Anatomical studies of the posterior petrous apex with regard to hearing preservation in acoustic neuroma removal

Some surgeons have shown that tumors of the internal auditory canal and cerebello-pontine angle may be removed with preservation of hearing through the suboccipital approach. If hearing is to be conserved, the cochlear division of the VIIIth cranial nerve and blood supply of the labyrinth must be preserved. In addition, surgical entry into the labyrinth, upon removal of the posterior wall of the internal auditory canal, must be avoided since it is likely to result in permanent sensorineural hearing loss. Careful anatomic dissection of 20 human temporal bones has shown that exposure of the lateral-most recess of the internal auditory canal from a suboccipital approach is impossible without injury to the endolymphatic duct, common crus, vestibule or ampulla of the posterior semicircular canal. Previous authors have suggested that exposure of the horizontal crest may be used as a safe landmark in avoiding labyrinthine injury. However, our study has shown that exposure of the horizontal crest usually leads to labyrinthine injury. In 19 out of 20 cases, the labyrinth would have been entered had the horizontal crest been used as a landmark for the lateral limit of bone removal. The application of the anatomical relationships quantified in this study may improve our ability to avoid labyrinthine injury in the suboccipital removal of acoustic neuromas.     

Retrosigmoid approach for acoustic tumor removal.

The retrosigmoid technique has evolved from the traditional suboccipital operation and, when combined with removal of the posterior wall of the internal auditory canal, affords a wide exposure of the cerebellopontine angle. This approach may be used for acoustic neuromas of all sizes, from intracanalicular to more than 4 cm from the porus acusticus. Hearing preservation may be attempted and is generally successful in a substantial minority of cases. The facial nerve is readily visualized at the lateral end of the internal auditory canal and is at no greater risk than in the translabyrinthine operation. We use this approach for all hearing preservation surgery as well as for tumors of more than 3 cm, regardless of hearing.     

Results of surgery by the modified extended middle cranial fossa approach in 100 patients with acoustic neuroma

The results of surgery by the middle cranial fossa (MCF) approach or the modified extended MCF approach in 100 patients with acoustic neuroma are reported. The rates of facial nerve preservation, tumor removal, and hearing preservation were reported and discussed. This surgical procedure can be applied to tumors of any size, from tumors confined to the internal auditory canal to those extending into the posterior fossa.     

听神经瘤术中听力监测及术后听力保留影响因素分析

目的探讨听神经瘤术中听力监测的应用及术后听力保留的可能影响因素。方法16例采用乙状窦后入路手术切除听神经瘤的成年患者,分为两组,术中采用听性脑干反应(auditory brainstem response,ABR)和耳蜗电图(electrocochleogram,ECochG)联合监测为监测组(8例),未监测者为未监测组(8例),比较两组患者术后听力保留情况,采用单因素分析,分析影响听力保留的可能因素,包括:年龄、病程、肿瘤大小、术前纯音听阈和言语识别率、术中是否行ABR和ECochG联合监测、内听道是否扩大、肿瘤和神经是否粘连等。结果前庭诱发肌源性电位(VEMP)提示16例患者肿瘤来源于前庭上神经,监测组中6例术中及术毕ABR波Ⅰ、Ⅴ和复合动作电位(CAP)持续存在,术后听力保留;1例术中ABR波Ⅰ、Ⅴ和ECochG CAP持续存在,但术后无可用听力;1例术中切除肿瘤时ECochG与基线重复性良好,ABR波V消失,手术结束波V仍未恢复;监测组术后听力保留率为75.0%(6/8),未监测组术后无一例保留听力,差异有统计学意义(P=0.007)。单因素分析显示,年龄、病程、肿瘤大小、术前纯音听阈以及内听道扩大与术后听力保留率无关(P>0.05),术前言语识别率、术中ABR和ECochG联合监测、肿瘤和神经粘连与否与术后听力保留率相关(P<0.05)。结论听神经瘤切除术中ABR和ECochG连续监测对指导手术和提高术后听力保留率有重要意义,肿瘤与神经粘连是术后听力保留的重要影响因素,手术技巧、术前听力、肿瘤大小、内听道扩大等是否是术后听力保留的影响因素需扩大样本进一步研究验证。     

Recurrence rates of acoustic neuroma in hearing preservation surgery.

Several authors have detailed the microscopic appearance of the acoustic neuroma/cochlear nerve interface. Others have highlighted the anatomic relationships existent between the lateral end of the internal auditory canal (fundus) and the otic capsule, as viewed from the posterior fossa. Based on these findings, several have suggested that hearing preservation attempts are likely associated with tumor persistence. They therefore question the feasibility of hearing preservation surgery. In this study, computerized tomography or magnetic resonance imaging was carried out on 28 patients having previously undergone excision of an acoustic neuroma with intraoperative sparing of the cochlear and facial nerves. Scans were done at least 5 years following surgery. Results of this study and a discussion of the literature follow.     

Surgical anatomy of the petrous canal and posterior cranial fossa in relation to hearing preservation in surgery of acoustic neuroma

The suboccipital (retrosigmoid) and the middle fossa routes are recommended for preservation of hearing in surgery for acoustic neuroma. We carried out a comparative study of the critical distances and bony landmarks on 520 petrous bones. Unlike the transtemporal route, the suboccipital approach offers no landmarks for identification of the common crus, the vestibule or the facial nerve; the lateral portion of the internal auditory canal is not always seen. The fundus can be seen in only 50% of bones without opening the labyrinth.     

Modified transotic approach to the cerebellopontile angle

A modification of the transotic approach to the cerebellopontile angle involves complete removal of the otic capsule bone, obliteration of the middle ear cleft, and removal of the posterior external auditory canal wall, while leaving the fallopian canal intact. The major advantage of this technique is that it allows more direct visualization of the most vulnerable portion of the facial nerve medial to the anterior wall of the internal auditory canal during acoustic tumor removal.     

Middle fossa acoustic tumor surgery: results in 106 cases

Although the middle cranial fossa approach has been used less frequently in recent years than in the past, it continues to be a useful technique for the removal of small acoustic tumors with possible hearing preservation. The approach provides complete exposure of the contents of the internal auditory canal, thus allowing positive facial nerve identification and facilitating total tumor removal. This paper reports the results of 106 middle fossa acoustic tumor removals over a 25-year period. Measurable postoperative hearing remained in 59% of cases. In 89% of cases, normal or near-normal postoperative facial nerve function was obtained. Total tumor removal was achieved in 98% of cases. Preoperative selection criteria are discussed, and postoperative complications are reported.     

Transcanal approach to the singular nerve.

OBJECTIVES/HYPOTHESIS: Intractable benign paroxysmal positional vertigo is rare, and surgery is indicated in only a very small number of cases. Transcanal singular neurectomy is considered a difficult and risky procedure possibly leading to hearing loss and vertigo. The objective of this study was to evaluate the feasibility of the singular neurectomy through the external ear canal in an attempt to explain the contradictory results of previous reports of anatomists and of surgeons who abandoned the technique, considering that the singular neurectomy could not be reached via the external auditory canal without damaging the labyrinth. MATERIALS AND METHODS: Anatomical study on 100 halves of human heads in which the canal of the singular nerve (SN) was identified and opened at its extremities, the internal auditory canal and the ampulla of the posterior semicircular canal, via a posterior fossa approach. Next, the canal of the SN was dissected via the external auditory canal, at the floor of the round window (RW) niche. The relation of the SN canal to the ampulla of the posterior semicircular canal was evaluated. RESULTS: In 90 cases, the canal was transected medially to and away from the ampulla of the posterior semicircular canal, and in 8, at its emergence from the posterior ampullary recess. In these 98 cases, the RW membrane and the bony labyrinth were kept intact. In two cases, the canal of the SN could not be reached at the floor of the RW niche. CONCLUSION: Singular neurectomy is feasible via the external auditory canal, without damaging the RW membrane or the labyrinth in 98% of the cases. Because singular neurectomy is indicated in a very small number of cases, it is difficult to master this particular surgical procedure. This may explain why most surgeons abandoned the technique after a few attempts, followed by an unacceptable rate of sensorineural hearing loss.     

Function-saving microsurgery in suboccipital removal of large acoustic neuromas

Various successful approaches are available for acoustic neurinoma surgery, permitting total tumor removal and preservation of cranial nerve function. In smaller and medium sized tumors excellent results can be achieved with respect to facial and cochlear nerve function using the transtemporal approach. For larger tumors similar results can be achieved by the suboccipital approach. The results of 45 completely removed large acoustic neurinomas all operated upon via the suboccipito-lateral approach with microsurgical techniques and neurophysiological monitoring are presented. The average tumor size, excluding the portion within the internal auditory canal was 3 cm. Anatomical preservation of the VIIth cranial nerve was achieved in nearly all cases. Satisfactory to excellent facial nerve function was preserved in 70% of all cases. Initial hearing was preserved in 29%. However delayed postoperative hearing loss was encountered in 13%. Therefore definite hearing preservation was achieved in 16% of the cases. Intraoperative monitoring, especially of auditory evoked potentials, was very helpful in achieving these functional results.     

Evaluation of the results of endoscope-assisted acoustic neuroma surgery through posterior fossa approach

The objective of this study was to evaluate the results of endoscope-assisted acoustic neuroma surgery in posterior fossa approach. Between 1996 and 2002, 60 consecutive patients with acoustic neuroma were operated via the retrosigmoid suboccipital approach. Standard 4-mm sinus endoscopes at different angles were used during the surgeries either for inspection or tumor endoscopic dissection. Clinical parameters and treatment outcome were evaluated retrospectively. Tumor sizes were small, medium and large in 46.6, 45 and 8.3% of the patients, respectively. The hearing preservation rate, which did not correlate with tumor size (p > 0.05), was 24.4%. The need for facial reanimation surgery, which was needed in 5% of patients, was significantly higher in the large tumors than in the small and medium tumors (p < 0.001). Cerebrospinal fluid fistula rate, which was not related to tumor size (p > 0.05), was 13.3%. Tumor recurrence or residual tumor was not encountered at all. In conclusion, endoscopes give accurate information about the relationship between the tumor and the adjacent structures and help control the fundus of the internal auditory canal to ensure complete tumor removal. It is also helpful in visually verifying the continuity of the facial and cochlear nerves. The use of endoscopes does not appear to increase the hearing preservation rate, but is very helpful in complete tumor removal in the posterior fossa approach.     

Medial acoustic neuromas. A new clinical entity.

The medial acoustic neuroma, a new clinical entity, is defined as an extrameatal tumor without tumor mass laterally in the internal acoustic meatus. During a 12-year period in Denmark, in a prospective analysis of 400 acoustic neuromas on which surgery was performed by the translabyrinthine approach, 48 tumors (12%) were medial tumors, corresponding to an incidence of 0.8 tumors per million inhabitants per year. Analysis of the symptoms and results showed that medial tumors are generally larger with more severe involvement of the cerebellum, the trigeminal nerve, and the brain stem, compared with the 352 nonmedial tumors. Because of its onset in the medial part of the vestibular nerve, the tumor may grow silently and to a considerable size without any widening of the internal auditory canal and with relatively good hearing. Meaningful hearing preservation is impossible in medial tumors, because the smallest medial tumor (subject's hearing, 40 dB or better) measured 3 cm, and the majority of tumors are giant tumors.     

An anatomic and radiologic evaluation of access to the lateral internal auditory canal via the retrosigmoid approach and description of an internal labyrinthectomy.

OBJECTIVE: The retrosigmoid approach to the posterior petrous bone may be used as a hearing preservation operation for extirpation of posterior fossa and internal auditory canal (IAC) lesions. However, it is usually not possible to remove tumor from the most lateral portions of the IAC even after removing the retrolabyrinthine bone down to the posterior semicircular canal. Our goal was to examine the advantages and disadvantages of the retrosigmoid approach with respect to approaching the lateral IAC, to find what is gained by drilling down the retrolabyrinthine bone, and to describe an internal labyrinthectomy whereby the labyrinth is removed via a retrosigmoid approach. MATERIALS AND METHODS: Using a 3.5-cm craniotomy, a retrosigmoid approach was performed on one side in each of five whole fresh cadaveric heads. The IAC was identified, and the length of the IAC and the depth of the acoustic porus from the center of the craniotomy were measured. The bone posterior to the labyrinth was removed, and the length of the portion of the IAC still inaccessible was measured. These measurements were compared with measurements using computed tomography (CT) scans of each cadaveric head. Additionally, we present the findings of 11 patients who had an internal labyrinthectomy performed by the senior author. RESULTS: The average length of the IAC based on CT scanning was 11.2 mm (SD, 0.84 mm; range, 10-12 mm, 95% confidence interval [CI], 9.44-12.96). The potential gain predicted by the CT scans was 8.4 mm (SD, 0.9 mm; range, 7-9 mm). In the anatomic study, the average length of the IAC was 11.0 mm (SD, 2.3 mm; range, 8.0-13.0 mm; 95% CI, 9.26-12.78). The average length of the IAC that was inaccessible after removing the retrolabyrinthine bone was 6.7 mm (SD, 1.5 mm; range, 5.0-7.8 mm), or 61%. The average gain in access to the lateral IAC was 4.3 mm (SD, 1.0 mm; range, 2.7-5.2 mm). After performing an internal labyrinthectomy, the fundus was accessible, as was the labyrinthine segment of Cranial Nerve VII and the geniculate ganglion. CONCLUSION: The retrosigmoid approach provides access to the posterior petrous bone, and removal of the bone posterior to the labyrinth provides some additional access to the lateral IAC. However, an internal labyrinthectomy is necessary to provide access to the fundus of the IAC via the retrosigmoid approach.     

Topographical anatomy of the internal auditory canal. Implications for functional surgery in the cerebello-pontine angle

For exposure of the cerebello-pontine angle by an enlarged middle-fossa approach without destruction of the inner ear, bone removal anterior and posterior to the internal auditory meatus (c.a.i.) can be performed with orientation at landmarks. Based on the experience of more than 300 interventions and documented by a series of 10 temporal bone micro-dissections, rules have been established for reliable localization of the following structures: geniculate ganglion, Fallopian canal, vertical crest at the fundus of the c.a.i., basal coil of the cochlea, and ampulla of the superior semicircular canal. The surgical technique has enabled the authors to remove acoustic neurinomas of up to 3.5 cm with preservation of hearing in 51%.     

122例听神经瘤患者听力受损相关因素分析

目的分析听神经瘤患者听力受损相关因素。方法回顾性分析122例单侧听神经瘤患者MRI,颞骨CT及听力学检查结果(纯音听阈及言语识别率),根据术前纯音听阈≤50dB、言语识别率≥50%的标准将患者分为实用听力组和无实用听力组,比较两组患者肿瘤大小、肿瘤囊变、脑干受压移位、瘤周有无水肿、内听道宽度、内听道长度、内听道底有无脑脊液等因素。结果内听道扩大程度与听力受损明显相关(P<0.001),肿瘤大小(P=0.25)、脑干受压移位(P=0.38)、瘤周有无水肿(P=0.91)、内听道长度(P=0.75)、内听道底有无脑脊液与听力受损无明显相关(P=0.18);肿瘤囊变可能有相关,但两组患者无统计学意义(P=0.08)。结论内听道扩大的程度与听神经瘤患者听力受损明显相关,其在一定程度上反应了内听道内耳蜗神经长时间受压情况。     

The efficacy of brainstem auditory evoked potentials in acoustic tumor surgery

As the identification of patients with small acoustic neuromas and salvageable hearing increases, intraoperative auditory nerve monitoring has been used increasingly in an attempt to improve the hearing preservation rate. Far-field recordings obtained by brainstem auditory evoked potentials (BAEP), at times enhanced by electrocochleography, have become a standard method of intraoperative auditory nerve assessment. To evaluate the usefulness of this monitoring technique, the hearing preservation results of a series of unmonitored acoustic tumor removals were compared to a series of patients monitored via the standard brainstem auditory evoked potentials. With comparable average tumor sizes, 4 of 7 unmonitored patients had hearing preserved at preoperative levels compared to 4 of 9 monitored patients. Neither preoperative BAEP assessments nor absolute tumor size were predictive of hearing preservation. This report brings into question the effectiveness of far-field intraoperative BAEP monitoring during acoustic tumor resection and suggests that direct auditory nerve monitoring may be more appropriate.     

Extended middle fossa and retrolabyrinthine approaches in acoustic neuroma surgery: case reports.

Hearing conservation surgery for small acoustic neuromas is well accepted. At present, two approaches are primarily used: the suboccipital and the middle fossa. The middle fossa approach to the internal auditory canal has the advantage of using bony landmarks to identify and protect the facial nerve. Because of anatomic constraints presented by the superior semicircular canal however, its uses are limited to intracanalicular tumors or tumors protruding only slightly into the cerebellopontine angle. By extending the approach through the superior semicircular canal, a wide access to the cerebellopontine angle can be safely obtained. In this study three procedures, two through the superior semicircular canal and one through the posterior semicircular canal, were utilized for hearing conservation surgery. By immediately sealing off the canal ends, hearing preservation was accomplished in two out of three of these cases. The dictum that labyrinthine opening invariably leads to anacusis should be reconsidered. By utilizing approaches through the semicircular canal, it is possible that morbidity from this surgery may be reduced.     

Diagnosis and management of intralabyrinthine schwannomas.

OBJECTIVES/HYPOTHESIS: Describe the symptoms, signs, radiographic findings, and treatment results for four patients with intralabyrinthine schwannoma beginning either primarily within the labyrinth or extending secondarily into the labyrinth from the internal auditory canal. STUDY DESIGN: Retrospective review. METHODS: Review of clinic records, operative records, imaging studies with follow-up telephone interview, and when possible, repeat examination. RESULTS: Four patients with intralabyrinthine schwannoma treated by the first author were identified. Episodic vertigo, indistinguishable from Meniere's disease, was present in all but one of the patients in this study. A progressive unilateral hearing loss was also found in all of the patients. Magnetic resonance imaging revealed tumor isolated to the vestibule in two patients with the cochlea primarily involved in the other two patients. Intracochlear tumor extending into the internal auditory canal had been missed on preoperative imaging in one patient and was found during a translabyrinthine vestibular nerve section. In another patient with an intracanalicular schwannoma, tumor extending into the basal turn of the cochlea was not removed during a translabyrinthine approach to the internal auditory canal. The tumor subsequently recurred, necessitating a transotic approach for removal. A transmastoid/translabyrinthine approach was used to successfully remove tumor in one patient. Another patient with good hearing and no vestibular symptoms at time of this writing is being followed with serial imaging studies. As expected, the three patients who underwent surgery have anacusis in the operated ear and are free of vertigo at follow-up intervals of 12, 26, and 65 months. CONCLUSIONS: Intralabyrinthine schwannomas are rare tumors with optimal treatment being determined by the symptoms, tumor location, and hearing. Findings of an intralabyrinthine schwannoma on magnetic resonance imaging may be easily overlooked and attributed to inflammatory changes.     

Patulous internal auditory canal

OBJECTIVE: To investigate a rare anomaly of the internal auditory canal known as a patulous canal and its relationship to hearing impairment. METHODS: High-resolution computed tomographic scans of the temporal bones of patients who presented between August 2001 and August 2002 were reviewed. The patients' medical charts were evaluated for age, sex, and hearing impairment, and the computed tomographic scans were examined for the presence of a patulous canal. RESULTS: The study group included 645 patients who underwent high-resolution computed tomography of the temporal bones for various reasons, including sensorineural hearing loss (50% of patients). A patulous canal without any associated anomaly of the labyrinth was the only finding in 2 patients. Both patients had chronic middle ear disease along with conductive hearing loss. CONCLUSION: Patulous canal is a rare anatomical variant of the internal auditory canal (0.3%), and its association with inflammatory ear disease accompanied by conductive hearing impairment appears to have been incidental in both cases in the present study.     

Selective resection of the lateral semicircular canal with hearing preservation--an animal experiment study]

The goal of our investigation was to develop a method for removing parts of the vestibular labyrinth by surgery without inducing a loss of auditory function. Three different surgical lesions were created in the lateral semicircular canal of the rabbit: (1) fenestration; (2) fibrin glue perfusion of the canal following fenestration; and (3) destruction of the semicircular canal by drilling after fenestration and fibrin glue perfusion. Brain-stem auditory potentials were recorded repeatedly up to 3 months after operation. They demonstrated preservation of hearing in all rabbits in the first group, in 78% of the second and 67% of the third group. In the last group a 20 dB deterioration of hearing was regularly noticed. Histological study revealed the utmost importance of the fibrin glue perfusion of the perilymph space of the semicircular canal inducing an interruption of the peri- and endolymph flow. A precise microsurgical technique was crucial for hearing preservation.