Isolated lateral semicircular canal-facial nerve dehiscence as a cause of third window syndrome—an emerging phenomenon and outcomes of surgical management

Introduction

Third window syndrome (TWS) consists of a set of both auditory and vestibular symptoms that arises when there is a pathological third window in the bony labyrinth of the inner ear. This extra mobile window adds to the already existing oval window and round window (RW), disrupting the normal auditory and vestibular function of the inner ear (1).

Minor et al. first described superior semicircular canal dehiscence (SSCD) in 1998 as a cause of third window pathology (2). He highlighted anatomical defects in the bony structure of the inner ear. However, these defects were first explained by Tullio when he demonstrated sound-induced eye movements (nystagmus) by opening the semicircular canal bony duct in pigeon animal models, which is now as Tullio phenomenon (3).

TWS was a term originally used to primarily describe the clinical symptoms associated with SCCD (4). However, there are patients with Tullio phenomenon but without the typical anatomical findings of SCCD (1). These cases have been subsequently shown to have a variety of other anatomical abnormalities including dehiscence involving other semi-circular canals such as those involving the lateral and posterior canals (5). Other abnormalities that have been associated with the TWS include enlarged vestibular aqueduct (6), X-linked deafness with stapes gusher (7), Paget’s disease of the temporal bone (8) and dehiscences between the internal auditory canal and vestibule, between the carotid and cochlea (9), and between the carotid and facial nerve (7).

Since the use of high-resolution computed tomography (CT) images of the temporal bone to show dehiscence above the superior semicircular canal, this has also been used to assist in the diagnosis of TWS. In light of these other anatomical causes of TWS, the criteria for diagnosis have been proposed to include a combination of symptoms such as pulsatile tinnitus, sound induced symptoms, pressure induced symptoms as well as electrophysiological abnormalities (10). Patients typically also have a conductive hearing loss, also known as a pseudo conductive hearing loss (4).

Gianoli et al. recently published the first reported case series of 16 patients with a newly described dehiscence between the facial nerve and the ampullated end of the lateral semicircular canal (11). These patients have the typical clinical features and electrophysiological changes of superior canal dehiscence but without the CT findings. However, this study also identified concurrent CT abnormalities to explain the TWS in more than half the patients including lateral semicircular canal-facial nerve (LSCC-VII) dehiscence, cochlear-facial nerve dehiscence, and cochlear-carotid dehiscence. In addition, 10 patients had a history of head trauma including two patients with a history of barotrauma. Four patients underwent surgery including one patient who had TWS symptoms after repair of the SSCD. The presence of confounding anatomical findings means that it is difficult to determine if LSCC-VII dehiscence is a clinically significant and isolated new finding.

This study aims to describe the clinical and radiological characteristics of patients with third window symptoms without classical SSCD, and to evaluate the surgical outcomes of those undergoing RW reinforcement (RWR) surgery. To the best of our knowledge, this is the first time this subset of TWS has been described retrospectively in the literature in addition to the surgical outcomes.

Methods

The study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. The study was approved by institutional ethics committee at Sir Charles Gairdner Hospital (approval No. 46643). Patient confidentiality was maintained by anonymizing data, and individual consent was waived for this retrospective analysis. Patients who presented with symptoms consistent with TWS and with initial CT temporal bone scan reports that were not consistent with a LSCC diagnosis were identified retrospectively between January 2017 and June 2022. Patients with TWS were defined as having at least one of the following main presenting symptoms (sound or pressure induced vertigo, autophony or tinnitus) and/or an abnormal cervical vestibular evoked myogenic potential (cVEMP). Patients that had other anatomical abnormalities found on their CT were excluded from the study.

cVEMP testing was performed using the two channel ICS-CHART EP (Natus, Middleton, WI, USA) with the following electrode montage: ground electrode placed on the centre of forehead, active electrodes on the upper third of the sternocleidomastoid muscle (SCM) on each side, the active electrodes were jumpered and placed on the sternum and the electrode for electromyography (EMG) monitoring was placed on lower third of the SCM. We considered a reduced cVEMP thresholds as recording a response at a click presentation stimulus of 75 dB or lower with the normal range of response expected at a stimulus level between 90 and 120 dB. CT scans of temporal bones were performed using a multislice (64–256 slices) scanning protocol, using multiplanar reconstruction with 0.625 or 0.5 mm thin overlapping slices on a sharp bone algorithm.

Patient demographics including age and sex were recorded, as well as a comprehensive otology history including a history of hearing loss, otalgia, otorrhea, previous ear surgery, tinnitus and a history of neurological disorders including migraine. Physical examination findings were also collated including otoscopy findings, and tuning fork assessments for Webber’s and Rinne’s tests. A standard audiological test battery of pure tone audiometry (bone and air conduction testing including masking when necessary), tympanometry and stapedial reflexes were obtained. A comprehensive vestibular function battery test was conducted including videonystagmography (VNG) which comprised of gaze, saccades, smooth pursuit, static positional testing and bithermal caloric testing. Additionally, video head impulse test (VHIT) of all six semi-circular canals, cVEMP, and electrocochleography (ECOG) were also performed.

An assessment of the CT scan of the temporal bones were initially conducted by two independent head and neck radiologists. The following aspects of the temporal bones were assessed, including the presence or absence of superior, posterior or lateral canal dehiscence, enlarged vestibular aqueduct, cochlear-carotid and cochlear facial nerve dehiscence and other incidental causes such as vestibular schwannoma (within the limitation of soft tissue windows) or otosclerosis.

Patients who underwent surgery were treated with RWR surgery. All patients had surgery performed under a general anaesthesia. An endaural incision was used followed by elevation of a tympanomeatal flap, and preservation of the chorda tympani nerve. The middle ear space was entered and the RW niche was identified. Temporalis fascia was harvested from the endaural incision and cut into small pieces measuring up to 1mm in size. The graft pieces were then placed in the RW niche and gently pushed to interface with the RW membrane until the niche was filled completely. The tympanomeatal flap was replaced, and absorbable gelatinous sponge dressing was placed in the ear canal. The wound was closed with absorbable sutures and the patient was discharged home within 24 hours and advised to avoid strenuous activity or performing the Valsalva for 2 weeks. The surgical procedure is similar to that previously described by Wackym et al. (12).

Patients who had surgery underwent a series of post-surgical assessments including a 2- and 6-week and 6-month postoperative follow-up in which they were asked questions in regard to their post-surgical symptomatology. Physical examination findings were also collated including otoscopy findings, and tuning fork assessments for Weber and Rinne’s.

The same standard audiological tests were performed both prior to surgery and postoperatively with particularly interest for cVEMP testing which was performed to measure thresholds at least 4 months after surgery. All patients underwent a preoperatively a baseline air and bone conduction pure tone audiogram and hearing outcomes between 500–8,000 Hz were recorded. A pure tone average air-bone gap (PTA-ABG) was used between 500–3,000 Hz as the primary measure.

Results

Over the study period, ten patients fulfilled the inclusion criteria of having third window symptomatology without CT findings of SSCD. Baseline patient characteristics including demographics are shown in Table 1. The mean age of patients was 49.1 years with an age range of between 22 to 76 years. Four patients were male and six were female. The laterality was split between three patients with right sided symptoms, six with left sided symptoms and one patient had bilateral symptoms. All patients had at least one main presenting complaint of either sound induced symptoms, autophony or pressure induced vertigo. Table 2 illustrates the main symptoms and physical examination findings displayed by the patients. Of note, four patients had the main presenting complaint of both sound induced symptoms and autophony, whereas two patients presented with sound induced vertigo or imbalance. Patient nine presented with intermittent episodes of vertigo and imbalance however stated it was not necessarily sound induced. Physical examination findings showed three patients had lateralising Weber’s test, two patients had an abnormal Romberg’s test, one patient respectively had negative Rinne’s and a positive Dix Hallpike result.

The CT scans revealed three patients had no discernible findings on CT relating to a TWS and six patients had a LSCC-VII dehiscence. One patient had a finding of cochlear-carotid dehiscence. Table 2 shows these findings. All findings were concordant between both radiologists.

All ten patients had cVEMP thresholds measured with seven patients recording a reduced cVEMP threshold preoperatively, three patients had a normal cVEMP threshold. All patients had audiological testing with a mean ABG of 9.4 dB. The preoperative ABG ranged from −1.5 to 17.5 dB, with a standard deviation of 7.2 dB as shown in Table 3.

Of the ten patients, eight patients were offered RWR surgery with five patients opting to have surgery due to disabling symptoms. Two were not offered surgery due to their surgical comorbidities. Of the five operative candidates, three had no CT findings to explain their TWS, while two patients had LSCC-VII dehiscence. Patients were aware of the general risks of RWR surgery and were informed that the surgery may not resolve these disabling symptoms prior to proceeding. In our series there were six surgeries performed (with one patient having bilateral surgery), The patient who had bilateral surgery initially had unilateral RWR surgery but due to drastic improvements in symptoms unilaterally they opted to undergo surgery on the contralateral side. Two patients did not want to proceed with surgery but preferred to watch and wait given their symptoms, and one patient did not want to proceed with a general anesthetic. All six procedures were performed via endaural approach and in all cases temporalis fascial grafts were used. There were no immediate intraoperative complications associated with the six separate procedures. One candidate felt unwell, and repeatedly blew their nose within two weeks of the initial surgery, and although initially having improvement of their symptoms, these symptoms recurred once the coryzal symptoms resolved and they subsequently underwent revision surgery 6 months later, with the same operative method. One other patient was re-referred to clinic 1 year post-surgery due to mild sound induced symptoms returning, however symptoms were not as debilitating as pre-surgery. The other three remaining surgical candidates had complete resolution of symptomatology at the 6-week postoperative follow-up.

As represented in Table 4, of the five operative candidates, two patients had a normalization of their cVEMP threshold postoperatively. Two patients had ongoing reduced cVEMP thresholds after surgery with one of those patients requiring revision surgery as mentioned above. Following their revision surgery, a repeat cVEMP test was not performed. The second patient having subjective resolving of their symptoms despite having a reduced cVEMP threshold after RWR surgery.

PTA-ABG was measured postoperatively across the five operative candidates the mean preoperative ABG across 500–3,000 Hz was 10.7±7.16, which improved to 0.73±2.45 dB postoperatively. A paired t-test demonstrated a mean reduction of 9.98±7.46 dB following surgery; this difference approached statistical significance [t(4)=2.99, P=0.04]. These findings suggest a clinically meaningful improvement in conductive hearing function, although the small sample size may have limited statistical power.

Discussion

This study presents a cohort of ten patients with TWS, all of whom exhibited at least one disabling symptom—autophony, subjective hearing loss, sound-induced tinnitus, or sound induced vertigo—as their primary complaint. CT findings included the recently described LSCC–VII dehiscence and a single case of carotid-cochlear dehiscence.

Among our ten patients, eight were offered surgical management. Of the remaining two patients, one patient had an incidental finding of a vestibular schwannoma in the affected ear and the other patient was precluded from surgery due to age and comorbidities. Of the eight patients whom were offered surgery only five patients opted for surgery due to disabling symptoms affecting their occupation. Following surgery all five patients were asked specifically about their sound induced symptoms, their pulsatile tinnitus and subjective hearing and all five had complete resolution in subjective symptom outcomes with all patients followed-up at six weeks and repeat cVEMP testing was performed at least four months after surgery. The cVEMP threshold postoperatively normalised in three of the five patients. All patients had reductions in their ABG which may indicate that ABG may be a more sensitive and objective measure of surgical response than the cVEMP response. It is important to note that one of the patients who had surgery initially had complete resolution of their symptoms initially but contracted COVID and had multiple coughing and sneezing fits within the first 2 weeks of surgery and had a recurrence of their symptoms and had a subsequently reduced cVEMP threshold. The patient opted to undergo revision surgery but a repeat cVEMP after the revision surgery was not performed however subjectively the patient had resolution of their symptoms after the second revision surgery.

LSCC-VII dehiscence has only recently been described (11). The images in Figure 1 are from the patients in our cohort who have had this finding identified. Anatomically the tympanic segment of the facial nerve runs inferior to the LSCC. These images show the dehiscence and in the case of the image on the left, complete absence of the bone between the facial nerve and the LSCC.

Figure 1 Coronal reconstruction (A) of high-resolution temporal bone CT of the right temporal bone of a 47-year-old woman with right autophony, demonstrating dehiscence (arrow) of bone separating the superior surface of the proximal tympanic segment of the facial nerve canal from the inferior surface of the anterior limb of the lateral semicircular canal (lateral semicircular canal-facial canal dehiscence). This patient also had dehiscence of bone along the undersurface of the tympanic segment of the facial nerve canal (arrowhead), a very common normal variant. Oblique sagittal reconstruction (B) from the same CT reconstructed along the plane of the tympanic and mastoid segments of the facial nerve canal (diamonds) showing the same small focal dehiscence (arrow) of bone superior to the tympanic segment of this canal, separating it from the lateral semicircular canal (dot). CT, computed tomography.

Figure 2 shows the one patient who had cochlear-carotid dehiscence. Thinning of the bony plate separating the carotid canal from other anatomic structures can occur anywhere along the course of the canal, including the cochlear-carotid bony plate (9).

Figure 2 Sagittal reconstruction (A) and the axial image (B) showing dehiscence (green line in A, and green arrow in B) of the bone between the carotid canal and the apical turn of the cochlea.

It is noted that RWR surgery via the endaural approach can be limited by an obstructive RW overhang or the presence of adhesions. However, these challenges can often be overcome with a limited posterior or anterior canalplasty. In the cases described, the RW overhang was intentionally preserved, as it provided structural support for securing the tissue graft placed into the RW niche.

TWS is a relatively new entity, with Minor et al. first describing SCCD in 1998. Since then, there has been a plethora of new pathologies found as potentially as a cause for TWS. Initially CT imaging was considered the gold standard for diagnosis for TWS pathology (7). Due to recent findings of various pathologies and even “CT negative” TWS, there appears to be emerging opinions amongst several authors that for diagnosis there has to be a collective approach involving symptoms, electrophysiological findings and CT findings rather than relying on CT findings alone. In a retrospective study by Mikulec it was found that 94% of patients with SCCD or symptoms consistent with SCCD experienced autophony and aural fullness and 86% had pseudoconductive hearing loss (13).

Over the past few decades there have been many new CT findings and tools to diagnose TWS (9). Despite new findings patients present with similar symptoms and signs of physical examination regardless of the specific site of dehiscence (11). It is important to use a comprehensive approach when offering surgery, not just using CT findings or electrophysiological testing. Thus, taking a full history and identifying the lifestyle impact for the patient is imperative.

Although considered to be accurate it should be noted that cVEMP testing can be prone to error as this test requires subjects to tense their neck muscles to potentiate the response and often this can be difficult to perform consistently if patients have musculoskeletal neck issues (14). Interestingly, Fife et al. noted in their study that non-SCCD causes of third window pathology can be poorly represented on cVEMP findings (15). This study was limited by a small sample size of 10 patients with only five operative candidates. A multicentre study and collaborative efforts are likely required to obtain a larger cohort of patients to assess imaging, electrophysiological findings and surgical outcomes. This would also assist in defining the true incidence of a LSCC-VII amongst a cohort of patients with TWS.

However, despite extensive research, there is currently no formal consensus on what should constitute TWS which makes comparing patient cohorts difficult. Newly recognised dehiscences as described in this study further complicate attempts to unify diagnostic criteria and we anticipate that with improvements in imaging techniques, the detection of known and yet to be described otic capsule dehiscences may be further described in the future (7,16).

Despite the lack of uniform diagnostic criteria, in our cohort of patients, all patients had at least one major symptom of autophony, subjective hearing loss or sound induced tinnitus, or vertigo as their main presenting complaint and that these symptoms were disabling. This presentation tends to guide the need for further investigation including the relevant electrophysiological and audiological studies to be performed. Objective audiological testing findings can also be variable with a wide variety of ABGs and the fact that reduced cVEMP thresholds were not present in all patients, may reflect the challenges of c-VEMP testing but also the variable size of the dehiscence which may be too small to characterise on routine high resolution temporal bone CT scans. Modern cone-beam CT scans of the temporal bone may offer more detail of the otic capsule in this regard. As the spectrum of TWS continues to grow, multicenter collaborations will be essential to improve diagnostic accuracy, validate new anatomical findings, and refine patient selection for surgery. Such efforts will also help determine the true incidence and clinical significance of rare entities like LSCC-VII dehiscence.

Conclusions

TWS symptoms are increasingly recognized as not solely due to SCCD syndrome and should prompt investigation with audiometry and cVEMP testing as well as high resolution CT scans of the temporal bones. The absence of SCCD should prompt clinicians to assess for other otic capsule dehiscences including a newly recognized entity of LSCC-VII dehiscence. Whilst the true incidence of this condition is unknown, its presence in the majority of our patients with TWS and previously deemed “negative CT” requires closer examination of the CT scan by a head and neck radiologist familiar with this condition. Whilst broad consensus amongst the literature is required to define CT negative TWS, in our experience, RWR surgery could potentially be an effective treatment for patients with disabling symptoms, however more research needs to be conducted in this area.

Acknowledgments

Our abstract was accepted for presentation at the ASOHNS 72^nd Annual Science Meeting in Adelaide 2022 presented between 10–12^th of June 2022, a similar abstract was also accepted 15^th Asia Oceana ORL-HNS congress in Brisbane between 10–12^th of March 2023. A version was presented at the Royal Australia and New Zealand College of Radiologist 72^nd ASM in 2022 between 27–30^th of October 2022.

Footnote

Data Sharing Statement: Available at https://www.theajo.com/article/view/10.21037/ajo-24-81/dss

Peer Review File: Available at https://www.theajo.com/article/view/10.21037/ajo-24-81/prf

Funding: None.

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://www.theajo.com/article/view/10.21037/ajo-24-81/coif). J.K. serves as an unpaid editorial board member of Australian Journal of Otolaryngology from January 2019 to December 2027. The other authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. The study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. The study was approved by institutional ethics committee at Sir Charles Gairdner Hospital (approval No. 46643). Patient confidentiality was maintained by anonymizing data, and individual consent was waived for this retrospective analysis.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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