A Series of 112 Fully Endoscopic Resections of Vestibular Schwannomas
By Mohamed S. Kabil & Hrayr K. Shahinian
We report a consecutive series of 112 patients with unilateral vestibular schwannoma (VS) having undergone fully endoscopic resection of their tumors in the period from October, 2001 to January, 2005. Patients' outcomes were evaluated especially with regards to cochlear nerve (hearing) preservation, facial nerve preservation, postoperative complications and completeness of the resection.
The patient population consisted of 112 consecutive cases with unilateral, "De Novo" VS(s); patients with neurofibromatosis type 2 (NFT2) or with a recurrent tumor were excluded from this study. Tumors ranged in size from 0.6-5.7 cms, most tumors were less than 3 cm in diameter (mean: 2.6). This shift towards smaller and also less symptomatic tumors may be due to an increase in the awareness of patients and earlier detection of their tumors (MRI era). Tumors were removed via 1.5 cm "keyhole" retrosigmoid craniotomies.
Utilizing the fully endoscopic technique, 106/112 (95%) tumors were completely removed; subtotal removal was performed in 6/112 (5%) patients in an attempt to preserve their hearing. Anatomical preservation of the facial nerve was achieved in all of the patients and of the cochlear nerve in 83/101 (82%) hearing ears. Functionally, measurable hearing (serviceable/some) was preserved in 59/101 (58%) cases that had either "serviceable" or "some" hearing preoperatively, 2 patients that had "some" hearing preoperatively had an improvement that was more than 30 db in their hearing postoperatively. There were no major neurological complications such as quadriparesis, hemiparesis, bacterial or aseptic meningitis, lower cranial nerve deficits, or deaths.
From our experience we conclude that the endoscope is ideally suited for a minimally invasive approach for resection of vestibular schwannomas.
Endoscopic, Acoustic Neuroma, Vestibular Schwannoma
Vestibular schwannoma (VS), previously and incorrectly known as acoustic neuroma or neurinoma, is a benign overproliferation of the Schwann cells of the eighth cranial nerve sheath, usually starting at the junction between peripheral and central myelin sheath . The rate of growth of VS(s) has been categorized into one of three patterns: slow, medium, and fast. These tumors can reach a remarkable size causing severe compression of the brain stem and inducing bony changes with invasion of the surrounding pneumatic cells and marrow spaces.
The estimated incidence of VS(s) based on cadaveric dissections by Schunecht is said to be 570/100,000 . However, many of these tumors fail to become symptomatic during a patient's lifetime. A consensus statement by the National Institutes of Health in 1991 estimated that between 2-3,000 vestibular schwannomas are diagnosed each year in the Untied States, representing a symptomatic incidence of only 1/100,000. VS(s) are reported to occur in all races, they account for 6 to 10% of all primary intracranial tumors[22,30,31], and for about 71 to 90% of all CPA tumors. Most commonly VS(s) occur during the fourth and fifth decades of life and it is about two times as common in females as in males .
There are two distinct clinical presentations for VS(s): sporadic unilateral VS and hereditary bilateral VS. Bilateral schwannoma of the internal auditory canal (IAC) is often an expression of neurofibromatosis type 2 (NFT2). Neurofibromatosis type 1 (NFT1) or von Recklinghausen's neurofibromatosis is a separate genetic disease resembling NFT2, and in extremely rare occasions can cause VS [19,31].
Symptoms and signs associated with VS(s) have been known for over 150 years and are due to direct tumor compression, invasion or vascular compromise of the surrounding structures . VS(s) are commonly associated with gradual (over months or years) hearing loss indicating damage to the cochlear nerve . They may manifest with other symptoms and signs depending on the site of origin and the growth pattern of the tumor; including tinnitus, vertigo, cerebellar dysfunction, cranial nerve dysfunction, and secondary obstructive hydrocephalus .
The first successful removal of an acoustic tumor was accomplished by either Sir Charles Ballance in 1891 or Thomas Annandale in 1895 . Since that time VS surgery has greatly progressed. At the beginning of the last century, neurosurgeons were content when they merely managed to save the life of the VS patient and not about the quality of life. During the last few decades, the advent of microsurgery advanced imaging technology, and intra-operative electrophysiological monitoring has shifted the focus to facial nerve preservation and preservation of serviceable hearing; and to the target of gross total tumor removal . The introduction of the fully endoscopic technique at the turn of the last century has followed the same path, that is making VS surgery even less invasive and optimizing its safety and outcome.
Several reports have highlighted the utility of the endoscope to assist microscopic removal of acoustic neuromas [20,21]. These articles suggest that the endoscope provides improved recognition of exposed air cells and allows for more complete tumor removal by direct visualization of the IAC to remove any residual tumor out of the view of the operating microscope[20,21].
Our group has been performing endoscope assisted and fully endoscopic surgery of the CPA for Trigeminal Neuralgia, Hemifacial Spasm, Glossopharyngeal Neuralgia and other CPA tumors since 1996 [9,15-17]. We began using the endoscope to supplement our microsurgical resection of VS(s) in l998. Using the endoscope in this region we found that the maneuverability and angled lenses of the endoscope provide significant advantages in visualizing and accessing the entire tumor, while avoiding injury to the surrounding neurovascular structures and eliminating the need for any cerebellar retraction. Late in 2001 our group converted to a fully endoscopic technique for resection of VS(s). In this report we present our experience with 112 cases where the endoscope was used as the sole imaging modality in performing VS surgery.
Patients and Methods
The subjects consisted of 112 consecutive patients with unilateral VS that have undergone fully endoscopic surgical resection of their tumors in the period from October, 2001 to January, 2005. Patients with NFT2 or with a recurrent tumor were excluded from this study. This series included 2 cases that have undergone previous gamma knife radiosurgery with no symptomatic relief for one year, and 9 cases with purely intracanalicular VS(s). Assessment was based upon clinical, radiological, and audiometric examinations. Pre- and postoperative data was collected and evaluated in a database for 112 patients that have undergone fully endoscopic VS resections at the Skull Base Institute, in Los Angeles, California.
All patients were prepared for surgery by thorough clinical investigations, including audiometry, brainstem auditory evoked response (BAER), contrast-enhanced CT with bone window, and gadolinium-enhanced MRI. Postoperatively; clinical, otorhinolaryngological, and contrast-enhanced MRI follow-up examinations were scheduled at 3-6 months, 1 year, and then annually for the rest of the follow-up period. The completeness of tumor removal was judged by surgical records and postoperative MRI.
Outcomes were evaluated especially with regards to cochlear (hearing) and facial nerve preservation, postoperative complications, and the completeness of resection.
The following audiometric parameters defined the terms "serviceable" hearing, "some" hearing and "total deafness" (Table 1). All patients underwent audiometry and BAER preoperatively and again one week after the operation.
The basic surgical concept is always to debulk the tumor from within in order to relieve the pressure on the surrounding neurovascular structures. Having completed this step it is easier to appreciate the full anatomical course of the related cranial nerves and vessels and to protect them from potential damage. A space for maneuverability is thus provided as the tumor is decompressed, CSF is drained and brain becomes lax. The most adherent points between the tumor and nerves are recognized and addressed last.
The operation begins with the patient placed in a lateral "park-bench" position; the patient's head is secured in a Mayfield 3-pin head clamp. The head is then flexed and slightly rotated away from the side of the tumor.
A 3cm retroauricular incision is performed; this is followed by dissection of the soft tissues of the scalp carried down to the cranium, using electrocautery and periosteal elevators. Hooks are used to retract the skin and soft tissues. Using the Asterion as a bony anatomical landmark, a 1.5 cm craniotomy is made just inferior and medial to the confluence of the sigmoid and transverse sinuses using an Anspach- drill. Bone wax is used to fill any air cells entered during the bone dissection. A curvilinear incision is made in the dura, which is then retracted anteriorly and is held in place with sutures. The CSF is allowed to slowly drain and a combination of mild hyperventilation, mannitol and positioning allows the cerebellum to spontaneously retract, opening up a narrow path to the CPApontine angle. A 2.7 or 4mm zero degree endoscope (Storz, Culver City, CA) endoscope is then guided atraumatically along this path with minimal dissection and almost no retraction to visualize the tumor. An irrigation sheath attached to the endoscope clears blood and debris from the lens, eliminating the time consuming and unsafe practice of removing and re-inserting the endoscope. A rigid pneumatic holding arm secures the endoscope in position, allowing bimanual surgical dissection.
Upon entering the CPA the surgeon conducts a preliminary survey of the surrounding structures including the trigeminal, facial, and lower cranial nerves, as well as the regional vascular anatomy. The facial nerve is then stimulated, and its response is measured via a facial nerve monitor (Xomed, Jacksonville, FL), which remains in place for the duration of the operation to avoid injury to the facial nerve. Once the surrounding critical structures are identified tumor dissection takes place guided by a zero-degree endoscope in much the same manner as the microsurgical procedure.
Using microdissecting instruments as well as the CUSA ultrasonic dissector the interior of the tumor is excised. The dura overlying the IAC is cauterized and incised and a diamond bur is used to open the IAC, following the tumor extent laterally within the canal. In cases of smaller tumors with patients having "serviceable" hearing preoperatively, this portion of the dissection should be performed with extreme caution. Following entry into the IAC the zero-degree endoscope is removed and the thirty-degree endoscope is introduced. Tumor dissection within the IAC is guided by the angled endoscope, allowing complete visualization of the lateral extent of the tumor as it is separated from the facial nerve.
Once tumor dissection is complete, the facial nerve is once again stimulated to confirm its function. The dura is re-approximated; the bone flap replaced and secured with a resorbable plate, and the scalp is closed in anatomical layers without the use of any drains.
Following the operation, patients are typically transferred to a step-down unit or to the intensive care unit (ICU) for overnight monitoring and then discharged within 48 hrs postoperatively.
Pre- and postoperative gadolinium-enhanced MRI of a 37 years old gentleman, showing a large (4.5x3.6 cms), right sided, inhomogenously enhancing VS that was removed totally by the fully endoscopic technique.
This study included 112, unilateral, "De Novo" cases of VS; neurofibromatosis and recurrent VS(s) cases were not included in this study. Mean follow-up period was 17 months; mean operative time was 132 minutes. The size of the tumors ranged from 0.6-5.7 cms in diameter, the vast majority being less than 3 cm in diameter (mean: 2.6 cm). There were two post Gamma-Knife-radiosurgery cases, referred for surgery, after having no symptomatic relief and with tumors that demonstrated continuous growth for more than one year. There were nine totally intracanalicular tumors in this series. Total excision was the primary aim in all cases and was given the highest priority. Recurrence occurred in only 1 out of 112 patients. Demographic information is presented in Table 2.
The presenting clinical manifestations of the patients are shown in Table 3. The most common presentations were hearing deterioration or loss in 95/112 (85%) patients; postural instability in 78/112 (70%) patients; persistent headaches in 69/112 (62%) patients; tinnitus in 66/112 (59%) patients; trigeminal paresthesia or hypesthesia in 16/112 (14%); and trigeminal neuralgia in 3/112 (3%) patients. None of the latter had trigeminal manifestations on follow-up.
Only 2 patients had symptoms that could be attributed to lower cranial nerve affection, one patient had difficulty in swallowing and the other complained of persistent pharyngeal pain. There were 4 patients that had secondary mild to moderate hydrocephalus; there was no evidence of ventricular dilatation on follow-up MRI.
Facial nerve function
Facial nerve function was evaluated using House-Brackmann (H-B) grades 1-6, and was categorised as excellent (H-B grade 1/2), intermediate (H-B grade 3/4), or poor (H-B grade 5/6). Out of the 112 patients, 6 patients were initially presented with various degrees of facial paresis that ranged from Grade 1 to Grade 3. There were no cases that had a complete facial palsy at time of presentation.
Anatomical preservation of the facial nerve was achieved in 112/112 (100%) cases. One year after the tumor removal, facial nerve function and functional outcome was re-assessed. Out of 112 patients with anatomically preserved facial nerves, 106/112 (95%) showed excellent facial nerve function (97/112: HB-G1, 9/112: HB-G2); while 6/112 (5%) showed intermediate function (HB-G3).
Cochlear nerve (Hearing) preservation
With regards to hearing preservation, anatomical preservation of the cochlear nerve was possible in 83/101 (82%) cases who had preoperative "serviceable" or "some" hearing; while functional preservation, being defined as measurable hearing (serviceable/some), was possible in 59/101 (58%) of the same group. One patient with a right sided intracanalicular VS, 10mm in diameter, and with a small portion of it protruding into the CPA, initially had "some" hearing, and regained a "serviceable" hearing one week postoperatively. Another patient had an improvement of > 30 db in his hearing postoperatively ( Table 4).
The vestibular nerve was completely resected in the majority of cases; partial vestibular nerve preservation was attempted whenever possible without compromising the major goal of complete tumor removal. Subjective results of this retrospective study showed that 103/112 (92%) patients considered their gait normal after one year of the operation, thus the impairment of postural stability was mostly slight. Out of the remaining 7 patients , 5 of them had good postural stability with eyes open, but with eyes closed the postural stability was poor, 2 patients reported mild difficulties with postural stability with eyes open.
Postoperative complications, unrelated to cranial nerves are shown in Table 5, as the most important ones related to cranial nerves are separately discussed. CSF leak from the wound occurred in 2 cases, re-suturing of the wound stopped the leak (lumbar drain was not required). One patient had delayed (1 month after surgery) retrograde CSF rhinorrhea (through a patent Eustachian tube) and eventually needed re-exploration of the wound and waxing of the mastoid air cells.
One patient developed postoperative ventricular dilatation on the second day of surgery and was obtunded; a temporary ventricular catheter for diversion of CSF pathway was placed for 5 consecutive days till the condition resolved. Exposure keratitis occurred in 2 patients and was treated with a "gold-weight" placed over the affected eyelid till the facial nerve function eventually improved. Superficial wound infection was encountered in 2 patients and was treated conservatively in 1 patient the other required re-opening of the wound for disinfection and trimming of its edges; one patient had low grade infection that manifested itself 20 days postoperatively and required debridement of the wound.
There were 69 patients that presented with persistent headaches. On follow-up, 9 patients still complained of headaches, while in 60 patients the headaches resolved within the early postoperative period. Patients that did not complain of headache preoperatively did not have a headache complaint postoperatively or on follow-up.
There were no major neurological complications such as postoperative hemorrhage, quadriparesis, hemiparesis, bacterial or aseptic meningitis, lower cranial nerve deficits, or deaths.
The diagnosis of VS has changed considerably during the last years as MRI has become the preferred diagnostic toolComputerized tomography (CT) was previously used as a reliable method for detection and assessment of VS(s), but contrast medium is needed to detect the tumor on CT because of the minimal density differences between VS(s) and the brain tissue. Magnetic Resonance Imaging (MRI) is today the standard in assessing patients with possible VS, MRI is superior to CT because on MRI bony structures appear black and cause less artifacts .
In our series we have realized that the dominance is shifting towards smaller tumors with minor symptoms accompanied by a corresponding increase in the awareness of patients. The development of operative and imaging techniques during the years of the present study along with the smaller tumor sizes, has definitely influenced the postoperative results.
Since major complications such as hemorrhage, meningitis, brainstem compression injuries and cerebrospinal fluid (CSF) leak are becoming less common, quality of life issues such as disequilibrium, headache, and hearing preservation are becoming more important . Furthermore, there is a general agreement that completeness of resection and preservation of the facial nerve are the major goals [8,23] and they are being met at increasing rates.
The pioneering effort of William House has paved the way for renewed optimism in the successful management of VS(s) with a low mortality and acceptable morbidity. Drake, Rand and Kurze, and others showed that acoustic tumors could be removed totally, safely, and with preservation of the facial nerve. Rand and Kurze discussed the possibility of anatomic preservation of the cochlear nerve, which they subsequently demonstrated in 1968. Subsequent reports on preservation of cochlear nerve function have since appeared in the literature.
The retrosigmoid approach to the posterior fossa, first described by Cohen in 1992 is actually a modification of the suboccipital craniotomy or craniectiomy. Its advantages include that it gives a wide view of the posterior fossa and offers a good chance of cranial nerve preservation. The fully endoscopic approach is based on the very same principles of this approach. However, disadvantages of the operating microscope with its direct forward view include the inability to completely visualize the lateral extent of the tumor within the IAC. Using the operating microscope, it is virtually impossible to "look around the corner" due to the oblique angle of the canal in relation to the trajectory of the dissection[20,21], as well as incomplete visualization of exposed air cells, which may lead to CSF rhinorrhea.
We have found that the fully endoscopic approach allows for the possibility of hearing preservation, provides excellent visualization of the entire tumor, avoids blind dissection behind the facial nerve, and is well tolerated by patients with minimal discomfort. We believe that the endoscopic approach allowed for smaller craniotomies, required less dissection, and virtually no cerebellar retraction. During the last 3 years, the fully endoscopic approach has been performed routinely for all patients with VS at our institute.
Facial nerve preservation
In a study by Hardy et al., in 1989 using a Translabyrinthine (TL) approach, they were able to preserve the facial nerve in 82% of the 98 patients with intact nerve preoperatively. Postoperatively 77 patients assessed their life quality to be excellent.
In this series, at the time of discharge from the hospital most of the patients had satisfactory facial nerve function with complete eye closure. After one year 106/112 (95%) still showed excellent facial nerve function (97/112: HB-G1, 9/112: HB-G2), and 6/112 (5%) showed intermediate function (HB-G3).
Inspite of the high rate of facial nerve preservation in this study; anatomical preservation of the facial nerve with complete tumor removal, especially in patients with large tumors, is still a challenge. Facial nerve monitoring has greatly aided separation of the facial nerve from the tumor[20,21,2,18,27]. When 10% of the functioning motoneurons are intact normal facial nerve function is preserved .
Cochlear nerve (Hearing) preservation
Cochlear nerve preservation has been reported by many authors [18,20,21,27,34] in the recent past but there is a lot of ambiguity on the criteria for 'useful hearing'. In addition, there are some who believe that the goal of 'gross total tumor removal' cannot be achieved with cochlear nerve preservation .
In this report, our relatively high rate of functional hearing preservation (59/101) (58%) patients does indeed reflect the better outcome associated with the better visualization of the angled and zero-degree endoscope. By opening the IAC under direct visualization and by gradual tumor reduction as long as brain stem auditory evoked potentials were maintained, we continued to perform total removal of the tumor. The fact that most tumors in this series were less than 3 cm in diameter and presented early with minimal symptoms does have a direct impact on the surgical outcome and prognosis. However, the same principles have been applied to larger tumors.
Although vestibular nerve preservation is virtually impossible in the majority of casesas most of VS(s) arise from the vestibular nerve, this may not constitute a problem. According to Wiet et al (1992) vestibular function has already been considerably reduced or even totally lost before VS surgery and immediate postoperative vertigo is usually minimal and transient and it seldom causes significant disability.28 Vertigo and imbalance postoperatively may be caused by vestibular or cerebellar dysfunction and may persist longer in older patients [23,33].
Completeness of resection
In the majority of cases a VS originates from the vestibular nerve and only compresses the cochlear nerve . More than 95% of VS(s) arise from vestibular fascicles; Samii et al in their series of 1000 acoustic neuromas have observed that only 1.1% of the CPA tumors arise from cochlear nerve. Therefore, resection of a macroscopically intact cochlear nerve in an attempt to seek complete tumor removal is not advised by many authors and this idea is supported by growing evidence; as that nerve never shows any tumor recurrences and would function well for decades[2,3].
In our series, by opening the IAC under direct visualization of the zero-degree and the angled endoscope, and by gradual reduction of the tumor as long as brain stem auditory evoked potentials were satisfying we continued to perform total removal of the tumor. Relevant tumor remnants were not left behind for the sake of hearing preservation but rather were followed and carefully excised. Subtotal tumor removal in this study was done for 6/112 (5%) patients and was performed only when a tumorous cochlear nerve was encountered in a patient that has "serviceable" or "some" hearing. In these cases the benefit of total removal should be carefully weighed against the risk of loosing a functioning cochlear nerve.
The importance of complete tumor removal and the effect it has on recurrence is well known. In 1989, Hardy et al.  reported only 3 perioperative deaths among 100 TL VS(s) operations, and the postoperative morbidity was low. Complete tumor excision was achieved in 97% and no recurrences were seen during follow up of 1 to 7 years. In our endoscopic series, although the follow-up period has obviously been short (mean: 17 months), the first results are encouraging and deserve to be studied further in a more comprehensive survey. The minor incidence of recurrences 1/112 suggests that total removal of these tumors, facilitated by direct visualization of the endoscope offers a high chance of not recurring.
Cerebellar and brain stem injuries are the major and most feared complications of the retrosigmoid approach[27,33] Recurrence of the tumor occurred in a single case after an 18 month period. Post operative complications, excluding those related to cranial nerves were minimal (Table 5). Major complications such as postoperative hemorrhage, quadriparesis, hemiparesis, bacterial or aseptic meningitis, lower cranial nerve deficits, or deaths did not occur.
Advances in fiber optic technology, microinstrumentation and minimally invasive techniques have allowed the field of skull base surgery to evolve from traditional neurosurgical, neuro-otologic and craniofacial techniques, to more functional minimally invasive endoscopic approaches that have resulted in shorter hospitalizations and overall excellent outcomes. Two important factors with regards to predicting the preservation of cranial nerves VII and VIII are tumor size (less than 3 cms) and preoperative hearing status.
Improved diagnostic screening with MRI and also a better informed population have resulted in the diagnosis of smaller and even non symptomatic VS(s)[26,28]. Moreover, new applications for intracranial endoscopic surgery continue to evolve. The endoscope provides improved visualization of the skull base, where narrow recesses and angled trajectories impair the direct forward view of the operating microscope. Endoscopic surgery allows for smaller craniotomies, less dissection and minimal retraction, without compromising the goals of the operation.
Notwithstanding the fact that microsurgical techniques have made a significant contribution in the advancement of skull base surgery, the fully endoscopic techniques continue to follow the same path. The success of angled endoscopes to assist microscopic removal of VS(S) has ultimately encouraged the progress to fully endoscopic brain surgery.
We believe that the improved exposure of the entire tumor provided by the endoscope with minimal or no retraction reduces the risk of injury to the brainstem and the surrounding cranial nerves, and results in a complete tumor removal. The more direct route "keyhole" approach has significantly decreased the time required for exposure of the tumor and the overall operative time (132 minutes) in this series. This minimally invasive technique allowed rapid recovery of the patients (mean LOS: 2.2 days) and resulted in minimal postoperative discomfort.
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