The variations of osseous structure of the ınternal acoustic canal: an anatomical study

OBJECTIVES

The internal acoustic meatus is an osseous canal that connects the inner ear to the posterior cranial fossa. It is located in the petrous portion of the temporal bone. A thin cribriform osseous plate known as the fundus is situated at the lateral end of the canal. This study assesses the structural and numerical variations of the fundus formations.

METHODS

Fifty-four temporal bones of unknown gender and age were examined with the surgical microscope.

RESULTS

The temporal bones analyzed were 46.2% right-sided and 53.7% left-sided. Only one temporal bone had two parallel transverse crests, while three had a single anterior crest that split into two branches posteriorly. The number of foramina at the transverse crest varied, with 29.6% having none, 48.1% having a single foramen, and 22.2% having several foramina. An anterior crest structure was seen in 53.7% of the temporal bones, with 5% having a slightly constricted entry to the facial canal. In cases with a single nerve foramen, 48.1% had one, while 51.8% had more than one, including examples with three or four foramina. A crest was found between the foramina of the single nerve in 7% of patients. Furthermore, a crest between the saccular nerve foramen and the high fiber foramina was seen in 25.9% of cases, and 5% had two saccular nerve foramina.

CONCLUSION

We think that revealing the anatomical, structural and numerical variations in the fundus will be useful in explaining the disease-symptom relationship.

LEVEL OF EVIDENCE

Level 4.

Evaluation of subarcuate canal on CT images in the perspective of clinical basis

OBJECTIVES

The aim of our study to contribute to the field of morphometrics by including measurements of the SAC and SAF and their distances from surrounding structures, particularly for surgeons involved in retrosigmoid approach for internal acoustic meatus tumor surgery and cerebellopontine angle surgery. Although there is limited information in the literature regarding the role of the subarcuate fossa (SAF) and subarcuate canal (SAC), it has been suggested that the SAC may be a potential pathway for infection from the middle ear to the posterior cranial fossa, and cerebellar abscesses may have this origin.

METHODS

For the images of our study, computerized tomography images of 118 individuals (59 females and 59 males) between the ages of 18-65 who applied to Bayındır Health Group.

RESULTS

The width of the cranial opening of the subarcuate canal was determined as 44 ± 0.54 mm, width of the labyrinth opening of the subarcuate canal was determined as 60 ± 0.42 mm, Length of the subarcuate canal was determined as 8.79 ± 2.31 mm, width of the subarcuate canal was determined as 5.54 ± 1.75 mm, and depth of subarcuate fossa was determined as 1.67 ± 0.69 mm. The distance of the cranial opening of the subarcuate canal to the superior semicircular canal (SSC-SAC/C) was measured as 5.33 ± 1.81 mm, The distance of the labyrinth opening of the subarcuate canal to the superior semicircular canal (SSC-SAC/L) was measured as 3.90 ± .98 mm, length of the petrous part of the temporal bone medial to the anterior semicircular canal measured from the apex to the SSCD (PLM) was measured as 33.56 ± 0.42 mm. No statistically significant differences were found between the right and left sides.

CONCLUSIONS

The morphometric measurements obtained in this study can provide useful information for neurosurgeons, neurotologist and otolaryngologists involved in retrosigmoid approach for internal acoustic meatus tumor surgery and cerebellopontine angle surgery, and for patients undergoing cochlear implant planning with a retrofacial approach.

Internal Acoustic Meatus Localization During Anterior Petrosectomy - A Novel Technique

OBJECTIVE

Anterior petrosectomy demands localization of the IAC for safe bone-drilling and maximum exposure. Various techniques have been described in the literature, each with shortcomings. We propose a new technique to localize the IAM using more consistent anatomical landmarks.

METHODS

The study was done in three phases. In phase-I(radiological), CT scan heads of fifty patients (100 sides) were analysed. Arcuate eminence-GSPN bifurcation angle(Garcia-Ibanez technique), Arcuate eminence-IAC angle(Fisch technique) and a new angle formed between foramen ovale and foramen spinosum line, and FS and IAM line (FO-FS-IAM angle) was measured. The mean, SD, and variance were calculated. In Phase-II(cadaveric), the FO-FS-IAM angle was measured on five (10 sides) dry skulls. In Phase-III(clinical), the IAM was localized using the FO-FS-IAM angle in 13 patients.

RESULTS

The mean angle between arcuate eminence and GSPN (Garcia-Ibanez technique) was 126.20±11.63 degrees(range 106-156) with a variance of 135.20. The mean bifurcation angle was 63±5.81 degrees(range 53-78). By the Fisch technique, the mean arcuate-IAM angle was 73.5±11.70 degrees(range 51-105) with a variance of 137.18. By our technique, the mean FO-FS-IAM angle was 94.72±5.89 degrees(range 84-108). The variance was 34.73. The mean FO-FS-IAM angle on dry skulls was identical (95±1.97 degrees) to our radiological measurements. This angle was reproduced reliably in clinical cases for localizing the IAM during anterior petrosectomy.

CONCLUSIONS

The FO-FS-IAM angle variance was much lower than the analogous angles measured by Garcia-Ibanez and Fisch techniques, making it a more reliable and effective tool for localizing the internal auditory meatus.

Glioblastoma multiforme at internal auditory canal

Background

Glioblastomas are the most common adult primary brain tumor present supratentorially. The presence of true extra-axial GBM infratentorially, especially in the internal auditory canal, is extremely rare with only three cases reported previously in the literature. We report the fourth case of primary internal auditory canal/cerebellopontine angle (CPA) glioblastoma which initially mimicked vestibular schwannoma on the basis of its location and presentation.

Case Description

A 65-year-old male presented with headache, vertigo, and progressive right ear deafness for 5 months. His preoperative magnetic resonance imaging findings were consistent with vestibular schwannoma. Maximum safe resection (near total) was done. The final histopathology report showed glioblastoma multiforme.

Conclusion

As per our knowledge, this is the fourth reported case of an extra-axial VIII cranial nerve glioblastoma located in internal auditory canal. Hence, despite being very rare, they should be considered as a differential in tumors at CPA.

Dissection of the Petrosal Presigmoid-Retrolabyrinthine Approach for the Petroclival Region on a Cadaver: 2-Dimensional Operative Video

Skull base epidermoid tumors, meningiomas, and schwannomas can be accessed by different techniques depending on the location and size of the lesion. Small lesions located anterior to the internal acoustic meatus (IAM) can be accessed via the subtemporal approach, and lesions located posterior to the IAM can be approached via retrosigmoid craniotomy. However, expansive lesions that are located anterior to the IAM and extend posteriorly toward the lower clivus can be accessed via the petrosal approach. The petrosal approach (presigmoid-retrolabyrinthine) is centered on the petrous ridge of the temporal bone and is mainly performed for intradural lesions located at the clivus and petroclivus junction area. Patients with intact hearing can benefit from this technique, as the labyrinth is untouched and yet the middle and posterior fossa compartments are connected. Additionally, extension of the lesion from the suprasellar area/cavernous sinus to the foramen magnum can be dissected and removed. There are variations of the petrosal approach, such as translabyrinthine, transotic, and transchoclear, with which hearing cannot be preserved, and the "transcrusal" approach, wherein posterior and superior semicircular canals are sacrificed yet hearing preserved. The endolymphatic duct is usually transected and not reapproximated. Neurotology input is always helpful when dealing with inner ear structures. This complex approach demands exhaustive practice with temporal bone dissection in a cadaver laboratory. Although this approach can be extended anteriorly, combination with an anterior petrosal approach permits more rostral exposure. In this video, we demonstrate the stepwise dissection of the posterior petrosal approach only, showing procedure nuances in a cadaver.1-8Used with permission from Barrow Neurological Institute, Phoenix, Arizona.

The influences of porus acusticus internus on ethnicity and importance in preoperative and intraoperative approaches

PURPOSE

The influences of porus acusticus internus (PAI) on ethnicity and differences between populations have not been investigated so far. Therefore, we performed this study to elucidate further the relationship between the different morphologies of PAI and ethnicity and to discuss their effects on surgery.

METHODS

One hundred twenty dry adult human temporal bones (61 male, 59 female) were investigated in the study. Their horizontal diameter (HD), vertical diameter (VD), shape, prevalence of the shapes of PAI, and the distance from the sulcus for the sigmoid sinus (SSS), sulcus for superior petrosal sinus (SSPS), and jugular foramen (JF) of dry Turkish temporal bones were recorded.

RESULTS

Forty-three (35.8%) elliptical (or transverse) and 77 (64.1%) oval (or round) PAI were detected in the Turkish dry temporal bones. On both sides, round PAI were identified more frequently than elliptical ones (32; 26.6% (left); 39; 32.5% (right)) (p < 0.05). There was a similarity between the Indian and Turkish dry temporal bones in terms of distance from the PAI to the SSPS (7-8 mm) and SSS (19-20 mm) (p = 0.01) and prevalence of morphology of PAI (oval and elliptical) (p = 0.04, p < 0.05).

CONCLUSION

The findings of the present study provided a detailed understanding of the preoperative and intraoperative identification of different morphologies of PAI and ethnicity. The ethnicity might contribute to morphology of the PAI and it can be explain the similar forms and distances between the various ethnic populations.

Descriptive and topographical analysis of the labyrinthine artery in human fetuses

Hearing or/and balance impairments may be caused by disorders of the labyrinthine artery (LA) and their branches. Most findings regarding the LA anatomy have been acquired through investigation of the cerebellopontine angle (CPA) in animal or adult human specimens. Eighty-eight CPAs and LAs of human fetuses were investigated using angio-techniques and microdissections. We found 15 intricate forms of distribution of LA. The LA usually originated from the extra-meatus loop in the anterior inferior cerebellar artery (AICA). The distribution of its terminal branches was 53.42% uni-arterial, 44.31% bi-arterial, and 2.27% tri-arterial systems. In the uni-arterial system, the LA described an anterior superior path to the cochlear nerve (CN) and originated its terminal branches in the gap between CN and the inferior part of the vestibular nerve. In the bi-arterial system, the anterior LA was located anterior and superior to the CN while the posterior LA appeared posterosuperior to the superior part of the vestibular nerve. In the tri-arterial system, the terminal branches originated directly from the AICA loop. Our results provide anatomical support to explain how compressions in the LA branches inside the internal acoustic meatus, as evoked by Schwannomas in the VII and VIII nerves, can lead to hearing and balance loss. The zone of the posterior vestibular nerve appeared to be a "safe area" for invasive procedures in these specimens.

Hearing preservation in vestibular schwannoma surgery via retrosigmoid transmeatal approach

BACKGROUND

Advances in various diagnostic and/or treatment modalities, including radiological imaging, neuromonitoring, and microsurgical techniques, have resulted in treatments of vestibular schwannomas being aimed at preserving facial and hearing functions while achieving optimal tumor control.

METHOD

We describe our surgical technique for hearing preservation in vestibular schwannoma surgery.

CONCLUSION

The retrosigmoid transmeatal approach under continuous neuromonitoring (auditory brainstem response, cochlear nerve action potentials, and continuous facial nerve monitoring) enables gross-total resection of vestibular schwannomas, while preserving hearing and facial functions. Radiological assessment and microsurgical techniques, such as meticulous tumor dissection, are also essential for functional preservation with sufficient tumor removal.

[Measurement of morphological parameters of internal acousticmeatus using thin-section CT]

Objective: This study aimed to measure the morphological parameters of the internal acoustic meatus(IAM) and its adjacent structures using temporal-bone thin-section CT(computed tomography). Methods: CT images were obtained from 50 Chinese adult patients (25 males and 25 females, 100 sides) which had no visible lesion in the petrous part of the temporal bone and inner ear, the morphological parameters of all inner ear parts were sectionally measured on the specified plane using SPSS 22.0 software for statistical analysis. Results: The integral morphological characteristics of the IAM were observed. These results revealed that anterior-posterior diameter of the internal acoustic poer(IAP)(CD) was (6.93±1.85)mm, the superior-inferior diameter of the IAP(EF) was (4.40±0.86)mm, the length of the IAM(AB) was (9.30±1.60)mm, the superior-inferior diameter of the IAM(the intersection of inner 1/3 section and middle 1/3 section) was (4.13±0.83)mm, the superior-inferior diameter of the IAM(the intersection of middle 1/3 section and outer 1/3 section) was (4.61±1.02)mm, the anterior-posterior diameter of the IAM(the intersection of inner 1/3 section and middle 1/3 section) was (6.62±1.92)mm, the anterior-posterior diameter of the IAM(the intersection of middle 1/3 section and outer 1/3 section) was (6.28±1.65)mm, the depth of transverse crest (superior wall) was (3.10±0.75)mm, the depth of transverse crest (interior wall)the was (1.46±0.59)mm, the distance from transverse crest vertex A to the superior wall of the IAM was (2.05±0.42)mm, the distance from transverse crest vertex A to the interior wall of the IAM was (2.93±0.41)mm, the thickness of the superior bone wall of the IAM (the intersection of inner 1/3 section and middle 1/3 section) was (4.45±1.34)mm, the thickness of the superior bone wall of the IAM (the intersection of middle 1/3 section and outer 1/3 section) was (4.32±1.12)mm, the thickness of the superior bone wall of the IAM (the intersection of outer 1/3 section and transverse crest vertex) was (4.37±1.28)mm, and the appearance ratio of the cells in the whole IAM superior wall was 32%.The whole IAM assumed the shape of short cylinder, inclining about 1 cm outward, with the upper-lower diameter and anterior-posterior diameter about 5 mm. Conclusion: It is necessary for carrying out preoperative the temporal-bone thin-section CT to obtain the morphological parameters of the IAM, determine its basic morphology, and provide references to avoid damaging the other important structures during IAM surgeries.

Influencing Factors Analysis of Facial Nerve Function after the Microsurgical Resection of Acoustic Neuroma

Objective

To explore and analyze the influencing factors of facial nerve function retainment after microsurgery resection of acoustic neurinoma.

Methods

Retrospective analysis of our hospital 105 acoustic neuroma cases from October, 2006 to January 2012, in the group all patients were treated with suboccipital sigmoid sinus approach to acoustic neuroma microsurgery resection. We adopted researching individual patient data, outpatient review and telephone followed up and the House-Brackmann grading system to evaluate and analyze the facial nerve function.

Results

Among 105 patients in this study group, complete surgical resection rate was 80.9% (85/105), subtotal resection rate was 14.3% (15/105), and partial resection rate 4.8% (5/105). The rate of facial nerve retainment on neuroanatomy was 95.3% (100/105) and the mortality rate was 2.1% (2/105). Facial nerve function when the patient is discharged from the hospital, also known as immediate facial nerve function which was graded in House-Brackmann: excellent facial nerve function (House-Brackmann I–II level) cases accounted for 75.2% (79/105), facial nerve function III–IV level cases accounted for 22.9% (24/105), and V–VI cases accounted for 1.9% (2/105). Patients were followed up for more than one year, with excellent facial nerve function retention rate (H-B I–II level) was 74.4% (58/78).

Conclusion

Acoustic neuroma patients after surgery, the long-term (≥1 year) facial nerve function excellent retaining rate was closely related with surgical proficiency, post-operative immediate facial nerve function, diameter of tumor and whether to use electrophysiological monitoring techniques; while there was no significant correlation with the patient’s age, surgical approach, whether to stripping the internal auditory canal, whether there was cystic degeneration, tumor recurrence, whether to merge with obstructive hydrocephalus and the length of the duration of symptoms.

Exposing the Fundus of the Internal Acoustic Meatus without Entering the Labyrinth Using a Retrosigmoid Approach: Is It Possible?

OBJECTIVES

To evaluate the feasibility of performing a labyrinth-sparing neuronavigation-assisted retrosigmoid approach to the fundus of the internal acoustic meatus (IAM) and to describe the anatomy of the structures embedded in the posterior meatal wall.

METHODS

Ten surgical dissections were performed bilaterally on 5 fresh cadavers. Cadavers were subjected to preoperative computed tomography scans and spatial coordinates of inner ear structures were recorded. A retrosigmoid craniectomy was performed. The IAM was drilled towards the fundus until no more than 1 mm of bone covered the labyrinthine structures. Specimens underwent a new computed tomography scan to verify the length of opened IAM and the status of the labyrinth. We then opened the labyrinthine structures and recorded their coordinates using navigation. These were compared with the radiologic coordinates to verify the neuronavigation accuracy.

RESULTS

In 9 sides, the IAM was opened to the fundus without injuring the labyrinth; in 1 side, the vestibule was opened. The mean residual bone on the fundus was 0.97 mm. The average length of the accessible IAM was 88.95%. The best accuracy of the navigation was for the identification of the common crus, with a mean value of 0.73 mm.

CONCLUSIONS

This surgical technique could facilitate the opening of the IAM with preservation of inner ear structures. We opened a mean of 88.95% of the IAM without entering the labyrinthine structures in 90% of cases. These results confirm the feasibility of the retrosigmoid approach for the exposure of the IAM fundus with preservation of labyrinthine structures.

Endoscopic extradural subtemporal approach to lateral and central skull base: a cadaveric study

OBJECTIVE

Endoscopy has provided a less invasive approach to skull base surgery, mainly through endonasal routes, but has been limited in its applications due to potential complications. The aims of this study were to evaluate the feasibility of the purely endoscopic extradural transcranial approach to lateral and central skull base through a subtemporal keyhole and to better understand potential distortions of the related anatomy via endoscopy.

METHODS

Ten fresh cadaver heads were studied with 4-mm 0° and 30° endoscopes to develop the surgical approach and to identify surgical landmarks.

RESULTS

The endoscopic extradural subtemporal approach was divided into 3 sections after exposure of the extradural space in the middle cranial fossa: 1) exposure of the lateral wall of the cavernous sinus and the preauricular infratemporal fossa; 2) anterior petrosectomy and posterior cranial fossa exploration; and 3) unroofing of the tympanic cavity and exposure of the facial nerve. This keyhole endoscopic technique clearly visualized anatomical landmarks of the lateral and central skull base via an extradural subtemporal route.

CONCLUSIONS

The endoscopic extradural subtemporal approach was feasible. This approach could display a wide range of lateral and central skull base structures with minimal invasiveness. The use of extradural space would be key to performing safe and effective endoscopic skull base surgery.