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Suboccipital resection of a medial acoustic neuroma with hearing preservation
Suboccipital Resection of a Medial Acoustic Neuroma with Hearing Preservation William E. Snyder, M.D., Michael B. Pritz, M.D., PH.D., and Richard R. Smith, M.D. Sections of Neurological Surgery and Neuroradiology, Indiana University School of Medicine, Indianapolis, Indiana
Snyder WE, Pritz MB, Smith RR. Suboccipital resection of a medial acoustic neuroma with hearing preservation. Surg Neurol 1999;51:548 –53. BACKGROUND
Many reports indicate that acoustic neuromas greater than 2.0 cm should be removed without hearing preservation attempted, even if hearing is present preoperatively. These studies advocate a translabyrinthine approach because the likelihood of hearing preservation is low. Medial acoustic neuromas, unlike the more common lateral tumors that involve the internal auditory canal, originate medial to that portion of the eighth nerve complex where the cochlear and vestibular nerves are fused. This anatomical feature suggests that these tumors may be amenable to resection with hearing preservation. METHODS
A patient with a 3.5 cm medial acoustic neuroma and useful preoperative hearing is presented. RESULTS
Gross total tumor removal with functional hearing was achieved after a two-stage procedure using a suboccipital approach.
location can vary even in the same individual [12]. This anatomic site is frequently located in the internal auditory canal (IAC) or at the porus acousticus, resulting in the tumor filling and eroding the canal and porus as it enlarges. However, the Obersteiner-Redlich zone may be medial to the porus resulting in a medial vestibular schwannoma that does not erode the porus and thus allows the tumor to reach a large size with brainstem compression before it causes recognizable symptoms [5,14]. Review of the literature reveals that large, medial vestibular schwannomas are uncommon and have been most frequently removed via a translabyrinthine approach that sacrifices hearing [11,14] because tumor size was such that hearing preservation was thought to be unlikely [13]. We present a case in which a medial vestibular schwannoma, 3.5 cm in maximal diameter, was resected in a twostage procedure with preservation of useful hearing.
CONCLUSION
Based on the anatomico-pathologic features in this case, we believe that, if a patient has reasonable preoperative hearing (speech discrimination score . 70%) and a medial acoustic neuroma, an approach to preserve hearing should be considered regardless of tumor size. © 1999 by Elsevier Science Inc. KEY WORDS
Acoustic neuroma, hearing preservation, medial tumor, suboccipital approach, vestibular schwannoma.
estibular schwannomas arise from the junction of the central and peripheral myelin known as the Obersteiner-Redlich zone [5]. This junction is typically 10 –13 mm from the brainstem in males and 7–10 mm in females. However, its
V
Address reprint requests to: Dr. Michael B. Pritz, Section of Neurological Surgery, Indiana University School of Medicine, 545 Barnhill Drive, Emerson 139, Indianapolis, Indiana 46202-5124. Received February 18, 1997; accepted November 3, 1997. 0090-3019/99/$–see front matter PII S0090-3019(97)00528-4
CASE REPORT A 51-year-old, right-handed woman was evaluated because of a 6-week history of periodic vertigo brought on by head turning, which was also associated with ataxia. In addition, the patient had decreased hearing in her right ear, nausea and vomiting at night, and a 15-year history of occipital headaches. She was referred for evaluation after a brain magnetic resonance (MR) scan showed an enhancing mass, 3.5 cm in maximal diameter, involving the pons and extending into the cerebellopontine angle on the right side. This lesion was associated with significant surrounding edema and ventriculomegaly (Figures 1 and 2). The porus acousticus was not involved (Figure 3). The visible eighth nerve was normal in size and configuration and did not enhance with contrast administration (Figure 2B). The preoperative diagnosis was pon© 1999 by Elsevier Science Inc. 655 Avenue of the Americas, New York, NY 10010
Medial Acoustic Neuroma Resection
Surg Neurol 549 1999;51:548 –53
Axial computed tomographic images through each IAC (asterisks) show bilateral symmetry and no abnormality.
3
tine exophytic glioma or ependymoma with extension into the right cerebellopontine angle. Brainstem auditory-evoked responses (BAERs) andaudiometry were performed preoperatively to obtain baseline levels. The BAERs showed prolongation of the right I-III interval with monaural stimulation, which was consistent with a right pontine lesion. Audiometry (high-frequency pure tone hearing, speech discrimination score, and speech reception thresholds) was normal (Figure 4).
Preoperative axial MR. T1-weighted images (A and B) demonstrate a hypointense mass in the region of the brachium pontis and foramen of Luschka with distortion of the fourth ventricle. T2-weighted images (C and D) show a heterogeneous hyperintense mass with surrounding edema in the right cerebellar white matter. The right internal auditory canal (arrow in D) is normal. Axial T1weighted postcontrast images (E and F) show heterogeneous tumor enhancement.
1
FIRST OPERATION Because the tumor was thought to be a malignancy and complete tumor excision unlikely, a right frontal ventriculostomy was placed first. Then, a standard suboccipital craniectomy with the patient in the lateral position was performed (MBP). Evoked potentials (BAERs and somatosensory-evoked potentials [SSEPs]) were used intraoperatively. The tumor indented the pons and arose from the eighth nerve complex medially as it left the brainstem. Frozen sections revealed that the tumor was a schwannoma. The tumor was substantially debulked using standard microsurgical technique until a change in the BAERs occurred that persisted despite removal of the retractors and 10 minutes of waiting. Therefore, surgery was electively terminated and the wound closed. The BAERs subse-
Preoperative coronal MR. T2-weighted image (A) shows the normal right IAC (arrow). Postcontrast T1-weighted image (B) illustrates the enhancing tumor (asterisk) separate from the normal IAC (arrow).
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550 Surg Neurol 1999;51:548 –53
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Preoperative (A) and postoperative (B) audiometry.
quently returned to baseline by the completion ofsurgery. Postoperatively, the patient’s hearing was unchanged in the right ear and her neurologic examination was otherwise normal. Her postoperative course was unremarkable. The ventriculostomy was removed on the fourth postoperative day and she was discharged 2 days later. SECOND OPERATION The patient returned to the operating room approximately 3 weeks after her initial procedure. Immediately before her second surgery, BAERs were repeated and showed continued asymmetry with right sided I-III interval prolongation with monaural stimulation consistent with a right-sided pontine lesion. Audiometry was also performed, which showed 96% speech discrimination score and a 10decibel speech receptive threshold on the right with only a slight deficit in high-frequency pure tone hearing at 6 megahertz in the right ear. Using the previous right suboccipital craniectomy, gross total tumor resection was achieved (MBP). Intraoperatively, SSEPs were unchanged, but BAERs showed right-sided monaural stimulation changes in Waves III-V until there was total absence of these waves. Frozen and permanent sections confirmed the diagnosis of schwannoma. Postoperatively, the patient again had clinically functional hearing in the right ear, no facial weakness, and no new neurologic deficits. Her postoperative course was unremarkable and she was discharged on the sixth postoperative day.
Brainstem auditory-evoked potentials were repeated 4 months after her second surgery and were found to be normal and symmetrical. Audiometry was also repeated. It showed a 96% speech discrimination score and 10-decibel speech reception threshold reading and only slight decrease in the high frequency pure tone hearing in the right ear at 6 megahertz (Figure 4B). Brain MR done 1 year (Figure 5) and 2.5 years after the second operation showed no evidence of tumor.
DISCUSSION Vestibular schwannomas, the most common cerebellopontine angle tumor, arise from the Obersteiner-Redlich zone, which is located at the porus acousticus or in the meatus in 44% of patients, or just medial to the porus in 56% of patients [6]. As the tumor grows to expand in the IAC, it widens the IAC and porus before it grows in the path of least resistance into the cerebellopontine angle [5]. A small percentage of vestibular schwannomas arise from the eighth nerve medial to the porus acousticus and do not extend into the IAC [5,14]. In a recent review of 400 consecutive cases of acoustic neuromas, 48 tumors (12%) were found to have little or no involvement of the IAC. Instead, they were located extra-meatally and involved the cerebellopontine angle [14]. Fifteen of these medial tumors had a completely normal porus and IAC, representing 3.7% of this series. These tumors were
Medial Acoustic Neuroma Resection
Surg Neurol 551 1999;51:548 –53
Postoperative axial postcontrast T1-weighted images. Complete tumor resection (A) and normal IAC and eighth nerves (arrows in B) are shown.
5
termed medial acoustic neuromas and were believed to represent a new clinical entity because of the lack of literature describing these lesions [14]. All of these tumors were removed via a translabyrinthine approach that sacrificed hearing because the authors believed that functional hearing could not be preserved [14]. Although functional hearing may not be preserved after resection of a medial acoustic neuroma, certain anatomical features of the eighth nerve complex may make hearing preservation favorable in this group of patients even if the tumor is greater than 2.0 cm in diameter. Histologic analysis has found that the vestibular and cochlear nerves are fused into a common trunk 4.5 mm medial to the vertical crest. This relationship is maintained until the nerve reaches the brainstem [6]. Where the cochlear and vestibular nerves are fused into a single trunk, no boundary to tumor infiltration of the cochlear nerve is present. However, where distinct separation of the cochlear and vestibular nerves is present, tumor infiltration would be less likely. Accordingly, tumors arising laterally in the IAC or medially adjacent to the brainstem would be the least likely to infiltrate the cochlear nerve. Thus, tumors located at the extremes of the course of the eighth nerve complex would have the greatest potential for hearing preservation because cochlear nerve infiltration by tumor would be least likely. Although surgical dissection of a medial acoustic neuroma from the brainstem is no different a procedure than removal of a tumor originating from any other part of the vestibular nerve, this anatomical feature of separation of the cochlear and vestibular nerves medially suggests that a medial acoustic neuroma, regardless of size, may be removed with hearing preserved. Despite the lack of correlation between tumor size and preoperative hearing, considerable evidence indicates that tumor size does affect postoperative hearing [1– 4,7,9,10,15]. Nevertheless, a medial acoustic neuroma may allow for hearing preservation because the tumor is medial to that
portion of the eighth nerve complex where the nerves are fused [6,8]. Our patient had a speech discrimination score of 96% with a speech reception threshold of 10 decibels preoperatively. After a staged suboccipital procedure, she had gross total resection of a 3.5 cm medial acoustic neuroma with normal audiometry postoperatively. Accordingly, we feel that a patient with a medial acoustic neuroma and good preoperative hearing should be offered a procedure that can preserve hearing. REFERENCES 1. Ebersold MJ, Harner SG, Beatty CW, Harper CM Jr, Quast LM. Current results of the retrosigmoid approach to acoustic neurinoma. J Neurosurg 1992;76: 901–9. 2. Fischer G, Fischer C, Remond J. Hearing preservation in acoustic neurinoma surgery. J Neurosurg 1992;76: 910 –17. 3. Glasscock ME, Hays JW, Minor LB, Haynes DS, Carrasco VN. Preservation of hearing in surgery for acoustic neuromas. J Neurosurg 1993;78:864 –70. 4. Gormley WB, Sekhar LN, Wright DC, Kamerer D, Schessel D. Acoustic neuromas: Results of current surgical management. Neurosurgery 1997;41:50 – 60. 5. Lanser MJ, Sussman SA, Frazer K. Epidemiology, pathogenesis, and genetics of acoustic tumors. Otolaryngol Clin North Am 1992;25:499 –519. 6. Neely JG. Is it possible to totally resect an acoustic tumor and conserve hearing? Otolaryngol Head Neck Surgery 1984;92:162–7. 7. Post KD, Eisenberg MB, Catalano PJ. Hearing preservation in vestibular schwannoma surgery: What factors influence outcome. J Neurosurg 1995;83:191– 6. 8. Rasmussen AT. Studies of the VIIIth cranial nerve of man. Laryngoscope 1940;50:67– 83. 9. Samii M, Matthies C. Management of 1000 vestibular schwannomas (acoustic neuromas): hearing function in 1000 tumor resections. Neurosurgery 1997;40:248 – 62. 10. Silverstein H, McDaniel AB, Norrell H. Hearing preservation after acoustic neuroma surgery using intraoperative direct eighth cranial nerve monitoring. Amer J Otol 1985;[Suppl]6:99 –106. 11. Silverstein H, McDaniel A, Norrell H, Haberkamp T. Hearing preservation after acoustic neuroma surgery with intraoperative direct eighth cranial nerve mon itoring: Part II. A classification of results. Otolaryngol Head Neck Surg 1986;95:285–91.
552 Surg Neurol 1999;51:548 –53
Snyder et al
12. Skinner HA. The origin of acoustic nerve tumours. Br J Surg 1929;16:440 – 63. 13. Smith MFW. Conservation of hearing in acoustic schwannoma surgery. Amer J Otol 1985;[Suppl]6:161– 3. 14. Tos M, Drozdziewicz D, Thomsen J. Medial acoustic neuromas. A new clinical entity. Arch Otolaryngol Head Neck Surg 1992;118:127–33. 15. Whittaker CK, Luetje CM. Vestibular schwannomas. J Neurosurg 1992;76:897–900.
COMMENTARY
The authors are to be commended on the excellent results they have achieved in this case report. I would add to their conclusion that an approach to save hearing in these circumstances should also be done when the preoperative speech discrimination is less than 70% and there is poor or absent hearing in the opposite ear [1]. Presumably the authors did not expose the medial part of the internal auditory canal. In my experience, most of the types of tumors that they describe have at least a small extension into the canal, which may not be seen on MRI, that requires a limited bone exposure for removal. This report again raises the question of the value of intraoperative monitoring of auditory function. A change in the BAEPs (details not specified) caused the first operation to be terminated, although the potentials had returned to baseline by the end of the operation and there was no change in hearing. At the second operation, Waves III-V were lost (presumably Wave I was still present), but apparently these changes did not alter the dissection. Four months later, BAEPs were normal. In our experience, when using a transtympanic electrode and BAEPs, if Wave I was present at the end of the operation but Wave V was lost or never detected, the results of hearing were unpredictable [2]. In a small number of patients, changes in the potentials have led to a change in the surgical procedures, but monitoring has not made a difference when the potentials are abruptly or rapidly lost. The value of monitoring in those patients in whom there is no change in the intraoperative potentials is unknown. Robert G. Ojemann, M.D. Department of Neurosurgery Massachusetts General Hospital Boston, Massachusetts REFERENCES 1. Ojemann RG. Comment on: Fischer G, Costantini JL, Mercier P. Improvement of hearing after microsurgical removal of acoustic neurinoma. Neurosurgery 1980;7: 158 –9.
2. Ojemann RG. Management of acoustic neuromas (vestibular schwannoma). Clin Neurosurg 1993;40:498 –535.
Based on a case report with hearing preservation in a large, so-called “medial” acoustic neurinoma, the authors hypothesize an increased chance of functional cochlear nerve preservation in this entity, and they advocate a surgical approach with a nerve conserving strategy in such situations. More and more of our colleagues are describing hearing preservation in acoustic neurinomas— even in large ones. This is one more report in that series, definitely a positive development. The theory regarding the improved chance of hearing preservation in medial neurinomas (i.e., in cases that do not involve the meatus) is probably a complex matter. Apparently the connection of the cochlear nerve to the vestibular nerve (the tumor origin) is closest just medial to the porus; therefore, separation of the tumorous vestibular nerve from the cochlear nerve seems most difficult, while it would be easier medially toward the brainstem or lateral to the fundus where the nerves enter separate channels. Regarding the pathoanatomical situation, the same is true for the facial nerve: it is most easily separated from the tumorous vestibular nerves within the lateral segment of the meatus and at the brainstem, while the greatest difficulty is found just medial to the porus. These tight connections are not necessarily attributable to tumorous infiltration, rather to the lack of a separating arachnoid sheath between these nerves. The authors’ conclusion that tumors located purely medial or lateral to this area of tight connection are best for conserving hearing at surgery is not quite true. In fact, the chance of hearing preservation in pure intrameatal tumors is less than in intrameatal tumors with small or medium extrameatal extension; the chance is greatly reduced in tumors with far lateral extension. One reason is the sensitivity of the cochlear nerve origin at the fundus and its vulnerability to medial stretching; another is the harder consistency of intrameatal tumors. Still, the relationship of the tumor and brainstem plays a major role. We have encountered many cases of functional deterioration or loss of auditory brainstem responses at medial dissection after successful lateral tumor mobilization. The importance of the medial acoustic tumor is based on its incidence, which has been reported to be as high as 3.7%. Up to now, we have not encountered it in our vestibular schwannomas. A lack of intrameatal extension has only been found in some large tumors extending up to the VII and VIII nerve bundle and originating from the caudal cranial
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nerves. It should be stressed that widening of the IAC is not the only sign of intrameatal involvement; in many cases shortening of the posterior auditory wall is the only objective sign. Intrameatal contrast uptake is often different from that of extrameatal tumors and therefore direct tumor visualization may be difficult even on MRI. We would like to encourage the increased awareness of the chance of hearing preservation in large
schwannomas produced by this case report, and the challenge it poses to others to find favorable options for treating them. Madjid Samii, M.D. Matthies Cordula, M.D. Department of Neurosurgery Krankenhaus Nordstadt Hannover, Germany
epression frequently co-occurs with a variety of other physical illnesses, including heart disease, stroke, cancer, and diabetes, and also can increase the risk for subsequent physical illness, disability, and premature death. Depression in the context of physical illness, however, is often unrecognized and untreated. Furthermore, depression can impair the ability to seek and stay on treatment for other medical illnesses. NIMH research suggests that early diagnosis and treatment of depression in patients with other physical illnesses may help improve overall health outcome.
D
—Depression Fact Sheet National Institute of Mental Health
Snyder WE, Pritz MB, Smith RR. Suboccipital resection of a medial acoustic neuroma with hearing preservation. Surg Neurol 1999;51:548 –53. BACKGROUND
Many reports indicate that acoustic neuromas greater than 2.0 cm should be removed without hearing preservation attempted, even if hearing is present preoperatively. These studies advocate a translabyrinthine approach because the likelihood of hearing preservation is low. Medial acoustic neuromas, unlike the more common lateral tumors that involve the internal auditory canal, originate medial to that portion of the eighth nerve complex where the cochlear and vestibular nerves are fused. This anatomical feature suggests that these tumors may be amenable to resection with hearing preservation. METHODS
A patient with a 3.5 cm medial acoustic neuroma and useful preoperative hearing is presented. RESULTS
Gross total tumor removal with functional hearing was achieved after a two-stage procedure using a suboccipital approach.
location can vary even in the same individual [12]. This anatomic site is frequently located in the internal auditory canal (IAC) or at the porus acousticus, resulting in the tumor filling and eroding the canal and porus as it enlarges. However, the Obersteiner-Redlich zone may be medial to the porus resulting in a medial vestibular schwannoma that does not erode the porus and thus allows the tumor to reach a large size with brainstem compression before it causes recognizable symptoms [5,14]. Review of the literature reveals that large, medial vestibular schwannomas are uncommon and have been most frequently removed via a translabyrinthine approach that sacrifices hearing [11,14] because tumor size was such that hearing preservation was thought to be unlikely [13]. We present a case in which a medial vestibular schwannoma, 3.5 cm in maximal diameter, was resected in a twostage procedure with preservation of useful hearing.
CONCLUSION
Based on the anatomico-pathologic features in this case, we believe that, if a patient has reasonable preoperative hearing (speech discrimination score . 70%) and a medial acoustic neuroma, an approach to preserve hearing should be considered regardless of tumor size. © 1999 by Elsevier Science Inc. KEY WORDS
Acoustic neuroma, hearing preservation, medial tumor, suboccipital approach, vestibular schwannoma.
estibular schwannomas arise from the junction of the central and peripheral myelin known as the Obersteiner-Redlich zone [5]. This junction is typically 10 –13 mm from the brainstem in males and 7–10 mm in females. However, its
V
Address reprint requests to: Dr. Michael B. Pritz, Section of Neurological Surgery, Indiana University School of Medicine, 545 Barnhill Drive, Emerson 139, Indianapolis, Indiana 46202-5124. Received February 18, 1997; accepted November 3, 1997. 0090-3019/99/$–see front matter PII S0090-3019(97)00528-4
CASE REPORT A 51-year-old, right-handed woman was evaluated because of a 6-week history of periodic vertigo brought on by head turning, which was also associated with ataxia. In addition, the patient had decreased hearing in her right ear, nausea and vomiting at night, and a 15-year history of occipital headaches. She was referred for evaluation after a brain magnetic resonance (MR) scan showed an enhancing mass, 3.5 cm in maximal diameter, involving the pons and extending into the cerebellopontine angle on the right side. This lesion was associated with significant surrounding edema and ventriculomegaly (Figures 1 and 2). The porus acousticus was not involved (Figure 3). The visible eighth nerve was normal in size and configuration and did not enhance with contrast administration (Figure 2B). The preoperative diagnosis was pon© 1999 by Elsevier Science Inc. 655 Avenue of the Americas, New York, NY 10010
Medial Acoustic Neuroma Resection
Surg Neurol 549 1999;51:548 –53
Axial computed tomographic images through each IAC (asterisks) show bilateral symmetry and no abnormality.
3
tine exophytic glioma or ependymoma with extension into the right cerebellopontine angle. Brainstem auditory-evoked responses (BAERs) andaudiometry were performed preoperatively to obtain baseline levels. The BAERs showed prolongation of the right I-III interval with monaural stimulation, which was consistent with a right pontine lesion. Audiometry (high-frequency pure tone hearing, speech discrimination score, and speech reception thresholds) was normal (Figure 4).
Preoperative axial MR. T1-weighted images (A and B) demonstrate a hypointense mass in the region of the brachium pontis and foramen of Luschka with distortion of the fourth ventricle. T2-weighted images (C and D) show a heterogeneous hyperintense mass with surrounding edema in the right cerebellar white matter. The right internal auditory canal (arrow in D) is normal. Axial T1weighted postcontrast images (E and F) show heterogeneous tumor enhancement.
1
FIRST OPERATION Because the tumor was thought to be a malignancy and complete tumor excision unlikely, a right frontal ventriculostomy was placed first. Then, a standard suboccipital craniectomy with the patient in the lateral position was performed (MBP). Evoked potentials (BAERs and somatosensory-evoked potentials [SSEPs]) were used intraoperatively. The tumor indented the pons and arose from the eighth nerve complex medially as it left the brainstem. Frozen sections revealed that the tumor was a schwannoma. The tumor was substantially debulked using standard microsurgical technique until a change in the BAERs occurred that persisted despite removal of the retractors and 10 minutes of waiting. Therefore, surgery was electively terminated and the wound closed. The BAERs subse-
Preoperative coronal MR. T2-weighted image (A) shows the normal right IAC (arrow). Postcontrast T1-weighted image (B) illustrates the enhancing tumor (asterisk) separate from the normal IAC (arrow).
2
550 Surg Neurol 1999;51:548 –53
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Snyder et al
Preoperative (A) and postoperative (B) audiometry.
quently returned to baseline by the completion ofsurgery. Postoperatively, the patient’s hearing was unchanged in the right ear and her neurologic examination was otherwise normal. Her postoperative course was unremarkable. The ventriculostomy was removed on the fourth postoperative day and she was discharged 2 days later. SECOND OPERATION The patient returned to the operating room approximately 3 weeks after her initial procedure. Immediately before her second surgery, BAERs were repeated and showed continued asymmetry with right sided I-III interval prolongation with monaural stimulation consistent with a right-sided pontine lesion. Audiometry was also performed, which showed 96% speech discrimination score and a 10decibel speech receptive threshold on the right with only a slight deficit in high-frequency pure tone hearing at 6 megahertz in the right ear. Using the previous right suboccipital craniectomy, gross total tumor resection was achieved (MBP). Intraoperatively, SSEPs were unchanged, but BAERs showed right-sided monaural stimulation changes in Waves III-V until there was total absence of these waves. Frozen and permanent sections confirmed the diagnosis of schwannoma. Postoperatively, the patient again had clinically functional hearing in the right ear, no facial weakness, and no new neurologic deficits. Her postoperative course was unremarkable and she was discharged on the sixth postoperative day.
Brainstem auditory-evoked potentials were repeated 4 months after her second surgery and were found to be normal and symmetrical. Audiometry was also repeated. It showed a 96% speech discrimination score and 10-decibel speech reception threshold reading and only slight decrease in the high frequency pure tone hearing in the right ear at 6 megahertz (Figure 4B). Brain MR done 1 year (Figure 5) and 2.5 years after the second operation showed no evidence of tumor.
DISCUSSION Vestibular schwannomas, the most common cerebellopontine angle tumor, arise from the Obersteiner-Redlich zone, which is located at the porus acousticus or in the meatus in 44% of patients, or just medial to the porus in 56% of patients [6]. As the tumor grows to expand in the IAC, it widens the IAC and porus before it grows in the path of least resistance into the cerebellopontine angle [5]. A small percentage of vestibular schwannomas arise from the eighth nerve medial to the porus acousticus and do not extend into the IAC [5,14]. In a recent review of 400 consecutive cases of acoustic neuromas, 48 tumors (12%) were found to have little or no involvement of the IAC. Instead, they were located extra-meatally and involved the cerebellopontine angle [14]. Fifteen of these medial tumors had a completely normal porus and IAC, representing 3.7% of this series. These tumors were
Medial Acoustic Neuroma Resection
Surg Neurol 551 1999;51:548 –53
Postoperative axial postcontrast T1-weighted images. Complete tumor resection (A) and normal IAC and eighth nerves (arrows in B) are shown.
5
termed medial acoustic neuromas and were believed to represent a new clinical entity because of the lack of literature describing these lesions [14]. All of these tumors were removed via a translabyrinthine approach that sacrificed hearing because the authors believed that functional hearing could not be preserved [14]. Although functional hearing may not be preserved after resection of a medial acoustic neuroma, certain anatomical features of the eighth nerve complex may make hearing preservation favorable in this group of patients even if the tumor is greater than 2.0 cm in diameter. Histologic analysis has found that the vestibular and cochlear nerves are fused into a common trunk 4.5 mm medial to the vertical crest. This relationship is maintained until the nerve reaches the brainstem [6]. Where the cochlear and vestibular nerves are fused into a single trunk, no boundary to tumor infiltration of the cochlear nerve is present. However, where distinct separation of the cochlear and vestibular nerves is present, tumor infiltration would be less likely. Accordingly, tumors arising laterally in the IAC or medially adjacent to the brainstem would be the least likely to infiltrate the cochlear nerve. Thus, tumors located at the extremes of the course of the eighth nerve complex would have the greatest potential for hearing preservation because cochlear nerve infiltration by tumor would be least likely. Although surgical dissection of a medial acoustic neuroma from the brainstem is no different a procedure than removal of a tumor originating from any other part of the vestibular nerve, this anatomical feature of separation of the cochlear and vestibular nerves medially suggests that a medial acoustic neuroma, regardless of size, may be removed with hearing preserved. Despite the lack of correlation between tumor size and preoperative hearing, considerable evidence indicates that tumor size does affect postoperative hearing [1– 4,7,9,10,15]. Nevertheless, a medial acoustic neuroma may allow for hearing preservation because the tumor is medial to that
portion of the eighth nerve complex where the nerves are fused [6,8]. Our patient had a speech discrimination score of 96% with a speech reception threshold of 10 decibels preoperatively. After a staged suboccipital procedure, she had gross total resection of a 3.5 cm medial acoustic neuroma with normal audiometry postoperatively. Accordingly, we feel that a patient with a medial acoustic neuroma and good preoperative hearing should be offered a procedure that can preserve hearing. REFERENCES 1. Ebersold MJ, Harner SG, Beatty CW, Harper CM Jr, Quast LM. Current results of the retrosigmoid approach to acoustic neurinoma. J Neurosurg 1992;76: 901–9. 2. Fischer G, Fischer C, Remond J. Hearing preservation in acoustic neurinoma surgery. J Neurosurg 1992;76: 910 –17. 3. Glasscock ME, Hays JW, Minor LB, Haynes DS, Carrasco VN. Preservation of hearing in surgery for acoustic neuromas. J Neurosurg 1993;78:864 –70. 4. Gormley WB, Sekhar LN, Wright DC, Kamerer D, Schessel D. Acoustic neuromas: Results of current surgical management. Neurosurgery 1997;41:50 – 60. 5. Lanser MJ, Sussman SA, Frazer K. Epidemiology, pathogenesis, and genetics of acoustic tumors. Otolaryngol Clin North Am 1992;25:499 –519. 6. Neely JG. Is it possible to totally resect an acoustic tumor and conserve hearing? Otolaryngol Head Neck Surgery 1984;92:162–7. 7. Post KD, Eisenberg MB, Catalano PJ. Hearing preservation in vestibular schwannoma surgery: What factors influence outcome. J Neurosurg 1995;83:191– 6. 8. Rasmussen AT. Studies of the VIIIth cranial nerve of man. Laryngoscope 1940;50:67– 83. 9. Samii M, Matthies C. Management of 1000 vestibular schwannomas (acoustic neuromas): hearing function in 1000 tumor resections. Neurosurgery 1997;40:248 – 62. 10. Silverstein H, McDaniel AB, Norrell H. Hearing preservation after acoustic neuroma surgery using intraoperative direct eighth cranial nerve monitoring. Amer J Otol 1985;[Suppl]6:99 –106. 11. Silverstein H, McDaniel A, Norrell H, Haberkamp T. Hearing preservation after acoustic neuroma surgery with intraoperative direct eighth cranial nerve mon itoring: Part II. A classification of results. Otolaryngol Head Neck Surg 1986;95:285–91.
552 Surg Neurol 1999;51:548 –53
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12. Skinner HA. The origin of acoustic nerve tumours. Br J Surg 1929;16:440 – 63. 13. Smith MFW. Conservation of hearing in acoustic schwannoma surgery. Amer J Otol 1985;[Suppl]6:161– 3. 14. Tos M, Drozdziewicz D, Thomsen J. Medial acoustic neuromas. A new clinical entity. Arch Otolaryngol Head Neck Surg 1992;118:127–33. 15. Whittaker CK, Luetje CM. Vestibular schwannomas. J Neurosurg 1992;76:897–900.
COMMENTARY
The authors are to be commended on the excellent results they have achieved in this case report. I would add to their conclusion that an approach to save hearing in these circumstances should also be done when the preoperative speech discrimination is less than 70% and there is poor or absent hearing in the opposite ear [1]. Presumably the authors did not expose the medial part of the internal auditory canal. In my experience, most of the types of tumors that they describe have at least a small extension into the canal, which may not be seen on MRI, that requires a limited bone exposure for removal. This report again raises the question of the value of intraoperative monitoring of auditory function. A change in the BAEPs (details not specified) caused the first operation to be terminated, although the potentials had returned to baseline by the end of the operation and there was no change in hearing. At the second operation, Waves III-V were lost (presumably Wave I was still present), but apparently these changes did not alter the dissection. Four months later, BAEPs were normal. In our experience, when using a transtympanic electrode and BAEPs, if Wave I was present at the end of the operation but Wave V was lost or never detected, the results of hearing were unpredictable [2]. In a small number of patients, changes in the potentials have led to a change in the surgical procedures, but monitoring has not made a difference when the potentials are abruptly or rapidly lost. The value of monitoring in those patients in whom there is no change in the intraoperative potentials is unknown. Robert G. Ojemann, M.D. Department of Neurosurgery Massachusetts General Hospital Boston, Massachusetts REFERENCES 1. Ojemann RG. Comment on: Fischer G, Costantini JL, Mercier P. Improvement of hearing after microsurgical removal of acoustic neurinoma. Neurosurgery 1980;7: 158 –9.
2. Ojemann RG. Management of acoustic neuromas (vestibular schwannoma). Clin Neurosurg 1993;40:498 –535.
Based on a case report with hearing preservation in a large, so-called “medial” acoustic neurinoma, the authors hypothesize an increased chance of functional cochlear nerve preservation in this entity, and they advocate a surgical approach with a nerve conserving strategy in such situations. More and more of our colleagues are describing hearing preservation in acoustic neurinomas— even in large ones. This is one more report in that series, definitely a positive development. The theory regarding the improved chance of hearing preservation in medial neurinomas (i.e., in cases that do not involve the meatus) is probably a complex matter. Apparently the connection of the cochlear nerve to the vestibular nerve (the tumor origin) is closest just medial to the porus; therefore, separation of the tumorous vestibular nerve from the cochlear nerve seems most difficult, while it would be easier medially toward the brainstem or lateral to the fundus where the nerves enter separate channels. Regarding the pathoanatomical situation, the same is true for the facial nerve: it is most easily separated from the tumorous vestibular nerves within the lateral segment of the meatus and at the brainstem, while the greatest difficulty is found just medial to the porus. These tight connections are not necessarily attributable to tumorous infiltration, rather to the lack of a separating arachnoid sheath between these nerves. The authors’ conclusion that tumors located purely medial or lateral to this area of tight connection are best for conserving hearing at surgery is not quite true. In fact, the chance of hearing preservation in pure intrameatal tumors is less than in intrameatal tumors with small or medium extrameatal extension; the chance is greatly reduced in tumors with far lateral extension. One reason is the sensitivity of the cochlear nerve origin at the fundus and its vulnerability to medial stretching; another is the harder consistency of intrameatal tumors. Still, the relationship of the tumor and brainstem plays a major role. We have encountered many cases of functional deterioration or loss of auditory brainstem responses at medial dissection after successful lateral tumor mobilization. The importance of the medial acoustic tumor is based on its incidence, which has been reported to be as high as 3.7%. Up to now, we have not encountered it in our vestibular schwannomas. A lack of intrameatal extension has only been found in some large tumors extending up to the VII and VIII nerve bundle and originating from the caudal cranial
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nerves. It should be stressed that widening of the IAC is not the only sign of intrameatal involvement; in many cases shortening of the posterior auditory wall is the only objective sign. Intrameatal contrast uptake is often different from that of extrameatal tumors and therefore direct tumor visualization may be difficult even on MRI. We would like to encourage the increased awareness of the chance of hearing preservation in large
schwannomas produced by this case report, and the challenge it poses to others to find favorable options for treating them. Madjid Samii, M.D. Matthies Cordula, M.D. Department of Neurosurgery Krankenhaus Nordstadt Hannover, Germany
epression frequently co-occurs with a variety of other physical illnesses, including heart disease, stroke, cancer, and diabetes, and also can increase the risk for subsequent physical illness, disability, and premature death. Depression in the context of physical illness, however, is often unrecognized and untreated. Furthermore, depression can impair the ability to seek and stay on treatment for other medical illnesses. NIMH research suggests that early diagnosis and treatment of depression in patients with other physical illnesses may help improve overall health outcome.
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—Depression Fact Sheet National Institute of Mental Health