-------------------------------------------------------------------------------- TITLE: INTERNAL AUDITORY CANAL TUMORS - SURGICAL APPROACHES SOURCE: Dept. of Otolaryngology, UTMB, Grand Rounds DATE: February 23, 1994 RESIDENT PHYSICIAN: Michael D. Bryan, M.D. FACULTY: Jeffrey T. Vrabec, M.D. DATABASE ADMINISTRATOR: Melinda McCracken, M.S. -------------------------------------------------------------------------------- "This material was prepared by resident physicians in partial fulfillment of educational requirements established for the Postgraduate Training Program of the UTMB Department of Otolaryngology/Head and Neck Surgery and was not intended for clinical use in its present form. It was prepared for the purpose of stimulating group discussion in a conference setting. No warranties, either express or implied, are made with respect to its accuracy, completeness, or timeliness. The material does not necessarily reflect the current or past opinions of members of the UTMB faculty and should not be used for purposes of diagnosis or treatment without consulting appropriate literature sources and informed professional opinion." ANATOMY - The internal auditory canal (IAC) is primarily a nerve conduit and traverses the petrous portion of the temporal bone in a medial to lateral direction, roughly parallel to the external auditory canal. The canal is typically about 4 mm in diameter and 8 mm in length, but this is variable, and landmarks are more important to the surgeon than absolute dimensions. The medial opening into the posterior cranial fossa on the cerebellar surface of the temporal bone is the internal auditory meatus or porus acousticus. This opening is found about at the junction of the middle third and lateral two thirds of the petrosa, slightly closer to the superior margin than the inferior. The canal is like a wedge between the cochlea and the vestibule, a concept that becomes important in surgical dissections of the lateral end of the IAC. The IAC encases cranial nerves VII and VIII, with VIII consisting of the cochlear nerve, the superior vestibular nerve, and the inferior vestibular nerve, which further separates into the posterior ampullary nerve and the saccular nerve. Its lateral terminus is called the fundus and contains orifices that transmit the various nerves to & from the IAC anteriorly. CN VII exits the fundus through the fallopian canal, marking the beginning of the labyrinthine segment. The posterior ampullary nerve (aka singular nerve) travels through the singular canal to innervate the posterior semicircular canal. The superior vestibular nerve innervates the superior and horizontal semicircular canals and the utricle. The saccular nerve (the terminal end of the remaining inferior vestibular nerve fibers) innervates the saccule, which has no known function in man. The nerves that traverse the IAC are not only functionally divided, but physically divided at the fundus. The "transverse crest" spans the lateral end of the IAC in a horizontal plane. This structure is also known as the falciform crest, and it transitions into the walls of the canal medially. CN VII and the superior vestibular branch of CN VIII lie above this septation, with the cochlear and inferior vestibular branches of CN VIII below. The cochlear division of VIII lies anterior to the inferior vestibular branch, and CN VII lies anterior to the superior vestibular nerve. Another osseous septum lies between VII and the superior vestibular nerve at the lateral end of the IAC. This is known as "Bill's bar" in deference to William House, who focused attention on the importance of this structure in identifying CN VII laterally during surgery involving the IAC. Note that the inferior vestibular nerve is not partitioned from the cochlear nerve, and this predisposes the cochlear nerve to a higher likelihood of injury if a lesion of the inferior vestibular nerve is surgically treated. The cochlear and vestibular branches of CN VIII tend to fuse as they are traced out of the IAC toward the brainstem, although some surgeons still can identify a cleavage plane between them in the cerebellopontine angle (CPA). The nerves do not maintain the same spatial relationship throughout the IAC. They "rotate" as they travel through the canal and on to the brainstem. The cochlear and inferior vestibular nerves rapidly make a 90o rotation; viewed from the medial direction, counterclockwise on the right and clockwise on the left. This causes the vestibulocochlear cleavage plane to transition, from a superior-inferior plane in the lateral IAC, to a anterior-posterior (horizontally oriented) plane in the posterior fossa. CN VII also rotates to lie medial and slightly inferior to CN VIII as they exit the porus, causing VII to be hidden from the lateral view until just before the brainstem is entered. Accompanying the cranial nerves through the IAC is the artery of the IAC, which is a branch of the anterior inferior cerebellar artery. This is the primary blood supply to the cochlea and vestibular system. It can be very difficult to discern at surgery, and according to Fisch, is variable in its site of origin and number of branches. The meninges accompany the nerves into the IAC, with the dura firmly adherent to the bony walls and becoming continuous with the thin periosteal layer at the lateral terminus of the IAC. The porus marks the "glial-schwann cell junction", which will be important in the origin of the majority of intracanalicular tumors. CLINICAL PRESENTATION - Tumors of the IAC would typically be expected to present with a predictable series of symptoms and signs. The natural history of neoplasms of the IAC (and subsequently the CPA) was elucidated by Cushing in 1917. Only the history and physical examination were reliably available at that time, although Cushing did perform calorics on many of the patients suspected of having CPA/IAC lesions. He described the progression of symptoms as follows: (1) auditory and labyrinthine dysfunction; (2) occipitofrontal pain; (3) cerebellar ataxia; (4) adjacent cranial nerve involvement; (5) increased intracranial pressure; (6) dysphagia and dysarthria; and (7) brain stem compression with depression of respiratory drive and death. It is obvious that the progression of symptoms parallels the stages of tumor growth: intracanalicular, cisternal, and brain stem compressive. IAC tumors (and CPA tumors) are usually diagnosed and treated prior to the final stages described by Cushing. Early clinical signs and symptoms correspond to the intracanalicular growth phase, with unilateral hearing loss the most common feature. In almost every pathologic entity of the IAC, hearing loss is the most common feature, although it is most prevalent in acoustic neuromas. The hearing loss can be and usually is gradual in onset, probably due to gradual compression of the cochlear nerve. However, a significant number (10 -20 %) of patients with intracanalicular lesions will experience sudden SNHL. This is believed to be caused by occlusion of the IAC artery by the lesion. The sudden onset may be attributable vascular occlusion secondary to new onset tumor edema, intratumoral hemorrhage, etc. This sudden SNHL is frequently reversible, especially in response to steroid treatment, and no doubt may be dismissed as idiopathic in such cases. This is a problem because most of these occurrences are thought to occur with small intracanalicular tumors, and early diagnostic opportunities are potentially lost. Lack of early diagnosis bears a negative impact on hearing preservation (see below). Schucknect has proposed other theories of hearing loss mechanisms, including cochlear nerve fiber destruction because of tumor invasion or pressure atrophy, direct invasion of the cochlea by tumor (as noted in some pathologic specimens), and/or biochemical degradation of the perilymph/endolymph through an unknown process. Tinnitus is a common complaint, although seen more often in acoustic neuromas than other entities. It is second only to SNHL as the most common symptom in IAC lesions. Vertigo, while certainly present in a significant number of cases, is surprisingly not prominent in most. A much more common complaint is a persistent dysequilibrium that patients experience but seem to tolerate fairly well. The dysequilibrium is believed to be related to the loss of vestibular input from the affected side, and/or cerebellar compression from an expanding tumor outside the IAC in the CPA. The lack of severe vestibular symptoms is believed to be secondary to central compensation that has occurred as the tumor gradually impaired the vestibular inputs from the affected ear. Acute and severe vertigo will cause a patient to seek medical attention. If the patient with vertigo and an IAC lesion is not correctly diagnosed, he or she might be treated symptomatically, while their acute symptoms will likely subside as compensation occurs and the tumor grows. This correlates with the findings of a higher incidence of vertigo as a complaint in patients whose tumors are small at the time of diagnosis, versus an increasing incidence of complaints of dysequilibrium with larger tumors (ie - those extending outside the IAC). For those patients who never experienced true vertigo, it is theorized that their tumor grew gradually enough that they did not experience marked sudden imbalance in their vestibular inputs and therefore not any acute vertigo. This could explain the lack of consistency of patient complaints in this respect. Tumors confined to the IAC obviously would not be expected to cause some of the above noted symptoms, such as trigeminal, glossopharyngeal, or vagal nerve signs/symptoms, nor would one expect increased ICP or brainstem compression. However, complaints related to facial nerve compression or invasion might be expected. The occurrence of this clinically turns out to be dependent largely on the pathologic entity. A minority (10-20%) of patients with acoustic neuromas demonstrate facial nerve motor dysfunction, and when present it is subtle in most. However, a positive Hitselberger's sign is fairly common in acoustic neuromas. Conversely the majority of patients with a facial nerve schwannoma exhibit progressive facial palsies. There does not appear to a reliable correlation between tumor size threshold and development of CN VII signs. However, it is generally believed that motor fibers (i.e. CN VII) are less sensitive to compression than sensory fibers (CN VIII). Overall, lack of consistency of clinical presentation is well documented. In 1993, Selesnick, et. al. published a review of the symptomology of 126 acoustic neuroma patients, as well as an overview of previous similar series. He noted "one of the more striking observations in (the) study was the frequency with which ANs present in a fashion at variance with classical descriptions." Despite the increased ability to accurately diagnose tumors of just a few millimeters size with MRIs, the time from the patient's first symptoms until the time of diagnosis in the aforementioned study averaged 4 years. This is in line with the results of similar previous comparisons. Selesnick comments: "The absence of progress in diagnostic delay over the past 30 years emphasizes that earlier diagnosis does not rely solely on technological advances. Many tumors remained undiscovered either because clinicians did not suspect the presence of an AN or because they were falsely assured by less accurate diagnostic studies. However, an irreducible minimum of these tumors will undoubtedly continue to achieve large sizes as some produce only trivial symptoms which are easily ignored by the patient, and human nature assures that certain individuals will choose to ignore potentially ominous symptoms." DIAGNOSTIC TESTS - Audiologic testing- Audiometric findings - a unilateral SNHL is found in the great majority of patients. The SNHL may be in almost any configuration, but a downsloping high frequency is predominant. The speech reception threshold (SRT) is elevated, but the speech discrimination scores (SDS) is reduced disproportionately. A small percentage (up to 5%) of patients will demonstrate normal hearing, or bilateral symmetric SNHL. Retrocochlear testing - variable, but frequently positive, with tone decay, suprathreshold adaptation, and absent facial nerve reflex. In some series, the facial nerve reflex was absent in 88 - 95% of patients with known IAC tumors. ABR - widely accepted as the most sensitive audiological screening exam to detect abnormalities associated with intracanalicular mass lesions. A sensitivity of 95% has been cited in more than one series, but Wilson et. al., and Levine et. al reported a false negative rates pf 33% and 30% respectively in two series of patients with small intracanalicular tumors (as compared to a 4% rate with tumors extending extracanalicularly). The predictive value of an abnormal ABR in diagnosing IAC lesions is about 10 - 20%. An abnormal ABR is designated in most reports as a interaural latency between waves V of greater than 0.4 msec, or a unilateral wave I to V latency of greater than 4.40 msec. Vestibular testing - ENG - because the superior vestibular nerve is the most surgically accessible portion of the VIIIth nerve, and this branch innervates the horizontal semicircular canal, some investigators assert that an abnormal ENG (hypoactive calorics) on the affected side is a better prognostic sign, indicating that the tumor involves the superior division of VIII. Conversely, a normal ENG would suggest involvement of the inferior division of CN VIII, a more difficult surgical problem, with increased potential for cochlear nerve involvement and injury during removal of the tumor. The sensitivity of the ENG in IAC tumors has been quite variable in the literature, with Brackmann reporting an 82% sensitivity in a total of more than 500 patients with acoustic neuromas, and 96% sensitivity in a smaller series reported by Ojemann. Others (Mathew et. al.) have found this to be less reliable, with a less than 60% sensitivity. A normal ENG should at least make one think of another pathology. Radiographic studies - Far and away, this is the area of greatest diagnostic advancement in the past few years. The advent of widely available CT and MRI scanning has made the early detection of intracanalicular tumors much more likely. However, the test still has to be ordered, and as noted by Selesnick, the key to diagnosis given the variability of presentation, is a high index of suspicion by the examiner. CT Scanning - excellent for evaluating the IAC itself for erosion and asymmetry, and identifying intralesional calcifications to help in the differentiation of pathology. CT scanning is fairly sensitive in picking up lesions > 1 cm in size, but will miss smaller lesions usually. MRI Scanning - the new gold standard in the diagnosis of intracanalicular tumors. Diagnostic sensitivity of lesions as small as 2-3 mm reported. T1 vs T2 enhancement and post-gadolinium enhancement imaging allows for the identification & differentiation of IAC masses in most cases, and also evaluation of the contents of the IAC for surgical planning. PATHOLOGIC ENTITIES - Tumors intrinsic to the IAC are almost always benign, with only 2% being malignant Benign tumors - acoustic neuroma meningioma hemangioma facial nerve schwannoma lipoma arachnoid cyst Malignant tumors metastatic disease "malignant" meningioma malignant schwannoma extension of local disease Acoustic neuromas - Sporadic unilateral acoustic neuromas (AN) (bilateral acoustic neuromas occurring in association with neurofibromatosis type 2 will be discussed separately): 8-10% of all intracranial neoplasms 85 - 90% of IAC tumors The incidence is not well defined, and varies in the current day literature from estimates of 0.1 per 100K population to 2.5 per 100K population Patients are most often diagnosed in the fifth and sixth decades of life. However, symptoms have usually been present for a number of years prior to diagnosis. This correlates with the typically slow growth rate of these tumors and the relatively quiescent period that follows the initial intracanalicular phase. In fact, Silverstein et. al., reported a series of patients who were observed longitudinally rather than treated surgically. 25% of the group had growth rates of between 0.01 and 0.59 cm per year, 25% showed no growth at one year, 30% actually shrank, and one patient's tumor involuted completely. However, one patient experienced a 2.5 cm increase in just one year. In that series of 20 patients selected for observation, this patient was the only one ending up requiring surgery (because of marked dysequilibrium which did not improve after surgery). Pathologically, neuroma is a misnomer, as the lesion arises from schwann cells of CN VIII. The superior vestibular nerve is the site of origin in the majority, followed by the inferior vestibular nerve, and least often, the cochlear division. Rarely, the lesion arises in the intralabyrinthine portion of the nerve. These tumors can undergo degenerative changes with intratumoral hemorrhage, cyst formation, or malignant transformation. Malignant transformation is exceedingly rare. As noted above, involution has been noted on rare occasions. The etiology of the neoplastic growth of ANs is not clear in the case of sporadic unilateral cases, but there is suspicion that spontaneous mutations involving the long arm of chromosome 22 may be involved. Recent investigations have isolated not only the location of this chromosomal aberration in patients with NF2, but have actually identified the amino acid substitution error that apparently leads to neoplastic growth in these patients. Whether the same defect occurring in isolation is responsible for unilateral cases is no known yet. The clinical presentation of patients with ANs has been discussed above. The primary complaint is usually decreased hearing, with a disproportionate reduction in speech discrimination scores. However, clinicians have to be aware of other presenting symptoms. Early diagnosis offers the best hope in hearing preservation. Any patient presenting with a asymmetric SNHL should be suspected of having an AN. Physical examination in the majority of AN patients will be normal in early lesions. As lesions enlarge, the physical signs and symptoms discussed above (e.g. - facial palsy or twitch, ataxia, Hitselberger's sign, positional nystagmus, etc.) may begin to appear. Routine evaluation in most institutions includes full audiometry testing including retrocochlear tests, ABR & sometimes ENG. If suspicion persists after these evaluations, an MRI is performed. Some authors note elimination of routine retrocochlear audiologic tests in favor of ABR. The ABR has been noted in some studies to be quite sensitive, but there is some evidence that the false negative rate is higher than thought, especially in the case of small intracanalicular tumors. Similar questions have plagued the reliability of ENG in identifying small lesions through reduced vestibular responses. A linear relationship has been reported between the ENG result and the tumor size. This creates a dilemma for the clinician. The MRI is believed to be the most sensitive test, especially for very small lesions, but the expense of routine MRI evaluation of every patient presenting with a symptom associated with ANs is prohibitive. The patients reliability in follow-up may be the deciding factor in determining whether MRI is warranted in cases where suspicion is elevated but ABR and ENG are normal. The relevance is that failure to diagnose lesions early in their course has a direct impact on the prognosis of hearing preservation if surgery is eventually considered. Fortunately, the slow growth rate of ANs affords some flexibility. The suspected mechanisms of hearing loss in ANs are discussed above. MRI scanning is the preferred imaging study. Small tumors can be identified and treatment optimized. ANs are dark on normal T1 weighted imaging, usually bright on T2 images, and bright in post-gadolinium T1 imaging. CT scanning is not the preferred diagnostic imaging modality. Although lesions extending outside the IAC > 1 cm are usually identifiable, intracanalicular lesions will be missed even with high resolution thin cut methods. However, when an AN is noted on CT it will appear iso- or hypointense without contrast, and should enhance with contrast Management of ANs is a controversial area. The reason for this is the issue of hearing preservation. Studies of post-operative results show variability, but overall, preservation attempts have been statistically disappointing. Most series reveal a minority of attempts to be successful. Analysis of multiple series of patients has provided a number of favorable prognostic factors in predicting the post-operative result in hearing preservation attempts: small tumor size "good" pre-operative hearing "normal" ABR reduced caloric response on ENG intact stapedial reflex lack of extension of tumor to fundus of the canal lack of IAC expansion on imaging studies. Most of the studies that were used to derive these results included all sizes of tumors. The most significant variables appear to tumor size and preoperative hearing level. Some management algorithms have been developed to assist in clinical decision making. These algorithms usually key on tumor size, as defined by largest dimension of tumor outside the IAC. Intracanalicular tumors by definition would fit in the "small" size category in all algorithms, so the decision to pursue a hearing sparing operation would hinge on the hearing, and other prognostic factors, as well the patients wishes. For patients over 65 years of age, some experts advocate watchful waiting with small tumors, based on the observation that the growth rate of the tumors is so slow that they usually don't develop more serious symptoms before they die of other causes. In fact, Silverstein does not advocate hearing preservation procedures at all in patients over 65. Other surgeons modify the algorithm, excluding age as an absolute determinant, but incorporating the patients "health status" instead. In either case, surgery would certainly be offered to patients suffering symptoms suggesting brainstem or other cranial nerve compromise. To evaluate the results of preservation attempts when dealing with small intracanalicular tumors, Shelton et. al. conducted a retrospective a review of 39 patients treated at the House Ear Clinic over nearly three decades. All of the patients included had no tumor extension outside of the IAC, and all were treated with hearing preservation procedures (middle fossa approach). Overall, hearing preservation was successful in 60% of the cases, with a successful result being "serviceable or better" hearing. Of these 76% actually had "good" hearing results. Serviceable hearing was defined as a SRT<50dB, with a SDS of >50%, with "good" defined as 30dB and 70% in the same categories (this classification appears fairly consistently in the literature and is called the Silverstein Hearing Classification). In this series, there was only one patient who ended up with a notable facial nerve paresis, and this was a grade III/VI result in a patient operated on before routine use of facial nerve monitoring. Based on the series of patients reviewed in the literature, a general guideline for intracanalicular tumors has been offered by Jackler & Pitts. For patients with "good" preoperative hearing, a hearing preserving operation is proposed based on the anatomic location of the mass within the canal, as well as some of the prognostic factors listed above. If the only the lateral IAC is involved, a middle fossa approach is advocated; if only the medial end of the IAC is involved, a retrosigmoid (suboccipital) approach is offered. If the whole canal is occupied by the AN, the patient is offered a choice, with recommendation to undergo a translabyrinthine procedure if the ABR is abnormal, the caloric response is reduced, and/or the IAC is expanded on imaging studies. Patients without the unfavorable factors (ABR, ENG, etc.) are encouraged to undergo a middle fossa approach to the tumor. Patients with "poor" preoperative hearing are generally treated via the translabyrinthine procedure. Exceptions to any of the algorithms are patients in whom the AN affects an only- hearing ear, or in patients with NF2. Patients affected in the only hearing ear are generally not operated on, but are observed longitudinally. Should they develop untoward complications or lose all usable hearing, they are typically treated surgically, with a translabyrinthine or retrosigmoid approach depending on the the tumor location and the surgeon's experience. Patients with NF2 are discussed below. Its generally accepted that surgical treatment should completely remove the tumor, but this is not always possible or advisable. When tumors are large, densely adherent to important structures, or located in areas where access is limited despite good surgical exposure. In cases like these, debulking surgery, sub-total removal or even simple decompression of the IAC can be carried out. In retrospective studies, cases wherein sub-total removal (90-95% of tumor removed) was performed were followed postoperatively. Recurrence was noted in about 50%, with about half of these patients becoming symtomatic subsequently. Reoperation was required in about 14%. While recurrence is not optimal, it should be noted that the rate of facial nerve paresis was significantly lower than a similar group of patients in whom total excision was carried out. The real value of debulking is prolongation of useful hearing, allowing patients time to acquire training in preparation for loss of all useful hearing. The middle fossa approach is frequently used when a debulking procedure is used. Neurofibromatosis type 2 - NF2 is a highly penetrant autosomal dominant disease. In a minority of cases, no family history can be found and these cases are believed to arise sporadically. Recently, the chromosomal defect that apparently causes the disease has been identified. A defect in the long arm of chromosome 22 leads to the inappropriate substitution of tyrosine in a polypeptitde that is in a family of cytoskeletal-associated proteins. 75% of NF2 victims develop other central nervous system tumors, most commonly meningiomas, astrocytomas, gliomas, and ependymomas. The development of these tumors is somehow related to the loss of function of this single protein. The incidence is estimated to be 1 - 2 cases per 100K population Development of bilateral acoustic neuromas is the hallmark (occurs in more than 90% of NF2 cases) and is considered pathognomonic. Signs and symptoms typically appear in young adults in the 2nd and 3rd decades of life. The presentation is usually indistinguishable from that of sporadic unilateral acoustic neuroma patients, but occasional advanced presentations are seen with only a brief duration of mild symptoms. This may be related to the observation that these schwannomas are frequently more aggressive, grow faster, and intimately involve more nervous and vascular structures than there unilateral cousins. Grobman et. al. found that these schwannomas infiltrated the vestibular and cochlear nerves, separating and encasing the fibers rather than compressing them or their vasculature against the IAC wall. This tendency to entangle more adjacent structures makes them more difficult to remove surgically and may explain the higher incidence of severe hearing loss following excision. Management decisions concerning whether to operate on a patient with bilateral ANs are not straightforward. However a reasonable management protocol is elucidated by Glasscock, et. al. If a patient has bilateral "good" hearing, with both schwannomas less than 1.5 cm in their extracanalicular diameter, proceeding to surgery early is advisable in an attempt to preserve hearing in the ear with the`smaller tumor. The rationale is that the disease progress is going to progress, and eventually the hearing will be destroyed on both sides. It is also likely that surgery will be required for other reasons, so early intervention is acceptable, given that the chances for hearing preservation decline over time. The clinical course is followed expectantly, monitoring for deterioration of the non-operated side, as well as for evidence of recurrence on the treated side. If the second side deteriorates, conservative management is the rule, with surgery reserved for handing progressive lesions. If bilateral tumors are both larger than 1.5 cm (extracanalicular), a hearing preserving operation is not likely to be successful and therefore observation is the rule usually. If the hearing is destroyed in any ear, or if progressive symptoms develop, a translabyrinthine operation can be done to prevent complications of continued tumor growth. Meningiomas 12.5 % of diagnosed intracranial neoplasms (incidence is much higher on autopsy studies) 10-15% of diagnosed intracanalicular tumors Most arise from arachnoidal fibroblasts within arachnoid villi, which are concentrated along intracranial sinuses and dural investments of cranial nerves. They are typically slow growing and frequently silent (hence the larger incidence at autopsy than incidence of diagnosis). They can invade bone, and surrounding hyperostosis is common. There is a slight predominance of frequency in middle aged females. 4 subtypes: meningotheliomatous - abundant benign appearing lobulated arachnoid tissue fibroblastic - long spindle shaped cells, sometimes with psammoma bodies, this types can be very difficult to distinguish from an acoustic neuroma, radiologically or histologically transitional - also known as psammomatous meningioma because of whorls of neoplastic cells, surrounding numerous psammoma bodies angioblastic - the most aggressive variant, tends to recur and spread more rapidly Rarely, an anaplastic "malignant" variant is found, basically indistinguishable from spindle cell sarcoma. These metastasize to distant sites. Clinically, meningiomas involving the IAC are less likely to present with hearing loss or tinnitus than acoustic neuromas, although they often do so. They frequently present with headaches, trigeminal symptoms (pain), or lower cranial nerve complaints (CN IX, X) because they tend to spread away from the IAC on a broad based dural attachment. Radiologically, they often have prominent calcifications noted on CT scanning, and are otherwise hyperdense with respect to cerebellar tissue. When strictly isolated to the IAC, they are not easily distinguishable from acoustic neuromas or ossifying hemangiomas. When they are in the IAC but based on dural attachments at or around the porus, they are sometimes more easily differentiated because of their broad attachment that enhances on T1 post-gadolinium MRI ("meningeal sign"), but this is not always present. Finally, meningiomas rarely cause erosion of the porus acousticus although acoustic neuromas frequently do. Angiography, although not routinely performed, usually can differentiate meningiomas from acoustic neuromas. Meningiomas typically are supplied by a prominent feeder vessel and display a tumor blush with subtraction angiograms. Treatment of meningiomas is surgical excision when possible. XRT response is poor but can be tried when surgery not possible. Post surgical hearing results in those patients where conservation is attempted are statistically better than with acoustic neuromas with similar preoperative hearing levels. Hemangioma - Intratemporal hemangiomas involving the IAC most often are hamartomatous vascular malformations that arise from the area of Scarpa's or the geniculate ganglia (about equal in incidence). They are predisposed to form in these areas because of the rich blood supply. They are sometimes called facial nerve hemangiomas because they occur along the path of CN VII, or ossifying hemangiomas because of frequent bony spicule formation intratumorally. The incidence in not clear, but they are rare. In a series of 34 patients at the House Ear Institute, the average age at presentation was 40 years. The histologic features of these lesions resemble features of cavernous hemangiomas, with large vascular spaces lined with a thin layer of endothelium, surrounded by a thick fibrous tissue capsule. The bony spicules are evident and believed to be reactive metaplasia. There is frequently some erosion or involvement of the surrounding bone. Although they are associated with the ganglia, they are extraneural tumors. However, they are so intimately associated with the nerve that removal is extremely difficult in some cases, resulting in sacrifice of the facial nerve in a significant portion of cases. In the House series, 4 of 18 patients wherein removal was attempted required sacrifice of CN VII with primary cable grafting. Clinical presentation is usually a progressive SNHL when the lesion is intracanalicular. Tinnitus and vestibular complaints are less often seen than with most other lesions of this area. Retrocochlear signs are frequently present. Gross facial nerve function is impaired in about 20% of isolated IAC cases, whereas nearly 100% of lesions at the geniculate ganglion present with progressive facial weakness. Inital diagnosis can be difficult. Because these tumors become symptomatic early , they are small and difficult to visualize with imaging techniques. The optimal diagnostic study for detecting small intracanalicular hemangiomas is once again the MRI. Hemangiomas appear hyperintense on T2 weighted images. This is similar to ANs. High resolution CT scanning is useful to identify intratumoral bony spicules, if they are present. The characteristic CT finding is of a mass with indistinct margins and a "honeycomb" appearance secondary to bony formations in the tumor. CT is actually better than MRI if the geniculate ganglion is the site of involvement. These tumors are generally aggressive and surgical excision is recommend, usually via the middle fossa approach because of the location. Postoperative hearing after preservation attempts is generally more likely to be well maintained than in cases of AN. This is not surprising given that the VIIIth nerve is not intimately involved, and the VIIth nerve is partitioned by bony septa in portions of the IAC. In House's series, 64% had hearing preservation to within 10db and 15% of their preoperative levels of SRT and SDS respectively. Preservation is more frequently successful if the lesion is completely intracanalicular. Facial nerve schwannomas Arise anywhere along the course of the facial nerve. A review of 255 consecutive cases found the most common locations in descending frequency are: tympanic segment mastoid segment vertical segment meatal segment Schwannomas that originate at the geniculate ganglion can extend back into the IAC. This is much more common than a de novo tumor of the IAC. Tumors that originate in the IAC cannot be easily distinguished clinically or radiologically from acoustic neuromas. The most common presentation includes a slowly progressive facial paralysis, wheras 20% of ANs also are associated with facial weakness. Other symptoms are along the lines as previously discussed. The site of origin is thought to be the sensory fibers of the nervus intermedius. The accompanying sensory deficits are common (Hitselberger's sign, hypogeusia, etc.). Surgical excision is recommended for symptomatic lesions. The sacrifice of VII is frequently required. Frozen sections are required to assure complete resection. If necessary primary repair or grafting of VII is carried out. The surgeon should be prepared to deal with a tumor larger than estimated form pre-operative imaging studies. More than one approach may be needed. Surgery should not be rushed into. Even with small tumors, VII is frequently damaged at operation. The result of a cable graft will be no better than a grade III/VI, so some advocate waiting until that degree of facial weakness exists before operating. However, this frequently can not be predicted because of uncertainty of the diagnosis and the presence of other symptoms. The middle fossa affords the best view of the geniculate and IAC areas if the hearing is intact. If more access is needed, a mastoidectomy can be performed. Hearing preservation attempts are more successful than with AN excisions. In a poorly or non-hearing ear, a translabyrinthine excision is recommended. Lipomas Very common in other anatomic locations, lipomas are exceedingly rare intracranially, comprising 0.1% of diagnosed tumors. They make up less than 0.1% of tumors of the CPA & IAC, with only about 30 cases reported in the literature, 9 of these specific for the IAC. They are congenital malformations derived form the neural crest. Most are discovered incidentally at autopsies. CPA and IAC lipomas often produce the classic VIIIth nerve symptomatology associated with ANs. Long histories of symptoms typically preceded diagnosis in the reported cases. Hearing loss on the affected side was the predominant complaint. These tumors are encapsulated and encase vessels and nervous structures adherently. When surgical removal has been performed, the sacrifice of hearing, and well as facial paralysis has been quite high because of the difficulty encountered in dissecting the tumor free. MRI imaging is virtually diagnostic, with bright imaging on T1 and post-gadolinium T1, but hypointense to sometimes isointense on T2 weighting. CT scan images reveal a homogenous hypodense nonenhancing smooth sided lesion. There is frequently erosion of the IAC. Conservative management is recommended, reserving surgery for relief of symptomatic patients who are fully informed of the high risk of post-operative defecits. Arachnoid cysts Rare lesions Consists of a thin walled CSF containing cyst between two layers of arachnoidal tissue. They are thought to be develop from the outer arachnoid cells which have secretory ability. The reason for their formation is not clear; some are associated with trauma, inflammation, or neoplastic processes.. Preoperative imaging should reliably differentiate these low density masses from other entities. CT scans reveal a low density non-enhancing mass with smooth edge, and MRI imaging reveals a non enhancing lesion on both T1 and post gadolinium T1 weighting, with a bright T2 weighted image. These cysts can expand and stretch nerves or vasculature, but majority are relatively asymptomatic. Because of their benign nature, they are treated only if symptomatic. The treatment is surgical drainage into the posterior fossa (cistern) via a retrosigmoid approach. Metastatic disease Metastatic disease of the IAC is the result of hematogenous spread. The IAC is involved less often than petrous apex (more marrow located in apex). Etiologies, in descending order of frequency: breast adenocarcinoma renal cell carcinoma lung cancer (adeno- or squamous cell carcinoma) prostate cancer gastric adenocarcinoma laryngeal cancer thyroid carcinoma The initial clinical presentation is often not distinguishable from other CPA & IAC lesions. The distinction is usually in time course of symptom progression, or in the presence of a known malignancy with a tendency to metastasize. If both CN VII and CN VIII are obviously involved early, then a malignancy should be considered. Diagnosis usually requires biopsy. Petrous apex and IAC metastatic disease defies surgical cure and the only recourse in most cases is chemotherapy and/or XRT TREATMENT - Nonsurgical vs Surgical - The goal of treatment is the complete removal of tumor with preservation of life, facial nerve function, and if possible hearing, with the minimum of morbidity. The decision on treatment options is made based on the degree of hearing loss preoperatively, severity of other symptoms, the location and size of the tumor, the patients clinical physical condition and age, and the suspected pathological diagnosis. Nonsurgical treatment is available for those patients in whom you do not reasonably believe the goals of surgical treatment can be attained, or for those patients who do not consent to surgery. Some reasons to advocate nonsurgical conservative treatment are extremes of age, poor health with intercurrent physical problems that makes surgery prohibitive, tumor involvement in an only-hearing ear, or bilateral tumors of 1.5 cm diameter or larger (without intolerable symptoms). The use of age ( > 65 years) as a prohibitive factor in pursuing a surgical treatment has been widely discussed. It would seem that clinical judgement concerning the expected longevity, and or the patient's ability to withstand the operative procedure, should be a better guideline. The need for surgical treatment for each pathologic entity and the role of debulking or subtotal surgical resection was addressed earlier. Nonsurgical treatment consists primarily of observation and/or stereotactic radiotherapy. Chemotherapy has been reported once, but results are not conclusive enough to be able to make a reasonable recommendation to a patient to try this, given the non innocuous nature of cytotoxic therapy. Follow-up guidelines for patients whom observation is selected provide for repeat MRI imaging 6 months after intiation of the observation, followed by yearly MRIs to monitor tumor size and allow for intervention should the situation change and surgical treatment be considered. Audiometry should be included as part of the monitoring program in patients with useful hearing. Stereotactic radiation has its place for patients who desire treatment but are not medically fit for surgery, or in those whom surgery may pose excessive risk (eg. Jehovah's witnesses). The risk of complete deafness after treatment with this modality is significant. Facial weakness or paralysis does occur but has been reported as transient, lasting several months. Surgical treatment - The surgical treatment of tumors of the IAC can be generally divided between those procedures that allow for preservation of hearing and those that result in sacrifice of the otologic and or vestibular end organs, leaving the patient deaf in the affected ear. The selection of procedure in the case of tumors of the IAC most often depends on the degree of hearing loss preoperatively, the location and size of the tumor, the patients physical condition and age, the suspected pathological diagnosis, and the experience of the surgeon with the procedure. The primary surgical approaches to the IAC are retrosigmoid, middle fossa, translabyrinthine, and transotic (or transcochlear) methods. The retrosigmoid and middle fossa approaches can be employed when hearing preservation is a goal, whereas the other procedures result in deafness. One procedure that should be mentioned, although it has not gained wide popularity is the "hearing sparing" translabyrinthine approach, wherein the membranous labyrinth is removed, the vestibule sealed with bone wax, and lost perilymph replaced with lactated Ringer's solution. McElveen, et. al. reported a case wherein this technique was used and useful hearing was preserved. Intraoperative facial nerve monitoring and ABR are routinely performed in hearing preservation surgery. Obviously the ABR monitoring would be unnecessary in hearing ablative procedures. The translabyrinthine approach - Advantages - 1. Minimized morbidity due to lack of cerebellar or temporal lobe retraction. 2. Positive identification of the facial nerve at a consistent anatomic location. 3. Good exposure of facial nerve for repair or cable graft in the event of injury or sacrifice. 4. Lower incidence of CSF leaks vs. the retrosigmoid approach. Disadvantages - 1. No chance of hearing preservation. 2. Some argue that inadequate exposure of the CPA is a problem. Procedure - The IAC meatus and CPA are approached through a mastoidectomy and labyrinthectomy. A complete simple mastoidectomy is carried out, with thorough skeletonization of the sigmoid sinus except for an island of thin bone covering the anterolateral aspect. This is called "Bill's island." Retrofacial air cells are removed to the level of the jugular bulb, leaving a thin layer of bone covering it. The sinodural angle is further developed and the superior petrosal sinus skeletonized. The horizontal canal, the fundamental landmark for the labyrinthectomy, is identified and the antrum opened. The labyrinthectomy is carried out and the facial nerve identified and skeletonized from the posterior aspect along its entire mastoid course. The vestibule is opened and drilled to the depth where cribiform bone (level of entry of the superior vestibular nerve) is noted. The plane between the facial nerve and the drill should be kept in view at all times to avoid injury to the facial nerve. Bone removal continues parallel and below the superior petrosal sinus until the dura of the IAC is recognized (initially leaving bone over it). Then bone is removed posterior to the IAC until the posterior fossa dura is identified. A thin layer of bone is posterior fossa dura is developed with retraction of the sigmoid sinus to allow dissection posteriomedial to it (use Bill's island for the retraction). The vestibular aqueduct and endolymphatic are removed when they are encountered. The next goal is continued removal of bone inferior to the IAC until the cochlear aqueduct is identified. This is a key landmark, as it marks the inferior limit of the dissection. Just deep to this lie CNs IX,X, and XI (where you don't want to be). The IAC is now skeletonized along its length in a medial to lateral direction, taking care to remove as much of the porus as possible (about 2/3 of the circumference). Leaving excess bone here will make identification of the facial nerve difficult medially. The lateral end of the canal is now skeletonized and dissected inferiorly to superiorly. The important landmarks (transverse crest and Bill's bar) are exposed, allowing safe identification and skeletonization of the facial nerve into its labyrinthine segment. The IAC dura is now completely skeletonized and the soft tissue dissection proceeds with opening of the dura at the midportion of the IAC. The tumor is exposed and carefully dissected away from the facial nerve in a lateral to medial direction. This is tedious and difficulty often is encountered near the porus. The tumor, along with the the vestiblar nerve and frequently the cochlear nerve, are excised once the facial nerve is completely separated from the mass. The dura is closed, and an abdominal fat graft is used to obliterate the mastoid cavity. Temporalis muscle is used by some surgeons to obliterate the aditus. Layered watertight closure of soft tissues is next, followed by a mastoid dressing. Most do not use a lumbar drain or a wound drain. The retrosigmoid (aka suboccipital) approach - Advantages - 1. Considered by proponents to be the most versatile access to the CPA & IAC. 2. Arguably better exposure to the inferior cerebellopontine cistern for treatment of large tumors. Disadvantages - 1. Poor access to the most lateral aspect of the IAC. 2. More difficult exposure for repair or grafting of the facial nerve in the event of injury. 3. Historically, increased incidence of intraoperative air embolism via the transverse sinus. (This is primarily a disadvantage when operating with the patient in a seated position, something that most surgeons no longer do.) 4. Higher incidence of postoperative CSF leaks than translabyrinthine approach. 5. Increased morbidity in some patients due to cerebellar retraction. 6. Persistent postoperative headaches - sometimes for months Procedure - The patient is positioned by most surgeons in the supine or intermediate horizontal position to avoid problems with air embolus previously complicating the procedure when a "park bench" or sitting posture was used in the past. The soft tissues are incised approximately a cm posterior to a translabyrinthine incision. A 4 x 5 cm craniotomy is made extending from the nuchal line superiorly and to the sigmoid sinus anteriorly. The posterior fossa dura is opened in cruciate manner. The cerebellum is allowed to fall back, and if adequate exposure is not available, the cerebellum is r retracted to increase the exposure. If tumor is seen in the posterior fossa, it can be debulked before proceeding to open the IAC. The dura on the posterior fossa surface of the petrous bone is elevated. The endolymphatic sac and vestibular aqueduct are identified and provide the lateral boundaries of the dissection. If you go lateral to this you will open the posterior semcircular canal which will likely deafen the patient. Medial to the sac and aqueduct, the lip of the porus is drilled away. The dissection continues laterally along the course of the IAC until the tumor can be seen entirely. The dura is opened and the tumor removed. The IAC artery needs to be left intact to preserve hearing. Any open, exposed air cells of the petrous bone are packed tightly with bone wax to avoid postoperative CSF leaks, and the dural incisions closed in a watertight manner. The craniotomy and soft tissues are then closed. The middle fossa approach - Advantages - 1. Hearing preservation possible, with slightly better hearing preservation in some series 2. Entire IAC is viewed, allowing access to lateral canal directly. 3. The operation is extradural almost in its entirity. 4. Early identification of the facial nerve. 5. Low incidence of post operative CSF leaks. Disadvantages - 1. Historical incidence of facial nerve paralysis slightly higher in some reports, but this is not as big a problem with current day intraoperative nerve monitoring. 2. Temporal lobe retraction increases morbidity, especially in older population who have a thins fragile temporal lobe dura. 3. Technically challenging 4. Very limited exposure of posterior fossa. Procedure - A pretragal incision is made, extending rostrally to allow exposure of the squamous portion of the temporal bone. A 3 x 4 cm craniotomy is made. The surgeon sits above the head of the be, looking caudally. The middle fossa dura is elevated off of the temporal bone in a posterior to anterior direction. The middle meningeal artery is identified and just medial to this as the dura is retracted, the greater superficial petrosal nerve is seen. This is the reference point to begin skeletonization of the nerve back to the geniculate ganglion. This is continued, exposing the labyrinthine facial nerve segment and the fundus of the IAC. The IAC is unroofed to the porus after the facial nerve is positively identified. The dura of the IAC is divided and the superior vestibular nerve sectioned in most cases. The tumor is identified and carefully dissected free. A key to the operation is to preserve the IAC artery. The dural defect is closed and an abdominal fat graft is used to assure closure. The temporal lobe is allowed to return to position and the craniotomy closed. The transotic approach - Advantages - 1. Increased exposure 2. The facial nerve is not translocated as it would be in a traditional translabyrinthine- transcochlear procedure. 3. Low incidence of CSF leaks. Disadvantages- 1. Increased operative time by 1-2 hrs. vs. a translabyrinthine approach. 2. Technically more difficult. Procedure - This procedure differs from the translab dissection in that a complete subtotal petrosectomy is performed first, with preservation of the fallopian canal bony framework. The cochlea and entire otic capsule are then drilled out. The eustachian tube is obliterated at the level of the isthmus, and the external canal permanently sutured closed. At the end of the bony dissection, the jugular bulb and carotid arteries are skeletonized. The tumor is then removed, starting laterally to medially, preserving the facial nerve. The posterior fossa dura is opened if necessary to get the last portion of the tumor if it extends that far. The dural defect is closed with fascia, and abdominal fat used to obliterate the dead space of the temporal bone defect, and the wounds closed. A pressure dressing is maintained for 5 days. Selection of surgical approach - In summary, the selection of procedure should take into account each of the following: Anatomical factors - tumor location, mastoid size, etc. Patient status - age, health Hearing preservation goals - preop hearing, prognostic factors, status of other ear Surgeon's preference - Suspected pathology Patient's wishes - patients may insist on an operation against the "algorithm" Postoperative Complications - CSF leaks, meningitis, deafness, VII paralysis, etc. ------------------------------------------------------------------------------- BIBLIOGRAPHY Proctor B Surgical Anatomy of the Ear and Temporal Bone. New York, N.Y.: Theime Medical Publishers, Inc; 1989. Valvassori GE, Applebaum EL "Imaging of the Temporal Bone." In: Nadol JB Jr., Schucknect HF, eds. Surgery of the Ear and Temporal Bone. New York, N.Y: Raven Press, Ltd; 1993. Mathew, GD, Facer GW, Suh KW, Houser OW, O'Brien PC "Symptoms, findings, and methods of diagnosis in patients with acoustic neuromas." Laryngoscope 1978; 88: 1893-1903. Valvassori GE "Growth rate of acoustic neuromas." Am J Otol 1989; 10: 174-176. Lalwani AK "Meningiomas, Epidermoids, and Other Nonacoustic Tumors of the Cerebellopontine Angle." In: Jackler RK, ed. Acoustic Neuroma II Diagnosis, Nonoperative Management and Results of Therapy; The Otolaryngologic Clinics of North America, April 1992; 25: 707-728. Rhoton AL, Tedeschi H "Microsurgical Anatomy of Acoustic Neuroma." In: Jackler RK, ed. Acoustic Neuroma I Surgical Management; The Otolaryngologic Clinics of North America, April 1992; 25: 257-294. Cohen NL "Retrosigmoid Approach for Acoustic Tumor Removal." In: Jackler RK, ed. Acoustic Neuroma I Surgical Management; The Otolaryngologic Clinics of North America, April 1992; 25: 295-310. Glasscock ME III, Vrabec JT "Management of Bilateral Acoustic Neuroma." In: Jackler RK, ed. Acoustic Neuroma I Surgical Management; The Otolaryngologic Clinics of North America, April 1992; 25: 449-470. Brackmann DE, Green JD "Translabyrinthine Approach for Acoustic Tumor Removal." In: Jackler RK, ed. Acoustic Neuroma I Surgical Management; The Otolaryngologic Clinics of North America, April 1992; 25: 311-330. Browne JD, Fisch U "Transotic Approach to the Cerebellopontine Angle." In: Jackler RK, ed. Acoustic Neuroma I Surgical Management; The Otolaryngologic Clinics of North America, April 1992; 25: 331-346. House WF, Shelton C "Middle Fosssa Approach for Acoustic Tumor Removal." In: Jackler RK, ed. Acoustic Neuroma I Surgical Management; The Otolaryngologic Clinics of North America, April 1992; 25: 347-360. Jackler RK, Pitts LH "Selection of Surgical Approach to Acoustic Neuroma." In: Jackler RK, ed. Acoustic Neuroma I Surgical Management; The Otolaryngologic Clinics of North America, April 1992; 25: 361-388. Selsnick SH, Jackler RK, "Clinical Manifestations and Audiological Diagnosis of Acoustic Neuromas." In: Jackler RK, ed. Acoustic Neuroma II Diagnosis, Nonopertive Management and Results of Therapy; The Otolaryngologic Clinics of North America, April 1992; 25: 521-552. Yanagihara N, Asai M "Sudden hearing loss induced by acoustic neuroma: significance of small tumors." Laryngoscope 1993; 103: 308-311. Wilson DF, Hodgson RS, Gustafson MF, et. al. "The sensitivity of auditory brainstem response testing in small acoustic neuromas." Laryngoscope 1992; 102: 961-964. Silverstein H, Rosenberg SI, Flazner JM, Wanamaker HH "An algorithm for the management of acoustic neuromas regarding age, hearing, tumor size, and symptoms." Otolaryngol Head Neck Surg 1993; 108: 1-10. Selesnick SH, Jackler RK "Atypical hearing loss in acoustic neuroma patients." Laryngoscope 1993; 103: 437-441. Selesnick SH, Jackler RK, Pits LW "The changing clinical presentation of acoustic tumors in the MRI era." Laryngoscope 1993; 103: 431-436. Jackson CG, Netterville JL, Glasscock ME III, et. al. "Defect reconstruction and cerebrospinal fluid management in neurotologic skull base tumors with intracranial extension." Laryngoscope 1992; 102: 1205-1214. Gavilan J, Nistal M, Gavialn C, Calvo M "Ossifying hemangioma of the temporal bone." Arch Otolaryngol Head Neck Surg 1990; 116: 965-967. Press GA, Hesselink JR "MR imaging of cerebellopontine angle and internal auditory canal lesions at 1.5 T." AJR 1988; 150: 1371-1381. Gardner G, Robertson JH, Clark WC "Transtemporal approaches to the cranial cavity." Am J Otol 1985; 11: 114-120. Howard JD, Elster AD, May JS "Temporal bone: three-dimensional CT; Part I . Normal anatomy, techniques, and limitations." Radiology 1990; 177: 421-425. Howard JD, Elster AD, May JS "Temporal bone: three-dimensional CT; Part II. Pathologic alterations." Radiology 1990; 177: 427-430. Shelton C, Hitselberger WE, House WF, Brackmann DE "Hearing preservation after acoustic tumor removal: long-term results." Laryngoscope 1990; 100: 115-119. Shelton C, Brackmann DE, Lo WWM, Carberry JN "Intratemporal facial nerve hemangiomas." Otolaryngol Head Neck Surg 1991; 104: 116-121. Brackmann DE, Green JD Jr. "Cerebellopontine Angle Tumors." In: Bailey BJ, ed. Head & Neck Surgery - Otolaryngology. Philadelphia, PA. : J.B. Lipppincott Co; 1993. Atlas MD, Fagan PA, Turner J "Calcification of internal auditory canal tumors." Ann Otol Rhinol Laryngol 1992; 101: 620-622. Gadre AK, Kwartler JA, Brackmann DE, House WF, Hitselberger WE "Middle fossa decompression of the internal auditory canal in acoustic neuroma surgery: A therapeutic alternative." Laryngoscope 1990; 100: 948-952. Grobman LR, Fisch U, Pollack A "Central neurofibromatosis: A clinical-pathological correlation." Am J Otol 1990; 2: 108-112. Gagnon NB, Lavigne F, Mohr G, et. al. "Extracranial and intracranial meningiomas." J Otolaryngol 1986; 15:6. 380-384. Glasscock ME III, Thedinger BA, Cueva RA "An analysis of the retrolabyrinthine vs. the retrosigmoid vestibular nerve section." Otolaryngol Head Neck Surg 1991; 104: 88-95. Tos, M, Thomsen J, Yousesef M, Turgut S Causes of facial nerve paresis after translabyrinthine surgery for acoustic neuroma." Ann Otol Rhinol Laryngol 1992; 101: 821-826. Shankar L, Hawk M, Leekam RN, Rutka J "Computed tomographic analysis of the intratemporal facial nerve and facial nerve neuromas." J Can Assoc Radiol 1989; 40: 150-155. Silverstein H, Norrell H, Haberkamp T "A comparison of retrosigmoid IAC, retrolabyrinthine, an middle fossa vestibular neurectomy for the treatment of vertigo." Laryngoscope 1987; 97: 165-173. -----------------------------------END-----------------------------------------