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Lesions of the cerebellopontine angle and internal auditory canal: Diagnosis and differential diagnosis
Lesions of the Cerebellopontine Angle and Internal Auditory Canal: Diagnosis and Differential Diagnosis Joel D. Swartz, MD Schwannoma of the eighth cranial nerve (acoustic neuroma) is, by far, the most common cerebellopontine angle lesion. There is, however, a wide variety of differential diagnostic possibilities that must be considered when viewing images of patients with sensorineural hearing loss (SNHL), vertigo, and dizziness. This communication is intended to provide the reader with an approach to this problem. Detailed anatomy of the region is also included in this communication. © 2004 Elsevier Inc. All rights reserved.
PERTINENT ANATOMY
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he internal auditory canals (IACs) are oriented perpendicular (or near perpendicular) to the sagittal plane of the skull.1 Often, the mediolateral orientation is such that the meatus (porus) is more superior as seen on coronal images. This is exaggerated in individuals with skull base disorders such as Paget’s disease. Axial images often reveal that the meatus is on a more posterior plane. We have observed a wide variation in the width and length of the canal. There is also significant normal variation with respect to the degree of flaring of the meatus (porus). In most cases, there is a strong tendency toward symmetry of the canals in the same individual; this was the basis for acoustic tumor diagnosis in the 1950s and 1960s with conventional polytomography. Most observers now realize that asymmetry is not rare. The facial nerve, the nerve of the second branchial arch (Reichert’s cartilage), arises from three brainstem nuclei: the motor nucleus (caudal pons), the superior salivatory nucleus (caudal pons, more dorsal), and the solitary tract nucleus (rostral medulla). The superior salivatory and solitary tract nuclei are responsible for parasympathetic secretory stimuli as well as taste and cutaneous sensory stimuli. These form the tiny intermediate nerve (Wrisberg) that accompanies the much larger motor branch through the IAC and temporal bone, eventually subtending the greater superficial petrosal and chorda tympani branches.1-3 The dorsal and ventral cochlear nuclei are located within the posterolateral aspect of the upper medulla and have been referred to collectively as
Address correspondence to: Joel D. Swartz, MD, P.O. Box 248, Gladwyne, PA 19035; e-mail: [email protected] © 2004 Elsevier Inc. All rights reserved. 0887-2171/04/2504-0004$30.00/0 doi:10.1053/j.sult.2004.04.003 332
the cochlear nuclear complex (CNC). There are four vestibular nuclei: superior, lateral, medial, and descending. These are located medial to the CNC in direct apposition to the floor of the fourth ventricle.4 The facial and vestibulocochlear nerves traverse the cerebellopontine angle cistern.1-4 The contents of the canal are anatomically consistent especially at the level of the fundus, the lateral most aspect of the IAC (Fig 1). The facial nerve consistently occupies the anterosuperior quadrant of the IAC at this level. The vestibulocochlear nerve enters the IAC intact but divides into vestibular and cochlear components at approximately the mid portion of the canal, where the IAC is subdivided by the crista falciformis, a horizontally oriented bony crest, into the superior and inferior compartments. The crista is consistently seen on coronal computed tomographic (CT) images. The superior IAC is further subdivided at this level by a thinner vertically oriented bony crest referred to as Bill’s Bar after the world-famous otologist, William F. House, MD. The vestibular nerve divides into its superior and inferior components just medial to the falciform crest. Thus, at the fundus of the IAC, the neural relationships are relatively consistent: the facial (and intermediate) nerves reside anterosuperiorly, the cochlear nerve anteroinferiorly, and the superior and inferior vestibular nerves within the posterosuperior and posteroinferior quadrants respectively. The superior vestibular nerve subtends the superior and lateral semicircular canals as well as the utricle. The saccule and posterior semicircular canal are innervated by the inferior vestibular nerve. The singular nerve is often identified separate from the inferior vestibular nerve within the posterior inferior quadrant. This nerve supplies the posterior semicircular canal and occupies a separate and distinct bony orifice, the singular foramen, which is consistently seen coursing parallel to the
Seminars in Ultrasound, CT, and MRI, Vol 25, No 4 (August), 2004: pp 332-352
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Fig 1. Internal auditory canal, cross sectional. Anatomy at the level of the fundus. (Color version of figure is available online.)
fundus of the IAC on both axial and coronal CT images. SCHWANNOMA OF THE EIGHTH CRANIAL NERVE (ACOUSTIC NEUROMA)
Schwannoma of the eighth cranial nerve (acoustic neuroma) is by far the most common cerebellopontine angle (CPA) neoplasm. They are noncalcifying, slow-growing, well-encapsulated lesions of midlife that account for 6% to 10% of all intracranial tumors and 60% to 90% of CPA tumors.5-7 They are somewhat more common and tend to be larger in women. There are strong indications that pregnancy may accel-
erate the growth of this tumor. Schwannomas most commonly present as a combined intracanalicular/CPA lesion; however, they may be entirely intracanalicular in nature. Interestingly, this latter subgroup of lesions is more common in men. Schwannomas arising from the extracanalicular portion of the nerve may involve only the CPA cistern (Fig 2), sparing the IAC and masquerade as meningioma, metastasis, or exophytic brainstem tumor. Intralabyrinthine Schwannomas are rare but may occur within the cochlea or vestibule. Eighth nerve Schwannomas typically present with progressive unilateral high frequency retroco-
Fig 2. Schwannoma of the 8th cranial nerve, extracanalicular. Contrast-enhanced axial T1-weighted image demonstrates an intensely enhancing mass within the cerebellopontine angle in direct apposition to the porus (arrow) of the internal auditory canal. Note that the lesion has not altered or invaded the canal. This is a somewhat atypical appearance for an acoustic tumor.
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Fig 3. Schwannoma of the 8th cranial nerve, intracanalicular. (A) Contrast-enhanced coronal T1-weighted image demonstrates intense enhancement at the porus of the internal auditory canal. (B) Axial T2-weighted fast-spin-echo image demonstrates a mass at the porus of the internal auditory canal (arrow) resulting in a subtle degree of bony expansion. This is a typical location for this lesion. Note the normal CSF intensity within the fundus of the IAC. The absence of tumor in this specific location has significance with respect to the choice of operative approach (see text). (Images courtesy H. Ric Harnsberger, MD.) C and D: Schwannoma of 8th cranial nerve, atypical. (C) Non-enhanced axial T1-weighted image. (D) Contrast-enhanced axial T1-weighted image. There is a focus of enhancement within the fundus of the left internal auditory canal. Surgical exploration revealed a small Schwannoma of the 8th cranial nerve. The fundus location is atypical. These lesions are much more commonly located at the porus (medially). Please see text. (E) Schwannoma of 8th cranial nerve arising from cochlear portion. Contrast-enhanced axial T1-weighted image demonstrated an intensely enhancing mass in the fundus (arrow) extending into the cochlea (compared to opposite side). A Schwannoma was found at the time of surgery arising from the cochlear portion of the vestibulocochlear nerve. F and G: Schwannoma 8th cranial nerve extending into vestibule. (F) Non-enhanced axial T1-weighted image. (G) Enhanced axial T1weighted image. There is an intensely enhancing left-sided intracanalicular mass (arrows). There is pathologic enhancement within the vestibule as well indicating a continuation of the neoplasm.
chlear sensorineural hearing loss presumably resulting from compression/infiltration of auditory (cochlear) nerve fibers.7 This hearing loss is often clinically manifest by disproportionately deficient speech discrimination (compared to pure tone loss)
as measured by brainstem electric response audiometry. Sudden worsening of hearing deficit occurs commonly (26%) and is presumably associated with occlusion of the internal auditory branch of the anterior inferior cerebellar artery (AICA). A
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Fig 3.
small number of patients have normal hearing (or symmetric loss). The clinician must maintain suspicion when nonauditory symptoms predominate such as tinnitus or vertigo. Tinnitus in this context is typically high pitched and unilateral (tumor side). Vertigo, an episodic illusion of motion, and horizontal nystagmus are manifestations of peripheral vestibular dysfunction that typically occur early in the course of tumor development, possibly due to disruption of vascular supply. Disequilibrium, a continuous sense of instability, occurs with larger lesions, possibly due to deafferentation. The presence of vertical nystagmus also implies a large
(continued)
tumor with brainstem compression. Secondary facial nerve involvement is unusual as this cranial nerve may undergo substantial torsion without losing its functional integrity. Recall that although the facial nerve is predominately motor, there is a sensory component that supplies a portion of the external auditory canal and pinna. Diminished sensation in this distribution is referred to as Hitselberger’s sign. Large lesions are more likely to cause fifth nerve symptoms (facial numbness, diminished corneal reflex). Facial pain, presumably due to vascular impingement on the trigeminal root entry zone, is uncommon but not rare. Intention
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Fig 4. Large cystic Schwannoma of the 8th cranial nerve. Axial contrast-enhanced T1weighted image demonstrates an intensely enhancing mass within the right internal auditory canal extending into the cerebellopontine angle (arrows). Note a large non-enhancing area in the periphery representing cystic/necrotic changes in the lesion. This is not an uncommon finding in very large acoustic tumors. Note the dramatic displacement of the 4th ventricle.
tremor and ataxia result from cerebellar compression. Despite the progressive nature of the hearing deficit, eighth nerve Schwannomas arise from the vestibular rather than the cochlear portion of the nerve in approximately 85% of these patients. The superior and inferior vestibular nerves are equally likely to be the site of origin. The IACs have a strong tendency to be symmetric, similar to the optic canals. This was the basis for radiologic diagnosis of Schwannoma with plain film and polytomography, modalities that required precise comparative measurements and identification of subtle erosive change.1,5,6 With the development of CT, direct visualization of the CPA component of the tumor could be accomplished and it was simultaneously discovered that a significant percentage of normal individuals have asymmetric IACs. Despite advancements in CT, the diagnosis of intracanalicular lesions remained difficult until the development of gas CT cisternography. This technique required the placement of a small amount (2 to 3 mL) of intrathecal air or CO2 via lumbar puncture. The patient was imaged in the decubitus position with the affected ear up. Utilizing this technique, very small lesions were first diagnosed and an appreciation of CPA/IAC neuro-
vascular anatomy was mandated. The emergence of thin-section gadolinium-enhanced T1-weighted MRI sequences revolutionized diagnosis as very small lesions could be easily diagnosed less invasively and larger lesions were more readily characterized (Fig 3A). More recently, thin-section T2-weighted fast-spin-echo (FSE) and gradientecho techniques have been developed that provide highly sensitive screening for these lesions without the use of gadolinium. FSE images have the advantage of far less tendency for magnetic susceptibility artifact and resolution is further increased via the use of phased array receiver coils.8,9 Cerebrospinal fluid (CSF) provides an excellent contrast in this context as virtually all Schwannomas are of lower T2 signal intensity (Fig 3B). This is more apparent with longer TR (4000-5000 msec), which is practical because of the decreased acquisition time resulting from the long echo trains inherent to this technique. There is some disagreement regarding the precise site of origination of eighth nerve Schwannomas.7 It has been postulated that they arise in most cases at the transition zone between the myelin produced by the oligodendroglia (central myelin) and that produced by Schwann cells (peripheral myelin). This neuroglial/neurilemmal junction, re-
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Fig 5. Bilateral vestibulocochlear Schwannomas, NF II. (A) Axial CT. Relative flaring of porus on right (arrow). (B) Axial T1-weighted MR image, precontrast. (C) Axial T1-weighted MR image, postcontrast. Bilateral lesions are demonstrated. Koos I on left. Koos IV on right. (Courtesy David M. Yousem, MD.)
ferred to as the Obersteiner-Redlich zone, occurs most commonly at the Scarpa’s (vestibular) ganglia typically (but not invariably) located at or in close proximity to the meatus (porus) of the IAC.
Others have suggested that these tumors originate from Scarpa’s ganglion because this is the area of the highest concentration of Schwann cells. Extracanalicular lesions invariably arise in close prox-
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Fig 6. Postoperative enhancement. Coronal contrast-enhanced T1-weighted image demonstrates faint but clearly discernible plaque-like enhancement (arrows) along the margins of the canal in this patient who has undergone Schwannoma removal via a retrosigmoid approach. This type of enhancement is an expected postoperative consequence. Nodular enhancement is more suspicious of recurrent/residual tumor, although the findings are not entirely specific. This examination nonetheless provides an excellent baseline examination.
imity to the meatus. No vestibular Schwannoma has ever been diagnosed at the brainstem root entry zone or even within the medial one-half of the cisternal segment of the nerve. Schwannomas only uncommonly arise within the lateral IAC (fundus), and therefore, imaging diagnosis of intracanalicular lesions in this specific location must be viewed with suspicion. Fundal tumors extending to the cochlea or vestibule are even more uncommon (Fig 3C-G). False positive examinations have been documented most commonly in the presence of relatively faint fundal enhancement. Nonneoplastic conditions encountered during surgical exploration of these “lesions” has lead to speculation that perhaps a “wait and see” approach should be advocated in this circumstance, with reimaging in 6 months. Disappearance of this pathologic enhancement has been documented in several patients. Schwannomas are composed of compact Antoni type A tissue and loose textured, often cystic, Antoni type B tissue.1,6,7 The predominance of the Antoni type A variety helps explain this tendency toward T2 hyposignal relative to CSF. As Schwannomas enlarge, regions of internal necrosis/cyst formation may result in a heterogeneous appearance (Fig 4). These are of higher signal intensity than CSF presumably due to the presence of hemorrhagic byproducts, necrotic material, or colloid-rich fluid. These larger lesions have a predominance of Antoni type B cells. Extramural (arachnoid) cysts are also associated with large lesions presumably secondary to elevation and deformation of the leptomeninges, which results in the formation of peritumoral adhesions thereby creating a pseudo-duplication of the arachnoid,
trapping fluid between the leptomeninges and the mass.10 Extramural cysts are also associated with meningiomas via a similar mechanism. Intratumoral hemorrhage is rare and associated with head injury or vigorous physical exertion. Both intratumoral hemorrhage and cystic expansion may result in a rapid increase in tumor volume. These patients often develop a sudden severe worsening of symptomatology. Eighth nerve Schwannomas are typically benign and slow growing. Malignant degeneration is rare and usually associated with neurofibromatosis I (NF I). These patients have an increased incidence of Schwannomas; however, fewer than 5% have eighth nerve Schwannoma.1 By contrast, the identification of bilateral eighth nerve Schwannomas is cornerstone of imaging diagnosis of the far less common neurofibromatosis II (NF II), so-called central neurofibromatosis. (Fig 5) These patients typically also have multiple meningiomas, neurofibromas, and glial tumors. Eighth nerve lesions may develop at a young age and magnetic resonance imaging (MRI) screening is recommended. The gene responsible for this neoplasm has been isolated to the long arm of chromosome 22. The reader should be aware that very large IACs may occur in the neurofibromatosis patient in the absence of neoplasm presumably secondary to dural ectasia. The collaboration of the imaging specialist and the surgical team is required in the choice of a definitive surgical procedure when an eighth nerve tumor is diagnosed. Three major approaches are currently employed: translabyrinthine (TL), retrosigmoid (RS), and middle fossa (MF). Factors
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Fig 7. CPA epidermoid (digital). (A) Axial T1-weighted MR image: heterogeneous predominantly isointense right cerebellopontine angle mass. (B) Axial T2-weighted MR image: heterogeneous predominantly hyperintense right cerebellopontine angle mass. This lesion did not enhance with contrast. (Courtesy H. Christian Davidson, MD.)
influencing the decision include the depth of tumor extension into the IAC (fundal involvement), the presence of a CPA component, and the prognosis for hearing conservation.7
If there is no serviceable hearing or the prognosis for acceptable hearing is dim, the TL approach is most desirable as surgical morbidity is greatly reduced.1,7 There is a much lower incidence of
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Fig 8. Arachnoid cyst, right cerebellopontine angle. Axial T2-weighted image demonstrates a clearly definable CSF intensity extra-axial mass displacing the cerebellar hemisphere. Differential diagnosis includes epidermoid (congenital cholesteatoma).
Fig 9. Meningioma, left cerebellopontine angle. (A) Contrast-enhanced axial T1-weighted image demonstrates an intensely enhancing mass within the left cerebellopontine angle. The epicenter is posterior to the porus of the internal auditory canal (arrowheads). (B) Axial T2-weighted image demonstrates that the mass is hypointense to the surrounding cerebellar parenchyma. This is typical for most histopathologic varieties of meningioma regardless of intracranial location.
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Fig 10. Intracanalicular lipoma. (A) Non-enhanced magnified axial T1-weighted image left ear. There is a small, well-marginated hyperintense mass at the level of the fundus of the internal auditory canal (arrow) in direct apposition to the approximate location of the cochlear nerve. (B) Fat suppressed magnified axial T1-weighted image. The lesion is no longer seen.
headache and postoperative CSF leak. The TL approach is also required when an intralabyrinthine component is present. Despite common misconception, the surgical exposure necessary for removal of large CPA components can be accomplished with the TL approach in experienced hands. Hearing conservation requires either an MF or RS approach. In this context, the imaging findings become especially crucial. The presence of a fundal component precludes the RS technique because a portion of the labyrinth must be removed to expose the lateral third of the IAC (Fig 3B). There are strong indications that FSE MR sequences utilizing T2 weighting are more sensitive to the presence of fundal tumor because bright CSF provides better contrast than the dark CSF visualized on gadolinium MRI (T1 weighting). The presence of a significant CPA component (greater than 1.5 cm.) precludes the MF technique. The MF technique affords the greatest likelihood of hearing conservation but does so at the expense of the need for greater facial nerve manipulation as the anterosuperior location of the nerve within the IAC places the nerve between the surgeon and the tumor. This is especially true when the neoplasm arises from the inferior (IVN) instead of the superior vestibular nerve (SVN). The likelihood of accomplishing satisfactory hearing conservation is
also reduced when the IVN is the source of the tumor due to its proximity to the cochlear nerve (inferior one-half of IAC). IVN tumors are also in closer proximity to the internal auditory artery. Compromise of this vessel (manipulative, occlusion, vasospasm) is an important contributor to postoperative hearing dysfunction. The imaging diagnosis of recurrent Schwannoma may be challenging. Postoperative intracanalicular enhancement with gadolinium is very common (Fig 6). Such enhancement may be due to recurrent tumor, adhesions, or meningeal irritation. Temporalis muscle and fascia, commonly used to seal the bony IAC defect created with the MF approach, results in a “globular” enhancement pattern. A linear enhancement pattern within the postoperative IAC is also an expected postoperative appearance.11 The current recommendation is to perform axial and coronal thin-section T1weighted pre- and postcontrast sequences within 2 months of surgery to serve as a baseline study. Follow-up studies that demonstrate a change in the enhancement pattern should be viewed with suspicion especially when this enhancement becomes nodular. Intracanalicular enhancement may also occur within the facial nerve as the result of surgical manipulation of this structure and must not be confused with tumor recurrence.
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Fig 11. Metastatic renal cell carcinoma, right cerebellopontine angle. (A) Non-enhanced axial T1-weighted image. (B) Enhanced T1-weighted image. (C) Axial T2-weighted image. There is an intensely enhancing mass in the right cerebellopontine angle with significant edema infiltrating the right cerebellar hemisphere. The lesion is of low signal intensity on T2-weighted images. This is often seen in metastatic lesions resulting from adenocarcinoma. (Courtesy Robert Tien, MD; In: Swartz and Harnsberger. Imaging of the Temporal Bone. Thieme Medical Publishers, 1998. Reprinted by permission.)
DIFFERENTIAL DIAGNOSIS—NEOPLASM
Cerebellopontine Angle Epidermoids Epidermoids (congenital cholesteatoma) consist of a sac of exfoliated keratin lined by stratified squamous epithelium and are derived from ectodermal rests.12-14 They occur at numerous intraosseous, intradural, and extradural locations. They grow very slowly via desquamation and therefore often do not present until adulthood despite their congenital etiology. They account for 3% to 7% of CPA masses and may become quite large before becoming symptomatic because they have a strong tendency to grow along paths of least
resistance and, as such, conform to the surface of the brain and tend to surround (encase) normal vascular structures rather than displace or invade them (Fig 7). Extension into the middle cranial fossa results in a dumbbell shape. On CT, their density is similar to CSF and there is little, if any, contrast enhancement. Hyperdense epidermoids have been reported but are extremely rare.15 This may be due to hemorrhage or the presence of keratinized debris with saponification to calcium soaps. An occasional lesion will have a faintly enhancing border or flecks of calcification. MRI examination usually reveals characteristically long T1- and T2-relaxation times similar to CSF because of high protein content and because the
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Fig 12. Exophytic brainstem glioma. (A) Contrast-enhanced axial T1-weighted image demonstrates an intensely enhancing mass filling the right internal auditory canal and extending to involve the cerebellopontine angle. There is a faint area of enhancement just ventral to the compressed fourth ventricle. (B) Axial T2-weighted image demonstrates that this mass infiltrates the brainstem. Indeed, the mass originated in the brainstem and grew exophytically throughout the cerebellopontine angle into the internal auditory canal. Diagnosis: Astrocytoma.
cholesterol in epidermoid tumors is in the solid/ crystalline state rather than the liquid state, which results in bright T1 signal. Craniopharyngioma often demonstrates the latter signal characteristic; epidermoids rarely do. Epidermoids often have a lamellated appearance because of surface desquamation. They demonstrate little if any enhancement. Malignant transformation of an epidermoid (squamous cell carcinoma) is rare but should be considered if follow-up scans demonstrate a significant increase in the amount of enhancement. Arachnoid cysts have a similar appearance to epidermoid on MR (and CT) examinations; however, the lamellated appearance and the tendency to insinuate rather than displace are far more characteristic of epidermoid. Arachnoid cysts usually result from congenital splitting of the arachnoid membrane and the MRI diagnosis is predicated on signal characteristics identical to CSF on all pulse sequences (Fig 8). Differentiation of the epidermoid from arachnoid cyst is typically accomplished utilizing FLAIR (flow attenuated inversion recovery) sequence.14 Typically, the arachnoid cyst will follow CSF intensity and the epidermoid will become bright. Recently, in difficult cases, diffusion imaging has provided additional support. Epidermoids are bright on diffusion imaging because of low water diffusability (low apparent diffusion coefficient, or ADC). Arachnoid cysts have high ADC.
Meningiomas Meningiomas usually arise from the posterior petrous surface or the underside of the tentorium.1,6 Intense enhancement is the rule. Relative T2 hyposignal is characteristic of most of these lesions but depends upon the precise histologic makeup (Fig 9). Differentiation from acoustic tumor is based primarily on remoteness of the epicenter from the meatus of the IAC. Occasionally, meningiomas will masquerade as acoustic tumors when the epicenter is indeed at the meatus. Purely intracanalicular meningiomas are rare but have been reported. They may behave in an aggressive fashion and extend throughout the petrous bone from this location. Interestingly, these lesions displace this facial nerve posteriorly rather than anteriorly. Both of these characteristics are atypical for Schwannoma. Extension of tumor into the IAC is not rare in this context. The presence of a dural tail was once thought to be pathognomonic of meningioma and represent tumor extending along the adjacent dura; however, not only have histologic studies proven that the “tail” is often merely an inflammatory response, but numerous neoplasms and even nonneoplastic entities have been discovered to exhibit this nonspecific imaging characteristic. Facial Nerve Schwannomas Facial nerve Schwannomas may arise anywhere along the course of the nerve from the
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Fig 13. Cavernous hemangioma, left cerebellopontine angle. (A) Non-enhanced axial T1-weighted image. (B) Enhanced axial T1-weighted image. There is a heterogeneous mass involving the left anterolateral aspect of the brainstem with a significant exophytic component into the left cerebellopontine angle and some pathologic enhancement within the internal auditory canal as well. (C) Axial T2-weighted images. Multiple areas of low signal are identified within the lesion likely representing hemosiderin deposition. This lesion is typical of cavernous hemangioma occurring at multiple other intracranial sites.
CPA to the stylomastoid foramen. Lesions arising within the CPA or IAC may be impossible to differentiate from acoustic tumors because their imaging characteristics are identical.16 If an intracanalicular lesion extends along the intratemporal facial canal, the diagnosis of facial nerve tumor is secured.
Cerebellopontine Angle/Internal Auditory Canal Lipomas CPA/IAC lipomas are rare lesions but are important to discuss for several reasons.1,6 They represent mesodermal inclusions, distinguishing them from epidermoid, which are ectodermal in nature. Surgery for these lesions is reserved for progres-
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Fig 14. Choroid plexus papilloma, right cerebellopontine angle. (A) Contrast-enhanced axial T1-weighted image demonstrates an enhancing mass extending along the right lateral recess of the 4th ventricle into the inferior portion of the cerebellopontine angle (arrows). (Note the normal enhancement of the choroids plexus in the lateral recess on the left.) Biopsy revealed choroids plexus papilloma. (B) Axial contrast-enhanced T1-weighted image. Normal image for comparison. There is clearly definable enhancement of the normal choroid plexus within the lateral recess of the 4th ventricle (arrowheads). Case courtesy H. Ric Harnsberger, MD.
sive neurological abnormality only because functional nerve fibers are typically intertwined within the mass, which makes surgical extirpation difficult or impossible. T1 hypersignal is characteristic as it is for fatty structures elsewhere (Fig 10). These lesions must be differentiated from hemorrhagic masses. T2-signal characteristics are usually diagnostic in this regard. Retained pantopaque (iophendylate) has signal characteristics identical to fat. Clinical history of previous myelogram is needed to suggest this diagnosis. On occasion, lipomas may be entirely intracanalicular and, despite their rarity, mandate routine precontrast thin-section T1-weighted images in the context of complete IAC examinations. Otherwise, a Schwannoma (presumably enhancing) may be diagnosed. Metastatic Disease CPA metastatic disease may be leptomeningeal or parenchymal (brainstem, cerebellum). Meningeal metastases may be diffuse or nodular and mass-like. Parenchymal lesions result from involvement of the peripheral of the brainstem or cerebellar hemisphere. With large lesions, the precise site of origin will usually not be apparent (Fig
11). Breast and lung primaries predominate. However, metastatic lesions in this location may result from numerous entities. Primary Parenchymal Neoplasms Similarly, primary parenchymal neoplasms may rarely have an exophytic component resulting in a CPA mass (Fig 12). Careful attention to all MR pulse sequences will usually reveal an infiltrative intra-axial component that clarifies the diagnosis. Childhood neoplasms such as medulloblastoma, astrocytoma, and ependymoma may present in this fashion. Astrocytoma is the most common lesion to result in this phenomenon in the adult patient. Cavernous hemangioma may occur anywhere within the neuraxis. Lesions of the brainstem and cerebellum are typically small and well circumscribed; however, large lesions with an exophytic component have been reported. To make the imaging diagnosis in this context, the observer must keep in mind the well-known heterogeneous (popcorn) appearance that occurs as the result of numerous hemorrhagic byproducts including hemosiderin (Fig 13).
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Fig 15. Ossifying hemangioma, internal auditory canal. (A) Axial T1-weighted MR image. There is a slightly hyperintense lesion identified within the fundus of the right internal auditory canal (arrow). (B) Contrast-enhanced axial T1-weighted image. The mass intensely enhances with contrast (arrow). (C) Magnified axial CT image, right ear. (D) Magnified coronal CT image, right ear. Note that the mass is eccentrically located (attention to coronal image) and contains multiple bony spiculations virtually diagnostic for ossifying hemangioma.
Choroid Plexus Papillomas
Ossifying Hemangioma
Choroid plexus papillomas most commonly occur in the lateral ventricle in children and in the fourth ventricle in the adult. Intense homogeneous enhancement with iodinated contrast (CT) and gadolinium (MRI) is the rule. The choroid plexus extends into the lateral recess of the fourth ventricle via the foramen of Lushka and extends as far as the CPA; therefore, this diagnosis must be kept in mind when a CPA mass is identified (Fig 14).
Ossifying hemangioma is a well-known, albeit uncommon, temporal bone lesion, most commonly originating along the facial nerve canal, particularly at the first genu. This lesion is characterized by intense contrast enhancement as well as bony speculations similar to hemangiomas noted elsewhere. The IAC is an unusual site for this lesion but has been reported (Fig 15).
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Fig 16. Meningeal metastases, intracanalicular. (A) Axial thin-section T1-weighted image, precontrast. (B and C) Axial thinsection T1-weighted images, postcontrast. Pathologic enhancement suspicious for acoustic tumor is identified within both internal auditory canals (arrows). Importantly, there is diffuse leptomeningeal enhancement noted on these images as well reflecting the diffuse nature of this process. This patient has widely disseminated metastatic lung carcinoma.
DIFFERENTIAL DIAGNOSIS— MENINGEAL DISEASE
Anatomy The meninges are composed of three discrete layers: the dura, which is dense connective tissue firmly attached to the calvarium, the pia, which is a clear membrane firmly attached to the surface of the brain extending deeply into the sulci, and the arachnoid, which is a complex network of reticular fibers interposed between the pia and the dura, closely applied to the latter.17 Importantly, the pia and arachnoid are collectively referred to as the leptomeninges; the dura is referred to as the pachy-
meninges. The leptomeninges are ectodermal in origin (neural crest); the pachymeninges are derived from mesoderm. The pachymeninges consists of an inner and outer layer between which the dural venous sinuses travel. The inner (meningeal) layer forms the falx cerebri, tentorium, and falx cerebelli. The outer (endosteal) layer is firmly attached to the inner table of the skull, especially at sutural sites. The latter anatomic characteristic explains why epidural collections are restricted from growing past the sutures and instead become convex toward brain, resulting in greater local mass effect than the subdural variety. Because the me-
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Fig 17. Lymphocytic meningitis, sensorineural hearing loss. (A) Contrast-enhanced magnified axial T1-weighted image demonstrates pathologic enhancement within the fundus of the internal auditory canal and within the cochlea (arrows). (B) Axial contrast-enhanced T1-weighted image (at level of pons). Note the enhancement and thickening of the cisternal segment of the 5th cranial nerve on the left as well as pathologic enhancement in vicinity of the cavernous sinus bilaterally (arrows). These findings are a manifestation of diffuse leptomeningeal disease. Diagnosis was lymphocytic meningitis.
ninges lack a blood-brain barrier, some enhancement with gadolinium on T1-weighted MR images is expected. The normal pachymeninges enhance with gadolinium in a predictable thin linear, discontinuous pattern. Disease of the pachymeninges results in enhancement, which is thicker, possibly somewhat nodular, and extends over a wider area. Some leptomeningeal enhancement may also be normal, but it is difficult to differentiate expected enhancement from relatively slowly flowing blood within medium-sized arteries and veins. The leptomeninges follow the second cranial (optic) nerve through the optic canal as the optic nerve sheath. Similarly, the leptomeninges also follow the seventh (facial) and eighth (vestibulocochlear) cranial nerves into the IAC. As such, any leptomeningeal disease process may manifest within the IAC and often causes related clinical symptomatology, particularly retrocochlear SNHL. Pathologic enhancement within the CPA and IAC may occur as a consequence of the entire gamut of meningeal disease including neoplastic and inflammatory/infectious conditions. Neoplastic entities include meningioma and meningeal metastases.17
Pathology Meningiomas Meningiomas arise from arachnoidal cap cells and are well known to occur within the CPA/ IAC. The histopathology is identical to those which develop elsewhere.1,6,17 Virtually all meningiomas enhance intensely and homogeneously with gadolinium (Fig 9). T2 signal appears to vary with histologic make-up: the angioblastic and syncitial varieties are relatively hyperintense secondary to higher water content. By contrast, the transitional and fibroblastic varieties demonstrate relative hypointensity due to densely packed collagen and/or psammomatous calcifications. Meningeal Metastasis Meningeal metastasis is referred to by a number of ambiguous terms including leptomeningeal carcinomatosis, meningeal carcinomatosis and carcinomatous meningitis. These terms are inexact as these neoplasms are not always carcinomas, often involve the pachymeninges as well as the leptomeninges and may not contain an inflammatory (-itis) component. Meningeal
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Fig 18. Sarcoidosis. (A) A progressive sensorineural hearing loss on right. Contrast-enhanced axial T1-weighted image demonstrates a faintly enhancing mass within the right internal auditory canal (arrow) involving primarily the fundus. Note the pathologic dural enhancement along the surface of the clivus (arrowheads). B and C: Different patient: Sarcoidosis, right cerebellopontine angle: (B) Contrast-enhanced coronal T1-weighted image. (C) Contrast-enhanced axial T1-weighted image. There is a clearly definable intensely enhancing mass involving the right cerebellopontine angle extending (on coronal images) to level of foramen magnum and into the right internal auditory canal. Adjacent foci of PS/arachnoid enhancement (arrowheads) indicate the presence of associated leptomeningeal disease and are the tip-off to the diagnosis of sarcoidosis.
metastases from solid tumors (i.e. breast) and lymphoproliferative malignancy (i.e. lymphoma) involve both the pachymeninges and the leptomeninges. Seeding from primary central nervous system (CNS) tumors involve only the leptomeninges. Leptomeningeal neoplasm may be diffuse or discrete and nodular (Fig 16). Nodular disease may masquerade as a primary neoplasm such as Schwannoma or meningioma. Involvement of multiple cranial nerves is common. In this context, both facial nerve dysfunction and vertigo/hearing loss may result from CPA/IAC dissemination.
Meningitis Meningitis may result in pathologic meningeal enhancement identical to that caused by neoplasm and has a similar incidence of CPA/IAC involvement (Fig 17). Clinical correlation is therefore necessary for diagnosis. Bacterial and fungal disease are more predisposed to this manifestation than viral disease. Meningeal disease is an extremely common manifestation of neurosarcoidosis. The leptomeninges are involved somewhat more commonly than the dura. This granulomatous process may result in both diffuse and nodular enhancement
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Fig 19. Intracranial hypotension, severe headaches. (A) Contrast-enhanced axial T1-weighted image demonstrates diffuse pathologic contrast enhancement of the leptomeninges especially in the prepontine and cavernous sinus region. There is also enhancement within the internal auditory canals (arrows). (B) Coronal contrast-enhanced T1-weighted image demonstrates enhancement within the internal auditory canals bilaterally. Differential diagnosis includes sarcoidosis as well as multiple other disorders of the leptomeninges. This patient had the classic headache syndrome of intracranal hypotension.
patterns. CPA/IAC disease is relatively common and results in retrocochlear sensorineural hearing loss (Fig 18). Involvement of other cranial nerves is very common. Pathologic enhancement of the meninges may also result from a variety of additional conditions. Segmental enhancement is routinely seen in postcraniotomy patients. On occasion, such enhancement is extensive and quite thick and may persist for several years. This enhancement exclusively involves the pachymeninges. Intracranial hypotension may be spontaneous or result from lumbar puncture or head trauma. The spontaneous variety is associated with a distinctive postural headache. Occult CSF leak is the presumed cause. The meningeal enhancement associated with this condition is most commonly dural but may be leptomeningeal as well (Fig 19). All the aforementioned leptomeningeal diseases may be associated with subcortical curvilinear enhancing strands implying involvement of the Virchow-Robin (perivascular) spaces. Communicating (extraventricular obstructive) hydrocephalus results from inhibition of CSF resorption at the level of the arachnoid granulations.
ADDITIONAL DIFFERENTIAL DIAGNOSTIC ENTITIES
Bell’s Palsy Bell’s palsy is the most common cause of facial palsy. Most observers favor a viral etiology. Thickening and enhancement of the facial nerve has been identified on MRI by several observers. Such enhancement typically involves the intratemporal facial nerve and must be distinguished from normal enhancement that occurs most commonly in the vicinity of the first genu (perigeniculate). Enhancement of the intracanalicular (IAC) segment of the nerve (Fig 20) is always abnormal and may be secondary to Bell’s palsy or other facial nerve maladies such as perineural spread of neoplasm (especially parotid), primary neoplasms, and postoperative/ posttraumatic etiologies. Facial palsy occurring in the context of clinical evidence of herpetic eruption within the external auditory canal is diagnostic of Ramsey Hunt syndrome. Intracanalicular facial and vestibulocochlear nerve enhancement may occur in this circumstance; membranous labyrinth enhancement may also be associated.
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Fig 20. Bell’s palsy. Axial (A) and coronal (B) contrast-enhanced T1-weighted images demonstrate pathologic contrast enhancement within the anterosuperior quadrant of the left internal auditory canal consistent with the location of the 7th cranial nerve in a patient with acute onset of facial weakness.
Fig 21. Siderosis. (A) Axial T2-weighted conventional spin-echo image diffuse leptomeningeal linear hyposignal. Cochlear nerve involvement on right (arrows). (B) Axial T2-weighted images, more inferior. Dramatic linear hyposignal again noted. (Courtesy Jonathan Lewin, MD.)
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Siderosis Siderosis is a condition characterized by intracellular and extracellular hemosiderin deposition along the leptomeninges, subpial tissue and cranial nerves, resulting from previous massive or recurrent subarachnoid hemorrhage that overwhelms the ability of the blood stream to remove the hemosiderin-laden
macrophages.18 Involvement of the IAC is classically associated with sensorineural hearing loss. Siderosis results in rapid T2 relaxation because of increased magnetic susceptibility and is manifest by pathologic hyposignal best seen on spin-echo T2-weighted images or gradient-echo sequences (Fig 21). There is no contrast enhancement.
REFERENCES 1. Swartz JD, Harnsberger HR: Imaging of the Temporal Bone. New York, Thieme Medical Publishers, 1998 2. Rubinstein D, Sandberg EJ, Cajade-Law AG: Anatomy of the facial and vestibulocochlear nerves in the internal auditory canal. AJNR Am J Neuroradiol 17:1099-1105, 1996 3. Gulya AJ: Anatomy and embryology of the ear, in Hughes GB, Pensak ML (eds): Clinical Otology, 2nd Edition. New York, Thieme Medical Publishers, 2000, Chapter 1, pp 3-34 4. Gebarski SS, Tucci DL, Telian SA: The cochlear nuclear complex: MR location and abnormalities. AJNR Am J Neuroradiol 14:1311-8, 1993 5. Smirniotopolous JG, Yue NC, Rushing EJ: Cerebellopontine angle masses: Radiologic-pathologic correlation. Radiographics 13:1131-1147, 1993 6. Maya MM, Lo WWM, Kovanlikaya I: Temporal bone tumors and cerebellopontine angle lesions, in Som PM, Curtin HD, Head and Neck Imaging (eds.): Fourth Edition, MosbyYearbook, St. Louis, 2003, Chapter 25, pg. 1275-1360 7. Jackler RK, in Jackler RK, Brackman DE (eds): Acoustic neuroma (vestibulocochlear schwannoma), in Neurotology. St Louis, Mosby Yearbook, 1994, Chapter 41, pp 729-785 8. Allen RW, Harnsberger HR, Shelton C, et al: Low-cost high resolution fast spin-echo MR of acoustic Schwannoma: An alternative to enhanced conventional spin-echo MR. AJNR Am J Neuroradiol 17:1205-1210, 1996 9. Fukui MB, Weissman JL, Curtin HD, Kanal E: T2weighted MR characteristics of internal auditory canal masses. AJNR Am J Neuroradiol 17:1211-1218, 1996 10. Tali ET, Yuh WTC, Nguyen HD, et al: Cystic acoustic
Schwannomas: MR characteristics. AJNR Am J Neuroradiol 14:1241-1247, 1993 11. Mueller DP, Gantz BJ, Dolan KD: Gadolinium-enhanced MR of the postoperative internal auditory canal following acoustic neuroma resection via the middle fossa approach. AJNR Am J Neuroradiol 13:197-200, 1992 12. Gao P, Osborn AG, Smirniotopolous JG, Harris CP: Epidermoid tumor of the cerebellopontine angle: Radiologicpathologic correlation. AJNR Am J Neuroradiol 13:863-872, 1992 13. Knorr JR, Ragland RL, Smith TW, et al: Squamous carcinoma arising within a cerebellopontine angle epidermoid: CT and MR findings. AJNR Am J Neuroradiol 12:1182-1184, 1991 14. Dutt SN, Mirza S, Chavda SV, Irving RN: Radiologic differentiation of intracranial epidermoids from arachnoid cysts. Otol Neurotol 23:84-92, 2002 15. Braun IF, Naidich TP, Leeds NE, et al: Dense intracranial epidermoid tumors. Radiology 122:717-719, 1977 16. Donnelly MJ, Cass AD, Ryan L: False positive MRI in the diagnosis of small intracanalicular vestibular Schwannomas. J Laryngol Otol 986-988, 1994 17. Cinnamon J, Sharma M, Gray D, et al: Neuroimaging of meningeal disease. Semin Ultrasound CT MRI 15:466-498, 1994 18. Hsu WC, Loevner LA, Forman MS, Thaler EA: Superficial siderosis of the CNS associated with multiple cavernous malformations. AJNR Am J Neuroradiol 20:1245-1248, 1999
PERTINENT ANATOMY
T
he internal auditory canals (IACs) are oriented perpendicular (or near perpendicular) to the sagittal plane of the skull.1 Often, the mediolateral orientation is such that the meatus (porus) is more superior as seen on coronal images. This is exaggerated in individuals with skull base disorders such as Paget’s disease. Axial images often reveal that the meatus is on a more posterior plane. We have observed a wide variation in the width and length of the canal. There is also significant normal variation with respect to the degree of flaring of the meatus (porus). In most cases, there is a strong tendency toward symmetry of the canals in the same individual; this was the basis for acoustic tumor diagnosis in the 1950s and 1960s with conventional polytomography. Most observers now realize that asymmetry is not rare. The facial nerve, the nerve of the second branchial arch (Reichert’s cartilage), arises from three brainstem nuclei: the motor nucleus (caudal pons), the superior salivatory nucleus (caudal pons, more dorsal), and the solitary tract nucleus (rostral medulla). The superior salivatory and solitary tract nuclei are responsible for parasympathetic secretory stimuli as well as taste and cutaneous sensory stimuli. These form the tiny intermediate nerve (Wrisberg) that accompanies the much larger motor branch through the IAC and temporal bone, eventually subtending the greater superficial petrosal and chorda tympani branches.1-3 The dorsal and ventral cochlear nuclei are located within the posterolateral aspect of the upper medulla and have been referred to collectively as
Address correspondence to: Joel D. Swartz, MD, P.O. Box 248, Gladwyne, PA 19035; e-mail: [email protected] © 2004 Elsevier Inc. All rights reserved. 0887-2171/04/2504-0004$30.00/0 doi:10.1053/j.sult.2004.04.003 332
the cochlear nuclear complex (CNC). There are four vestibular nuclei: superior, lateral, medial, and descending. These are located medial to the CNC in direct apposition to the floor of the fourth ventricle.4 The facial and vestibulocochlear nerves traverse the cerebellopontine angle cistern.1-4 The contents of the canal are anatomically consistent especially at the level of the fundus, the lateral most aspect of the IAC (Fig 1). The facial nerve consistently occupies the anterosuperior quadrant of the IAC at this level. The vestibulocochlear nerve enters the IAC intact but divides into vestibular and cochlear components at approximately the mid portion of the canal, where the IAC is subdivided by the crista falciformis, a horizontally oriented bony crest, into the superior and inferior compartments. The crista is consistently seen on coronal computed tomographic (CT) images. The superior IAC is further subdivided at this level by a thinner vertically oriented bony crest referred to as Bill’s Bar after the world-famous otologist, William F. House, MD. The vestibular nerve divides into its superior and inferior components just medial to the falciform crest. Thus, at the fundus of the IAC, the neural relationships are relatively consistent: the facial (and intermediate) nerves reside anterosuperiorly, the cochlear nerve anteroinferiorly, and the superior and inferior vestibular nerves within the posterosuperior and posteroinferior quadrants respectively. The superior vestibular nerve subtends the superior and lateral semicircular canals as well as the utricle. The saccule and posterior semicircular canal are innervated by the inferior vestibular nerve. The singular nerve is often identified separate from the inferior vestibular nerve within the posterior inferior quadrant. This nerve supplies the posterior semicircular canal and occupies a separate and distinct bony orifice, the singular foramen, which is consistently seen coursing parallel to the
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Fig 1. Internal auditory canal, cross sectional. Anatomy at the level of the fundus. (Color version of figure is available online.)
fundus of the IAC on both axial and coronal CT images. SCHWANNOMA OF THE EIGHTH CRANIAL NERVE (ACOUSTIC NEUROMA)
Schwannoma of the eighth cranial nerve (acoustic neuroma) is by far the most common cerebellopontine angle (CPA) neoplasm. They are noncalcifying, slow-growing, well-encapsulated lesions of midlife that account for 6% to 10% of all intracranial tumors and 60% to 90% of CPA tumors.5-7 They are somewhat more common and tend to be larger in women. There are strong indications that pregnancy may accel-
erate the growth of this tumor. Schwannomas most commonly present as a combined intracanalicular/CPA lesion; however, they may be entirely intracanalicular in nature. Interestingly, this latter subgroup of lesions is more common in men. Schwannomas arising from the extracanalicular portion of the nerve may involve only the CPA cistern (Fig 2), sparing the IAC and masquerade as meningioma, metastasis, or exophytic brainstem tumor. Intralabyrinthine Schwannomas are rare but may occur within the cochlea or vestibule. Eighth nerve Schwannomas typically present with progressive unilateral high frequency retroco-
Fig 2. Schwannoma of the 8th cranial nerve, extracanalicular. Contrast-enhanced axial T1-weighted image demonstrates an intensely enhancing mass within the cerebellopontine angle in direct apposition to the porus (arrow) of the internal auditory canal. Note that the lesion has not altered or invaded the canal. This is a somewhat atypical appearance for an acoustic tumor.
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Fig 3. Schwannoma of the 8th cranial nerve, intracanalicular. (A) Contrast-enhanced coronal T1-weighted image demonstrates intense enhancement at the porus of the internal auditory canal. (B) Axial T2-weighted fast-spin-echo image demonstrates a mass at the porus of the internal auditory canal (arrow) resulting in a subtle degree of bony expansion. This is a typical location for this lesion. Note the normal CSF intensity within the fundus of the IAC. The absence of tumor in this specific location has significance with respect to the choice of operative approach (see text). (Images courtesy H. Ric Harnsberger, MD.) C and D: Schwannoma of 8th cranial nerve, atypical. (C) Non-enhanced axial T1-weighted image. (D) Contrast-enhanced axial T1-weighted image. There is a focus of enhancement within the fundus of the left internal auditory canal. Surgical exploration revealed a small Schwannoma of the 8th cranial nerve. The fundus location is atypical. These lesions are much more commonly located at the porus (medially). Please see text. (E) Schwannoma of 8th cranial nerve arising from cochlear portion. Contrast-enhanced axial T1-weighted image demonstrated an intensely enhancing mass in the fundus (arrow) extending into the cochlea (compared to opposite side). A Schwannoma was found at the time of surgery arising from the cochlear portion of the vestibulocochlear nerve. F and G: Schwannoma 8th cranial nerve extending into vestibule. (F) Non-enhanced axial T1-weighted image. (G) Enhanced axial T1weighted image. There is an intensely enhancing left-sided intracanalicular mass (arrows). There is pathologic enhancement within the vestibule as well indicating a continuation of the neoplasm.
chlear sensorineural hearing loss presumably resulting from compression/infiltration of auditory (cochlear) nerve fibers.7 This hearing loss is often clinically manifest by disproportionately deficient speech discrimination (compared to pure tone loss)
as measured by brainstem electric response audiometry. Sudden worsening of hearing deficit occurs commonly (26%) and is presumably associated with occlusion of the internal auditory branch of the anterior inferior cerebellar artery (AICA). A
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Fig 3.
small number of patients have normal hearing (or symmetric loss). The clinician must maintain suspicion when nonauditory symptoms predominate such as tinnitus or vertigo. Tinnitus in this context is typically high pitched and unilateral (tumor side). Vertigo, an episodic illusion of motion, and horizontal nystagmus are manifestations of peripheral vestibular dysfunction that typically occur early in the course of tumor development, possibly due to disruption of vascular supply. Disequilibrium, a continuous sense of instability, occurs with larger lesions, possibly due to deafferentation. The presence of vertical nystagmus also implies a large
(continued)
tumor with brainstem compression. Secondary facial nerve involvement is unusual as this cranial nerve may undergo substantial torsion without losing its functional integrity. Recall that although the facial nerve is predominately motor, there is a sensory component that supplies a portion of the external auditory canal and pinna. Diminished sensation in this distribution is referred to as Hitselberger’s sign. Large lesions are more likely to cause fifth nerve symptoms (facial numbness, diminished corneal reflex). Facial pain, presumably due to vascular impingement on the trigeminal root entry zone, is uncommon but not rare. Intention
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Fig 4. Large cystic Schwannoma of the 8th cranial nerve. Axial contrast-enhanced T1weighted image demonstrates an intensely enhancing mass within the right internal auditory canal extending into the cerebellopontine angle (arrows). Note a large non-enhancing area in the periphery representing cystic/necrotic changes in the lesion. This is not an uncommon finding in very large acoustic tumors. Note the dramatic displacement of the 4th ventricle.
tremor and ataxia result from cerebellar compression. Despite the progressive nature of the hearing deficit, eighth nerve Schwannomas arise from the vestibular rather than the cochlear portion of the nerve in approximately 85% of these patients. The superior and inferior vestibular nerves are equally likely to be the site of origin. The IACs have a strong tendency to be symmetric, similar to the optic canals. This was the basis for radiologic diagnosis of Schwannoma with plain film and polytomography, modalities that required precise comparative measurements and identification of subtle erosive change.1,5,6 With the development of CT, direct visualization of the CPA component of the tumor could be accomplished and it was simultaneously discovered that a significant percentage of normal individuals have asymmetric IACs. Despite advancements in CT, the diagnosis of intracanalicular lesions remained difficult until the development of gas CT cisternography. This technique required the placement of a small amount (2 to 3 mL) of intrathecal air or CO2 via lumbar puncture. The patient was imaged in the decubitus position with the affected ear up. Utilizing this technique, very small lesions were first diagnosed and an appreciation of CPA/IAC neuro-
vascular anatomy was mandated. The emergence of thin-section gadolinium-enhanced T1-weighted MRI sequences revolutionized diagnosis as very small lesions could be easily diagnosed less invasively and larger lesions were more readily characterized (Fig 3A). More recently, thin-section T2-weighted fast-spin-echo (FSE) and gradientecho techniques have been developed that provide highly sensitive screening for these lesions without the use of gadolinium. FSE images have the advantage of far less tendency for magnetic susceptibility artifact and resolution is further increased via the use of phased array receiver coils.8,9 Cerebrospinal fluid (CSF) provides an excellent contrast in this context as virtually all Schwannomas are of lower T2 signal intensity (Fig 3B). This is more apparent with longer TR (4000-5000 msec), which is practical because of the decreased acquisition time resulting from the long echo trains inherent to this technique. There is some disagreement regarding the precise site of origination of eighth nerve Schwannomas.7 It has been postulated that they arise in most cases at the transition zone between the myelin produced by the oligodendroglia (central myelin) and that produced by Schwann cells (peripheral myelin). This neuroglial/neurilemmal junction, re-
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Fig 5. Bilateral vestibulocochlear Schwannomas, NF II. (A) Axial CT. Relative flaring of porus on right (arrow). (B) Axial T1-weighted MR image, precontrast. (C) Axial T1-weighted MR image, postcontrast. Bilateral lesions are demonstrated. Koos I on left. Koos IV on right. (Courtesy David M. Yousem, MD.)
ferred to as the Obersteiner-Redlich zone, occurs most commonly at the Scarpa’s (vestibular) ganglia typically (but not invariably) located at or in close proximity to the meatus (porus) of the IAC.
Others have suggested that these tumors originate from Scarpa’s ganglion because this is the area of the highest concentration of Schwann cells. Extracanalicular lesions invariably arise in close prox-
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Fig 6. Postoperative enhancement. Coronal contrast-enhanced T1-weighted image demonstrates faint but clearly discernible plaque-like enhancement (arrows) along the margins of the canal in this patient who has undergone Schwannoma removal via a retrosigmoid approach. This type of enhancement is an expected postoperative consequence. Nodular enhancement is more suspicious of recurrent/residual tumor, although the findings are not entirely specific. This examination nonetheless provides an excellent baseline examination.
imity to the meatus. No vestibular Schwannoma has ever been diagnosed at the brainstem root entry zone or even within the medial one-half of the cisternal segment of the nerve. Schwannomas only uncommonly arise within the lateral IAC (fundus), and therefore, imaging diagnosis of intracanalicular lesions in this specific location must be viewed with suspicion. Fundal tumors extending to the cochlea or vestibule are even more uncommon (Fig 3C-G). False positive examinations have been documented most commonly in the presence of relatively faint fundal enhancement. Nonneoplastic conditions encountered during surgical exploration of these “lesions” has lead to speculation that perhaps a “wait and see” approach should be advocated in this circumstance, with reimaging in 6 months. Disappearance of this pathologic enhancement has been documented in several patients. Schwannomas are composed of compact Antoni type A tissue and loose textured, often cystic, Antoni type B tissue.1,6,7 The predominance of the Antoni type A variety helps explain this tendency toward T2 hyposignal relative to CSF. As Schwannomas enlarge, regions of internal necrosis/cyst formation may result in a heterogeneous appearance (Fig 4). These are of higher signal intensity than CSF presumably due to the presence of hemorrhagic byproducts, necrotic material, or colloid-rich fluid. These larger lesions have a predominance of Antoni type B cells. Extramural (arachnoid) cysts are also associated with large lesions presumably secondary to elevation and deformation of the leptomeninges, which results in the formation of peritumoral adhesions thereby creating a pseudo-duplication of the arachnoid,
trapping fluid between the leptomeninges and the mass.10 Extramural cysts are also associated with meningiomas via a similar mechanism. Intratumoral hemorrhage is rare and associated with head injury or vigorous physical exertion. Both intratumoral hemorrhage and cystic expansion may result in a rapid increase in tumor volume. These patients often develop a sudden severe worsening of symptomatology. Eighth nerve Schwannomas are typically benign and slow growing. Malignant degeneration is rare and usually associated with neurofibromatosis I (NF I). These patients have an increased incidence of Schwannomas; however, fewer than 5% have eighth nerve Schwannoma.1 By contrast, the identification of bilateral eighth nerve Schwannomas is cornerstone of imaging diagnosis of the far less common neurofibromatosis II (NF II), so-called central neurofibromatosis. (Fig 5) These patients typically also have multiple meningiomas, neurofibromas, and glial tumors. Eighth nerve lesions may develop at a young age and magnetic resonance imaging (MRI) screening is recommended. The gene responsible for this neoplasm has been isolated to the long arm of chromosome 22. The reader should be aware that very large IACs may occur in the neurofibromatosis patient in the absence of neoplasm presumably secondary to dural ectasia. The collaboration of the imaging specialist and the surgical team is required in the choice of a definitive surgical procedure when an eighth nerve tumor is diagnosed. Three major approaches are currently employed: translabyrinthine (TL), retrosigmoid (RS), and middle fossa (MF). Factors
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Fig 7. CPA epidermoid (digital). (A) Axial T1-weighted MR image: heterogeneous predominantly isointense right cerebellopontine angle mass. (B) Axial T2-weighted MR image: heterogeneous predominantly hyperintense right cerebellopontine angle mass. This lesion did not enhance with contrast. (Courtesy H. Christian Davidson, MD.)
influencing the decision include the depth of tumor extension into the IAC (fundal involvement), the presence of a CPA component, and the prognosis for hearing conservation.7
If there is no serviceable hearing or the prognosis for acceptable hearing is dim, the TL approach is most desirable as surgical morbidity is greatly reduced.1,7 There is a much lower incidence of
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Fig 8. Arachnoid cyst, right cerebellopontine angle. Axial T2-weighted image demonstrates a clearly definable CSF intensity extra-axial mass displacing the cerebellar hemisphere. Differential diagnosis includes epidermoid (congenital cholesteatoma).
Fig 9. Meningioma, left cerebellopontine angle. (A) Contrast-enhanced axial T1-weighted image demonstrates an intensely enhancing mass within the left cerebellopontine angle. The epicenter is posterior to the porus of the internal auditory canal (arrowheads). (B) Axial T2-weighted image demonstrates that the mass is hypointense to the surrounding cerebellar parenchyma. This is typical for most histopathologic varieties of meningioma regardless of intracranial location.
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Fig 10. Intracanalicular lipoma. (A) Non-enhanced magnified axial T1-weighted image left ear. There is a small, well-marginated hyperintense mass at the level of the fundus of the internal auditory canal (arrow) in direct apposition to the approximate location of the cochlear nerve. (B) Fat suppressed magnified axial T1-weighted image. The lesion is no longer seen.
headache and postoperative CSF leak. The TL approach is also required when an intralabyrinthine component is present. Despite common misconception, the surgical exposure necessary for removal of large CPA components can be accomplished with the TL approach in experienced hands. Hearing conservation requires either an MF or RS approach. In this context, the imaging findings become especially crucial. The presence of a fundal component precludes the RS technique because a portion of the labyrinth must be removed to expose the lateral third of the IAC (Fig 3B). There are strong indications that FSE MR sequences utilizing T2 weighting are more sensitive to the presence of fundal tumor because bright CSF provides better contrast than the dark CSF visualized on gadolinium MRI (T1 weighting). The presence of a significant CPA component (greater than 1.5 cm.) precludes the MF technique. The MF technique affords the greatest likelihood of hearing conservation but does so at the expense of the need for greater facial nerve manipulation as the anterosuperior location of the nerve within the IAC places the nerve between the surgeon and the tumor. This is especially true when the neoplasm arises from the inferior (IVN) instead of the superior vestibular nerve (SVN). The likelihood of accomplishing satisfactory hearing conservation is
also reduced when the IVN is the source of the tumor due to its proximity to the cochlear nerve (inferior one-half of IAC). IVN tumors are also in closer proximity to the internal auditory artery. Compromise of this vessel (manipulative, occlusion, vasospasm) is an important contributor to postoperative hearing dysfunction. The imaging diagnosis of recurrent Schwannoma may be challenging. Postoperative intracanalicular enhancement with gadolinium is very common (Fig 6). Such enhancement may be due to recurrent tumor, adhesions, or meningeal irritation. Temporalis muscle and fascia, commonly used to seal the bony IAC defect created with the MF approach, results in a “globular” enhancement pattern. A linear enhancement pattern within the postoperative IAC is also an expected postoperative appearance.11 The current recommendation is to perform axial and coronal thin-section T1weighted pre- and postcontrast sequences within 2 months of surgery to serve as a baseline study. Follow-up studies that demonstrate a change in the enhancement pattern should be viewed with suspicion especially when this enhancement becomes nodular. Intracanalicular enhancement may also occur within the facial nerve as the result of surgical manipulation of this structure and must not be confused with tumor recurrence.
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Fig 11. Metastatic renal cell carcinoma, right cerebellopontine angle. (A) Non-enhanced axial T1-weighted image. (B) Enhanced T1-weighted image. (C) Axial T2-weighted image. There is an intensely enhancing mass in the right cerebellopontine angle with significant edema infiltrating the right cerebellar hemisphere. The lesion is of low signal intensity on T2-weighted images. This is often seen in metastatic lesions resulting from adenocarcinoma. (Courtesy Robert Tien, MD; In: Swartz and Harnsberger. Imaging of the Temporal Bone. Thieme Medical Publishers, 1998. Reprinted by permission.)
DIFFERENTIAL DIAGNOSIS—NEOPLASM
Cerebellopontine Angle Epidermoids Epidermoids (congenital cholesteatoma) consist of a sac of exfoliated keratin lined by stratified squamous epithelium and are derived from ectodermal rests.12-14 They occur at numerous intraosseous, intradural, and extradural locations. They grow very slowly via desquamation and therefore often do not present until adulthood despite their congenital etiology. They account for 3% to 7% of CPA masses and may become quite large before becoming symptomatic because they have a strong tendency to grow along paths of least
resistance and, as such, conform to the surface of the brain and tend to surround (encase) normal vascular structures rather than displace or invade them (Fig 7). Extension into the middle cranial fossa results in a dumbbell shape. On CT, their density is similar to CSF and there is little, if any, contrast enhancement. Hyperdense epidermoids have been reported but are extremely rare.15 This may be due to hemorrhage or the presence of keratinized debris with saponification to calcium soaps. An occasional lesion will have a faintly enhancing border or flecks of calcification. MRI examination usually reveals characteristically long T1- and T2-relaxation times similar to CSF because of high protein content and because the
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Fig 12. Exophytic brainstem glioma. (A) Contrast-enhanced axial T1-weighted image demonstrates an intensely enhancing mass filling the right internal auditory canal and extending to involve the cerebellopontine angle. There is a faint area of enhancement just ventral to the compressed fourth ventricle. (B) Axial T2-weighted image demonstrates that this mass infiltrates the brainstem. Indeed, the mass originated in the brainstem and grew exophytically throughout the cerebellopontine angle into the internal auditory canal. Diagnosis: Astrocytoma.
cholesterol in epidermoid tumors is in the solid/ crystalline state rather than the liquid state, which results in bright T1 signal. Craniopharyngioma often demonstrates the latter signal characteristic; epidermoids rarely do. Epidermoids often have a lamellated appearance because of surface desquamation. They demonstrate little if any enhancement. Malignant transformation of an epidermoid (squamous cell carcinoma) is rare but should be considered if follow-up scans demonstrate a significant increase in the amount of enhancement. Arachnoid cysts have a similar appearance to epidermoid on MR (and CT) examinations; however, the lamellated appearance and the tendency to insinuate rather than displace are far more characteristic of epidermoid. Arachnoid cysts usually result from congenital splitting of the arachnoid membrane and the MRI diagnosis is predicated on signal characteristics identical to CSF on all pulse sequences (Fig 8). Differentiation of the epidermoid from arachnoid cyst is typically accomplished utilizing FLAIR (flow attenuated inversion recovery) sequence.14 Typically, the arachnoid cyst will follow CSF intensity and the epidermoid will become bright. Recently, in difficult cases, diffusion imaging has provided additional support. Epidermoids are bright on diffusion imaging because of low water diffusability (low apparent diffusion coefficient, or ADC). Arachnoid cysts have high ADC.
Meningiomas Meningiomas usually arise from the posterior petrous surface or the underside of the tentorium.1,6 Intense enhancement is the rule. Relative T2 hyposignal is characteristic of most of these lesions but depends upon the precise histologic makeup (Fig 9). Differentiation from acoustic tumor is based primarily on remoteness of the epicenter from the meatus of the IAC. Occasionally, meningiomas will masquerade as acoustic tumors when the epicenter is indeed at the meatus. Purely intracanalicular meningiomas are rare but have been reported. They may behave in an aggressive fashion and extend throughout the petrous bone from this location. Interestingly, these lesions displace this facial nerve posteriorly rather than anteriorly. Both of these characteristics are atypical for Schwannoma. Extension of tumor into the IAC is not rare in this context. The presence of a dural tail was once thought to be pathognomonic of meningioma and represent tumor extending along the adjacent dura; however, not only have histologic studies proven that the “tail” is often merely an inflammatory response, but numerous neoplasms and even nonneoplastic entities have been discovered to exhibit this nonspecific imaging characteristic. Facial Nerve Schwannomas Facial nerve Schwannomas may arise anywhere along the course of the nerve from the
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Fig 13. Cavernous hemangioma, left cerebellopontine angle. (A) Non-enhanced axial T1-weighted image. (B) Enhanced axial T1-weighted image. There is a heterogeneous mass involving the left anterolateral aspect of the brainstem with a significant exophytic component into the left cerebellopontine angle and some pathologic enhancement within the internal auditory canal as well. (C) Axial T2-weighted images. Multiple areas of low signal are identified within the lesion likely representing hemosiderin deposition. This lesion is typical of cavernous hemangioma occurring at multiple other intracranial sites.
CPA to the stylomastoid foramen. Lesions arising within the CPA or IAC may be impossible to differentiate from acoustic tumors because their imaging characteristics are identical.16 If an intracanalicular lesion extends along the intratemporal facial canal, the diagnosis of facial nerve tumor is secured.
Cerebellopontine Angle/Internal Auditory Canal Lipomas CPA/IAC lipomas are rare lesions but are important to discuss for several reasons.1,6 They represent mesodermal inclusions, distinguishing them from epidermoid, which are ectodermal in nature. Surgery for these lesions is reserved for progres-
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Fig 14. Choroid plexus papilloma, right cerebellopontine angle. (A) Contrast-enhanced axial T1-weighted image demonstrates an enhancing mass extending along the right lateral recess of the 4th ventricle into the inferior portion of the cerebellopontine angle (arrows). (Note the normal enhancement of the choroids plexus in the lateral recess on the left.) Biopsy revealed choroids plexus papilloma. (B) Axial contrast-enhanced T1-weighted image. Normal image for comparison. There is clearly definable enhancement of the normal choroid plexus within the lateral recess of the 4th ventricle (arrowheads). Case courtesy H. Ric Harnsberger, MD.
sive neurological abnormality only because functional nerve fibers are typically intertwined within the mass, which makes surgical extirpation difficult or impossible. T1 hypersignal is characteristic as it is for fatty structures elsewhere (Fig 10). These lesions must be differentiated from hemorrhagic masses. T2-signal characteristics are usually diagnostic in this regard. Retained pantopaque (iophendylate) has signal characteristics identical to fat. Clinical history of previous myelogram is needed to suggest this diagnosis. On occasion, lipomas may be entirely intracanalicular and, despite their rarity, mandate routine precontrast thin-section T1-weighted images in the context of complete IAC examinations. Otherwise, a Schwannoma (presumably enhancing) may be diagnosed. Metastatic Disease CPA metastatic disease may be leptomeningeal or parenchymal (brainstem, cerebellum). Meningeal metastases may be diffuse or nodular and mass-like. Parenchymal lesions result from involvement of the peripheral of the brainstem or cerebellar hemisphere. With large lesions, the precise site of origin will usually not be apparent (Fig
11). Breast and lung primaries predominate. However, metastatic lesions in this location may result from numerous entities. Primary Parenchymal Neoplasms Similarly, primary parenchymal neoplasms may rarely have an exophytic component resulting in a CPA mass (Fig 12). Careful attention to all MR pulse sequences will usually reveal an infiltrative intra-axial component that clarifies the diagnosis. Childhood neoplasms such as medulloblastoma, astrocytoma, and ependymoma may present in this fashion. Astrocytoma is the most common lesion to result in this phenomenon in the adult patient. Cavernous hemangioma may occur anywhere within the neuraxis. Lesions of the brainstem and cerebellum are typically small and well circumscribed; however, large lesions with an exophytic component have been reported. To make the imaging diagnosis in this context, the observer must keep in mind the well-known heterogeneous (popcorn) appearance that occurs as the result of numerous hemorrhagic byproducts including hemosiderin (Fig 13).
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Fig 15. Ossifying hemangioma, internal auditory canal. (A) Axial T1-weighted MR image. There is a slightly hyperintense lesion identified within the fundus of the right internal auditory canal (arrow). (B) Contrast-enhanced axial T1-weighted image. The mass intensely enhances with contrast (arrow). (C) Magnified axial CT image, right ear. (D) Magnified coronal CT image, right ear. Note that the mass is eccentrically located (attention to coronal image) and contains multiple bony spiculations virtually diagnostic for ossifying hemangioma.
Choroid Plexus Papillomas
Ossifying Hemangioma
Choroid plexus papillomas most commonly occur in the lateral ventricle in children and in the fourth ventricle in the adult. Intense homogeneous enhancement with iodinated contrast (CT) and gadolinium (MRI) is the rule. The choroid plexus extends into the lateral recess of the fourth ventricle via the foramen of Lushka and extends as far as the CPA; therefore, this diagnosis must be kept in mind when a CPA mass is identified (Fig 14).
Ossifying hemangioma is a well-known, albeit uncommon, temporal bone lesion, most commonly originating along the facial nerve canal, particularly at the first genu. This lesion is characterized by intense contrast enhancement as well as bony speculations similar to hemangiomas noted elsewhere. The IAC is an unusual site for this lesion but has been reported (Fig 15).
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Fig 16. Meningeal metastases, intracanalicular. (A) Axial thin-section T1-weighted image, precontrast. (B and C) Axial thinsection T1-weighted images, postcontrast. Pathologic enhancement suspicious for acoustic tumor is identified within both internal auditory canals (arrows). Importantly, there is diffuse leptomeningeal enhancement noted on these images as well reflecting the diffuse nature of this process. This patient has widely disseminated metastatic lung carcinoma.
DIFFERENTIAL DIAGNOSIS— MENINGEAL DISEASE
Anatomy The meninges are composed of three discrete layers: the dura, which is dense connective tissue firmly attached to the calvarium, the pia, which is a clear membrane firmly attached to the surface of the brain extending deeply into the sulci, and the arachnoid, which is a complex network of reticular fibers interposed between the pia and the dura, closely applied to the latter.17 Importantly, the pia and arachnoid are collectively referred to as the leptomeninges; the dura is referred to as the pachy-
meninges. The leptomeninges are ectodermal in origin (neural crest); the pachymeninges are derived from mesoderm. The pachymeninges consists of an inner and outer layer between which the dural venous sinuses travel. The inner (meningeal) layer forms the falx cerebri, tentorium, and falx cerebelli. The outer (endosteal) layer is firmly attached to the inner table of the skull, especially at sutural sites. The latter anatomic characteristic explains why epidural collections are restricted from growing past the sutures and instead become convex toward brain, resulting in greater local mass effect than the subdural variety. Because the me-
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Fig 17. Lymphocytic meningitis, sensorineural hearing loss. (A) Contrast-enhanced magnified axial T1-weighted image demonstrates pathologic enhancement within the fundus of the internal auditory canal and within the cochlea (arrows). (B) Axial contrast-enhanced T1-weighted image (at level of pons). Note the enhancement and thickening of the cisternal segment of the 5th cranial nerve on the left as well as pathologic enhancement in vicinity of the cavernous sinus bilaterally (arrows). These findings are a manifestation of diffuse leptomeningeal disease. Diagnosis was lymphocytic meningitis.
ninges lack a blood-brain barrier, some enhancement with gadolinium on T1-weighted MR images is expected. The normal pachymeninges enhance with gadolinium in a predictable thin linear, discontinuous pattern. Disease of the pachymeninges results in enhancement, which is thicker, possibly somewhat nodular, and extends over a wider area. Some leptomeningeal enhancement may also be normal, but it is difficult to differentiate expected enhancement from relatively slowly flowing blood within medium-sized arteries and veins. The leptomeninges follow the second cranial (optic) nerve through the optic canal as the optic nerve sheath. Similarly, the leptomeninges also follow the seventh (facial) and eighth (vestibulocochlear) cranial nerves into the IAC. As such, any leptomeningeal disease process may manifest within the IAC and often causes related clinical symptomatology, particularly retrocochlear SNHL. Pathologic enhancement within the CPA and IAC may occur as a consequence of the entire gamut of meningeal disease including neoplastic and inflammatory/infectious conditions. Neoplastic entities include meningioma and meningeal metastases.17
Pathology Meningiomas Meningiomas arise from arachnoidal cap cells and are well known to occur within the CPA/ IAC. The histopathology is identical to those which develop elsewhere.1,6,17 Virtually all meningiomas enhance intensely and homogeneously with gadolinium (Fig 9). T2 signal appears to vary with histologic make-up: the angioblastic and syncitial varieties are relatively hyperintense secondary to higher water content. By contrast, the transitional and fibroblastic varieties demonstrate relative hypointensity due to densely packed collagen and/or psammomatous calcifications. Meningeal Metastasis Meningeal metastasis is referred to by a number of ambiguous terms including leptomeningeal carcinomatosis, meningeal carcinomatosis and carcinomatous meningitis. These terms are inexact as these neoplasms are not always carcinomas, often involve the pachymeninges as well as the leptomeninges and may not contain an inflammatory (-itis) component. Meningeal
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Fig 18. Sarcoidosis. (A) A progressive sensorineural hearing loss on right. Contrast-enhanced axial T1-weighted image demonstrates a faintly enhancing mass within the right internal auditory canal (arrow) involving primarily the fundus. Note the pathologic dural enhancement along the surface of the clivus (arrowheads). B and C: Different patient: Sarcoidosis, right cerebellopontine angle: (B) Contrast-enhanced coronal T1-weighted image. (C) Contrast-enhanced axial T1-weighted image. There is a clearly definable intensely enhancing mass involving the right cerebellopontine angle extending (on coronal images) to level of foramen magnum and into the right internal auditory canal. Adjacent foci of PS/arachnoid enhancement (arrowheads) indicate the presence of associated leptomeningeal disease and are the tip-off to the diagnosis of sarcoidosis.
metastases from solid tumors (i.e. breast) and lymphoproliferative malignancy (i.e. lymphoma) involve both the pachymeninges and the leptomeninges. Seeding from primary central nervous system (CNS) tumors involve only the leptomeninges. Leptomeningeal neoplasm may be diffuse or discrete and nodular (Fig 16). Nodular disease may masquerade as a primary neoplasm such as Schwannoma or meningioma. Involvement of multiple cranial nerves is common. In this context, both facial nerve dysfunction and vertigo/hearing loss may result from CPA/IAC dissemination.
Meningitis Meningitis may result in pathologic meningeal enhancement identical to that caused by neoplasm and has a similar incidence of CPA/IAC involvement (Fig 17). Clinical correlation is therefore necessary for diagnosis. Bacterial and fungal disease are more predisposed to this manifestation than viral disease. Meningeal disease is an extremely common manifestation of neurosarcoidosis. The leptomeninges are involved somewhat more commonly than the dura. This granulomatous process may result in both diffuse and nodular enhancement
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Fig 19. Intracranial hypotension, severe headaches. (A) Contrast-enhanced axial T1-weighted image demonstrates diffuse pathologic contrast enhancement of the leptomeninges especially in the prepontine and cavernous sinus region. There is also enhancement within the internal auditory canals (arrows). (B) Coronal contrast-enhanced T1-weighted image demonstrates enhancement within the internal auditory canals bilaterally. Differential diagnosis includes sarcoidosis as well as multiple other disorders of the leptomeninges. This patient had the classic headache syndrome of intracranal hypotension.
patterns. CPA/IAC disease is relatively common and results in retrocochlear sensorineural hearing loss (Fig 18). Involvement of other cranial nerves is very common. Pathologic enhancement of the meninges may also result from a variety of additional conditions. Segmental enhancement is routinely seen in postcraniotomy patients. On occasion, such enhancement is extensive and quite thick and may persist for several years. This enhancement exclusively involves the pachymeninges. Intracranial hypotension may be spontaneous or result from lumbar puncture or head trauma. The spontaneous variety is associated with a distinctive postural headache. Occult CSF leak is the presumed cause. The meningeal enhancement associated with this condition is most commonly dural but may be leptomeningeal as well (Fig 19). All the aforementioned leptomeningeal diseases may be associated with subcortical curvilinear enhancing strands implying involvement of the Virchow-Robin (perivascular) spaces. Communicating (extraventricular obstructive) hydrocephalus results from inhibition of CSF resorption at the level of the arachnoid granulations.
ADDITIONAL DIFFERENTIAL DIAGNOSTIC ENTITIES
Bell’s Palsy Bell’s palsy is the most common cause of facial palsy. Most observers favor a viral etiology. Thickening and enhancement of the facial nerve has been identified on MRI by several observers. Such enhancement typically involves the intratemporal facial nerve and must be distinguished from normal enhancement that occurs most commonly in the vicinity of the first genu (perigeniculate). Enhancement of the intracanalicular (IAC) segment of the nerve (Fig 20) is always abnormal and may be secondary to Bell’s palsy or other facial nerve maladies such as perineural spread of neoplasm (especially parotid), primary neoplasms, and postoperative/ posttraumatic etiologies. Facial palsy occurring in the context of clinical evidence of herpetic eruption within the external auditory canal is diagnostic of Ramsey Hunt syndrome. Intracanalicular facial and vestibulocochlear nerve enhancement may occur in this circumstance; membranous labyrinth enhancement may also be associated.
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Fig 20. Bell’s palsy. Axial (A) and coronal (B) contrast-enhanced T1-weighted images demonstrate pathologic contrast enhancement within the anterosuperior quadrant of the left internal auditory canal consistent with the location of the 7th cranial nerve in a patient with acute onset of facial weakness.
Fig 21. Siderosis. (A) Axial T2-weighted conventional spin-echo image diffuse leptomeningeal linear hyposignal. Cochlear nerve involvement on right (arrows). (B) Axial T2-weighted images, more inferior. Dramatic linear hyposignal again noted. (Courtesy Jonathan Lewin, MD.)
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Siderosis Siderosis is a condition characterized by intracellular and extracellular hemosiderin deposition along the leptomeninges, subpial tissue and cranial nerves, resulting from previous massive or recurrent subarachnoid hemorrhage that overwhelms the ability of the blood stream to remove the hemosiderin-laden
macrophages.18 Involvement of the IAC is classically associated with sensorineural hearing loss. Siderosis results in rapid T2 relaxation because of increased magnetic susceptibility and is manifest by pathologic hyposignal best seen on spin-echo T2-weighted images or gradient-echo sequences (Fig 21). There is no contrast enhancement.
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Schwannomas: MR characteristics. AJNR Am J Neuroradiol 14:1241-1247, 1993 11. Mueller DP, Gantz BJ, Dolan KD: Gadolinium-enhanced MR of the postoperative internal auditory canal following acoustic neuroma resection via the middle fossa approach. AJNR Am J Neuroradiol 13:197-200, 1992 12. Gao P, Osborn AG, Smirniotopolous JG, Harris CP: Epidermoid tumor of the cerebellopontine angle: Radiologicpathologic correlation. AJNR Am J Neuroradiol 13:863-872, 1992 13. Knorr JR, Ragland RL, Smith TW, et al: Squamous carcinoma arising within a cerebellopontine angle epidermoid: CT and MR findings. AJNR Am J Neuroradiol 12:1182-1184, 1991 14. Dutt SN, Mirza S, Chavda SV, Irving RN: Radiologic differentiation of intracranial epidermoids from arachnoid cysts. Otol Neurotol 23:84-92, 2002 15. Braun IF, Naidich TP, Leeds NE, et al: Dense intracranial epidermoid tumors. Radiology 122:717-719, 1977 16. Donnelly MJ, Cass AD, Ryan L: False positive MRI in the diagnosis of small intracanalicular vestibular Schwannomas. J Laryngol Otol 986-988, 1994 17. Cinnamon J, Sharma M, Gray D, et al: Neuroimaging of meningeal disease. Semin Ultrasound CT MRI 15:466-498, 1994 18. Hsu WC, Loevner LA, Forman MS, Thaler EA: Superficial siderosis of the CNS associated with multiple cavernous malformations. AJNR Am J Neuroradiol 20:1245-1248, 1999