You are in: eMedicine Specialties > Otolaryngology and Facial Plastic Surgery > SKULL BASE Skull Base, Acoustic Neuroma (Vestibular Schwannoma)Article Last Updated: Jun 9, 2006AUTHOR AND EDITOR INFORMATIONSection 1 of 12
  Author: Peter S Roland, MD, Chair, Professor, Department of Otolaryngology, University of Texas Southwestern Medical Center Peter S Roland is a member of the following medical societies: Alpha Omega Alpha, American Academy of Otolaryngic Allergy, American Academy of Otolaryngology-Head and Neck Surgery, American Otological Society, and Texas Medical Association Editors: Douglas D Backous, MD, Director of Listen for Life Center, Department of Otolaryngology-Head and Neck Surgery, Virginia Mason Medical Center; Francisco Talavera, PharmD, PhD, Senior Pharmacy Editor, eMedicine; Gerard J Gianoli, MD, Clinical Associate Professor, Department of Otolaryngology-Head and Neck Surgery, Tulane University School of Medicine; Christopher L Slack, MD, Otolaryngology-Facial Plastic Surgery, Private Practice, Associated Coastal ENT; Medical Director, Treasure Coast Sleep Disorders; Arlen D Meyers, MD, MBA, Professor, Department of Otolaryngology-Head and Neck Surgery, University of Colorado School of Medicine Author and Editor Disclosure Synonyms and related keywords: acoustic neurinoma, unilateral sensorineural hearing loss, Schwann cells, acoustic tumor, unilateral tinnitus INTRODUCTIONSection 2 of 12
  Acoustic neuromas are intracranial extra-axial tumors that arise from the Schwann cell sheath investing either the vestibular or cochlear nerve. As acoustic neuromas increase in size, they eventually occupy a large portion of the cerebellopontine angle. Acoustic neuromas account for approximately 80% of tumors found within the cerebellopontine angle. The remaining 20% are principally meningiomas. In rare cases, a facial nerve neuroma, vascular tumor, lipoma, or metastatic lesion is found within the cerebellopontine angle. History of the ProcedureOver the last 80-90 years, the operative mortality rate has dropped dramatically from 40% at the beginning of the century to less than 1-2% in the last decade. Postoperative facial paralysis, once the rule, is now an uncommon permanent sequela of acoustic tumor surgery. Attempts at hearing conservation, unimaginable at the beginning of the century, are increasingly successful. These very dramatic improvements are the result of the convergence of several factors. Vastly improved imaging techniques permitting early diagnosis, adaptation of the microscope to the operating theatre, development of facial and auditory nerve monitoring techniques, improved anesthesia, and improved perioperative management have all contributed to enhanced outcomes. Most important has been the relentless striving of hundreds of neurosurgeons and otologists. FrequencyClinically diagnosed acoustic neuromas occur in 0.7-1.0 people per 100,000 population. The incidence may be rising, a reflection of the increasing frequency with which small tumors are being diagnosed. Careful autopsy studies can detect small vestibular schwannomas in a higher percentage of elderly patients, which suggests that many acoustic neuromas never become clinically apparent. EtiologyFamilial neurofibromatosis type II occurs in individuals who have a defective tumor suppressor gene located on chromosome 22. Bilateral acoustic tumors are a principle clinical feature of neurofibromatosis type II, although other manifestations, including peripheral neurofibromata, meningioma, glioma, and juvenile posterior subcapsular ventricular opacities, are often present as well. Peripheral neurofibromatoma and café au lait spots, however, are much less frequently observed than is typical in neurofibromatosis type I. Many patients with neurofibromatosis type II present in late adolescence or early adulthood. PathophysiologyThe vast majority of acoustic neuromas develop from the Schwann cell investment of the vestibular portion of the vestibulocochlear nerve. Less than 5% arise from the cochlear nerve. The superior and inferior vestibular nerves appear to be the nerves of origin with about equal frequency. Overall, 3 separate growth patterns can be distinguished within acoustic tumors, as follows: (1) no growth or very slow growth, (2) slow growth (ie, 0.2 cm/y on imaging studies), and (3) fast growth (ie, >1.0 cm/y on imaging studies). While most acoustic neuromas grow slowly, some grow quite quickly and can double in volume within 6 months to a year. While some tumors adhere to one or another of these growth patterns, others appear to alternate between periods of no or slow growth and rapid growth. Tumors that have undergone cystic degeneration (presumably because they have outrun their blood supply) are sometimes capable of relatively rapid expansion because of enlargement of their cystic component. Because the cerebellopontine angle is relatively empty, tumors can continue to grow until they reach 3-4 cm in size before they come in contact with important structures. Growth is often sufficiently slow that the facial nerve can accommodate to the stretching imposed by tumor growth without clinically apparent deterioration of function. Tumors that arise within the internal auditory canal may produce relatively early symptomatology in the form of hearing loss or vestibular disturbance by compressing the cochlear nerve, vestibular nerve, or labyrinthine artery against the bony walls of the internal auditory canal. ClinicalUnilateral hearing loss is overwhelmingly the most common symptom present at the time of diagnosis and is generally the symptom that leads to diagnosis. Consider any unilateral sensorineural hearing loss an acoustic neuroma until proven otherwise. The tumor can produce hearing loss through at least 2 mechanisms, direct injury to the cochlear nerve or interruption of cochlear blood supply. Progressive injury to cochlear fibers probably accounts for slow progressive neurosensory hearing loss observed in a significant number of patients with acoustic neuromas. Sudden and fluctuating hearing losses are more easily explained on the basis of disruption of cochlear blood supply. Consistent with direct injury to cranial nerve VIII, a significant number of individuals with acoustic neuroma have speech discrimination scores reduced out of proportion to the reduction in the pure-tone average—a feature deemed typical for retrocochlear lesions. Such marked reductions in speech discrimination scores (often into the teens or twenties) are not invariable, however. A normal speech discrimination score does not rule out an acoustic tumor. A significant number of patients with acoustic tumors have normal or near-normal hearing or speech discrimination scores. Hearing loss associated with acoustic neuroma can be sudden or fluctuating in 5-15% of patients. Such hearing loss may improve spontaneously or in response to steroid therapy. Consequently, acoustic tumor should be considered in anybody with a sudden or fluctuating loss even if hearing returns to normal. Patients with acoustic neuroma have normal hearing at the time of diagnosis in 3-5% of cases. Surprisingly, patients with medium and large tumors are nearly as likely to have normal hearing as patients with very small tumors. Not surprisingly, the discovery of acoustic neuromas in persons with normal hearing has been increasing as gadolinium-enhanced MRI is becoming more common. The presence of unilateral tinnitus alone is a sufficient reason to evaluate an individual for acoustic tumor. Although tinnitus is most commonly a manifestation of hearing loss, a few individuals with acoustic tumors (around 10%) seek treatment for unilateral tinnitus without associated subjective hearing loss. Vertigo and dysequilibrium are uncommon presenting symptoms among patients with acoustic tumors. Rotational vertigo (the illusion of movement or falling) is much more common when tumors are small. In Samii's series of 16 patients with intracanalicular tumors, 75% of patients presented with vertigo. This is an atypically high percentage even for small tumors. Dysequilibrium (a sense of unsteadiness or imbalance), on the other hand, appears to be more common in larger tumors. Overall, if carefully questioned, approximately 40-50% of patients with acoustic tumors report some balance disturbance. However, balance disturbance is the presenting symptom in less than 10% of patients. The destruction of vestibular fibers apparently is sufficiently slow as to permit seamless compensation. Headaches are present in 50-60% of patients at the time of diagnosis, but fewer than 10% of patients have headache as their presenting symptom. Headache appears to become more common as tumor size increases and is a prominent feature in patients who develop obstructive hydrocephalus associated with a very large tumor. Facial numbness occurs in about 25% of patients and is more common at the time of presentation than facial weakness (about 10% of patients). The motor fibers in the facial nerve can accommodate very substantial stretching as long as it occurs slowly, and they seem much more resistant to injury than sensory fibers. Objective hypoesthesia involving the teeth, buccal mucosa, or skin of the face is associated with larger tumors, but a subjective reduction in sensation that cannot be documented on objective examination occurs commonly with medium-sized and small tumors. Decrease in the corneal reflex generally occurs earlier and more commonly than objective facial hypoesthesia. Even though approximately 50-70% of individuals with large tumors have objectively demonstrable facial hypoesthesia, they are often unaware of it, and it is uncommonly the presenting symptom. Facial weakness is sufficiently uncommon (5-10% of patients) that facial weakness associated with a small or medium-size tumor should raise suspicion that it is not an acoustic neuroma. Entertain other diagnoses, such as facial neuroma, meningioma, granuloma, arteriovenous malformation (AVM), or lipoma. Large tumors (>4.0 cm) can obstruct the flow of spinal fluid through the ventricular system by distorting and obstructing the fourth ventricle. In the early decades of this century, 75% of patients presented with hydrocephalus. INDICATIONSSection 3 of 12
Surgical removal is the most commonly accepted treatment for acoustic tumors. See Surgical therapy. RELEVANT ANATOMYSection 4 of 12
The cerebellopontine angle is a space filled with spinal fluid. It has the brain stem as its medial boundary, the cerebellum as its roof and posterior boundary, and the posterior surface of the temporal bone as its lateral boundary. The floor of the cerebellopontine angle is formed by the lower cranial nerves (IX-XI) and their surrounding arachnoid investments. The flocculus of the cerebellum may lie within the cerebellopontine angle and may be closely associated with cranial nerves VIII and VII as they cross the cerebellopontine angle to enter the internal auditory canal. Vascular structures within the cerebellopontine angle The most important vascular structure within the cerebellopontine angle is the anterior inferior cerebellar artery (AICA). It arises most commonly as a single trunk from the basilar artery but can arise as 2 separate branches. In rare cases, it originates as a branch of the posterior inferior cerebellar artery (PICA). As the AICA moves from anterior to posterior, it first follows the ventral surface of the brain stem, but within the cerebellopontine angle it takes a long loop laterally to the porus acusticus. In 15-20% of cases, the AICA actually passes into the lumen of the internal auditory canal before turning back on itself toward the posterior surface of the brain stem. The AICA can thus be divided into 3 different segments, the premeatal, meatal, and postmeatal segments. The main branch of the AICA passes over cranial nerves VII and VIII in only 10% of cases. The remainder of the time, it either passes below the VII and VIII cranial nerves or, in 25-50% of individuals, actually passes between them. Three branches that regularly arise from the meatal segment of the AICA can be identified. Small perforating arteries supply blood to the brain stem. The subarcuate artery passes through the subarcuate fossa into the posterior surface of the temporal bone, and the third regular branch is the internal auditory artery (labyrinthine artery). Cranial nerves VII and VIII receive their blood supply from small branches of AICA. Two venous structures must be kept in mind during surgical procedures on the cerebellopontine angle. The petrosal vein (of Dandy) brings returning venous blood from the cerebellum and lateral brain stem to the greater petrosal sinus. It is generally encountered in the area of the trigeminal nerve anterior to the porus acusticus. The petrosal vein often carries enough venous blood that its obstruction can lead to venous infarction and cerebellar edema, and it should be preserved if at all possible. Additional venous blood reaches the greater petrosal sinus through a series of bridging veins that cross the cerebellopontine angle. While every attempt should be made to preserve these veins, their sacrifice is generally inconsequential. The vein of Labbé carries returning venous blood from the inferior and lateral surface of the temporal lobe to either the greater petrosal sinus or the transverse sinus. Its configuration and anatomy is quite variable. However, obstruction, obliteration, or occlusion of the greater petrosal sinus may, in some cases, result in inclusion of the vein of Labbé. Sudden occlusion of the vein of Labbé carries with it high risk of venous infarction of the temporal lobe and rapid life-threatening cerebral edema. Nerves The facial nerve leaves the brain stem anterior to the foramen of Luschka. As it leaves the brain stem, the fibers are sheathed in oligodendroglia derived from the central nervous system. Within a few millimeters of leaving the brain stem, however, the nerve loses its oligodendroglial ensheathment and becomes ensheathed instead by Schwann cells. Throughout the remainder of its peripheral course, it remains within its Schwann cell investment. It passes directly across the cerebellopontine angle for about 20 cm, accompanied by the vestibulocochlear nerve. It consistently enters the internal auditory canal by crossing the anterior superior margin of the porus acusticus. The vestibulocochlear nerve arises from the brain stem slightly posterior to the facial nerve. It remains sheathed in oligodendroglia for approximately 15 mm (almost to the point at which it passes into the internal auditory canal). It has the longest oligodendroglial investment of any peripheral nerve. The junction between oligodendroglia and Schwann cells thus occurs just medial to the porus acusticus. Because acoustic neuromas arise from Schwann cells, they arise most commonly within the most lateral portions of the cerebellopontine angle or the internal auditory canal. The nervus intermedius (nerve of Wrisberg) leaves the brain stem together with the cochleovestibular nerve. At some point within the cerebellopontine angle, the nervus intermedius crosses over to become associated with the facial nerve. It may do so as several separate rootlets. At what point the nervus intermedius crosses to become associated with the facial nerve is considerably variable, but in 22% of individuals, it is adherent to the vestibulocochlear nerve for 14 mm or more. As the vestibulocochlear and facial nerve reach the porus acusticus (medial opening of the internal auditory canal) they pass into it together with the nervus intermedius and, sometimes, a loop of AICA. Internal auditory canal The internal auditory canal is approximately 8.5 mm in length (range 5.5-10.5 mm), lined with dura, and filled with spinal fluid. Its medial end is oval in shape and is referred to as the porus acusticus. Its lateral end is a complicated structure referred to as the fundus or lamina cribrosa. The fundus is divided into a superior and inferior half by the transverse crest. The upper half is further subdivided into an anterior and posterior segment by a vertical crest, often referred to as the Bill Bar after William House, who popularized its importance as a surgical landmark. Temporal bone The anatomy of the superior surface of the temporal bone must be mastered if middle fossa approaches are to be undertaken successfully. Laterally, the irregular superior surface of the temporal bone transitions relatively smoothly to the temporal squama. The free edge of the tentorium and the greater petrosal sinus attach to the medial edge of the superior surface of the temporal bone. The arcuate eminence represents the most superior portion of the superior semicircular canal, which often rises slightly higher than the plane of the superior surface of the temporal bone. It is often difficult to identify, especially in well-pneumatized temporal bones. The geniculate ganglion generally lies within the substance of the temporal bone just medial to and a few millimeters anterior to the head of the malleus. The geniculate ganglion may be sitting right on the surface of the temporal bone with no bony covering, or alternatively, it may lie several millimeters beneath the superior surface of the bone. The head of the malleus is generally easy to identify if the thin bone of the tegumen is removed so as to enter into the middle ear space from above. In difficult surgical situations, the head of the malleus can be used to identify the geniculate ganglion. The geniculate ganglion gives off the greater superficial petrosal nerve, which courses anteriorly and erupts through the superior surface of the temporal bone at the facial hiatus. The facial hiatus is generally 4-8 mm anterior to the geniculate ganglion. The greater superficial petrosal nerve can generally be identified in this area. It can then be followed retrograde to the geniculate ganglion. A centimeter or so lateral to the greater superficial petrosal nerve lies the foramen spinosum, through which the middle meningeal artery and associated veins pass. A few millimeters anterior and lateral to the foramen spinosum is the foramen ovale, which accommodates the third (mandibular) division of the trigeminal nerve. The horizontal portion of the carotid canal courses through the anterior temporal bone medial to the foramen spinosum and foramen ovale. The cochlea cannot be identified from the surface appearance of the superior temporal bone. It lies just anterior to the labyrinthine segment of the facial nerve but is deep to the geniculate ganglion. CONTRAINDICATIONSSection 5 of 12
Few absolute contraindications to the surgical removal of an acoustic tumor exist. Serious medical illness may make surgical removal in some patients too risky. The risk-benefit ratio is especially likely to tilt against surgery for smaller tumors. Surgery must often be performed for large tumors with brainstem shift and obstructive hydrocephalus, even in the presence of significant medical illness. The decision to operate should be carefully considered when the tumor is within the internal auditory canal of a patient's only hearing ear. In some cases, observing the tumor until hearing has been lost is best, while in other cases, attempting surgical removal with hearing conservation is more prudent. WORKUPSection 6 of 12
Imaging Studies
Diagnostic Procedures
Histologic FindingsTwo histologic types of tissue have been identified in acoustic tumors. Antoni A tissue consists of elongated spindle cells in a regular string packed tightly together with a tendency to palisading. Antoni B tissue, on the other hand, has a loose spongy texture and markedly reduced cellularity. A given acoustic neuroma may contain areas with both Antoni A and Antoni B tissue. While the histologic appearance of acoustic tumors is fairly straightforward, they can occasionally be difficult to distinguish from meningiomas. TREATMENTSection 7 of 12
Medical therapyAcoustic neuromas are managed in one of 3 ways, (1) surgical excision of the tumor, (2) arresting tumor growth using stereotactic radiation therapy, or (3) careful serial observation. Observation 2) Patients with small tumors, especially if their hearing is good 3) Patients with medical conditions that significantly increase the risk of operation 4) Patients who refuse treatment 5) Patients with a tumor on the side of an only hearing ear or only seeing eye
Steriotactic Radiotherapy
Surgical therapySurgical removal remains the treatment of choice for tumor eradication. A variety of surgical approaches can be used to remove acoustic tumors. Each approach is discussed in detail in the following sections. Preoperative detailsThree different approaches are used in the management of acoustic neuromas, the retrosigmoid, translabyrinthine, and middle fossa approaches. All have advantages and disadvantages as indicated below. Advantages of the retrosigmoid approach
Disadvantages of the retrosigmoid approach
Advantages of the translabyrinthine approach
Disadvantages of the translabyrinthine approach
Advantages of the middle cranial fossa approach
Disadvantages of the middle cranial fossa approach
Approach selectionA variety of different considerations go into deciding which approach should be used for any individual patient. These variables are detailed below. Preoperative hearing level If the patient has no useful hearing, either the translabyrinthine or the suboccipital approach is selected, depending upon the experience and training of the surgeon. In most centers performing large numbers of surgeries for acoustic tumors, the translabyrinthine approach is preferred. Opinions vary considerably about what constitutes useful hearing. The 50/50 rule is frequently quoted. The rule suggests that individuals with a pure-tone average greater than 50 dB and speech discrimination less than 50% do not have useful or salvageable hearing. Other surgeons have stricter criteria and consider only individuals with better than a 30-dB pure-tone average and more than 70% discrimination for hearing conservation operations. Auditory brainstem response Normal preoperative ABR findings favor hearing conservation. Marked abnormalities of ABR wave morphology or increased wave I-III and I-V latencies make hearing conservation less feasible. Electronystagmography Allegedly, an abnormal finding on electronystagmography (ENG) increases the likelihood of successful hearing conservation surgery. The ENG tests the horizontal semicircular canal, which is innervated by the superior vestibular nerve. A normal ENG finding arguably demonstrates that the superior vestibular nerve is normal. Consequently, the acoustic tumor must have originated from the inferior vestibular nerve, which is directly adjacent to the cochlear nerve. Surgical removal, then, is more likely to directly injure the cochlear nerve or interfere with cochlear blood supply. Tumor size Opportunities for hearing conservation decrease as tumors become larger. Hearing is much more difficult to conserve when tumors are 1.5-2.0 cm in diameter than if they are small intracanalicular tumors. Consequently, some surgeons limit hearing conservation surgery to smaller tumors, preferring to use a translabyrinthine approach to maximize the chance of facial nerve conservation for larger lesions. Tumor position If hearing conservation is to be attempted and the tumor lies within the lateral portions of the internal auditory canal, many surgeons prefer a middle fossa approach. The middle fossa approach permits direct exposure of the lateral end of the internal auditory canal without sacrificing hearing. The approach is frequently used for any tumor lying completely within the internal auditory canal, although tumors limited to the medial portions of the internal auditory canal can be managed using a suboccipital approach. Some surgeons extend the use of the middle fossa technique to include tumors that extend as much as 0.5-1.0 cm into the cerebellopontine angle. Generally, however, tumors that have significant volume medial to the plane of the porus acusticus are extirpated using a retrosigmoid approach if hearing is to be conserved. If hearing conservation is not an issue, the retrosigmoid approach is sometimes preferred for tumors with significant inferior extension. Visualization of lower cranial nerves is better with a retrosigmoid approach. Often the retrosigmoid approach is combined with a translabyrinthine approach for such large acoustic neuromas. Relevant anatomy The following anatomic variations can make the translabyrinthine approach much more difficult and at times impossible.
Surgeon preference Some surgeons have vastly increased experience and are much more comfortable with one approach relative to another. Generally, such preferences should be indulged. However, if hearing conservation is a realistic option using an approach unfamiliar to the primary surgeon, consideration should be given to referring the patient to someone who is familiar with the appropriate approach. Patient preference Patient preferences should be carefully considered even when they do not conform to the surgeon's judgment. Some patients are adamant about going to any lengths for hearing conservation even when the treating physician is quite convinced that the patient's hearing is so poor as to be of little or no practical utility. Some patients cheerfully sacrifice even good hearing if doing so even slightly enhances the possibility of successful facial nerve preservation. Some patients have very clear-cut opinions about one type of incision versus another (sometimes based on cosmetic consideration). Intraoperative detailsRetrosigmoid approach The term suboccipital is still used to describe this approach, even though the craniotomy now used rarely extends inferior to the attachment of the cervical muscles at the uncal line. Retrosigmoid is an increasingly used term. Most surgeons simply place the patient in the supine position on the operating table and turn the head toward the contralateral shoulder. The true lateral or park-bench position is still used by some surgeons because it permits the occiput to be rotated a little bit more superiorly. This allows a slightly more direct view of the internal auditory canal. The operation is performed through either a vertically oriented linear incision or an anteriorly based U-shaped flap. An occipital craniotomy is then realized. Any mastoid air cells are carefully waxed off to prevent postoperative spinal fluid leak. The dura is opened and the arachnoid incised. The cerebellum frequently falls away from the posterior surface of the temporal bone. Hyperventilation, steroids, and intraoperative diuretics (principally mannitol) are used to reduce intracranial pressure and to provide easy surgical exposure with a limited amount of retraction. Nonetheless, gentle cerebellar retraction is frequently necessary. Once adequate exposure has been obtained, the tumor is clearly visualized along with the brain stem and lower cranial nerves. However, cranial nerves VII and VIII are rarely observed because they are almost always pushed forward and lie across the anterior surface of the tumor, which cannot be visualized. Debulking of the tumor is the next step and must be carefully performed so as to maintain the anterior portions of the capsule if injury to cranial nerve VII and/or VIII is to be avoided. Once the tumor has been substantially debulked, the posterior wall of the internal auditory canal can be removed using a high-speed drill. Great care must be taken to avoid injuring the labyrinth while removing the posterior wall of the internal auditory canal. Portions of the labyrinth quite commonly are medial to the lateral end of the internal auditory canal. Although no single anatomic landmark is completely reliable for prevention of injury to the labyrinth, the singular nerve and its canal, the vestibular aqueduct, and the cochlear aqueduct are all used as important surgical landmarks. Careful measurements taken from preoperative CT scans can provide useful information during drilling of the posterior canal wall. The length of the internal auditory canal varies considerably from individual to individual, and knowing exactly how much posterior canal wall needs to be removed to adequately expose the tumor can help limit inadvertent injury to the labyrinth. Blind extraction of tumor from the internal auditory canal without removing the posterior wall prevents significant risks of injury to the facial and/or auditory nerve (if hearing is to be saved) while at the same time increasing the chances of leaving tumor at the fundus. Use of intraoperative angled endoscopes has been of considerable help in performing this phase of the operation. Bone dust should be prevented from entering the subarachnoid space during this phase of the procedure. One probable cause for severe and intractable postoperative headache is spillage of bone dust into the subarachnoid space during tumor removal. Carefully place Surgicel, pieces of Gelfoam, Telfa pads, and/or cottonoid strips around the operative site so that bone dust from drilling adheres to them and is removed as they are removed. Once the internal auditory canal is exposed, the dura is opened and the tumor is removed from it. Although never proven, dissection from medial to lateral is thought to be less traumatic to both the cochlear nerve and to the vascular supply of the inner ear. The vestibular nerves are generally sacrificed, and unless hearing is to be preserved, the cochlear nerve is sacrificed as well. Eventually, the surgeon is left with the anterior portions of the capsule adhered to the brain stem and cranial nerve VII. As the tumor capsule is carefully removed from the brain stem, the root entry zone of cranial nerve VII can be identified. The capsule is then carefully removed from the facial nerve with as little trauma as possible. The facial nerve monitor is a great help in this portion of the dissection. A meaningful amount of data now shows that results are improved when facial nerve monitoring is employed. A variety of techniques has been used to monitor the cochlear nerve when hearing preservation is desired. The most commonly used method is intraoperative ABR audiometry, but it has a number of disadvantages. Most importantly, it requires summing a large number of repetitions in order to extract a response from background noise. Consequently, a delay occurs between surgical manipulations and ABR changes. Direct cochlear nerve monitoring offers the advantage of real-time feedback, but a fully satisfactory method of placing and securing the electrode still is lacking. Once tumor removal is complete and hemostasis is absolute, the dura is closed and the craniotomy defect is repaired, either by replacing the original bone flap or with methylmethacrylate or hydroxyapatite. Middle cranial fossa approach Although some surgeons use an extended middle cranial fossa approach for tumors that extend a centimeter or more outside the porus acusticus into the cerebellopontine angle, the middle cranial fossa approach is most frequently used for intracanalicular tumors. It is, by consensus, the approach of choice for small tumors that lie within the lateral portions of the internal auditory canal when hearing conservation is desired. The head must be in the true lateral position. In young individuals with a supple neck, this can often be accomplished by turning the head to the side with the patient in the supine position. But if neck mobility is limited or concern exists that forced head turning will limit posterior fossa circulation or aggravate cervical spine disorders, then a true lateral (park-bench) position should be used. Exposure must be centered over a vertically oriented line that passes approximately 1 cm anterior to the external auditory meatus. This is most easily accomplished through a linear incision. A posteriorly based U-shaped or curvilinear S-shaped incision can be used if concern exists about scar contracture. Depending upon the incision used, the temporalis muscle is incised or reflected inferiorly. A temporal craniotomy (approximately 3 cm by 3 cm) is performed with its base at the root of the zygoma. The dura is elevated up from the floor of the middle cranial fossa, and osmotic diuretics, head elevation, hyperventilation, and steroids are used to limit cerebral edema. The dura of the temporal lobe is then elevated off the superior surface of the temporal bone. The anterior extent of such elevation is usually the foramen spinosum, but the middle meningeal artery can be divided between clips and elevation continued anteriorly to foramen ovale if additional exposure is desired. Dural elevation should proceed from posterior to anterior to avoid injury to an exposed greater superficial petrosal nerve or geniculate ganglion. Bleeding from the veins associated with the middle meningeal artery is often quite brisk but can generally be controlled with Surgicel packing. Medial dissection continues to the free edge of the temporal bone. The superior petrosal sinus is attached to the posterior surface of the temporal bone but not always at its superior edge. Care must be taken to avoid injuring it. If inadvertent injury occurs, bleeding can generally be controlled with intraluminal Surgicel packing, electrocautery, or hemoclips. When extended middle cranial fossa approaches are employed, the superior petrosal sinus is deliberately divided between clips. When it can be detected easily, the arcuate eminence is an extremely helpful landmark. Careful drilling can often identify the blue line of the superior canal within it. Because the most difficult exposure to achieve during middle fossa surgery is the lateral posterior end of the internal auditory canal, perform dissection as close to the superior semicircular canal as possible. The greater superficial petrosal nerve is generally easy to visualize and can be followed retrograde to the geniculate ganglion. It lies approximately 1.0 cm directly medial to the foramen spinosum. Once the area of the geniculate is identified, small diamond burrs are used to completely expose it. If the greater superficial petrosal nerve cannot be located and no other landmarks are available, the middle ear space can be entered from above and the head of the malleus intercranial identified. The geniculate ganglion lies approximately 2-3 mm anterior and medial to the head of the malleus. Once the geniculate ganglion has been completely exposed, the labyrinthine portion of the nerve can be identified and followed medially and inferiorly into the internal auditory canal. Remember that the labyrinthine portion of the nerve takes a markedly vertical course as it moves from the lateral geniculate ganglion to the proximal fundus of the internal auditory canal, which lies 5 or more millimeters deep to the geniculate ganglion. Some surgeons prefer to identify the internal auditory canal medially. Once the medial end of the canal is completely identified, they follow the canal laterally to the fundus of the internal auditory canal. The internal auditory canal should be skeletonized approximately 270°. The most difficult area to expose is the point at which the superior vestibular nerve penetrates the labyrinthine bone to innervate the ampulla. However, exposure in this area is critical if the anatomy of the lateral end of the internal auditory canal is to be well visualized. If this area can be well visualized, then tumor removal is generally both successful and relatively straightforward. Larger tumors frequently have the facial nerve splayed out over the anterior superior portions of the tumor. Tumor removal begins, as with other approaches, by carefully debulking. Once the tumor is well debulked, enough room is created within the internal auditory canal to carefully remove the tumor capsule from the inferior surface of the facial nerve. Again, care must be taken to avoid torsion or twisting of the nerve during tumor removal. Once the tumor has been completely removed, the integrity of the facial nerve is tested using the intraoperative facial nerve monitor. Presumably, the monitor has been in use throughout the case. If the facial nerve can be stimulated with low stimulus intensities, chances of good postoperative facial nerve function increase. Fat is then packed into the internal auditory canal after using bone wax to fill obvious air cells to prevent postoperative cerebrospinal fluid leak. The facial nerve monitor generally alerts the physician if fat is being packed in so tightly that the integrity of the facial nerve is being compromised. Retractors are removed, and the temporal lobe is allowed to reexpand. The bone plate is replaced using miniplates, and the wound is closed in multiple layers. Postoperative detailsUnless a complication develops, postoperative care is straightforward. The patient is generally kept in the ICU overnight so that rapid intervention is available if postoperative intercranial pressure increases or bleeding occur. Vestibular rehabilitation should begin on the first postoperative day and continue twice daily throughout the hospital stay. Most patients can be discharged on the third or fourth postoperative day. Follow-upObtain follow-up MRI within 3-6 months of the surgical procedure to document the completeness of tumor removal and to serve as a baseline for further follow-up scans. Assuming complete tumor removal, follow-up MRI should be obtained at 5 years and at 10 years. If the findings on the 10-year scan are normal, further imaging should be performed only if clinical circumstances require it. COMPLICATIONSSection 8 of 12
Arterial injury Injury to the AICA (much less commonly to the PICA) fortunately occurs very rarely. Although the AICA may be loosely attached to the tumor capsule, separating it from the tumor is generally fairly easy. Sacrificing the AICA itself has variable consequences depending upon the details of individual patient anatomy. It can be catastrophic and lead to devastating neurologic injury or death. The branches of AICA most vulnerable to injury are, of course, the labyrinthine artery and branches going to the facial nerve. Some otherwise perplexing cases of postoperative facial nerve weakness may be related to interruption of facial nerve vascular supply due to coagulation of small branches of the AICA. Failure to conserve hearing is sometimes (perhaps frequently) due to disruption of cochlear blood supply. Because the labyrinthine artery may be very intimately associated with the tumor, sacrifice often cannot be avoided. Conservation of the internal labyrinthine artery becomes more difficult as tumor size increases, doubtlessly accounting in some measure for the reduced success in hearing conservation with larger tumors. Neurologic injury or cerebral edema secondary to venous injury usually occurs as a result of injury to the sigmoid sinus itself, the petrosal vein of Dandy, or to the vein of Labbé. Occlusion of the sigmoid sinus has variable effects depending largely upon the patient's unique venous anatomy. If the contralateral venous outflow tract is patent and communication through the torcula is good, complete occlusion may go entirely unnoticed. The size of the 2 sigmoid sinuses is generally asymmetrical, with a greater volume of blood flowing through one. Depending on how much more blood flows through the dominant sinus, occlusion of the dominant sinus can result in catastrophic increases in intracranial pressure, venous infarction, and even death. Because a number of potential collaterals exist between the torcula and the jugular bulb, occlusion of the sigmoid sinus close to the torcula is much more likely to have significant adverse effects than its occlusion close to the jugular bulb. The petrosal vein of Dandy is a single large outflow tract in some patients but consists of series of several large veins in others. Its occlusion can result in edema and infarction of either the temporal lobe or the brain stem. While neurologic injury secondary to occlusion of the Dandy vein is not inevitable, severe injury can occur, and the petrosal vein should be carefully preserved. Occlusion of the vein of Labbé (formally termed the anastomotic vein of Labbé) often results in severe edema of the temporal lobe and temporal infarct. The edema can be sufficiently severe to cause brain herniation and death. The vein of Labbé generally enters the superior petrosal or transverse sinus between the torcula and the point at which the greater petrosal sinus joins the transverse sinus. Thus, it is generally not directly in the field during acoustic tumor surgery. Occasionally, however, injury to the superior petrosal sinus results in its obliteration, and in some instances, the vein of Labbé is also injured or obstructed. Its presence and importance should be kept in mind during acoustic tumor surgery. Hemorrhage into the posterior fossa in the immediate postoperative period can produce brainstem compression and death quite rapidly. Death can occur within a few minutes. Rapid neurologic deterioration in the first 24 hours postoperatively should raise suspicion of posterior fossa hemorrhage and mandates rapid and decisive intervention. If time permits, a rapid unenhanced CT scan should be obtained to secure the diagnosis while the operating room is prepared for an immediate return to surgery. If neurologic deterioration is rapid, forgo CT scanning and take the patient directly back to the operating theatre. If deterioration is very rapid with loss of consciousness, decerebrate posturing, and signs of imminent death, open the wound at the bedside to permit a posterior fossa decompression prior to emergent translocation of the patient to the operating room for wound exploration, debridement, and extensive irrigation. Cerebellar injuries Injury to the cerebellum was common in the early decades of the century, but its incidence has dramatically diminished in recent decades. Cerebellar injuries still occur but are generally not troublesome. The rotating shaft of the surgeon's burr is often the culprit because surgeons usually look past the shaft to the head of the burr to control bone removal. The shaft is often outside the surgical field of view. Such small areas of injury rarely have noticeable sequelae. Bleeding can be controlled with Surgicel, cautery, or Gelfoam, and edema is limited. Direct injury to the cerebellar hemisphere from compression and retraction, intracerebral hemorrhage, infarction due to alteration of the arterial inflow, or venous engorgement with our without infarction can produce severe edema of the entire cerebellum. Brainstem compression and/or intracranial herniation can produce death. Obstruction of the fourth ventricle and aqueduct can produce significant hydrocephalus. Management should consist of aggressive use of osmotic diuretics, hyperventilation, and steroids. If medical management is unsuccessful, resection of part of the involved cerebellar hemisphere may be required. Facial paralysis Postoperative facial paralysis is sometimes unavoidable. The tumor may simply be attached too tightly to the thin attenuated facial nerve. Sometimes the tumor has wrapped around the facial nerve, and tumor removal cannot be accomplished without resection of a portion of the facial nerve. Eye care is critical to successful management of postoperative facial paralysis. Make liberal use of artificial tears specifically adapted to deal with dry eye (eg, Bion Tears). At night, place ocular lubricants (eg, Lacri-Lube) in the eye. If aggressive use of artificial tears during the day (q15-30min) and ointment at night is insufficient to maintain corneal hydration and exposure keratitis begins to develop, then consider use of an eye patch, placement of a gold weight, and lower lid shortening procedures. Tarsorrhaphy should be used only as a last resort and is only very rarely required. Coexisting injury to cranial nerve V with corneal hypesthesia or anesthesia vastly increases the problem in management. The lack of corneal sensation provides the patient no reliable guide as to the severity of corneal epithelial disruption. In such cases, tarsorrhaphy is much more likely to be required. Cerebrospinal fluid complications Transient abnormality of cerebrospinal fluid resorption may lead to mild transient postoperative hydrocephalus. While postoperative shunting can reduce difficulty controlling cerebrospinal fluid fistula, it is now rarely if ever required. Even when hydrocephalus is present in the preoperative period, it generally resolves without difficulty in the first few postoperative weeks. Postoperative meningitis occurs in 2 forms. Bacterial meningitis is potentially life threatening and occurs in 1-5% of patients with acoustic neuroma. It can occur within the first 24-36 hours postoperatively, or its appearance may be delayed for a couple of weeks. Once initiated, it can progress very rapidly, and individuals can lapse from a normal level of consciousness into a dense coma in a matter of a few hours. Consequently, intervention must be rapid. Diagnosis depends upon the presence of fever and, in the alert patient, the presence of headache, stiff neck, nuchal rigidity, and decreasing level of consciousness. If meningitis is suspected, perform an immediate lumbar puncture to obtain fluid for culture, but only after a CT scan has excluded the possibility of significant hydrocephalus, which could lead to brain herniation. Obtain spinal fluid for Gram stain, glucose, protein, and white blood cell count. If the Gram stain is positive, spinal fluid glucose is 40 mg% or less, or spinal fluid white blood cell count is higher than 2500, begin antibiotics immediately pending culture results. If the spinal fluid does not meet any of these criteria, closely observe the patient with the understanding that any deterioration of the condition requires prompt repuncture for additional spinal fluid. Aseptic meningitis has been reported in 7-70% of postoperative neurosurgical patients. It shares with bacterial meningitis the clinical signs of increasing headache, fever, nuchal rigidity, and elevation of cerebrospinal fluid pressure. Spinal fluid profile in such patients shows marked elevation of white blood cell count and cerebrospinal fluid protein levels, but cerebrospinal fluid glucose remains within the reference range, and culture results (when they are finally complete) are normal. Corticosteroids are extremely helpful in managing aseptic meningitis, and their prompt administration often results in marked decrease in headache and nuchal rigidity within a few hours. Spinal fluid leak through either the wound or the eustachian tube and middle ear occurs in 2-20% of patients. It can occur after translabyrinthine or retrosigmoid approaches and is less common after middle fossa craniotomy. When it follows retrosigmoid approaches, the path of egress is generally through pneumatized air cell tracts. Cerebrospinal fluid is produced within the ventricular system at a rate of 0.3 mL/min or at about 500 mL/d. It enters the subarachnoid space in the posterior fossa via the midline and lateral foramen of the fourth ventricle. Contamination of the cerebrospinal fluid circulation by blood, bone dust, and necrotic debris at the time of surgery often impairs cerebrospinal fluid absorption directly by mechanical interference in the arachnoid villi or indirectly by inciting an inflammatory response within the subarachnoid space. The syndrome may vary from brief asymptomatic elevation of cerebrospinal fluid pressure to clinically manifested aseptic meningitis (discussed above). Cerebrospinal fluid escaping through the wound can initially be managed by resuturing the wound. Sometimes this results in elimination of the difficulty, while at other times it merely produces cerebrospinal fluid rhinorrhea, as the spinal fluid finds an alternate means of egress. If the cerebrospinal fluid leak persists for more than 12-24 hours after initiation of conservative management, including pressure dressing and consistent head elevation, then consider reducing the cerebral spinal fluid pressure by 1 of 3 measures, including (1) multiple lumbar punctures, (2) continuous or intermittent drainage via lumbar intradural catheter, or (3) permanent cerebrospinal fluid diversion by means of an indwelling shunt. When cerebrospinal fluid diversion is selected, the most common method is an indwelling subarachnoid catheter placed into the lumbar subarachnoid space. The drain is opened episodically so as to remove 200-400 mL of spinal fluid in any given 24-hour period. Some surgeons observe a minimum drainage period of 2 days, others 5 days. General consensus is that, if the drain has been in place for more than 5 days, it should be replaced to avoid infection. Variation among surgeons is considerable as to when reexploration is required. Some centers reexplore after 24-48 hours of drainage; other centers use as many as two 5-day trials of continuous lumbar drainage before considering a second operation. Severe postoperative headache has long been associated with suboccipital procedures. This problem appears to have diminished considerably since the introduction of 2 intraoperative steps: (1) great care is taken to avoid contaminating the spinal fluid and subarachnoid space with bone dust, and (2) the bone flap is replaced and any residual bony defect is eliminated with methylmethacrylate or hydroxyapatite. The latter step eliminates the direct attachment of posterior cervical musculature to the dura. When postoperative headaches do occur, they should be managed with relatively high-dose nonsteroidal anti-inflammatory agents and aggressive regimens of manipulative physical therapy. OUTCOME AND PROGNOSISSection 9 of 12
Tinnitus Tinnitus becomes worse in only 6-20% of individuals after tumor removal. In a substantial number of individuals, the tinnitus remains unchanged. In about 25-60% of patients, tinnitus is eliminated or improved. Although 30-50% of patients who had no preoperative tinnitus develop it in the immediate postoperative period, such tinnitus only rarely becomes troublesome. Recurrence/residual tumor Recurrence is uncommon after acoustic tumor removal. Overall, the recurrence rate is 5-10% or lower. The vast majority of recurrences follow suboccipital removal. Presumably, a small amount of tumor is left in the lateral end of the internal auditory canal where intraoperative visualization is so difficult. Tumor recurrence may be signaled by recurring headache, altered sensation to the face, or dysarthria and dysphasia if the lower cranial nerves become involved. Inflammation in the tumor bed may persist for months and even years after acoustic tumor removal, and consequently, areas of contrast enhancement are present on postoperative gadolinium MRI. Distinguishing tumor recurrence from postoperative inflammation can be quite difficult. Tumor recurrences tend to be globular while postoperative inflammatory enhancement tends to be linear. Often, however, one must view serial scans to detect tumor recurrence. Surveillance for postoperative tumor recurrence should persist for 8-10 years postoperatively. Facial function Preservation of facial function continues to improve. It has steadily improved throughout the decade, and the recent introduction of facial nerve monitoring has produced additional improvements. However, facial nerve outcomes do continue to vary according to tumor size. When tumors are smaller than 1.5 cm, good facial nerve function can be expected (House-Brackmann grade I-II) in more than 90% of patients. Only 3.2-6.7% of patients with this size tumor have poor outcomes (House-Brackmann III-IV). In addition to tumor size, preoperative electrophysiologic testing can help predict postoperative outcome, although this testing is not commonly used. Demonstrable electrophysiologic abnormalities on nerve conduction studies, electromyography, and blink reflex testing correlate well with postoperative facial nerve deficits. Arriaga has shown that even patients with poor facial nerve function at the time of discharge (House-Brackmann V-VI) had a 25% chance of recovery of normal function (House-Brackmann I-II). Less optimistic is the report of Sterkers. In his series of patients, anyone who had House-Brackmann III function or worse at a 4- to 6-week postoperative evaluation was left with significant deficit and generally had some synkinesis. Facial nerve paralysis may be delayed and may develop within a few hours to a week or more after acoustic neuroma removal. Incidence of delayed facial palsy varies from 10-30%. The mechanism of action is unclear. Ischemia secondary to vasospasm, vascular injury, traction, nerve edema, stretching, and even a viral reactivation have been proposed. Unlike final facial nerve outcome, incidence of delayed facial paralysis does not appear to be related to tumor size. The vast majority of individuals who have delayed onset of facial paralysis make complete and total recoveries. If deterioration is severe (more than 3 House-Brackmann grades), some chance of poor long-term outcome exists. Perioperative steroids are widely used in an attempt to enhance both immediate and long-term postoperative facial nerve function, but unequivocal evidence for their effectiveness is lacking. Hearing outcome The ability to preserve hearing has increased substantially over the last decade or two. Depending on criteria for successful hearing conservation, hearing can be reserved in 30-80% of properly selected patients. Meta-analysis performed by Gardner and Robinson appears to show an overall average success rate of about 33%. Stereotactic radiation (the gamma knife) does not appear to have a significantly higher rate of hearing conservation than does properly conducted surgery when long-term results are compared. Rosenberg et al and Tucci et al have both shown reasonable stability of hearing over time. On the other hand, Shelton's study appears to show significant hearing deterioration in 30-50% of patients who originally had successful hearing preservation. FUTURE AND CONTROVERSIESSection 10 of 12
The relative roles of stereotactic radiation and surgery will remain controversial until the long-term results of radiation treatment protocols are clearly defined. MULTIMEDIASection 11 of 12
REFERENCESSection 12 of 12
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||
