Hearing Loss after Round Window Surgery in Mice Is due to Middle Ear Effusion

Abstract

Background: Delivery of therapeutic agents directly through the round window (RW) offers promise for treating sensorineural hearing loss. However, hearing loss can result from the surgical approach itself, and the reasons for this are poorly understood. We examined the hearing loss following the 3 major steps involved with the RW approach to access the mouse cochlea: bullostomy, RW puncture, and RW injection. Methods: Twenty-one adult CBA/J mice underwent bullostomy alone, 10 underwent RW puncture, and 8 underwent RW injection with PBS with 5% glycerol. Auditory brainstem responses (ABR) and otoscopy were performed preoperatively and up to 6 weeks postoperatively. Hair cells were stained, and survival was assessed using immunofluorescence. Results: One week postoperatively, mice in all groups showed significant threshold shifts. Otoscopy revealed approximately half of all mice had middle ear effusion (MEE), with a higher incidence of effusion in the RW puncture and RW injection groups. Those with MEE had significant ABR threshold shifts, whereas those without MEE had minimal hearing loss. MEE persisted through 6 weeks in a majority of cases, but in those mice with MEE resolution, there was at least partial improvement in hearing. Immunohistochemistry showed minimal loss of hair cells in all animals. Conclusion: MEE is highly correlated with hearing loss in mice undergoing RW surgery. Otoscopy is an important adjunct to consider after ear surgery in mice, as MEE may contribute to postsurgical hearing loss.

AAV2.7m8 is a powerful viral vector for inner ear gene therapy

“…The adult mouse otic capsule was completely ossified (in contrast to the neonatal mouse otic capsule, which is cartilaginous). Therefore, the adult mouse inner ear gene delivery was done using the round window approach 13,27,28 . A post-auricular incision was made using small scissors.…”

Antisense Oligonucleotides for the Treatment of Inner Ear Dysfunction

“…The latter 2 methods have an advantage in that they deliver directly to the perilymphatic system, bypassing adsorption barriers, though, at the same time, these may be considered more invasive and with more potential risks than systemic or intratympanic delivery approaches. Indeed, round window injection has variable efficacy, possibly due to the increased risk of middle ear effusion [31]. Canalostomy may have some advantages for drug delivery directly to the inner ear without the risk for hearing loss [22,32].…”

rAAV-Mediated Cochlear Gene Therapy: Prospects and Challenges for Clinical Application

“…The viral vectors are, thus, in contact with the basilar membrane, from where they can pass to the endolymphatic space and reach the targeted cells. However, a transient elevation of hearing thresholds is often observed after opening the RWM and injecting viral vectors in adult mice [65,66]. Xia et al [67] showed that opening the RWM followed by injection of viral vectors did not induce cochlear damage in neonatal mice.…”

“…For the successful clinical translation of gene therapy for treating or preventing inner ear genetic diseases, important questions such as off-target side effects, biodistribution of vector components, and the risk of carcinogenesis remain to be addressed through preclinical trials used in non-human primates or humanized models such as: (i) explants of human adult cochlear epithelium from surgical resection of the cochlea or from donors in the state of brain death, and (ii) inner-ear organoids generated from human-derived iPSCs [65,66] or from deaf patients. The generation of in vitro pathological models for each genetically deaf patient would allow transitions into personalized and precise medicine.…”