal meningoencephaloceles.
The normal bone marrow signal of the clivus is homogeneous and should match the white matter on non-contrast T1 non-fat-saturated images. There are some lesions arising primarily from the osteocartilaginous structure of the clivus that make this normal signal change.
Ecchordosis Physaliphora (EP)
EP or benign notochordal cell tumour (BNCT) is a remnant of the notochord located at the spheno-occipital synchondrosis of the clivus not bigger than 2 cm. It is a T1 hypointense, T2 hyperintense, non-enhancing and no erosive small bony stalk in the intradural space of the prepontine cistern. It is usually discovered incidentally and its radiological features overlap with small chordomas. Therefore, a follow-up MRI should be advised.
Fibrous dysplasia
Fibrous dysplasia is a congenital disorder characterised by the replacement and distortion of normal bone with poorly organised fibrous tissue. It can be isolated or enclosed in a syndrome such as McCune-Albright syndrome, and it also can be monostotic or polyostotic. Craniofacial fibrous dysplasia is one of the most common forms and commonly involves the sphenoid bone, which is related to the clivus.
Lesions are homogeneously dense and sclerotic in the skull bones, expansive, with intact cortex and loss of the corticomedullary differentiation on CT. They are T1 and T2 heterogeneous and show heterogeneous gadolinium enhancement, with no restricted diffusion.
Paget´s disease
Paget´s disease is a chronic idiopathic alteration of the bone remodelling process. It can affect the skull base bones and make scarce cyst-like changes. Bony CT demonstrates mixed sclerotic and lytic patterns with lytic expansive and cotton wool-appearing areas. On MRI they show T1 hypointensity and heterogeneous contrast enhancement.
There are some benign tumours involving the clivus.
Pituitary adenoma
Adenohypophyseal macroadenomas (>10 mm) may extend through the sellar floor and invade different structures, including the clivus. Large adenomas show the “snowman sign” due to the diaphragm sellae and displace laterally the carotid artery. They are heterogeneous with cystic and hemorrhagic areas, hypodense to the brain on CT, T1 iso-hypointense, T2 iso-hyperintense, and show variable restricted diffusion/ADC values and heterogeneous contrast-enhancement on MRI.
Petroclival meningioma
Central skull base meningiomas arise from the 2/3 upper of the clivus. They are hyperdense on non-contrast-CT, may have calcifications and show intense and homogeneous contrast-enhancement. Adjacent bone can show hyperostotic, permeative or sclerotic changes without displacement of the carotid arteries and with no “snowman sign”. In this case, the pituitary gland is preserved. Meningiomas are T1 ant T2 hypo- to isointense to grey matter, may show low or absent signal in calcified areas, rarely have haemorrhagic T1 hyperintense foci, T2 hyperintense CSF-vascular cleft at the periphery of large lesions and prominent homogeneous enhancement and dural tail.
Trigeminal schwannoma and Paraganglioma
Trigeminal schwannomas are slow-growing encapsulated benign tumours originating from the Schwann cells. They remodel the bone, are isodense to the brain, T1 hypointense, and T2 hyperintense, show heterogeneous but prominent enhancement, may have cystic areas and can extend from the Meckel cave to the skull base.
Jugular and tympanic paragangliomas grow in the petrous bone and can extend to the paraclival region. They are benign slow-growing neuroendocrine tumours that remodel the adjacent bone and may show a “salt and pepper” pattern in T1, rapid contrast-enhancement and also rapid wash-out.
There are also malignant lesions in this area, some primarily of the clivus and others related to surrounding structures.
Chordoma
Chordomas are locally aggressive tumours derived from the notochorda. They have a predilection for the skull base and the sacrococcygeal region. The symptoms caused by them at the skull base originate from the local growth and are usually VI nerve palsy (as it goes through the Dorello canal in the clivus), pain and rhinorrhoea due to a CSF leak.
Even if small chordomas appear as BNCT, big ones appear erosive, lobular, septate, and heterogeneous, T1 hypointense, containing high-signal foci and T2 hyperintensity with T2 hypointense septa. They show a heterogeneous gadolinium-enhancement, with myxoid areas enhancing poorly, and others with variable restricted diffusion.
Chondrosarcoma
Chondrosarcomas arise typically lateral to the petroclival synchondrosis and may show on CT “ring-and-arc” pattern of calcifications and chondroid tumour matrix. They are T2 hyperintense with hypointense calcified foci, heterogeneous contrast-enhancement and more extreme values on DWI/ADC sequences than chordomas.
Nasopharyngeal carcinoma
The invasion of the sphenoid sinus and retropharyngeal area gives T3 value to the nasopharyngeal local staging. Higher stages include intracranial extension that can involve clival structures. On CT, invasion of the clivus is seen as bone erosion contiguous to the tumour. MRI shows bone marrow invasion with T1 hypointense, solid contrast-enhancement and restricted diffusion next to the tumour which shows T2 hyperintensity to the muscle, T1 hypointensity and less contrast-enhancement to the mucosa. Perineural invasion through the trigeminal nerve may be observed.
The clivus can be the seat of other malignant lesions in a more global context such as metastases, lymphoma, myeloma, sinus histiocytosis or Rosai Dorfman disease.
Metastases
Although clival bone metastases are rare, prostate adenocarcinoma, thyroid carcinoma, hepatocarcinoma and breast cancer are the most common origins. Most of the cases are late-stage cancers with diffuse bone involvement and hematogenous spread. CT and MRI features are not specific and they may mimic a primary malignancy.
Plasmacytoma / Multiple Myeloma
Multiple myeloma and plasmacytoma are malignant proliferations of a single plasma cell clone. CT shows multiple lytic foci without a sclerotic rim, with variable coalescence and extramedullary extensions shown as expansive soft tissue density masses. Plasmacytoma is a single lytic bone lesion associated with a soft tissue mass, being very rare at the skull base. Multiple myeloma lesions are T1 hypointense, T2 hyperintense, homogeneous contrast-enhancement and restricted diffusion.
Lesions of inflammatory and infectious origin also affect the clivus and paraclival region. They usually originate in neighbouring structures such as the sphenoid sinus and middle and external ear.
Mucocele
Primary clival mucoceles are extremely rare. They are formed by pneumatization secondary to an obstructed communication with the sphenoid sinus and expansion of cystic masses into other cranial base structures, such as clivus. Mucoceles are T1 variable depending on protein contents, T2 hyperintense, without calcifications and with no contrast-enhancement.
Osteomyelitis
Skull base osteomyelitis is usually related to external malignant otitis or chronic suppurative otitis media. Its diagnosis is crucial to speed up the instauration of antimicrobial therapy. Those patients tend to have comorbidities such as diabetes mellitus, corticosteroids, HIV infection, and chronic rhinosinusitis. Bone erosion, demineralization, contrast-enhancing soft tissue, obliteration of fat planes, involvement of the skull base foramina, and vascular complications can be seen on CT. MRI shows focal or diffuse T1 hypointense clival bone marrow and inflammatory soft tissue with mild restricted diffusion.
