An acoustic neurinoma is the most common tumor of the cerebellopontine angle. Advancements in surgery of the cerebellopontine angle are directly reflected in the history of improvements used in both the diagnosis and treatment of acoustic neurinomas.
Sir Charles Ballance performed the first successful removal of an acoustic neurinoma in 1894. The tumor was approached through a suboccipital craniectomy and was removed by blunt finger enucleation. Cushing subsequently developed a subtotal intracapsular technique, which markedly decreased the operative mortality. Walter Dandy accomplished the first complete resection of an acoustic neurinoma in 1917. Dandy, in 1925 described what now serves as the basis for the current operative approach. He outlined a technique utilizing a unilateral suboccipital craniectomy with internal decompression of the tumor and stressed the importance of unroofing the internal auditory meatus for complete resection. Givre and Olivecrona pioneered preservation of the facial nerve during removal of acoustic tumors. Rand and Kurze reported preservation of the cochlear as well as the facial nerve in 1968.
Developments in microsurgical technique and early diagnosis through computed tomography (CT) and magnetic resonance imaging have resulted in the detection of smaller tumors that can be removed with more reliable preservation of cranial nerve function. The use of intraoperative auditory evoked potential monitoring to help preserve hearing and the use of direct intraoperative seventh cranial nerve stimulation has also played an important role in the successful resection of acoustic tumors. These advances in diagnostic and surgical techniques used for the treatment of acoustic neurinomas have also led to a progressively lower morbidity and mortality for the resection of all cerebellopontine angle lesions.
Clinical Features
The differential diagnosis of cerebellopontine angle lesions includes, in order of occurrence, acoustic neurinomas, meningiomas, epidermoid tumors and arachnoid cysts. Significantly less common lesions include neurinomas of other cranial nerves, lipomas, glomus tumors and vascular lesions.
Acoustic neurinomas arise from the Schwann cells of the vestibular nerve. The vestibular nerve is ensheathed in oligodendrocytes for much of its course through the cerebellopontine angle. However, as the nerve enters the internal auditory meatus the oligodendrocytes are replaced by Schwann cells in a region known as the zone of Obersteiner-Redlich. This transitional zone usually lies at the mouth of the internal auditory meatus and thus Schwann cells invest the vestibular nerve along virtually all of its length within the canal. It is these cells within the canal which are thought to give rise to the acoustic neurinoma.
A history of progressive unilateral hearing loss, usually over many months and sometimes years, is the hallmark of an acoustic neurinoma. In most cases it is associated with tinnitus. As the tumor enlarges, the patient complains of unsteadiness and loss of balance. True rotational vertigo is rare. The facial nerve usually functions normally until the tumor reaches a large size. When nerve function is compromised, it is usually mild. Total facial paralysis is rare. Involvement of the trigeminal nerve likewise occurs late and is seen primarily in tumors more than 3 cm in diameter. As the tumor grows upward into the superior aspect of the cerebellopontine angle, it encroaches upon the trigeminal nerve, producing a gradual decrease of the corneal reflex and facial analgesia and anesthesia. Tic douloureux occurs rarely.
It is unusual for patients with an acoustic neurinoma to present with complaints of swallowing dysfunction or hoarseness and lower cranial nerve involvement is unlikely unless the tumor is large. Cerebellar symptoms and signs also occur late in the clinical course of these tumors and are often found in association with compromised function of cranial nerves. Papilledema and symptoms of hydrocephalus can also be present and are usually secondary to compression of the brain stem and the fourth ventricle by a large tumor.
Meningiomas are the second most frequent tumor of the cerebellopontine angle. They constitute 3 to 13 percent of cerebellopontine angle tumors. These tumors produce the same general symptoms and signs as do acoustic tumors, with several exceptions. Often these lesions originate from the superior-anterior lip of the porus acousticus, and are associated with early involvement of the seventh nerve. Hearing loss, however, occurs later. Thus, in terms of facial and auditory function, meningiomas are the exact opposite of acoustic tumors. Involvement of the posterior root of the fifth cranial nerve may lead to numbness of the face and ticlike symptoms. These symptoms, preceding hearing loss, suggest that a meningioma may be present or, less likely, a trigeminal neurinoma. Meningiomas also cause a higher incidence of lower cranial nerve abnormalities compared to acoustic tumors. The growth downward of these lesions results in hoarseness, numbness of the throat or complaints of difficulty swallowing. As with acoustic tumors, large meningiomas can produce cerebellar symptoms and signs or hydrocephalus with increased intracranial pressure.
Epidermoid tumors and arachnoid cysts are both rare lesions of the cerebellopontine angle, accounting for 2 to 6 percent and 1 to 3 percent of all lesions, respectively. Epidermoid tumors are benign and grow slowly. They can present with multiple cranial nerve abnormalities or cerebellar symptoms and signs which develop over a number of years. Patients with arachnoid cysts can present with a complaint of unilateral hearing loss, headache or imbalance. Facial or trigeminal nerve dysfunction can occasionally be observed.
Anatomy
The cerebellopontine angle is an inverted triangular cistern in which the fifth, seventh and eighth cranial nerves, along with the anterior inferior cerebellar artery (AICA) and the superior petrosal vein are located. From a surgeon' s viewpoint, the cistern is bounded laterally by the back wall of the petrous bone, medially by the pons and cephalad by the tentorium. which forms the base of the triangle. This cistern communicates freely with the other cerebrospinal fluid (CSF) spaces within the posterior fossa, including a small diverticulum which extends into the porus acusticus.
At the upper aspect of the cistern, the fifth cranial appears as a broad white band, extending from the lateral aspect of the pons into Meckel's cave. The superior petrosal vein lies at the upper posterior edge of this nerve, and drains from the superior aspect of the cerebellum to the superior petrosal sinus. This vein is usually 1 to 2 mm in diameter and at times may be made up of a cluster of veins.
The seventh and eighth nerves course laterally from the pontomedullary junction to the internal auditory canal. They cross the cistern as an apparent single nerve, which is composed of four discrete nerves: the superior and inferior vestibular nerves, the cochlear nerve and the facial nerve. When viewed from the suboccipital approach. the vestibular nerves form the posterior aspect, or the portion closest to the surgeon. The facial nerve makes up the anterior superior portion within this bundle and the cochlear division of the eighth nerve makes up the anterior inferior portion. When one looks into the posterior fossa from the extreme lateral aspect of a suboccipital approach, the sixth nerve is occasionally seen, coursing from its origin at the pontomedullary junction to its entrance into the dura of the clivus (Dorello's canal). In situations where the tumor has rotated and displaced the brain stem, this nerve may be confused with the seventh nerve, inasmuch as it exits on the same plane as the seventh nerve and enters the dura at the same level as the internal auditory canal.
The ninth, tenth and eleventh nerves, although not specifically within the cerebellopontine angle cistern, are found immediately below its inferior margin. The most superior of these nerves, the ninth, appears round and shiny and is made up of a single filament. The tenth nerve consists of multiple filaments that are flat, whereas the eleventh nerve is unique in having a spinal root traversing the foramen magnum.
The anterior inferior cerebellar artery has a variable location within the cistern. In acoustic tumors, this vessel is usually located in the arachnoid over the cleft between the cerebellum and the dome of the tumor.
Operative Approaches
Suboccipital- Transmeatal Approach
A number of operative approaches to the cerebellopontine angle have been described, including a suboccipital-transmeatal, a translabyrinthine, a middle fossa, a translabyrinthine-transtentorial and a subtemporal-transtentorial approach. The authors prefer the suboccipital-transmeatal approach.
Operative Technique
Following the induction of general anaesthesia, the patient is placed on the operating room table in a lateral position. Some neurosurgeons insert lumbar spinal drain and the distal end of the tubing is brought out underneath the operating room table for the anaesthesiologist, who controls the CSF drainage during surgery. (In general the drain remains closed until the dura is exposed and the surgeon is ready to open the dura.) The patient is then moved into the final lateral position with a roll under the lower axilla. The shoulders and hips are taped to the table to allow manipulation of the table in all planes without risk of the patient slipping. It is important to place gentle traction on the shoulder of the upper arm parallel to the body to pull the shoulder out of the operative field. The head is then fixed with a Mayfield clamp with a moderate degree of flexion and slight rotation to bring the mastoid tip to the top of the operative field.
Table rotation is used to the advantage of the surgeon during the operation as the line of sight into the cerebellopontine angle is maximized by moving the patient rather than the surgeon, who usually sits in one position during the operation. The surgeon's chair is on casters and has adjustable arm rests adaptable for any surgeon and any patient. The combination of moving the chair and the operating room table brings almost all cerebellopontine angle lesions into a comfortable line of sight for the surgeon.
The skin preparation and draping are routine. Furosemide and mannitol are given prior to making the skin incision. In the unusual patient who presents with clinically manifested hydrocephalus, we perform a ventriculoperitoneal shunt 7 to 10 days before the tumor surgery. The time delay allows the wound to heal and the risk of infection is then minimal. It is wise to consider placing the shunt on the side opposite the tumor so the tubing does not encroach on the suboccipital surgical site.
A two-limbed incision that begins 2 to 3 cm below and 1 cm medial to the mastoid tip and which extends vertically to the level of the top of the pinna and then curves medially toward the external occipital protuberance. The medial limb can be shortened or lengthened depending on the degree of exposure needed. This incision gives adequate exposure of the occipital bone and at the time of closure, provides sufficient galea to cover most of the upper transverse aspect of the wound. It also eliminates the need to transect the suboccipital musculature in a nonanatomic plane. The scalp flap can be easily dissected off the occipital bone with monopolar electrocautery. Caution must be used when opening the inferior portion of the vertical limb of the incision. medial to the mastoid tip. Rarely an anomalous vertebral artery can be found coursing up between the muscles and the bone. A variable number of emissary veins, connecting the external venous system of the scalp with the underlying sinuses, can be encountered during the dissection. Although they may bleed profusely they are easily controlled with coagulation and bone wax.
The craniectomy is performed using multiple burr holes followed by bone removal with a rongeur or drill. The craniectomy should be large enough to expose the sigmoid sinus laterally and the transverse sinus superiorly. It is not uncommon to encounter large venous channels during the bony dissection, especially as the sigmoid sinus is approached. Bleeding from the bone is controlled with bone wax, whereas bleeding from the sinus can be controlled with a small piece of Gelfoam . Mastoid air cells overlying the sigmoid sinus are also frequently encountered during the craniectomy. These can serve as a guide as one approaches the sinus and should be waxed thoroughly before the dura is opened. As the craniectomy is completed, the spinal drain is opened, if it was inserted.
The dural opening is started in the center of the craniectomy and is opened first in the direction of the junction of the transverse and sigmoid sinuses. A second incision is made toward the inferior aspect of the craniectomy, completing a triangle with the sigmoid sinus as its base. The resulting flap of dura is then reflected back and tacked to the cervical musculature with nylon suture. The dural opening is completed by making two more radial cuts starting from the apex of the dural triangle. The first cut extends toward the transverse sinus and the second cut extends toward the inferior medial portion of the craniectomy. The resulting smaller leaves of dura are tacked back. The object, however, is to completely open the dura to provide adequate exposure regardless, of the number of dural incisions.
At the time of closure, lyodura is used if the dural leaves do not approximate easily. By the time the dura is opened, the combination of spinal drainage and gravity has pulled the cerebellum away from the petrous bone. The patient is then rolled toward the surgeon far enough to position the petrous bone vertically. This manoeuvre, plus slight retraction with a 1-cm blade of a Greenberg retractor, exposes the arachnoid of the cerebellopontine angle. The superior petrosal vein, extending from the cerebellum to the junction of the tentorium and the petrous bone is coagulated as soon as it is visualized. Traction on this vein. which can be made up of multiple smaller vessels, can lead to troublesome, although not dangerous bleeding. An effective way to manage a disrupted vein is to cover it with a small piece of Gelfoam and a cottonoid and after the bleeding has stopped, remove the pack and coagulate the vessel. Bleeding from these veins looks quite serious in the small confines of the cerebellopontine angle but is always low-pressure bleeding and can always be managed with conservative measures.
With slight retraction of the cerebellum at its junction with the tumor, the surgeon can pull the arachnoid tight and divide it between the surface vessels of the tumor. As the surface vessels are identified, they are coagulated and dissected carefully along the arachnoid. This simple manoeuvre of opening the arachnoid establishes the critical tissue planes between the tumor and the side of the pons, as well as the lower cranial nerves and the AICA. The latter is often found buried in the arachnoid at the junction of the cerebellum and the dome of the tumor. Once a clear view of the tumor is achieved, stimulation of the exposed surface in an attempt to locate the seventh nerve is performed because the relationship of the nerve and the tumor may be variable, especially with meningiomas. With acoustic tumors the course of the nerve can also be variable, but is usually located anterior to the tumor. After the seventh nerve leaves the pontomedullary junction, it may course directly toward the internal auditory canal under the lower pole of the tumor and at other times it may course superiorly along the side of the pons up toward the root entry zone of the trigeminal nerve and follow the course of this nerve back to the petrous bone . In large tumors, the nerve is often very thin and difficult to identify, but with stimulation it is possible.
There are several possible techniques applicable to the ultimate removal of the tumor. These include the laser, the ultrasonic aspirator and bipolar coagulation with concomitant suction. Most neurosurgeons use primarily a bipolar coagulation technique with continuous irrigation and suction. This allows for a bloodless field. In large tumors, the removal begins in the center of the surface facing the surgeon. The slow and meticulous removal of the tumor internally and gradually outward allows the capsule to fall inward. This decompression of the tumor in essence changes a large tumor into a small one and allows the eventual visualization of the cranial nerves and vessels and permits the surgeon to define their relationship to the tumor. After the center of the tumor has been decompressed, the dissection is best carried out rostrally in the region of the fifth nerve. This nerve is easily identified and tolerate dissection better than the lower cranial nerves.
By following this nerve medially to identify the entrance into the brain stem, this is then followed by exposure of cranial nerves IX, X, and XI, located at the lower pole of the tumor. As the interface between the pons and the tumor becomes apparent, small pieces of Gelfoam are inserted between the two to give gentle retraction and at the same time protect the side of the pons and the related vessels. Attempts to pull on the tumor or to manipulate it without adequate tumor decompression will, under most circumstances, result in serious problems.
Special care must be exercised in the management of the ninth and tenth cranial nerves. These nerves are extremely sensitive to traction or trauma and must be protected very carefully. As soon as possible, we free them from the underlying arachnoid and tumor surface with sharp dissection and cover them with cottonoids.
After the tumor has been reduced in volume to the point at which it is anatomically free of the fifth, ninth, tenth and eleventh cranial nerves and the lateral aspect of the pons, it becomes possible to identify the seventh nerve as it exits from the brain stem beneath the choroid plexus protruding from the foramen of Luschka. This location is ventral and slightly above the root entry zone of the vestibular nerves. The facial nerve has a distinct. silvery, shiny appearance. In contrast, the vestibular nerves are dull in colour and are somewhat tan.
The removal of smaller cerebellopontine angle lesions is easier because the relationship of the lesion to most of the important structures can be defined readily. In the removal of small acoustic tumors and in large acoustic tumors after they have been reduced in size, the internal auditory canal must then be unroofed. By drilling off the roof of the canal back to the fundus, it is possible to assure a complete removal of the tumor and of equal importance, to expose a portion of the seventh nerve that is free of tumor. This serves as an excellent starting point for developing the plane between the seventh nerve and the tumor.
Unroofing of the canal is started by coagulating the dura adherent to the petrous bone. The bone can be removed with any high-speed drill. The drilling must go far enough to expose the fundus of the canal and particularly the transverse crest (which is oriented vertically in the lateral position). When viewed in the lateral position, the transverse crest separates the superior and inferior vestibular nerves superficially in the canal and the facial and cochlear nerves deep in the canal. In general, the unroofing of the canal is started with a cutting burr: the surgeon switches to a diamond drill as the outline of the canal becomes apparent. Prior to drilling, the relationship between the jugular bulb and the canal should be ascertained from the CT scan because they may be in close proximity. During the unroofing process, air cells in the posterior wall of the internal auditory canal may be opened. It is essential that these be sealed prior to closure. either with bone wax, Gelfoam, muscle, or fibrin glue, for they are a potential source of postoperative CSF leakage. Following the bony dissection of the canal. the dura of the canal is opened with microscissors. starting at the medial end.
Commonly, there is a 1- to 2-mm area in the fundus that is free of tumor and, by gently dissecting in this area, the nerves may be exposed and identified. The intracanalicular portion of the tumor generally has a loose attachment to the seventh nerve, usually at the origin: subsequent sharp and blunt dissection allows easy separation along their anatomic plane. Once the tumor has been freed from its attachments within the canal it can be dissected back to the lip of the canal. Care must be exercised by the surgeon at this stage of the operation because the seventh nerve becomes broad and thin and sometimes takes on the appearance of thickened arachnoid, especially as it goes over the edge of the lip of the canal. This thinned out portion is the point where the seventh nerve is most vulnerable.
After removal of the last remnant of tumor, air cells that have been exposed during drilling must be sealed. The sealing process can be with bone wax applied with a Penfield dissector and tamped into place with a cottonoid, If a number of air cells are opened or are difficult to seal with bone wax. a piece of Gelfoam covered with muscle and held in place with fibrin glue provides an effective seal. Extra time spent during this phase of the operation is worthwhile because it minimizes the risk of postoperative CSF leakage.
Prior to dural closure, multiple Valsalva manoeuvres are performed to confirm venous haemostasis. The subarachnoid space is irrigated until clear. An attempt is always made to close the dura primarily: however closure with lyodura or thick periosteum harvested nearby, may be necessary.
In cases where a large tumor has been removed and a potential for cerebellar swelling exists, the dura is left patulous. It is preferable to monitor postoperative intracranial pressure by way of a subdural catheter. This catheter is tunnelled out from the incision though a separate stab wound above the horizontal limb of the opening. This catheter may also serve as a CSF drain.If a small bone flap has been turned during the opening, this can be replaced. Alternatively, if the patient is concerned about cosmesis, a cranioplasty can be performed.
The scalp wound is then closed in a two-layered fashion with 2-0 Vicryl for the galea and 3-0 nylon interrupted mattress or subcuticular sutures for the skin. Care must be taken to close the muscle and fascial layers of the inferior portion of the vertical limb of the incision because this region may leak CSF if not adequately closed.
Results
Mortality rate is ranging between 1.6 - 5 percent depending upon the patients categories. Anatomical preservation of the facial nerve is ranging between 60-90 percent depending upon various factors, such as tumor size. Preservation of sound sensation also ranging between 20-66 percent.
Middle Fossa Approach
The middle fossa approach, as described by House in 1961 involves an extradural subtemporal approach with microneurosurgical unroofing of the internal auditory canal. This approach is limited to the excision of small intracanalicular tumors that have not escaped the confines of the internal auditory canal. It is usually performed in patients in whom hearing remains at a functional level, providing a chance of hearing preservation.
Translabyrinthine Approach
The microsurgical translabyrinthine approach was described by House in 1964. It exposes the posterior fossa dura in the retromeatal trigone (Trautmann's triangle) formed by the sigmoid sinus, the jugular bulb and the superior petrosal sinus. This approach is usually reserved for patients with moderate-size tumors (1.0 to 2.5 cm in diameter). Unfortunately, any preoperative auditory function is lost as a consequence of this approach.
Translabyrinthine-transtentorial Approach
The combined translabyrinthine-transtentorial approach has been used for the removal of lesions of the cerebellopontine angle. Advantages of this approach, as described by Morrison and King, include better visualization of the upper pole of the tumor, trigeminal nerve and brain stem: easier identification of the facial nerve: and minimal cerebellar retraction. The surgery is performed through a small lateral scalp flap centered on the ear with a posterior limb extending down over the mastoid process. This flap is turned to expose the mastoid process and the upper margin of the external auditory canal. A small bone flap over the temporal lobe, which exposes the sigmoid sinus, is used. The labyrinthine dissection involves removal of all of the lateral and superior semicircular canals and most of the posterior canal together with the aqueduct of the vestibule and the bone medial and inferior to the vestibule. The part of the posterior canal that lies most near the descending portion of the facial nerve is left intact. The bony dissection is continued by exposing the whole length of the sigmoid sinus down to the jugular bulb and by uncovering the posterior fossa dura, superior petrosal sinus and middle fossa dura. Finally. the superior, posterior and inferior bony margins of the internal auditory canal are removed. The dura of the canal can now be opened and the tumor freed from the facial nerve and dissected from the medial portion of the canal. At this point, the remaining bone of the floor of the middle fossa overlying the translabyrinthine exposure is removed. The dura of the temporal lobe and the posterior fossa is then opened and the superior petrosal sinus and tentorium are divided. Removal of the remaining tumor resumes, with the surgeon mobilizing the tumor from the internal auditory canal and freeing the tumor from the dural margins of the porus. With both medium and large tumors, the arachnoid covering the tumor is divided completely, with dissection proceeding from the superior aspect. The dissection is completed by internally decompressing and mobilizing the remaining tumor. Complications described following this approach include dysphasia, seizures and subdural CSF collections.
Subtemporal-Transtentorial Approach
The subtemporal-transtentorial approach as described by Rosomoff, uses a craniotomy centered low over the petrous ridge, extending anteriorly over the middle cranial fossa, superiorly to the parietal boss and posteriorly to a point midway between the mastoid process and the inion. A U-shaped dural flap based on the transverse sinus is made. The temporal lobe is retracted anteriorly and the occipital lobe is retracted posteriorly. At this point it may be necessary to divide the vein of Labbe or possibly several smaller veins draining the temporal and occipital lobes. The petrous ridge and superior petrosal sinus are followed to the edge of the tentorium, where the trochlear nerve can be identified. The tentorium is opened close to the petrosal sinus and this opening is angled back to a point behind the entrance of the trochlear nerve. Retraction of the divided tentorium provides adequate exposure of the cerebellopontine angle. In the removal of an acoustic tumor, the superior petrosal sinus is ligated and a dural flap is turned over the acoustic meatus. The roof of the canal is drilled away and the tumor is dissected free of the nerves. A technique of internal decompression with mobilization is used to remove the remaining tumor. Complications of this approach include possible injury to the trochlear nerve, inadequate exposure of the lower pole of the tumor, postoperative seizures, and temporal lobe dysfunction.
Complications
The two most common complications following surgery of the cerebellopontine angle are CSF leakage and cranial nerve palsies. Less commonly encountered complications include bacterial and aseptic meningitis, wound infection, hydrocephalus and haemorrhage.
CSF leakage most often results from a mastoid air cell opened during the craniectomy or during the drilling of the posterior wall of the internal auditory canal. Fluid then drains from these cells into the middle ear and through the eustachian tube down into the pharynx or the nose. A CSF leak may not present immediately, but may start several days following surgery when the patient is mobilized. A small drainage of clear fluid occurring in the immediate postoperative period may represent fluid that has accumulated in the mastoid air cells at the time of surgery. A drainage that persists longer than 24 h, or one that worsens with a Valsalva manoeuvre, is more likely to be a true CSF leak and should be managed aggressively. This includes placement of a lumbar drain, administration of prophylactic antibiotics, and daily measurement of CSF cell counts. After 5 to 7 days, the drain is removed and, in usual conditions, the leakage does not recur. If that is unsuccessful, patients usually undergo a mastoidectomy for obliteration of the air cells and the eustachian tube without re-exploring of the surgical site.
A CSF leak can also occur from the wound. This is usually a result of poor wound-healing, hydrocephalus, or wound infection. The treatment of the leak actually begins at the time of the initial incision. A clean, sharp incision and careful handling of the wound edges is important. Also, the surgeon must obtain a meticulous closure of the fascial layers, especially along the inferior aspect of the wound overlying the mastoid tip, where clear fascial planes are not always present. In spite of good technique, a CSF leak may still occur if intracranial pressure is elevated or if a wound infection develops. In patients with hydrocephalus, simple stitching of the wound seldom solves the problem unless the hydrocephalus is treated simultaneously. If there is no underlying infection, a ventriculoperitoneal shunt is placed. If the patient has a concurrent infection, a ventriculostomy is placed until the infection has cleared and a shunt can be inserted.
Dysfunction of cranial nerves V, VII, VIII, IX, X and (rarely) VI can be encountered following surgery of the cerebellopontine angle. Although cranial nerves IX and X are not by definition in the cerebellopontine angle, their function can become impaired with the resection of large tumors.
Postoperative facial nerve paresis of various degrees can be evident immediately following surgery. Interestingly, if there is complete anatomic disruption of the nerve at the time of surgery, the patient may be able to close the eye for a period of 24 to 48 h following surgery with a subsequent progression to complete facial paralysis. More commonly, however, the patient has a variable degree of preserved eye closure and facial movement immediately postoperatively. This function also can decline between the third and the fifth postoperative days, which may be due to oedema or ischemia of the nerve. If the facial nerve paralysis is so severe that the cornea is inadequately covered, the eye should be covered with a protective shield, and artificial tears and a lubricant given every 2 to 4 h. Further therapy depends on whether the nerve is anatomically intact and whether there is adequate coverage of the cornea. If the nerve is intact but dysfunctional and the patient has adequate eye closure, the patient should be followed by recovery of the nerve. If after 12 months the facial nerve function has not returned, then a facial reanimation procedure can be planned. Various techniques and results for facial reanimation are discussed in more detail below. Poor lid coverage of the cornea in more severe cases of facial nerve paresis can be addressed with a tarsorrhaphy or with the placement of either a spring or weight in the lid. If the facial nerve is disrupted, facial reanimation is performed early.
Fifth cranial nerve injury can follow removal of a tumor of any size. The fifth nerve function should be evaluated immediately postoperatively. If corneal sensation is diminished, the eye should be covered with a protective shield and artificial tears applied every 2 to 4 h. If corneal sensation is completely absent, the patient is at an increased risk of developing a corneal abrasion, and a tarsorrhaphy should be considered strongly.
Dysphagia with aspiration or hoarseness due to impairment of the glossopharyngeal or vagus nerves can also occur following resection of large tumors in the cerebellopontine angle. If glossopharyngeal or vagus nerve impairment is suspected, then vocal cord and pharyngeal sensation and function should be assessed as soon after extubation as possible. A modified barium swallow with video fluoroscopy is often helpful in determining oral pharyngeal function and the patient's risk of aspiration. If there is evidence of aspiration, NGT or a feeding gastrostomy tube should be placed until there is adequate recovery of these nerves.
Diplopia can occur after resection of large tumors. It is usually from abducens nerve paresis and the majority of patients improve spontaneously. Patching the affected eye can give some relief to the patient until the nerve function returns.
A postoperative fever and/or headache, with or without nuchal rigidity suggests the possibility of either bacterial or aseptic meningitis. Patients with aseptic meningitis present with symptoms several weeks after surgery, usually as their steroid dose is being tapered. Evaluation of these patients should include a CT scan and immediate lumbar puncture with the CSF analyzed for cell counts, Gram stain and cultures. The glucose levels of the CSF and the serum should be measured as well. Broad-spectrum intravenous antibiotics with good gram-positive and gram-negative coverage should be started and the steroid dose increased. If the cultures are negative after 48 h. the antibiotics can be stopped and the steroids can be slowly tapered off over several weeks.
Although postoperative epidural, subdural and intracerebellar hematomas are rare, they represent the most serious complications and if not properly diagnosed and treated, may lead to catastrophe. The diagnosis is usually not difficult in the patient who has awakened from anaesthesia and then become stuporous or comatose. During the early postoperative period, monitoring of the intracranial pressure via a subdural posterior fossa monitor may help in the early detection of a developing hematoma. If the deterioration is slow, there may be time for a CT scan: however, if the deterioration is rapid, the patient is best taken to the operating room for re-exploration.
Hydrocephalus can occur in the early postoperative period, especially in patients with a large tumor and preoperative distortion of the fourth ventricle. The patient may become symptomatic from increased intracranial pressure or may develop a full flap at the surgical site. A CT scan can confirm the diagnosis and the patient can be treated initially with a ventriculostomy. Most patients recover spontaneously and rarely is a ventriculoperitoneal shunt required.
Facial Paralysis
Several procedures have been developed to improve facial tone and motor function in patients with facial nerve paralysis. The choice of the procedure is tailored to the individual patient. The ideal treatment is intracranial end-to-end anastomosis at the time of the initial surgery. Unfortunately. most often the nerve has been attenuated or destroyed, making this impossible. The alternatives then are hypoglossal-facial, spinal accessory-facial or phrenicfacial nerve anastomosis. In cases of longstanding facial paralysis (more than 2 years) other methods must be used, which include facial dynamic or static reanimation using muscle transfers, transposition or neuromuscular pedicle grafts.
Timing of Surgery
The timing of surgery depends on the state of integrity of the facial nerve. If it is anatomically severed and cannot be repaired intracranially. it is better to wait 3 to 4 weeks, then readmit the patient for hypoglossal-facial nerve anastomosis. If the nerve is anatomically and physiologically preserved during surgery but is without postoperative function, the anastomosis is generally not done for at least a year, because about 90 percent of the patients seem to have adequate, although delayed functional facial nerve recovery.
Hypoglossal-Facial Nerve Anastomosis
This procedure is performed under general anaesthesia, with the patient supine on the operating room table and with the head turned to the contralateral side. The ear lobe is stitched up anteriorly, out of the operative field. A postauricular incision is made from half an inch above the tip of the mastoid process down in front of the sternocleidomastoid muscle, for a length of approximately 10 cm. The skin and subcutaneous tissue are opened and the fascia and platysma muscle are then divided in a longitudinal fashion. The sternocleidomastoid muscle is identified and retracted laterally. Dissection continues superiorly and medially. The cervical fascia is identified and opened. Then the posterior belly of the digastric muscle is identified and dissection is carried around it until its anteriomedial tendinous portion is identified. The hypoglossal nerve is located beneath the posterior belly of the digastric muscle. It may be identified by following the descending ansa hypoglossi up until it meets with the hypoglossal nerve.
Attention is turned to the area of the mastoid tip. Using a periosteal elevator, the digastric muscle is partially separated from the periosteum of the mastoid process. The tip of the mastoid process is rongeured away, improving visualization of the area of the styloid process and the stylomastoid foramen. Sharp dissection for the exposure of the facial nerve at its exit from the stylomastoid foramen is done. Occasionally, the surgeon must dissect through the posterior part of the parotid gland to identify this nerve. Once both the facial and hypoglossal nerves have been identified, the hypoglossal nerve is sectioned sharply at the point where it begins to branch. The facial nerve is sectioned at the stylomastoid foramen. The proximal end of the hypoglossal nerve is swung upward and posteriorly, adjacent to the posterior belly of the digastric muscle to contact the distal end of the facial nerve. Using microsurgical technique, the two ends are anastomosed using 10-0 Prolene sutures. Care must be taken to ensure that the nerve is not angulated or under tension. After the anastomosis is complete, the wound is closed in a standard fashion. Another alternative is to section half of the hypoglossal nerve and perform anastamosing so as not to loose completely the essential function of the hypoglossal nerve.
Spinal Accessory-Facial Nerve Anastomosis
The incision is identical to the one used for the hypoglossal-facial nerve anastomosis. The sternocleidomastoid muscle is identified and retracted laterally and inferiorly, exposing the posterior belly of the digastric muscle, The spinal accessory nerve may be identified entering the posterior aspect of the sternocleidomastoid muscle. To expose the distal end of the facial nerve, the technique described above is used. Once the facial nerve has been divided at the stylomastoid foramen, the spinal accessory nerve is sectioned in its most distal portion, roughly where it enters the sternocleidomastoid muscle. The proximal spinal accessory nerve is swung around superiorly and posteriorly and is sutured to the distal facial nerve.
Phrenic-Facial Nerve Anastomosis
Two incisions are used. The first is similar to the one described for the hypoglossal-facial and the spinal accessory-facial nerve anastomoses; it is used to expose the distal facial nerve right at the stylomastoid foramen. The second incision is placed approximately two finger breadths above the clavicle in the supraclavicular fossa. The sternocleidomastoid muscle is retracted medially and superiorly, exposing the anterior scalene muscle. The phrenic nerve is in front of the anterior scalene muscle, underneath the fascia. Once the nerve is identified, it is cut at its lowermost end on the anterior scalene muscle. The proximal end of the phrenic nerve is brought up underneath the sternocleidomastoid muscle and is sutured to the cut end of the facial nerve. It is recommended that the phrenic nerve be cut and brought up first. to help judge the length of facial nerve that will be needed to perform the anastomosis without tension. If there is trouble obtaining the needed length, it is always possible to perform a mastoidectomy and expose the facial nerve higher up in the temporal bone.
Results of Facial Nerve Anastomotic Procedures
with a successful procedure the facial muscular tone show that with a successful procedure the facial muscular tone shows signs of recovery at 4 to 6 months, with restoration of a symmetrical face at rest. Movement of the ipsilateral side usually appears first about the oral commissure, then progresses to the cheek, lips, and orbits over the ensuing 18 months.
Bilateral Acoustic Tumors
Bilateral acoustic tumors are pathognomonic of central neurofibromatosis. In general, the goals for surgery are preservation of brain stem function followed by preservation of facial nerve function and hearing. It is not wise to remove both tumors at one operation. In general, the larger tumor is operated on first. Removal of the tumor is carried out using the technique outlined above. The patient only returns for surgery on the second side after completely recovering from the first procedure. This includes wound healing as well as recovery of facial nerve function. In the event of facial nerve paralysis following the first operation, the second one is delayed until the nerve recovers or a facial reanimation procedure can be performed. In general, tumor removal should be carried out as soon as the tumors are found because removal of smaller tumors is associated with better results for hearing preservation.
