Acoustic-Vestibular and Histologic Examinations in Guinea-Pigs Afterinterruption of Membranous Labyrinth in Semicircular Canals or Cochlea

The History and Evolution of Surgery on the Vestibular Labyrinth

The history of surgery on the vestibular labyrinth is rich but sparsely documented in the literature. The story begins over a century ago with the labyrinthectomy in an era that consisted exclusively of ablative surgery for infection or vertigo. Improved understanding of vestibular physiology and pathology produced an era of selective ablation and hearing preservation that includes semicircular canal occlusion for benign paroxysmal positional vertigo. An era of restoration began with a discovery of superior semicircular canal dehiscence and its repair. The final era of vestibular replacement is upon us as the possibility of successful prosthetic vestibular implantation becomes reality.

Sensitivity of the cochlear nerve to acoustic and electrical stimulation months after a vestibular labyrinthectomy in guinea pigs

Single-sided deafness patients are now being considered candidates to receive a cochlear implant. With this, many people who have undergone a unilateral vestibular labyrinthectomy for the treatment of chronic vertigo are now being considered for cochlear implantation. There is still some concern regarding the potential efficacy of cochlear implants in these patients, where factors such as cochlear fibrosis or nerve degeneration following unilateral vestibular labyrinthectomy may preclude their use. Here, we have performed a unilateral vestibular labyrinthectomy in normally hearing guinea pigs, and allowed them to recover for either 6 weeks, or 10 months, before assessing morphological and functional changes related to cochlear implantation. Light sheet fluorescence microscopy was used to assess gross morphology throughout the entire ear. Whole nerve responses to acoustic, vibrational, or electrical stimuli were used as functional measures. Mild cellular infiltration was observed at 6 weeks, and to a lesser extent at 10 months after labyrinthectomy. Following labyrinthectomy, cochlear sensitivity to high-frequency acoustic tone-bursts was reduced by 16 ± 4 dB, vestibular sensitivity was almost entirely abolished, and electrical sensitivity was only mildly reduced. These results support recent clinical findings that patients who have received a vestibular labyrinthectomy may still benefit from a cochlear implant.

Human experience with canal plugging

Canal occlusion/plugging is a very effective technique with a low risk to hearing. The series of posterior semicircular canal occlusions described in this review now becomes the largest in the literature to date. It continues to support this procedure as the treatment of choice for intractable benign paroxysmal positional vertigo (BPPV). All 44 operated ears were relieved of BPPV, with one patient having an atypical late recurrence. Of the 40 ears with normal preoperative hearing, one had a delayed (3-month) sudden and permanent profound loss, while one other had a mild (20 dB) loss. Six patients had protracted courses of imbalance and motion sensitivity. Canal plugging has led to several new and innovative developments including the partial labyrinthectomy for difficult-to-access skull base lesions and superior semicircular canal plugging for dehiscence. These new procedures and their development are reviewed in this paper.

Partial labyrinthectomy with hearing preservation: frequency-specific data using tone-burst auditory brain stem response

Surgical approaches to the inner ear with hearing preservation have valuable implications for neurotologic surgery. In a previous study in guinea pigs, we demonstrated that click-evoked auditory brain stem responses (ABRs) were preserved after transection and plugging of 1 or more semicircular canals but were lost after entering the vestibule (Smouha EE, et al. Otolaryngol Head Neck Surg 1996; 114:777-784). A limitation of that study was that click-evoked ABR might not represent the function of the entire cochlea. In this study we used tone-burst ABR to determine thresholds across a broad range of frequencies (2 to 24 kHz) before and after surgical entry into the labyrinth at the lateral semicircular canal, ampulla, and vestibule. Serial measurements were made, and a sham surgery group was used as a control. The results obtained with tone-burst ABR generally agreed with those previously obtained with click stimuli. Toneburst ABR thresholds were similar across most frequencies tested. Transection of the lateral semicircular canal resulted in preservation of ABR thresholds. Ampullectomy had a variable effect on ABR thresholds. Vestibulotomy usually resulted in substantial hearing loss. We conclude that hearing can be preserved across a range of frequencies after selective surgery of the inner ear. Surgical entry into the membranous labyrinth near the vestibule is the critical factor contributing to hearing loss after partial labyrinthectomy.

Partial Labyrinthectomy with Hearing Preservation: An Experimental Study in Guinea Pigs

Recent advances in neurotologic surgery have challenged the traditional belief that violating the labyrinth is incompatible with hearing. Our aim in this study was to define the conditions that result in hearing preservation and hearing loss after surgery on the labyrinth. A guinea pig model was developed. Click-evoked auditory brain stem responses were used to determine hearing thresholds. Animals underwent surgical destruction of part or ail of the vestibular labyrinth. Transection and plugging of the lateral semicircular canal resulted in normal hearing. Transection of multiple semicircular canals also resulted in hearing preservation. Intentional suctioning of perilymph from a transected canal led to transient hearing loss with complete recovery. Sequential destruction of the entire lateral semicircular canal resulted in preserved hearing as long as the vestibule was not opened. Wide vestibulotomy resulted in hearing loss. Preliminary histologic studies showed that cochlear hair cells were preserved in most cases. The results of our experiments demonstrate the feasibility of preservation of hearing after partial labyrinthectomy and provide physiologic criteria for developing new operations on the inner ear in human subjects.

Modifying the translabyrinthine approach to preserve hearing during acoustic tumour surgery

Removing an acoustic schwannoma using the translabyrinthine approach has previously been considered incompatible with hearing preservation. By modifying the approach and preventing the loss of endolymph, we have successfully removed an intracanalicular acoustic schwannoma, which originated from the inferior vestibular nerve, and preserved hearing in the operated ear. This report represents the preliminary findings using this particular technique in the management of an intracanalicular acoustic tumour.