Round And Oval Window Reinforcement

Anatomy and Physiology

The malleus, incus, and stapes, known collectively as the ossicles, are bones of the middle ear that mechanically convert the vibration of the tympanic membrane into fluid pressure waves in the cochlea. The malleus is divided into the head, neck, manubrium, anterior process, and lateral process. The incus is divided into the body, short crus, long crus, and lenticular process. The stapes is divided into the head, neck, crus anterius, crus posterius, and footplate.[20]

The inner ear comprises the bony and membranous labyrinths. The membranous labyrinth includes the cochlea, semicircular canals, utricle, and saccule. The cochlea is a spiral-shaped, fluid-filled organ with two openings into the middle ear space, the oval and round windows. The stapes footplate overlies the oval window, and the mechanical energy from ossicular chain movement is transmitted through the oval window membrane to initiate a fluid wave through the inner ear. The perilymph wave propagates through the scala vestibuli, helicotrema, and scala tympani, terminating at the round window. The round window is situated below the promontory within the round window niche.[21] The round window vibrates at an opposing phase to the oval window to counteract the fluid shift created within the cochlea. This mechanism permits free movement of fluid throughout the cochlea and improves sound transmission.[22]

As the mechanical vibrations from the middle ear move through the perilymph, the hair cells found within the organ of Corti in the cochlea are displaced, causing mechanically gated potassium channels to open and allowing depolarization of the spiral ganglion neurons. These neurons are tonotopically organized; the cells at the base of the cochlea close to the oval window respond to high-frequency sounds, and cells at the apex near the helicotrema respond to low-frequency sounds.[23] In this way, the cochlear nerve is activated, and the signal is sent to the central nervous system. A review of the central processing of sound is beyond the scope of this activity.

Indications

The goal of treating hyperacusis with round and oval window reinforcement is to reduce the mechanical energy that is transmitted from the ossicular chain to the inner ear, reduce the fluid displacement within the cochlea, and improve the LDL test score. The goal of the procedure is similar for patients with superior semicircular canal dehiscence (SSCD) and other third window pathologies that often are the root cause of autophony, as well as other bothersome audiological phenomena; employing the procedure in this population is more controversial. However, round and oval reinforcement is often used as a low-risk first-line option in patients with SSCD who are unsuitable for or uninterested in more invasive procedures.

Contraindications

The contraindications to round and oval window reinforcement include but are not limited to active infection of the middle ear, comorbidities precluding anesthesia, a preexisting diagnosis of anxiety or depression, and Ménière disease. If the ear that requires reinforcement is the sole functional or hearing ear, surgical window reinforcement is contraindicated.

Equipment

The equipment required to perform oval and round window reinforcement typically includes:

• Otoscopic microscope
• Ear speculum holder
• Otologic drill with 2-diamond and 1-diamond drill bits
• Otologic instrument tray
• 2- and 4-mm biopsy punches

Personnel

The personnel typically required to perform oval and round reinforcement include:

• Primary surgeon, typically a neurotologist or experienced general otolaryngologist
• Anesthesia provider
• Surgical technician or operating room nurse
• Circulating or operating room nurse

Preparation

The evaluation of patients with hyperacusis should include a comprehensive medical history and thorough physical examination, including a focused head and neck exam, otoscopic exam, and tuning fork evaluation. Preoperative pure tone audiogram and impedance audiometry are performed to assess the nature and degree of hearing loss. Thin-cut (<1 mm) computed tomography of the temporal bones with Poschl and Stenvers views is recommended for surgical planning and to rule out pathologies such as SSCD or perilymphatic fistulas. If SSCD is suspected, electrocochleography (ECoG), cervical vestibular evoked muscle potentials (cVEMP), and ocular evoked muscle potentials (oVEMP), while uncommonly performed, may be of diagnostic utility.

Before surgical intervention, patients must be screened for hyperacusis sensitivity using the Hyperacusis Questionnaire and undergo a Loudness Discomfort Level (LDL) test. The LDL test is an audiological assessment measuring sound sensitivity. Pure tone thresholds at 250, 500, 1000, 2000, 3000, 4000, and 8000 Hz are obtained. For each tested frequency, the decibels are slowly increased until the tone reaches an uncomfortable level. The average LDL in normal-hearing patients is approximately 100 dB HL.[22]

Patients must undergo preoperative counseling regarding the risks of the procedure, including but not limited to infection, transient taste disturbances, partial or complete sensorineural hearing loss, incomplete resolution of symptoms, and possible need for additional procedures. Patients should be offered alternative therapeutic options such as cognitive behavioral therapy or hearing aids.

Technique or Treatment

Round and oval window reinforcement is traditionally an outpatient procedure performed under general anesthesia. Electromyography (EMG) leads should be placed to monitor the facial nerve throughout the procedure. The ear should be prepped and draped in the normal sterile fashion. The external auditory canal should be examined and cleaned of debris using otomicroscopy. The procedure is performed via a transmeatal approach.

Transmeatal Approach

The tympanomeatal flap is elevated by making lateral incisions from the annulus towards the auditory meatus, superiorly at the 12-o'clock position and inferiorly at the 6-o'clock position. These incisions are joined by a transverse incision laterally to form a medially-based U-shaped flap. The canal skin is elevated toward the annulus, which is raised off the bony groove. The tympanomeatal flap is reflected forward to access the posterior mesotympanum. If further visualization of the stapes footplate and oval window is required, the posterior scutum can be taken down using a curette and 2- or 1-diamond burr. Care must be taken to limit injury to the chorda tympani traversing the anterior border of the scutum.

Temporalis Fascia Harvest

Execute a 2-cm horizontal incision above the hairline overlying the temporalis muscle. Place a Weitlaner retractor to facilitate visualization. Incise the superficial temporalis fascia to reveal the underlying deep temporalis fascia. Harvest a section of deep temporalis fascia measuring 2 x 2 cm and place it on a fascial press. Excise 10 to 13 pieces of this fascia using the 2-mm biopsy punch and 1 piece using the 4-mm biopsy punch.

Temporalis Fascia Placement

After accessing the middle ear cavity, visually examine the ossicle for malformations and gently palpate them to assess for mobility. The mobility of the stapes is classified as normal mobility, mild hypermobility, or severe hypermobility, correlating to a grade of 1, 2, or 3, respectively.[3] 

Visualize the round window niche. Roughen the anterior promontory and inferior surface of the niche with a curette or otologic drill fitted with a 2- or 1-diamond burr to increase the likelihood of scarring onto the fascia. Place 2 pieces of 2-mm temporalis fascia into the round window niche and drape a 4-mm fascial piece over the promontory. Place a piece of Gelfoam^® in the center of the 4-mm fascial piece to help tuck the fascia into the niche.

Address the oval window by placing multiple pieces of 2-mm fascia circumferentially around the oval window and on top of the stapes footplate. Gently return the tympanic membrane to its normal anatomical position. Place antibiotic-soaked Gelfoam^®  lateral to the tympanic membrane to secure its position.

Postoperative Care

The patient may be discharged from the postoperative care unit once discharge criteria are met; some practitioners perform and document a facial nerve exam before discharge, but this is not universal. Patients are only admitted to the hospital under extenuating circumstances, such as intraoperative complications or significant comorbidities.

Patients should be evaluated in the clinic 1 week after round and oval window reinforcement procedures. The Gelfoam^® should be removed from the external auditory canal at that time. Some practitioners will initiate a 7- to 14-day course of otologic ciprofloxacin or ciprofloxacin-dexamethasone at the 1-week postoperative visit. Audiogram and LDL testing should be repeated 3 months postoperatively to evaluate surgical outcomes.

Complications

Complications of oval and round window reinforcement are rare but may include persistent tympanic membrane perforation and transient taste disturbances. Employing a conservative operative technique further reduces the already very low risk of postoperative displacement of the stapes, severe or complete sensorineural hearing loss, facial nerve injury, and vertigo.

Clinical Significance

Oval and round window reinforcement has been shown to improve sound tolerance in greater than 80% of patients with hyperacusis.[3] While the procedural benefits are slightly lower in patients with autophony, oval and round window reinforcement is still a viable surgical intervention for patients who cannot or will not undergo more invasive procedures.[19]

Enhancing Healthcare Team Outcomes

Patients with hyperacusis or autophony frequently present to a primary care practitioner, who should refer patients with these audiological disorders resulting in worsening quality of life to audiologists for hearing evaluation. If audiometry reveals findings suspicious for hyperacusis, autophony, or third window pathologies, further evaluation will be required.

The management of these disorders requires a multidisciplinary team, including but not limited to otolaryngologists, neurotologists, cognitive behavioral therapists, and audiologists. Not all patients with hyperacusis or autophony will require surgical intervention. However, when conservative therapeutic interventions fail to improve quality of life, procedures such as round or oval window reinforcement may be beneficial. For patients who elect to pursue surgical therapy, regular audiometric testing is required to document surgical outcomes.