Surgical anatomy of the chorda tympani: a micro-CT study

Radiological Evaluation of Tympanic Segment of Chorda Tympani Nerve in Normal Ears: An Ultra-High-Resolution CT Study

BACKGROUND

To visualize the course of the tympanic segment of chorda tympani nerve (CTN) using ultra-high-resolution computed tomography (U-HRCT).

METHODS

A hundred and fourteen ears with no evident otologic pathologies were included. The tympanic segment of CTN was divided into four portions: periannular, posteromalleal, malleal, and anteromalleal. The length of the periannular portion running along the tympanic annulus was recorded. Four points of interest (the beginning and end of the posteromalleal and anteromalleal portions) were selected to perform distance measurements relative to the tip of the malleus manubrium. Differences in lengths and distances were compared in terms of ear sides and sexes.

RESULTS

The length of the periannular portion was 2.49 ± 1.16 mm. The beginning of the posteromalleal portion was located more laterally on the right side than on the left side (mean: 4.09 mm vs. 3.92 mm, P = 0.016). The end of the posteromalleal portion was located more inferiorly on the right (mean: 2.11 mm vs. 2.26 mm, P = 0.018). The beginning of the anteromalleal portion on the right was located more laterally than that on the left (mean: 2.60 mm vs. 2.45 mm, P = 0.027). The start and end of the anteromalleal portion were more posteriorly located in women than in men (both Ps < 0.001).

CONCLUSIONS

The course of the tympanic segment of normal CTN was comprehensively visualized by U-HRCT. Preoperative evaluation of the tympanic segment of CTN might be helpful in avoiding iatrogenic injury during middle ear surgery.

The Impact of the Location of Chorda Tymapni Nerve Origin on the Round Window Accessibility During Pediatric Cochlear Implantation: A Radioclinical Assessment

OBJECTIVES

This study assessed the impact of the location of the chorda tympani nerve (CTN) origin on the round window (RW) accessibility during pediatric cochlear implantation (CI). We also tried to validate the radiologic method to measure the length between the origin of the CTN from the facial nerve to the stylomastoid foramen (CF-SM).

STUDY DESIGN

It was a prospective observational case-series study.

SETTINGS

The included CI surgeries were performed at tertiary referral institutions from November 2018 to August 2021.

SUBJECTS

We included 146 pediatric patients who were candidates for CI.

INTERVENTION

We measured the CF-SM length in the parasagittal cut of the preoperative high-resolution computed tomography. We also classified the intraoperative RW according to the accessibility through the ordinary posterior tympanotomy approach into accessible or inaccessible.

MAIN OUTCOME MEASURE

We correlated the preoperative radiologic CF-SM length with the intraoperative RW accessibility.

RESULTS

The radiologic CF-SM length ranged from 2.9 to 7.4 mm with a mean of 4.9 ± 1.03 mm. The RW was accessible in 107 patients and inaccessible in 39 patients. Spearman's correlation coefficient revealed a significant relationship between the location of CTN origin and the RW accessibility as the p value was less than 0.0001.

CONCLUSIONS

We found a precise method to measure the CF-SM length in the parasagittal cut of the high-resolution computed tomography. We also found a significant impact of the location of the CTN origin on intraoperative RW accessibility. The radiologic CF-SM length of more than 5.4 mm had a powerful prediction capability of the RW inaccessibility.

Validation of the radiological detection of the chorda-facial angle: impact on the round window accessibility during pediatric cochlear implantation

OBJECTIVES

The facial recess, an essential landmark for the posterior tympanotomy approach, is limited by the facial nerve and the chorda tympani, with a complicated relationship. This study tried to find the most appropriate radiological method to evaluate the chorda-facial angle (CFA). We also checked the effect of this angle on the round window accessibility during cochlear implantation.

METHODS

It was a retrospective study that included cochlear implant surgeries of 237 pediatric patients, from September 2016 to April 2021. Two physicians evaluated the CFA in the para-sagittal cut of the preoperative HRCT. The round window accessibility was assessed in the unedited surgery videos.

RESULTS

The CFA ranged from 21° to 35° with a mean of 27.14 ± 3.5°. It was detected in all cases with a high agreement between the two CT reviewers' measurements. The CFA differed significantly between the accessible group and the group with difficult accessibility (p value < 0.001). Spearman's correlation coefficient revealed a strong correlation between the CFA and the intraoperative round accessibility. 25.5° was the best cutoff point; below this angle, difficult accessibility into the RW was expected, with high sensitivity, specificity, and accuracy CONCLUSIONS: Our study on a relatively large number of cases provided a precise, valid, reliable, and applicable method to evaluate the CFA in the HRCT scan. We found a significant-close relation between the CFA and the round window accessibility; the difficulty increased with a need for posterior tympanotomy modification when the angle decreased.

KEY POINTS

• Radiological detection of the chorda-facial angle was always problematic, without a previous straightforward method in the literature. • We used the para-sagittal cut of the high-resolution CT scans to evaluate the CFA. This cut was beneficial to seeing the chorda tympani nerve in every examined case. There was a high agreement between the two CT reviewers' measurements. • Preoperative evaluation of the CFA in the HRCT accurately predicted the round window accessibility. Patients with CFA less than 25.5^° were expected to have difficult accessibility into the round window during cochlear implantation.

Immune responses in the injured olfactory and gustatory systems: a role in olfactory receptor neuron and taste bud regeneration?

Sensory cells that specialize in transducing olfactory and gustatory stimuli are renewed throughout life and can regenerate after injury unlike their counterparts in the mammalian retina and auditory epithelium. This uncommon capacity for regeneration offers an opportunity to understand mechanisms that promote the recovery of sensory function after taste and smell loss. Immune responses appear to influence degeneration and later regeneration of olfactory sensory neurons and taste receptor cells. Here we review surgical, chemical, and inflammatory injury models and evidence that immune responses promote or deter chemosensory cell regeneration. Macrophage and neutrophil responses to chemosensory receptor injury have been the most widely studied without consensus on their net effects on regeneration. We discuss possible technical and biological reasons for the discrepancy, such as the difference between peripheral and central structures, and suggest directions for progress in understanding immune regulation of chemosensory regeneration. Our mechanistic understanding of immune-chemosensory cell interactions must be expanded before therapies can be developed for recovering the sensation of taste and smell after head injury from traumatic nerve damage and infection. Chemosensory loss leads to decreased quality of life, depression, nutritional challenges, and exposure to environmental dangers highlighting the need for further studies in this area.

Improved visualization of the chorda tympani nerve using ultra-high-resolution computed tomography

Background

Recognition of the anatomical course of the chorda tympani nerve (CTN) is important for preventing iatrogenic injuries during middle-ear surgery.

Purpose

This study aims to compare visualization of the CTN using two computed tomography (CT) methods: conventional high-resolution CT (C-HRCT) and ultra‐high-resolution CT (U-HRCT).

Materials and methods

We performed a retrospective visual assessment of 59 CTNs in normal temporal bones of 54 consecutive patients who underwent both C-HRCT and U-HRCT. After dividing CTN into three anatomical segments (posterior canaliculus, tympanic segment, and anterior canaliculus), two neuroradiologists scored the visualizations on a four-point scale.

Results

On C-HRCT, the visual scores of the posterior canaliculus, tympanic segment, and anterior canaliculus were 3.5 ± 0.7, 1.6 ± 0.6, and 3.1 ± 0.7, respectively. The respective values were significantly higher in all segments on U-HRCT: 3.9 ± 0.2, 2.4 ± 0.6, 3.5 ± 0.6 (p < 0.01). Although the difference in scores between methods was greatest for the tympanic segment, the visual score on U-HRCT was lower for the tympanic segment than for the anterior and posterior segments (p < 0.01).

Conclusion

Ultra‐high-resolution CT provides superior visualization of the CTN, especially the tympanic segment.

Endoscopic Anatomy of the Chorda Tympani: Systematic Dissection, Novel Anatomic Classification, and Surgical Implications

HYPOTHESIS

A transcanal endoscopic approach enables visualization of the variable course of the chorda tympani inside the middle ear.

BACKGROUND

The chorda tympani is the longest intrapetrous branch of the facial nerve. Despite having been investigated in several studies, a description of its tympanic tract from an endoscopic point of view is lacking in the literature.

METHODS

We performed transcanal endoscopic dissections of 44 human cadaveric head and ear specimens. The entry point of the chorda tympani into the middle ear was classified into four categories according to its location, and as covered or dehiscent according to its appearance. The chordal eminence (CE) was defined as absent, shallow, intermediate, prominent, or fused, based on its shape and extension. The relationship of the chorda tympani to adjacent bony and ligamental structures was assessed.

RESULTS

The tympanic tract of the chorda tympani was divided into three portions. The periannular segment was dehiscent in 54.5% of specimens, with type II being the most frequent entry point configuration (52.3%). In the interossicular segment, the nerve consistently passed lateral to the incus and medial to the malleus. The course of the intrapetrous segment was independent from the conformation of the tensor fold and supratubal recess.

CONCLUSION

The transcanal endoscopic approach allows a detailed description of tympanic segment of the chorda tympani. Novel anatomic classifications of the chorda tympani and CE are proposed herein to highlight their possible surgical implications during otologic procedures.

Classification of the Chorda Tympani: An Endoscopic Study

OBJECTIVES

To determine the ability of preoperative computed tomography (CT) to predict the variable surgical anatomy of the chorda tympani nerve (CTN) based on endoscopic tympanotomy.

STUDY DESIGN

Retrospective case review.

SETTING

Tertiary referral center.

PATIENTS

We identified 192 ears of 162 patients who underwent transcanal endoscopic ear surgery from August 2013 to June 2018. Patients with middle ear malformations, revision surgeries, myringoplasty, and cholesteatoma involving the CTN were excluded.

INTERVENTIONS

An intraoperative endoscopic image depicting the chorda tympani was selected for each patient and classified into one of five types. Preoperative CT images were analyzed to match the pictorial classification.

MAIN OUTCOME MEASURES

The visible tympanic segment of the chorda tympani was classified into the following five types: external auditory canal (EAC), detached, attached long, attached short, and ultrashort.

RESULTS

A total of 128 ears from 101 patients ranging in age from 2 to 81 years were enrolled. The EAC, detached, attached long, attached short, and ultrashort types of CTN were found in 7 (5.5%), 6 (4.7%), 84 (65.6%), 18 (14.0%), and 13 (10.2%) patients, respectively. The presence of the EAC type could be predicted by preoperative CT while the other four types could be predicted by binning into two groups, with a sensitivity of 0.61 and specificity of 0.72.

CONCLUSION

The variable anatomy of the chorda tympani nerve can be classified into five major groups based on endoscopic tympanotomy.

'Black Bone' magnetic resonance imaging as a novel technique to aid the pre-operative planning of posterior tympanotomy for cochlear implantation

Purpose: 'Black Bone' magnetic resonance imaging (BB MRI) is a novel sequence developed as an alternative to computed tomography (CT) for osseous imaging. We explored its potential utilisation in the pre-operative surgical planning of posterior tympanotomy for cochlear implantation through depiction of the mastoid facial nerve (mFN) canal and the posterior canaliculus of the chorda tympani (ChT), thus defining the facial recess. Methods: Twenty five adult patients were prospectively imaged with a dedicated BB MRI sequence. A consensus qualitative BB MRI 'visibility score' for the confidence of demonstration of the mFN canal and the posterior canaliculus of the ChT was recorded, as well as a 'corresponding score' to determine whether the neural structures on BB MRI corresponded to the paths of the nerves on a previous CT study. Results/discussion: The BB MRI sequence was able to clearly delineate the course of mFN in 100% of cases and that of ChT in 72%, with their courses corresponding to those depicted on CT in almost all cases. Maximum intensity projections with 7 mm slabs provided the optimal simultaneous demonstration of mFN, ChT and round window along the posterior tympanotomy surgical approach. Conclusion: The proposed BB MRI sequence reliably depicts mFN and ChT in the majority of cases, with a performance comparable to that of CT. It is proposed that it will be a useful adjunct to MRI protocols as part of cochlear implant assessment in those centres where CT is not routinely performed.

Blood supply to the chorda tympani: A review and clinical applications

The chorda tympani (CT) is important in gustatory sensation from the anterior two-thirds of the tongue and in secretomotor innervation to the submandibular and sublingual glands. Although the blood supply to the CT is not well delineated in the literature, some studies have shown that a posterior tympanic branch of the stylomastoid artery supplies CT at its origin from the mastoid segment of the facial nerve. We review the blood supply to the CT comprehensively. A better understanding of the vasculature involved is required to prevent iatrogenic injury during middle ear surgery and complications secondary to ischemia.

Does the Degree of the Mastoid Pneumatization Affect the Side of Bell Palsy?

OBJECTIVE

The aim is to investigate the impact of degree of mastoid pneumatization on the affected side of Bell palsy (BP).

STUDY DESIGN

Retrospective study in tertiary academic hospital.

METHODS

In total, 52 patients who were diagnosed with as BP were included in the study. Each patient was staged using House-Brackmann (HB) staging system. All patients underwent temporal bone computed tomography imaging. House-Brackmann scores, side of the BP, and mastoid pneumatization of all of patients were evaluated in the present study.

RESULTS

Regarding the degree of the mastoid pneumatization, there were no significant differences between the affected side and the unaffected side (P = 0.439). The degree of the mastoid pneumatization of the affected side and the unaffected side did not differ between males and females (P = 0.918 for the affected side, P = 0.765 for the unaffected side, respectively). A negative correlation between the age and mastoid pneumatization of each side was found (P = 0.001, P = 0.025, respectively). There was no significant correlation between HB score and the degree of the mastoid pneumatization of each side (P = 0.789, P = 0.703).

CONCLUSION

As a conclusion, the degree of the mastoid pneumatization is not one of the risk factors for BP. Further randomized studies with larger numbers of patients are needed to confirm these findings.

Population Statistics Approach for Safety Assessment in Robotic Cochlear Implantation

HYPOTHESIS

Descriptive statistics with respect to patient anatomy and image guidance accuracy can be used to assess the effectiveness of any system for minimally invasive cochlear implantation, on both an individual patient and wider population level.

BACKGROUND

Minimally invasive cochlear implantation involves the drilling of a tunnel from the surface of the mastoid to cochlea, with the trajectory passing through the facial recess. The facial recess anatomy constrains the drilling path and places prohibitive accuracy requirements on the used system. Existing single thresholds are insufficient for assessing the effectiveness of these systems.

METHODS

A statistical model of the anatomical situation encountered during minimally invasive drilling of the mastoid for cochlear implantation was developed. A literature review was performed to determine the statistical distribution of facial recess width; these values were confirmed through facial recess measurements on computed tomography (CT) data. Based on the accuracy of a robotic system developed by the authors, the effect of variation of system accuracy, precision, and tunnel diameter examined with respect to the potential treatable portion of the population.

RESULTS

A facial recess diameter of 2.54 ± 0.51 mm (n = 74) was determined from a review of existing literature; subsequent measurements on CT data revealed a facial recess diameter of 2.54 ± 0.5 mm (n = 23). The developed model demonstrated the effects of varying accuracy on the treatable portion of the population.

CONCLUSIONS

The presented model allows the assessment of the applicability of a system on a wider population scale beyond examining only the system's ability to reach an arbitrary threshold accuracy.

Subannular tube insertion: anatomical considerations

Objectives:

To assess the distance between the bony groove created during subannular tubes placement and the chorda tympani, and examine the depth of the hypotympanum and retrotympanum.

Method:

Grooves drilled in cadaver temporal bones at two levels were imaged to measure: the distance between the chorda tympani nerve and the tympanic sulcus, and the depth of the hypotympanum and the retrotympanum relative to the annulus.

Results:

The chorda tympani was between 0 and 5 mm from the groove cut across the annulus. The hypotympanum average depth was 2 mm (0.44–6.40 mm) and the retrotympanum average depth was 1 mm (0–2.53 mm).

Conclusion:

Grooves drilled across the tympanic sulcus should be placed at a point 20 per cent of the height of the tympanic membrane or lower; this will ensure least risk of injury to the chorda tympani nerve. The depth of the hypotympanum and retrotympanum dictates that the posteroinferior part of a subannular tube flange should be approximately 2 × 1 mm.

The morphology and morphometry of the fetal fallopian canal: a microtomographic study

Background

The canal for facial nerve (the fallopian canal, FC) is a bony structure passing through the petrous part of the temporal bone. The anatomy of this demanding and important for oto- and neurosurgeons structure is well described in literature. Among several studies on radiological anatomy of this region, still little papers focus on the developmental measurements in prenatal period.

Aim

Assessment of a microtomographic appearance of FC and dimensions based on available landmarks.

Method

The study was performed on 22 fetal temporal bones aged 16–27 Hbd. Specimens were scanned in micro-CT scanner. Length (FC1, FC2) and width (FC1W, FC2W) of the labyrinthine and tympanic portions of FC, angle of the first curve of FC (A1-2), length of the internal acoustic meatus (IAM), distance from FC to the basal cochlear turn (BCT) and to the lateral semicircular canal (LSC) were measured.

Results

The paper discusses problems and a value of micro-CT in neuroanatomical studies. FC was found in 20/22 cases. Average value of all distances measured was: FC1 1.38 ± 0.35 mm; FC2 6.68 ± 1.34 mm; FC1W 1.07 ± 0.1 mm; FC2W 1.25 ± 0.13 mm; A1-2 87.24 ± 4.05°; IAM 4.89 ± 0.60 mm; BCT 0.35 ± 0.05 mm; LSC 0.55 ± 0.05 mm.

Conclusions

Labyrinthine portion starts to ossify between 16th and 18th weeks of gestation and tympanic portion is fully ossified only after 20th week. Labyrinthine and tympanic portion of FC and the IAM elongate with age, whereas the angle of the first curve of FC and the distances to the BCT and the LSC remain stable and present no correlation with age.

Cochlear implantation surgery in patients with narrow facial recess

CONCLUSION

The new method of facial recess enlargement through suspending, antedisplacing, and adhering the chorda tympani nerve to the posterior wall of the auditory canal can expose the round window, make electrode insertion easier, and preserve the function of the facial nerve and chorda tympani nerve.

OBJECTIVE

To describe and report cochlear implantation surgery in patients with narrow facial recess, including surgical technique and postoperative outcomes.

METHODS

Cochlear implantation surgery was performed in our hospital in 39 cases with narrow facial recess by enlarging the facial recess. To enlarge the distance between the facial nerve and chorda tympani nerve, the chorda tympani nerve was suspended. The chorda tympani was anteplaced and adhered to the posterior wall of the auditory canal.

RESULTS

Among the 39 cases, the narrowest distance between the facial nerve and the chorda tympani nerve was less than 1.0 mm. All patients successfully underwent cochlear implantation surgery. No injuries of the facial nerve, chorda tympani nerve, or the posterior wall of the auditory canal were reported in any of the patients. There were significant differences in the injuries of the facial nerve, chorda tympani nerve, and posterior wall of the auditory canal in these cases compared with previous approaches to this operation.