Schwannomas occur less frequently on the 5th cranial nerve and rarely involve other cranial nerves.
The acoustic schwannoma takes origin from the vestibular component of the 8th cranial nerve near the internal auditory meatus, at the transition zone where the Schwann cells replace the oligodendroglia.
Great variations of growth rate from around 2 mm per year is usual, to as much as 17 mm per year
The facial and trigeminal nerves are more frequently affected than the bulbar nerves.
These lesions may occur entirely within the CPA but, in the case of trigeminal schwannomas, dumbbell extension through Meckel’s cave into the middle cranial fossa is usual.
Facial nerve neuromas are usually indistinguishable from acoustic lesions until the tumour is encountered at surgery.
Surgery is therefore performed either by the translabyrinthine or retrosigmoid approach according to the patient’s pre-operative hearing.
Neuromas of the trigeminal nerve often present with a dumbbell mass present in both the posterior and middle cranial fossae , usually removed via a pre-sigmoid combined posterior fossa/middle fossa approach.
Neuromas of the vagus, glossopharyngeal and spinal accessory nerve are exceedingly rare .
Melanocytes are normally present in the intracranial leptomeninges, usually in the posterior cerebral fossa, and can give rise to benign (meningeal melanocytoma) or malignant (malignant melanoma) tumours.
However, metastatic malignant melanomas greatly outnumber primary melanocytic neoplasms.
At CT, primary melanocytic lesions appear as well-circumscribed, isoattenuating to hyperattenuating extraaxial tumours with homogeneous enhancement.
Even if there is no hyperostosis, they can mimic a meningioma.
MR imaging demonstrates variable signal intensity on T1- and T2-weighted images in proportion to the amount of melanin within the tumo.
Therefore, melanocytic neoplasms are isointense or hyperintense to adjacent normal brain tissue on T1-weighted images and isointense or hypointense on T2-weighted images.
They enhance after intravenous administration of contrast material.
Imaging Melanoma in a 58-year-old woman with a left cerebellar syndrome. (a) Axial CT scan shows a hyperattenuating melanoma of the left CPA. (b) Axial T1-weighted MR image shows a well-defined extraaxial mass at the posterior edge of the petrous bone. The high signal intensity is suggestive of melanin. (c) Gadolinium-enhanced axial T1-weighted MR image shows a normal left internal auditory canal (arrow) and lack of dural tail enhancemen
In addition to carcinomatous meningitis, many infectious or inflammatory diseases can have focal nodular meningeal involvement and manifest as a CPA mass.
Leptomeningeal metastases from lung cancer, breast cancer, melanoma, or other neoplasms can mimic benign tumours of the CPA.
Therefore, when an unusual aggressive schwannoma-like mass is encountered and no primary cancer is known to be present, radiologists should be aware of the possibility of metastases and check the lungs and breast carefully.
Metastases in a 67-year-old man with lung cancer and right-sided hypoacusia, vertigo, and ear pain.
Axial T2-weighted MR image shows a metastasis of the right CPA that mimics a vestibular schwannoma but with unusual associated middle ear retention.
Contrast-enhanced axial T1-weighted MR image shows intense enhancement of the lesion, which extends into the cochlea (arrow). Note the presence of another enhancing lesion at the tip of the right petrous bone (arrowhead).
However, in most cases, there are numerous other intra and extraaxial masses, which are very suggestive of metastases when correlated with a clinical history of carcinoma.
Tuberculosis is one of the main causes of focal pachymeningeal thickening in the posterior cranial fossa, but this finding is not specific.
Neurosarcoidosis can also appear as a dura-based mass, like meningioma. The lesions of neurosarcoidosis are hyperattenuating on CT scans and isointense to gray matter on T1-weighted images with homogeneous enhancement; however, the possible low signal intensity on T2-weighted images may suggest the diagnosis.
The surgical approach is determined by the clinical presentation.
If hearing is preserved, a retrosigmoid approach is favored whereas a transpetrous approach is used if useful hearing has been lost pre-operatively
If the lesion extends far medially, the cochlea may require removal to provide sufficient access.
Cholesteatomas usually envelop a multitude of cranial nerves and vascular structures.
While the soft contents of the lesion can readily be removed, the capsule, which is usually adherent to vascular and neural structures, needs to be excised to avoid recurrence .
Although the mortality from surgery to remove these lesions is low, neurological morbidity in the form of post-operative cranial nerve lesions is frequent.
This is most commonly the facial nerve, and is most frequently confined to a House–Brackmann Grade II weakness, but complete lesions can occur.
Lower cranial nerve lesions were also present in around 30–40%.
House-Brackmann Grade (Facial Nerve Palsy) No movement Total paralysis 6 Barely perceptible motion Severe dysfunction 5 Obvious weakness and/or dysfiguring and assymetry Moderate-severe dysfunction 4 Obvious but not dysfiguring Moderate dysfunction 3 Slight weakness on close inspection Mild dysfunction 2 Normal function in all areas Normal 1 Description Function Grade
Like epidermoid cysts, dermoid cysts result from inclusion of ectodermal elements during neural tube closure but may originate a bit earlier.
Dermoid cysts are midline lesions that rarely invade the CPA laterally and contain elements from all layers of the skin.
Thus, fat, hair, sebaceous glands, and sweat glands can be found in addition to squamous epithelium.
Typically, dermoid cysts have negative attenuation values on CT scans and high signal intensity on T1-weighted images due to their fatty content, may have a very suggestive fat-fluid level, and contain calcifications.
Lipomas in the CPA are maldevelopmental masses that arise from abnormal differentiation of the meninx primitiva.
They are homogeneous fatty lesions surrounding and encasing normal adjacent neurovascular structures with very dense adhesions .
Nevertheless, lipomas are rarely symptomatic , and conservative follow-up is often preferred to aggressive and potentially risky resection.
Lipomas appear as fat: homogeneously hypoattenuating with a negative attenuation value on CT scans and as characteristic and suggestive homogeneous high signal intensity on T1-weighted images, which decreases on fat-suppressed images.
There is no enhancement after contrast material administration. Therefore, nonenhanced T1-weighted imaging should be performed when evaluating a CPA syndrome to look for spontaneous hyperintense lesions, such as lipomas.
Imaging Lipoma in a 7-year-old boy with a polymalformation syndrome. (a) Axial CT scan shows a welldefined hypoattenuating lipoma of the left CPA. (b) Axial T1-weighted MR image shows that the lipoma has signal intensity similar to that of subcutaneous fat.
In young adults, brainstem gliomas can manifest as asymmetric expansion of the brainstem with a possible pedicle into the CPA and even then can mimic an acoustic neuroma.
Gliomas appear as hypoattenuating masses at CT with variable enhancement depending on the glioma grade.
At MR imaging, T1-weighted images show hypointense masses, whereas T2- weighted images show hyperintense tumours as well as hyperintense adjacent edema.
In addition, the exophytic component in the cerebellopontine cistern is much better demonstrated than at CT.
Imaging Brainstem glioma in a 23-year-old man with vertigo and hypoacusia. Contrast-enhanced axial T1-weighted MR image shows an unusual round grade III glioma located in front of the porus. The tumour demonstrates central enhancement.
Hemangioblastomas are frequent in von Hippel– Lindau disease, but they may also occur in the absence of that disease in the posterior cranial fossa of young adults, where they rarely reach the cerebellar surface and invade the CPA.
Hemangioblastoma usually appears as a large, well-circumscribed, homogeneous cystic mass with a small hypervascular mural nodule, but it can also appear as a solid mass without a cystic component.
The nodule appears hypointense on T1-weighted images and hyperintense on T2-weighted images and enhances intensely after injection of gadolinium contrast material.
Because of the hypervascularity, possible high-flow vessels can be observed as flow voids at the periphery of the mass.
When present, cysts usually have smooth margins and homogeneous CSF-like signal intensity.
Finally, edema is usually slight or absent around this intraaxial tumour.
Ependymoma in a 24-year-old woman with vertigo, headaches, and left facial nerve palsy.
Contrast-enhanced axial T1-weighted MR image shows a heterogeneous ependymoma with a lobulated multicystic component in the left CPA. The tumour invades the internal auditory canal without widening the porus (arrow).
(b) Coronal T2-weighted MR image shows marked mass effect of the ependymoma on the brainstem.
Adult cerebellar medulloblastomas mostly occur during the 3rd or 4th decade of life.
They appear as round or ovoid tumours with smooth margins, are more common in the cerebellar hemisphere than in the vermis, and often extend to the brain surface with possible exophytic invasion of the CPA or internal auditory canal.
Irregularity of some portions of the tumour-brain interface is a clue to their intraaxial origin.
At nonenhanced CT, medulloblastomas are frequently hyperattenuating and uncommonly demonstrate calcification.
At MR imaging, these tumours are hypointense on T1-weighted images and iso- or hyperintense on T2-weighted images with possible cystic or necrotic components and enhance after injection of gadolinium contrast material
Dysembryoplastic neuroepithelial tumours arise from dysembryoplastic cortex, yet have rarely been reported in the posterior cranial fossa.
When such tumours occur in this location, the patients tend to be young adults with minor symptoms such as headache, vertigo, or tinnitus for several years but without seizures.
MR imaging shows a large, heterogeneous, slow-growing lesion with a predominant cystic component and an enhancing mural nodule of hamartomatous tissue.
Mass effect is less significant than would be expected, and no edema is seen in the brainstem. Erosion of adjacent bony structures is suggestive of dysembryoplastic neuroepithelial tumour when associated with a large cystic lesion of the CPA that manifests as mild, long-lasting symptoms.
Dysembryoplastic neuroepithelial tumour in a 39-year-old man with mild, longlasting
Axial T2-weighted MR image shows a large dysembryoplastic neuroepithelial tumour of the right CPA with a cystic component () associated with a possibly suggestive bone erosion (arrow). Note the lack of edema.
(b) Contrast-enhanced axial T1-weighted MR image shows enhancing hamartomatous tissue (*) and possibly suggestive slight mass effect with a normal left cerebellopontine cistern (arrow).
Imaging Choroid plexus papilloma in a 49-year-old woman with vertigo and intracranial hypertension. (a) Axial T2-weighted MR image shows a right CPA papilloma extending through the foramen of Luschka. The tumour contains massive hypointense calcification (arrowhead). (b) Contrast- enhanced axial T1-weighted MR image shows intense enhancement of the hypervascularized tumour. Note the normal choroid plexus in the left foramen of Luschka.
Cholesterol granulomas can occur in any obstructed air cells.
They often arise from the apex of the petrous bone and may enlarge enough to expand in the posterior cranial fossa and produce nerve disturbances.
At CT, cholesterol granulomas appear as sharply and smoothly marginated expansile lesions in the temporal bone, isoattenuating with brain tissue and nonenhancing.
At MR imaging, they characteristically have a large central region of increased signal intensity and a thin peripheral rim of decreased signal intensity on both T1- and T2-weighted images. The latter finding corresponds to expanded cortical bone and hemosiderin deposits.
Spontaneous and homogeneous central high signal intensity on T1- weighted images is very suggestive of a cholesterol granuloma.
Cholesterol granuloma in a 32-yearold man with right trigeminal neuralgia.
Axial T1-weighted MR image shows a cholesterol granuloma at the apex of the right petrous bone with typical high signal intensity. An additional suggestive feature is the thin hypointense rim (arrowheads), which represents expanded cortical bone of the petrous apex.
(b) Axial T2-weighted MR image shows that the granuloma has heterogeneous signal intensity surrounded by a hypointense rim (arrowheads).
(c) Contrast-enhanced coronal T1-weighted MR image shows the normal right trigeminal nerve (arrow) at the top of the mass.
Surgical excision offers a potential cure in the treatment of these patients.
A combined trans and infratemporal approach to Type C (tympanomastoid with infralabyrinthine or petrous apex destruction) and D (intradural extension) tumours as described by Fisch.
Pre-operative angiography with tumour embolization , where possible, is mandatory. Preliminary control of the internal carotid artery, the sigmoid sinus and the cavernous sinus is necessary to avoid a vascular catastrophe.
70% of patients had an “excellent”, 10% a “good” and 20% a “poor” outcome from surgery for large glomus tumours.
Long-term follow-up has shown that a small number of patients may develop local tumour recurrence.
Early post-operative MRI needed to assess residual disease and provide a baseline for future reference.
If tumour recurrence occurs, further surgery and/or radiotherapy needs to be considered.
Chordomas develop from remnants of the notochord and are located near the dorsum sellae, from which they can expand into the CPA.
At CT, chordomas appear hypoattenuating with possible slight calcification at the periphery of the tumour in association with irregular bone erosion.
At MR imaging, especially on T2-weighted images, chordomas usually appear as lobulated, large, hyperintense masses with septa of low signal intensity.
Slight enhancement is present.
The overall appearance can be quite similar to that of chondroma.
Imaging Chordoma in a 61-year-old man with left trigeminal neuralgia and headaches. Contrast-enhanced axial T1-weighted MR image shows a chordoma invading the left CPA with unusual sparing of the clivus. There are suggestive enhanced septa (arrowheads).
Endolymphatic sac tumours are papillary adenomatous tumours that originate from the endolymphatic sac, which is located in the distal portion of the vestibular aqueduct of the petrous bone.
These tumours occur sporadically but are frequent in von Hippel–Lindau disease.
At CT, the tumour destroys the retrolabyrinthine petrous bone with geographic or moth-eaten margins, and intratumoural spiculated or reticulated bone can be seen.
At MR imaging, endolymphatic sac tumours appear heterogeneous on both T1- and T2-weighted images with focal high signal intensity due to subacute hemorrhages and low signal intensity due to calcification or hemosiderin.
Blood-filled cysts and protein-filled cysts, both of which appear hyperintense on T1- and T2-weighted images, may be present and suggest the diagnosis.
Heterogeneous enhancement is seen after contrast material administration.
Apex petrositis (petrous apicitis) involves an inflammatory lesion of the petrous apex secondary to otitis media.
In addition to ear pain, patients experience fifth and sixth cranial nerve palsy; this clinical complex constitutes Gradenigo syndrome.
CT scans show a destructive lesion of the petrous apex, often in association with fluid in the middle ear. MR imaging can show an enhancing mass at the petrous tip and extension of this mass, as well as its relationship to cranial nerves V and VI
Imaging Apex petrositis in a 50-year-old woman with Gradenigo syndrome at clinical evaluation. (a) Axial T1-weighted MR image shows an irregular lesion at the tip of the petrous apex (arrow). (b) Contrast-enhanced axial T1-weighted MR image shows right-sided apex petrositis as an enhancing lesion along the courses of cranial nerves V and VI (arrow).
Basilar Artery Ectasia and posterior circulation aneurysms
The vertebral and basilar arteries and some of their branches pass through the cerebellopontine cistern, where a tortuous segment or ectasia or even an aneurysm can develop, present as a mass in the CPA.
Although not neoplasms, such lesions can cause mass effect on the neural structures of the CPA and thus produce neurologic symptoms
These lesions should be recognized at a stage that enables appropriate neurovascular management to be directed at them.
High-flow aneurysms appear as oval or round masses that have no signal (flow void) with all spin-echo sequences.
Extreme low signal intensity on T2-weighted images is very suggestive of such lesions and indicates that normal arteries need to be evaluated.
When an aneurysm is thrombosed, high signal intensity on T1-weighted images due to methemoglobin could be suggestive, but the signal intensity is variable; radiologists should be aware that enhancement of the mass can be observed due to organization of the thrombus, thus mimicking a schwannoma
Imaging Asymptomatic aneurysm in a 68-year-old man with lymphoma and right trigeminal neuralgia. a) Axial T2-weighted MR image shows an aneurysm of the left posterior inferior cerebellar artery with typical lack of signal (arrow). Note the lymphoma in the right pterygopalatine fossa (arrowheads), which explains the neuralgia. Aneurysm in a 75-year-old man with hypoglossal nerve palsy. b) Axial T2-weighted MR image shows a thrombosed aneurysm of the right posterior inferior cerebellar artery with focal calcification (arrowhead). Note the normal right hypoglossal canal (arrow), a finding inconsistent with a schwannoma. c) Contrast- enhanced coronal T1-weighted MR image shows homogeneous enhancement of the organized thrombus, which completely fills the aneurysm.