Pathophysiology, Incidence, Epidemiology, and Natural History of Disease

The ability to hear is often taken for granted, yet hearing loss is one of the most common human deficiencies, affecting an estimated 1.1 billion people worldwide. ¹ In the United States alone, nearly 30 million people, older than age 12 years are estimated to have some degree of bilateral hearing loss. ² Roughly 1 in 20 persons is currently classified as deaf or hearing impaired, ³ and nearly 60,267 deaf children were reported in 2015. ⁴

There are varying degrees of hearing loss severity. Patients with normal hearing have pure tone thresholds of up to 25 decibels hearing level; patients with severe hearing loss have thresholds of 70 to 90 decibels hearing level, and patients with profound hearing loss have thresholds greater than 90 decibels hearing level. Persons with profound hearing loss may not hear even the loudest of sounds, such as music at a live rock concert or the roar of a jet taking off, although they may feel the vibrations.

Hearing loss is currently classified by the location of the impairment within the peripheral auditory pathway. Hearing loss can arise from conductive losses, such as the malfunction of the outer or middle ear apparatus, or sensorineural hearing losses, such as the loss of cochlear hair cell function, or compromise of the auditory cranial nerve (CN) (vestibulocochlear nerve [CN VIII]) transmission to the brain. The maximum amount of hearing loss that a conductive loss can produce is 60 decibels hearing level; any greater loss indicates sensorineural pathway involvement.

Patients with profound hearing loss comprise a small but nevertheless important subset of hearing-impaired patients who have limited rehabilitative options for amplification and restoration of their auditory system, as their handicap is beyond the limit of assistance provided by most hearing aid devices. Most patients for whom a hearing aid is not enough can benefit substantially from a cochlear implant. However, some of these persons do not have a viable auditory nerve or else have a malformation distortion in the inner ear anatomy, rendering a cochlear implant less likely to be useful. For this group, an auditory brainstem implant (ABI) may be a viable solution, as it transmits auditory information from the external world directly to the auditory brainstem and bypasses the malfunctioning circuitry of the inner ear and the auditory nerve. The ABI uses electrical stimulation of the cochlear nucleus in the brainstem to enable deaf persons to be aware of environmental and speech sounds.

An ABI was implanted experimentally for the first time in 1979 by House and Hitselberger in an adult with neurofibromatosis type 2 (NF2). ⁵ It was designed to stimulate the cochlear nucleus in the brainstem of persons lacking a viable auditory nerve. However, the Nucleus ABI (Cochlear) was not approved by the US Food and Drug Administration (FDA) until 20 years later, and then only for patients 12 years and older who had NF2. The device functions by processing the incoming auditory information into a band-pass filter, allowing different electrodes to be activated by various frequencies of speech. As of this writing, the device manufacturer with current FDA marketing approval for an ABI is Cochlear Americas (Centennial, CO, USA). Although the manufacturer of the cochlear implant manufactured by MED-EL (Innsbruck, Austria) does not yet have FDA approval for use in adults or children, patients who received an implant outside the United States are eligible for a “compassionate use exemption” specifically for device programming in the United States.

It has been nearly 40 years since the first ABI was implanted. Since then, more than 2,000 people have received ABIs worldwide. ⁶ An estimated 2.1% of potential cochlear implant candidates younger than 18 years of age currently meet criteria for an ABI in the United States. ⁴ Although the FDA-approved ABI candidacy criteria have remained unchanged in the United States for several years, candidacy in Europe has been broadened to include both prelingually and postlingually deafened patients. With the increasing need for other forms of amplification in children for whom a cochlear implant is not an option, the FDA in 2013 cleared safety trials of ABIs in young children being treated at several medical centers across the United States. ^{7 , 8 , 9}

Clinical Presentation

There are several causes of profound hearing loss for which an ABI may be the only option to obtain sound awareness, and in some cases, speech perception and understanding. Until recently, patients with NF2 were the most common group of patients seen in clinics for an ABI evaluation. These patients have an autosomal dominant mutation that causes the development of bilateral acoustic neuromas by young adulthood. These tumors have progressive growth, often stretching and damaging the vestibulocochlear nerve, which necessitates surgical resection and causes profound sensorineural hearing loss.

ABI candidacy has also been suggested for patients with certain congenital cochlear anomalies, such as severe bilateral cochlear or vestibulocochlear nerve aplasia. ⁷ In such cases, the patients usually have prelingual deafness, which is defined as deafness that occurred before their development of spoken language when they were younger than 2 years old. Other indications for an ABI include postmeningitis ossification of the cochleae and rare cases of bilateral temporal bone trauma that severely damage the cochleae or the vestibulocochlear nerves, for which insertion of the cochlear implant electrode may not be feasible. ⁴ Indications for non-NF2 prelingually deafened children without NF2 are controversial at this time, and many neurosurgeons are of the opinion that a cochlear implant trial should precede an ABI. Some ABI candidates have been reported to have derived similar benefits with a cochlear implant. ^{10 , 11 , 12} Table 33.1 ^{7 , 9 , 13} summarizes the current well-defined ABI candidacy requirements recommended by the FDA. It also includes a summary of the European consensus for ABI in adults and children.

There are currently no audiologic criteria set forth for ABI candidacy. In cases of abnormal (small or absent) vestibulocochlear nerves, the decision of whether to implant a cochlear implant or an ABI is made on a case-by-case evaluation, as a cochlear implant trial is typically warranted before an ABI in patients with appropriate cochlear anatomy. ^{7 , 10} The ABI is not yet approved for use in children younger than the age of 12 years in the United States, but it is undergoing FDA-approved clinical trials in several institutions nationwide. However, other centers around the world are currently placing ABIs in children. It is well accepted that children with cochlear malformations, such as a severe incomplete partition or common cavity malformations, can benefit greatly from a cochlear implant. ¹³ However, until more data are collected about the hearing outcomes in children who receive ABIs, we advise that if a cochlear or vestibulocochlear nerve is present on magnetic resonance imaging (MRI), a cochlear implant should be attempted first in children with cochlear malformations.

Perioperative Multidisciplinary Evaluation

The evaluation for ABI candidacy is comprehensive, requiring the involvement of the family and a multidisciplinary team in every step of the process. The team typically consists of neurotologists, neurosurgeons, audiologists, speech-language pathologists, radiologists, educational specialists, and a neuropsychologist, all of whom contribute to the primary goal of safely rehabilitating patients with deafness and providing meaningful sound recognition for the ABI recipient. The team members review all the medical records and test results to determine whether the patient meets the candidacy criteria for ABI surgery. In young children, the inclusion criteria should also ensure that the child has some language competence (typically a visual-based language), as well as strong family support and access to rehabilitative resources, which will be essential for the child after ABI surgery.

On initial evaluation, a thorough physical evaluation and history are vital when determining whether the patient is eligible to receive an ABI. It is important to ask about birth and family history, records of immunizations, otologic trauma or infections, and any prenatal exposure to ototoxic drugs. In prelingually deafened children, it might be necessary to obtain genetic testing and assess for any congenital syndromes when appropriate.

Because the underlying pathology for ABI candidacy is specific to the anatomy of the inner ear or the vestibulocochlear nerve, imaging is a vital tool in the preoperative evaluation. Both high-resolution computed tomography images of the temporal bone and MRI of the otic capsule and internal auditory canal play an important role in enabling the surgeon to evaluate the integrity of the vestibulocochlear nerve, the inner ear, and the surrounding brain anatomy. It is the combined results of the computed tomography and MRI that will guide the surgeon in deciding whether a cochlear implant or an ABI should be attempted. ¹⁴ Furthermore, imaging will reveal the development of the lateral recess of the fourth ventricle, as the current recommendation holds that the side with the better developed lateral recess should be implanted first with an ABI. ⁷ For patients who already have one cochlear implant with limited benefit, an ABI will often be placed on the opposite side.

An audiologic evaluation is indispensable for documenting bilateral profound sensorineural hearing loss. Several testing techniques can determine the presence and severity of a hearing loss. In a very young child, objective and behavioral measures are administered. The objective measures include auditory brainstem response (ABR), otoacoustic emissions, and acoustic immittance tests. Behavioral audiometry involves air- and bone-conduction thresholds (pure tones or noise bands), and speech audiometry (detection/reception/identification). Standard behavioral audiometry is performed in children older than 6 years of age who have no developmental delays. The audiologist will program the ABI device postoperatively and continue to evaluate the patient’s audiologic performance.

Speech-language evaluation in children is performed to determine baseline communication abilities. The speech-language pathologist may evaluate receptive and expressive language, articulation, and oral-motor function. In prelingually deafened children, the communication mode is typically some form of sign language, whereas older patients with progressive hearing loss may have already well-developed spoken-language abilities. Options for other modes of communication, such as sign language, should be discussed since an ABI is usually a last resort procedure and often does not provide enough auditory information for the patient to achieve normal spoken-language abilities similar to those of persons with normal hearing and persons who receive cochlear implants. The speech-language pathologist may provide targeted therapy postoperatively to facilitate auditory learning and support language development.

A neuropsychologist or a psychologist experienced in assessing patients who are deaf and hard of hearing will administer a series of tests to uncover developmental, social-emotional, and cognitive delays. This evaluation is particularly important for younger patients, because these conditions may hinder progress after ABI surgery. About 40% of pediatric patients with congenital hearing loss have additional special needs and thus require proper evaluation by, and support from, skilled professionals. ¹⁵ The psychologist and other members of the care team help set realistic expectations and reiterate the possible outcomes after an ABI. Evaluation of family support, for both pediatric and adult patients, is crucial because the postoperative period may become an extensive process with multiple follow-up examinations.

Before surgery, all patients should receive an age-appropriate polyvalent pneumococcal vaccine. Children younger than age 24 months should be administered Prevnar (pneumococcal 7-valent conjugate vaccine), and those older than 24 months should receive Pneumovax (pneumococcal vaccine polyvalent). ¹⁶ An additional vaccination against Haemophilus influenzae type b is suggested for all children younger than age 5 years. Although appropriate vaccinations are currently being administered, cases of postimplantation meningitis still occur. ¹⁷

Postoperatively, the patient requires hospitalization and is initially monitored in the neurosurgical intensive care unit. The patient will then be transferred to an inpatient unit with discharge planned within days after surgery. Both the duration of surgery and the hospital stay can be affected by whether the patient required simultaneous tumor removal during the surgery, which usually lasts longer for patients with NF2.

The patient also requires ongoing postoperative evaluations with clinical follow-up visits, audiologic assessment, ABI device programming, and appropriate rehabilitation. Thus, continual multidisciplinary patient care is vital to achieve the best hearing, speech, and language outcomes.

Surgical Approaches

ABI surgery requires a craniotomy. There are two surgical approaches: the translabyrinthine craniotomy and the retro-sigmoid suboccipital craniotomy. For patients with NF2 who have an acoustic neuroma, the surgeon can spend considerable time removing the tumor before ABI placement.

In the past, most adult recipients of ABIs were offered a translabyrinthine craniotomy for both the tumor removal and the simultaneous electrode placement. This approach is preferred when removal of large cerebellopontine angle tumors (> 2.0 cm) is required and when careful facial nerve dissection is necessary, as it allows for simultaneous removal of the tumor and early identification of the facial nerve. ¹⁸ Furthermore, it places large cranial vessels posterior to the tumor, and it does not necessitate cerebellar retraction, minimizing the risk of postoperative ataxia due to cerebellar edema. The translabyrinthine craniotomy provides direct exposure of the foramen of Luschka, allowing for easy placement of the electrode paddle. The main disadvantages of a translabyrinthine craniotomy compared with the retrosigmoid approach for ABI placement are longer surgery time, extensive bony drilling, and destruction of the vestibular labyrinth. Another downside to this approach is that it requires the harvesting of abdominal fat to use in obliterating the large bony defect.

In patients who need only an ABI placement, without tumor removal, the retrosigmoid approach is usually favored. This approach preserves both the cochlea and the vestibular labyrinth. The retrosigmoid approach is also preferred for younger patients, as this population has a small temporal bone that makes translabyrinthine craniotomy more challenging. Although the retrosigmoid approach has been widely used in Europe for ABI recipients with and without tumors, the approach is slowly gaining prominence in the United States with reports of successful outcomes after tumor removal and ABI placement. ¹⁹ The downside to retrosigmoid craniotomy is the considerable cerebellar retraction necessary to provide access to the lateral recess, resulting in a 9.2% reported rate of cerebellar edema and contusion. ¹⁵

Before surgery, the patient is administered mannitol and furosemide to decrease intracranial pressure, as well as a dose of antibiotics and dexamethasone. After the patient is intubated, the electrophysiologist sets up electrodes to monitor facial and lower CNs during surgery. Electrically evoked ABR (EABR) monitoring enables the surgeon to determine the correct positioning of the ABI electrode array. The anesthesiologist should be notified to avoid the use of long-acting muscle relaxants, which interfere with the monitoring of facial and other CNs.

The intraoperative role of a good electrophysiologist cannot be overemphasized. An ABI electrode paddle is inserted through the fourth ventricle and positioned on the surface of the cochlear nucleus. Although the cochlear nucleus is tonotopically organized, this organization is not visible during surgery, and the placement of electrodes occurs in a somewhat “blind” approach, necessitating the use of intraoperative EABR to confirm positioning. Each electrode is tested separately with gradual increases in current, which helps to ascertain optimal electrode placement and improve postsurgical programming of the device. Furthermore, electrophysiologic tests allow the surgeon to monitor any adverse stimulation of neighboring CNs (trigeminal [CN V], facial [CN VII], glossopharyngeal [CN IX], vagus [CN X], and spinal accessory [CN XI] nerves) to decrease the chance of postoperative side effects. Careful monitoring of the glossopharyngeal nerve is essential to avoid cardiac arrhythmias.

Translabyrinthine Approach

The patient is placed in a supine position, and the table is turned 180 degrees. The head of the patient is turned away from the operative ear. The operative site, including the mastoid area and postauricular scalp, are clipped, prepared, and draped in a sterile fashion. A wide postauricular C-incision is made and taken down to the level of the temporalis fascia. The ear is reflected forward through a release incision in the mastoid periosteum. A silk 2–0 stay suture and a self-retaining retractor are applied to secure the ear in place.

Under direct visualization of the operating microscope, a wide-field cortical mastoidectomy is developed using a large diamond bur and constant suction irrigation. Bone overlying the dura of the middle and posterior fossae is decorticated, and the sigmoid sinus is decompressed. After the passage of the facial nerve is identified, a labyrinthectomy is completed down to the internal auditory canal. Smaller diamond burs and suction irrigation are used to skeletonize 270° of the superior and inferior portion of the bony internal auditory canal ( Fig. 33.1a ). The jugular bulb and the superior petrosal sinus serve as inferior and superior landmarks, respectively, for decompression of the cerebellopontine angle and for adequate exposure. Identification is now possible of the trigeminal, facial, vestibulocochlear, glossopharyngeal, vagus, and spinal accessory nerves, the flocculus, and the choroid plexus in the foramen of Luschka (the lateral recess).

The intraoperative electrophysiology data from the EABR and the anatomical landmark of the choroid plexus, CN VIII, and the origin of the glossopharyngeal nerve are used to introduce an electrode paddle into the lateral recess of the floor of the fourth ventricle where the cochlear nucleus is located ( Fig. 33.1b ). The taenia choroidea bisects the roof of the lateral recess and aids in the localization of the ventral cochlear nucleus. Placement of electrodes within the lateral recess and over the ventral portion of the cochlear nucleus is preferable, as it gives the least non-auditory side effects with the best auditory outcome. Once the optimum position is determined, Teflon is packed between the choroid and electrode paddle to secure it in place.

At this stage, the incus is removed from the middle ear, and the facial recess is opened with a 2-mm diamond bur. Cerebrospinal fluid (CSF) leaks are prevented by harvesting a piece of temporalis fascia and obliterating the lumen of the eustachian tube with a combination of fascia, Nu-Knit (Surgicel; Ethicon Endo-Surgery) absorbable hemostat and bone wax, followed by pieces of muscle. Simultaneously, an autologous fat strip is obtained from the lower abdomen using a linear incision, and it is placed into the mastoid defect after closure of the dura. A custom-made titanium plate is secured over the fat graft with titanium screws. The internal processor of the ABI is positioned and secured beneath the skin flap. The postauricular incision is then closed in multiple layers.

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