Posterior Semicircular Canal Dehiscence: Case Series and Systematic Review

When the Jugular Bulb Contacts the Facial Nerve

Dehiscences of the intrapetrous canal of the facial nerve (FN) and those of a high jugular bulb (HJB) have various incidences. However, the HJB-FN common dehiscent osseous wall is a scarce finding and rarely reported. Thus, this work aimed to study this peculiar intrapetrous morphology on computed tomograms (CTs). A batch of 60 archived CTs was used. In 28.33% of cases, no HJBs were found; in 51.67% of cases, there were unilateral HJBs, and in 20% of cases, bilateral HJBs were found. 83.87% of the unilateral HJBs were on the right side, and the rest on the left side. In 3 cases with right-sided HJBs, 1 male and 2 females, FNs were found directly contacting the HJB. Therefore, the contact HJB-FN occurred in 5% of cases and 2.5% of the petrous bones/sides. HJBs' dehiscences to the retrotympanum or the posterior cranial fossa were also found in those cases. A dehiscent posterior semicircular canal to the HJB was found in one of these. Incidences for the HJB and the HJB-FN contacts seem higher than in previous studies. However, as only a 60-case batch of study was used, an overestimation may have resulted. Although scarce, the HJB-FN contact could determine specific symptoms and must be documented before specific surgical procedures involving the mastoid, retrotympanum, and posterior cranial fossa.

Posterior Semicircular Canal Dehiscence with Vestibulo-Ocular Reflex Reduction for the Affected Canal at the Video-Head Impulse Test: Considerations to Pathomechanisms

Posterior semicircular canal dehiscence (PSCD) has been demonstrated to result in a third mobile window mechanism (TMWM) in the inner ear similar to superior semicircular canal dehiscence (SSCD). Typical clinical and instrumental features of TMWM, including low-frequency conductive hearing loss (CHL), autophony, pulsatile tinnitus, sound/pressure-induced vertigo and enhanced vestibular-evoked myogenic potentials, have been widely described in cases with PSCD. Nevertheless, video-head impulse test (vHIT) results have been poorly investigated. Here, we present six patients with PSCD presenting with a clinical scenario consistent with a TMWM and an impaired vestibulo-ocular reflex (VOR) for the affected canal on vHIT. In two cases, an additional dehiscence between the facial nerve and the horizontal semicircular canal (HSC) was detected, leading to a concurrent VOR impairment for the HSC. While in SSCD, a VOR gain reduction could be ascribed to a spontaneous "auto-plugging" process due to a dural prolapse into the canal, the same pathomechanism is difficult to conceive in PSCD due to a different anatomical position, making a dural herniation less likely. Alternative putative pathomechanisms are discussed, including an endolymphatic flow dissipation during head impulses as already hypothesized in SSCD. The association of symptoms/signs consistent with TMWM and a reduced VOR gain for the posterior canal might address the diagnosis toward PSCD.

Iatrogenic Third Window After Retrosigmoid Approach to a Vestibular Schwannoma Managed with Cochlear Implantation

OBJECTIVE

To present a case of an iatrogenic inner ear third window after vestibular schwannoma microsurgery.

PATIENTS

A 42-year-old male presented 9-months after left-sided retrosigmoid approach for an intracanalicular vestibular schwannoma with hearing-preservation attempt performed elsewhere. Immediately postoperatively, he developed the following disabling and persistent symptoms on the ipsilateral side: autophony, pulsatile tinnitus, high-pitched ringing tinnitus, and hearing his footsteps. He denied vertigo. Otoscopy was normal. Tuning fork (512-Hz) lateralized to the left and Rinne was negative on the left. Audiogram demonstrated a severe mixed hearing loss and 10% aided word-recognition score. High-resolution CT demonstrated violation of the common crus and dehiscence of bone along the medial vestibule suggestive of an iatrogenic inner ear third window.

INTERVENTIONS

Labyrinthectomy and concurrent cochlear implantation.

MAIN OUTCOME MEASURES

Resolution of third window symptoms, open-set speech recognition, tinnitus suppression.

RESULTS

Patient reported immediate resolution of third window symptoms after labyrinthectomy and cochlear implantation. He demonstrates open-set word recognition of 64% at 1-year postoperatively and tinnitus suppression with his cochlear implant on.

CONCLUSIONS

Iatrogenic third window symptoms can occur after hearing-preservation vestibular schwannoma microsurgery. Patients with sufficient hearing preservation who are disabled by third window symptoms yet lack sound clarity and useful hearing may be considered for labyrinthectomy and concurrent cochlear implantation. This intervention effectively extinguishes third window symptoms by destroying residual auditory function and simultaneously provides an opportunity to restore useful hearing and suppress tinnitus, thereby enhancing overall quality of life.

Rare Disorders of the Vestibular Labyrinth: of Zebras, Chameleons and Wolves in Sheep's Clothing

The differential diagnosis of vertigo syndromes is a challenging issue, as many - and in particular - rare disorders of the vestibular labyrinth can hide behind the very common symptoms of "vertigo" and "dizziness". The following article presents an overview of those rare disorders of the balance organ that are of special interest for the otorhinolaryngologist dealing with vertigo disorders. For a better orientation, these disorders are categorized as acute (AVS), episodic (EVS) and chronic vestibular syndromes (CVS) according to their clinical presentation. The main focus lies on EVS sorted by their duration and the presence/absence of triggering factors (seconds, no triggers: vestibular paroxysmia, Tumarkin attacks; seconds, sound and pressure induced: "third window" syndromes; seconds to minutes, positional: rare variants and differential diagnoses of benign paroxysmal positional vertigo; hours to days, spontaneous: intralabyrinthine schwannomas, endolymphatic sac tumors, autoimmune disorders of the inner ear). Furthermore, rare causes of AVS (inferior vestibular neuritis, otolith organ specific dysfunction, vascular labyrinthine disorders, acute bilateral vestibulopathy) and CVS (chronic bilateral vestibulopathy) are covered. In each case, special emphasis is laid on the decisive diagnostic test for the identification of the rare disease and "red flags" for potentially dangerous disorders (e. g. labyrinthine infarction/hemorrhage). Thus, this chapter may serve as a clinical companion for the otorhinolaryngologist aiding in the efficient diagnosis and treatment of rare disorders of the vestibular labyrinth.

Case Report: Could Hennebert's Sign Be Evoked Despite Global Vestibular Impairment on Video Head Impulse Test? Considerations Upon Pathomechanisms Underlying Pressure-Induced Nystagmus due to Labyrinthine Fistula

We describe a case series of labyrinthine fistula, characterized by Hennebert's sign (HS) elicited by tragal compression despite global hypofunction of semicircular canals (SCs) on a video-head impulse test (vHIT), and review the relevant literature. All three patients presented with different amounts of cochleo-vestibular loss, consistent with labyrinthitis likely induced by labyrinthine fistula due to different temporal bone pathologies (squamous cell carcinoma involving the external auditory canal in one case and middle ear cholesteatoma in two cases). Despite global hypofunction on vHIT proving impaired function for each SC for high accelerations, all patients developed pressure-induced nystagmus, presumably through spared and/or recovered activity for low-velocity canal afferents. In particular, two patients with isolated horizontal SC fistula developed HS with ipsilesional horizontal nystagmus due to resulting excitatory ampullopetal endolymphatic flows within horizontal canals. Conversely, the last patient with bony erosion involving all SCs developed mainly torsional nystagmus directed contralaterally due to additional inhibitory ampullopetal flows within vertical canals. Moreover, despite impaired measurements on vHIT, we found simultaneous direction-changing positional nystagmus likely due to a buoyancy mechanism within the affected horizontal canal in a case and benign paroxysmal positional vertigo involving the dehiscent posterior canal in another case. Based on our findings, we might suggest a functional dissociation between high (impaired) and low (spared/recovered) accelerations for SCs. Therefore, it could be hypothesized that HS in labyrinthine fistula might be due to the activation of regular ampullary fibers encoding low-velocity inputs, as pressure-induced nystagmus is perfectly aligned with the planes of dehiscent SCs in accordance with Ewald's laws, despite global vestibular impairment on vHIT. Moreover, we showed how pressure-induced nystagmus could present in a rare case of labyrinthine fistulas involving all canals simultaneously. Nevertheless, definite conclusions on the genesis of pressure-induced nystagmus in our patients are prevented due to the lack of objective measurements of both low-acceleration canal responses and otolith function.

Persistent Conductive Hearing Loss After Tympanostomy Tube Placement Due to High-Riding Jugular Bulb

High-riding jugular bulb (HRJB) is a rare condition not often observed in the clinical setting that occurs in 1% to 3% of cases. The jugular bulb is not present at birth, and the precise size and location likely depends on a myriad of postnatal events. This report describes the case of a male adolescent who experienced persistent conductive hearing loss (CHL) unilaterally following bilateral tympanostomy tube placement. Subsequent workup included computed tomography, which identified a very high jugular bulb eroding the posterior semicircular canal and occluding the round window niche. The patient had no hearing or vestibular symptoms aside from CHL and continues to be observed on a regular basis. HRJB is a rare disorder that has been known to erode the posterior semicircular canal, resulting in possible tinnitus, vertigo, dizziness, and/or sensorineural hearing loss. CHL has been reported in HRJB cases, although it is uncommon. HRJB may result in CHL through a third-window defect shunting hydromechanical energy away from the round window or due to middle ear blockage. Imaging is useful in ascertaining rare causes of CHL, such as HRJB. Because HRJB is not easily fixable, it is important to recognize it as a rare cause of CHL for appropriate patient counseling. Possible interventions should be tailored to the patient after careful consideration of contralateral anatomy and likely benefits. Laryngoscope, 131:E1272-E1274, 2021.

Concomitant Dehiscences of the Temporal Bone: A Case-Based Study

Otic capsule dehiscences create a pathological third window in the inner ear that results in a dissipation of the acoustic energy consequent to the lowered impedance. Superior semicircular canal dehiscence (SSCD) was identified by Minor et al in 1998 as a syndrome leading to vertigo and inner ear conductive hearing loss. The authors also reported the relation between the dehiscence and pressure- or sound-induced vertigo (Tullio's phenomenon). Prevalence rates of SSCD in anatomical studies range from 0.4% to 0.7% with a majority of patients being asymptomatic. The observed association with other temporal bone dehiscences, as well as the propensity toward a bilateral or contralateral "near dehiscence," raises the question of whether a specific local bone demineralization or systemic mechanisms could be considered. The present report regard a case of a patient with a previous episode of meningitis, with a concomitant bilateral SSCD and tegmen tympani dehiscence from the side of meningitis. The patient was affected by dizziness, left moderate conductive hearing loss, and pressure/sound-induced vertigo. Because of disabling vestibular symptoms, the patient underwent surgical treatment. A middle cranial fossa approach allowed to reach both dehiscences on the symptomatic side, where bone wax and fascia were used for repair. At 6 months from the procedure, hearing was preserved, and the vestibular symptoms disappeared.