Cerebellopontine Angle and Internal Auditory Canal Neoplasms

CHAPTER 36 Cerebellopontine Angle and Internal Auditory Canal Neoplasms



The cerebellopontine angle (CPA) cistern is a cerebrospinal fluid (CSF)–filled space whose boundaries are made up by the pons and cerebellum medially, the petrous portion of the temporal bone laterally, and the tentorium superiorly. The Cranial nerves V through VIII pass through the upper portion of the cistern, whereas the cranial nerves IX through XI pass through the lower portion. The presenting symptoms of CPA lesions are usually not related to the histology of the lesion but to the nerves and other structures that the lesion affects.


Neoplasms occurring in this region make up 6% to 10% of all intracranial tumors in adults and only 1% of lesions in children.1,2 The two most common lesions are the meningioma and the vestibular schwannoma. Indeed, meningiomas and schwannomas comprise approximately 90% of neoplasms in this region.3 However, a variety of other lesions also make their home in the CPA region. These lesions arise within the cerebellopontine cistern or from related structures and include arachnoid cysts, nonacoustic schwannomas, and meningeal lesions. Embryologic remnants may also be found, including lipomas, epidermoid cysts, and neurenteric cysts. Lesions, such as chondromatous tumors, chordomas, paragangliomas, and endolymphatic sac tumors, involve the CPA by extension from surrounding structures such as the skull base and petrous apex. Exophytic brain stem neoplasms or ventricular tumors can also involve the CPA. Locating a point of origin along with knowledge of a lesion’s morphology, CT density, MR intensity, and reaction of adjacent structures can help narrow the differential diagnosis.



SCHWANNOMA


Schwannomas are encapsulated lesions that arise from the Schwann cells of cranial, spinal, and peripheral nerves. Schwannomas are also referred to as neurilemmomas. Schwannomas arising from the vestibular nerves are currently referred to as vestibular or acoustic schwannomas but have also been known by the names acoustic neuroma, acoustic neurinoma, and neurilemmoma.







Imaging




MRI


The utilization of thin-section (3 mm) MRI is beneficial in evaluation of most lesions of the CPA. Administration of a contrast agent is a necessary part of the examination because smaller lesions may be missed if contrast enhancement is not utilized.


The appearance of schwannomas on MRI, like CT, may vary with the tumor size. As the lesions become larger they may undergo cystic change, and this alters their signal characteristics. Typically, schwannomas are slightly hypointense to isointense on T1-weighted (T1W) imaging. However, T1 hyperintensity may be seen if there has been hemorrhage into the schwannoma or if regions of cystic degeneration contain proteinaceous material. These lesions enhance intensely after gadolinium infusion, and the degree of homogeneous enhancement depends on the amount of cystic change (Fig. 36-4). Schwannomas are typically hyperintense on T2W imaging, and if cystic degeneration is present more focal areas of increased signal will be seen. Expansion of the IAC may be seen in up to 70% to 90% of vestibular schwannomas (Fig. 36-5).6 Approximately 5% of schwannomas may be associated with an arachnoid cyst that occurs between the neoplasm and the brain, suggesting that the mechanism of formation is peritumoral adhesions (Fig. 36-6).9





The major imaging differential diagnosis for schwannomas is the meningioma. Meningiomas may extend into the IAC and may be very difficult to distinguish from a schwannoma. However, the ability to center the lesion over the IAC greatly favors schwannoma. In general, schwannomas are usually more hyperintense on T2W images compared with meningiomas. They also tend to be more spherical as opposed to meningiomas, which are more often hemispheric. Meningiomas may occasionally result in expansion of the IAC, but this is much less common than with schwannomas.


If the lesion is large enough, MR spectroscopy has the potential to aid in differentiating the two lesions. Schwannomas have been noted to have elevated signal at 3.56 ppm representing myoinositol, as well as reduction in N-acetyl-aspartate (NAA) and elevation in choline. Meningiomas, on the other hand, demonstrate absence of NAA because they are not of neural origin, and an alanine peak may be noted at 1.55 ppm.10 Evaluation of the brain stem with a 3D fast spin-echo heavily T2W sequence may reveal a focus of increased signal intensity in the dorsal brain stem in the region of the vestibular nucleus unilateral to the lesion. This is thought to represent degeneration of the vestibular nucleus associated with a vestibular schwannoma and helps to suggest the diagnosis.11 Relative cerebral blood volume (rCBV) has been evaluated as a means of differentiating meningiomas from schwannomas; however, there is some overlap between the two lesions. Studies have compared the rCBV ratios (rCBV of the lesion divided by the rCBV of normal white matter) and found the highest reported rCBV ratio in schwannomas to be 4.4, whereas in meningiomas the ratio typically ranges from 6 to 9.12,13



MENINGIOMA


Meningiomas arise from meningothelial cells, also known as arachnoid cap cells, and are the most common nonglial primary neoplasms of the central nervous system (CNS). They are the second most common lesion in the CPA after schwannomas.







Imaging




MRI


Meningiomas are hypointense to isointense on T1W imaging and avidly and homogeneously enhance on postcontrast imaging owing to their vascular nature. They are isointense to hyperintense on T2W imaging (Fig. 36-10), but there may be areas of hypointensity that correlate to regions of calcification. T2 hyperintensity may be seen within the adjacent brain parenchyma secondary to tumor-induced vasogenic edema. On postcontrast imaging a “dural tail” may be seen, but this is not pathognomonic for meningiomas and has been reported in other lesions adjacent to the dura, including both schwannomas and hemangiopericytomas. In addition, the dural tail is only seen in up to 60% of meningiomas.19



Atypical imaging features such as heterogeneous enhancement, cyst formation, hemorrhage, and fatty degeneration may be seen in approximately 15% of histologically benign meningiomas.18 The degree of associated vasogenic edema does not appear to correlate with tumor grade or size. The etiology of the vasogenic edema is controversial and is probably the result of a combination of different mechanisms.18


MR perfusion demonstrates an elevated rCBV ratio, usually in the range of 6 to 9. Typically, there is less than 50% return to baseline because of the extra-axial nature of these lesions. This marked elevation in rCBV may be of some assistance in differentiating meningiomas from schwannomas, whose rCBV ratio values typically do not exceed 4, but there can be overlap.12 Because of susceptibility artifact in the posterior fossa, obtaining perfusion studies in this region may be limited. MR spectroscopy may demonstrate an alanine doublet at 1.4 ppm, but, again, like perfusion, there may be limitations of spectroscopy in the posterior fossa owing to the surrounding osseous structures. Alanine is found in tumors of meningeal origin and may be seen in 30% to 40% of meningiomas. NAA is not visualized because these tumors are not of neural origin, and choline may be elevated due to rapid membrane turnover.20


The major imaging differential diagnosis for a CPA meningioma is the schwannoma. Meningiomas may extend into the IAC and may be very difficult to distinguish from a schwannoma. However, meningiomas are less likely to be centered over the IAC than a schwannoma and they do not tend to enlarge the IAC. Also, schwannomas are usually more hyperintense on T2W images than meningiomas. Meningiomas are more hemispheric as compared with schwannomas, which tend to be rounded. In addition, schwannomas demonstrate elevated myoinositol and present, but reduced, NAA.




EPIDERMOID CYST


Epidermoid cysts are benign, slow-growing ectoderm-lined inclusion cysts that gradually expand over the years. They have been referred to as cholesteatomas, owing to their high cholesterol content. However, this term is confusing because of the presence of other intracranial entities that are referred to by the same name; thus, use of this term should be avoided.







Imaging



CT


Epidermoid cysts are well-demarcated, unilocular lesions. The margins are typically scalloped and irregular due to molding around adjacent structures. On CT they are hypodense, nonenhancing mass lesions that have attenuation similar to CSF. The hypodensity is thought to be due to the high cholesterol and keratin content of the desquamated debris. Occasionally, they may have slightly lower attenuation numbers (−10 to −20), but these numbers are never low enough to be confused with fat. Even rarer, they may be hyperdense, which, like bronchogenic cysts, may be related to high protein content.23 Hemorrhage within the cyst and elevated protein content are both suggested causes for this. On postcontrast imaging, epidermoid cysts typically do not enhance. On occasion, thin peripheral enhancement may be seen along the lining that may be due to an inflammatory response. The lobulated, irregular margins help distinguish them from arachnoid cysts, which usually have smooth margins. Epidermoid cysts tend to insinuate themselves around structures rather than displacing them; however, they may become adherent to these structures, making resection difficult.



MRI


MRI of epidermoid cysts usually demonstrates T1 and T2 intensity that is similar to CSF. However, high T1 signal intensity has been reported in some epidermoid cysts (“white epidermoids”). These lesions have a high lipid content composed of mixed triglycerides that contain unsaturated fatty acid residues.24 Occasionally, low signal may be seen on T2W images that may be related to elevated protein content and increased viscosity, and these may also have increased signal on T1W images. Hemorrhage into arachnoid cysts has been reported, which alters the signal characteristics.25 On fluid-attenuated inversion recovery (FLAIR) imaging, epidermoid cysts do not follow CSF, and they demonstrate reduced diffusion on diffusion-weighted imaging (Fig. 36-13). The finding of reduced diffusion is useful in postsurgical follow-up to assess for the presence of residual material. Both the FLAIR signal and reduced diffusion help to differentiate epidermoid cysts from arachnoid cysts, which follow CSF signal on all sequences. Unlike arachnoid cysts, epidermoid cysts tend to engulf nerves and vessels, rather than displace them, which can make surgical resection difficult. On MR spectroscopy, elevated lactate levels are seen within epidermoid cysts.26



Epidermoid cysts may have a lamellated appearance on MRI owing to layering of the accumulated desquamated material. The squamous cell lining of epidermoid cysts is usually too thin to be recognized by MRI. Peritumoral parenchymal edema is typically not seen. After the administration of gadolinium, these cysts may demonstrate thin rim enhancement, which may represent an inflammatory reaction.



LIPOMA


Lipomas are non-neoplastic developmental lesions developing from the meninx primitiva, which is the precursor to the pia mater and arachnoid.







ARACHNOID CYST


Arachnoid cysts are intra-arachnoid “pouches” that are filled with CSF.






Jan 22, 2016 | Posted by in NEUROLOGICAL IMAGING | Comments Off on Cerebellopontine Angle and Internal Auditory Canal Neoplasms
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