GENETIC HEARING LOSS

The etiology of the hearing loss is an important consideration. Profound hearing loss and deafness, although clear symptoms, have a heterogeneous group of causes. Of the genetic causes, more than 400 forms of syndromic hearing loss have been described, and the list of nonsyndromic loci now exceeds 80,¹ many of which are discussed elsewhere in this book.

Congenital deafness occurs in approximately 1 in every 1000 children, and at least 60% of these children have hereditary causes of the deafness.² It is estimated that 70% of all hereditary hearing losses are nonsyndromic, nearly 80% of which are inherited in an autosomal recessive fashion.³ To date, 54 autosomal recessive, 24 autosomal dominant, and 8 X-linked loci have been characterized for nonsyndromic sensorineural hearing loss (NSHL).¹ Several mitochondrial DNA variants have also been implicated.¹ Studies indicate that up to 50% of all NSHL cases are due to a mutation in a single gene encoding connexin 26 (Cx26).⁴ The gene coding for Cx26 (gap junction protein beta 2 or GJB2) is located at locus DFNB1 on human chromosome 13q12. The coding sequence of the protein is contained in a single exon that can be easily analyzed using sequencing methods.⁵ Children with nonsyndromic genetic hearing loss, if implanted early, are typically excellent performers with their devices.

Genetic syndromal deafness represents a small proportion of all profound hearing impairment; however, there are typically other considerations to be made when these individuals are being considered for cochlear implantation. Although there are more than 400 genetic syndromes that include hearing loss, most syndromic deafness is confined to a limited number of syndromes.⁶ Only two common autosomal recessive forms of syndromic deafness exist: Pendred’s syndrome (deafness, wide vestibular aqueduct, and thyroid dysfunction) and Usher’s syndrome (deafness, blindness due to retinitis pigmentosa, with or without vestibular dysfunction). Jervell and Lange-Nielsen syndrome (deafness and sudden death syndrome due to prolonged QT interval) occurs in families with strong histories or in population isolates. It is, however, important to consider when preparing to bring a child to the operating room when there is a family history of deafness and cardiac death. Unfortunately, the electrocardiogram (ECG) is not entirely sensitive or specific for this syndrome⁷; however, referral to a cardiologist for evaluation and treatment is essential for any deaf child with prolongation of the QT interval on an ECG, history of syncopal episodes, or a family history of prolonged QT interval. Neurofibromatosis type 2 (NF2) is usually diagnosed between the ages of 10 and 30 years, and these individuals can express the phenotype of bilateral acoustic neuromas, which is an important issue to consider when developing a management algorithm. If one of the tumors is removed while small and if the cochlear nerve is preserved, despite losing functional hearing, cochlear implantation is appropriate if a several-year interval has been observed indicating that a recidivistic tumor will not compromise the cochlear nerve. Otherwise, auditory brainstem implantation is an appropriate alternative.

The previous four syndromic disorders are not readily diagnosed at birth by history and physical examination. The most common dominant syndromes resulting in deafness are Stickler syndrome, branchio-oto-renal syndrome, and Waardenburg syndrome. When physical examination or history suggests a syndromic hearing loss, online resources have been developed to help the physician during the evaluation process.⁸

AUDITORY NEUROPATHY/AUDITORY DYS-SYNCHRONY

Over several years evidence has emerged regarding the existence of a hearing disorder that does not fit into the standard conductive, mixed, or sensorineural hearing loss categories. Relatively recently, the diagnosis of auditory neuropathy/auditory dys-synchrony (AN/D) has been specified as a hearing disorder in which present cochlear outer hair cell function is found in conjunction with absent or abnormal auditory neural responses, which is indicative of poor neural synchrony.⁹ Reports on the incidence of AN/D symptoms range from 0.5¹⁰ to 1.3%¹¹ of the population suspect for hearing loss, and 15% of those with absent auditory brainstem response,¹¹ which is otherwise consistent with severe to profound sensorineural hearing loss. Behavioral audiometric thresholds may or may not be within normal limits and can fluctuate over time, and speech perception in AN/D patients is often much poorer than the behavioral thresholds would predict. There are many possible reasons for poor auditory neural synchrony including, but not limited to, dysfunction of cochlear inner hair cells, of the inner hair cell/spiral ganglion nerve synapse, or of the auditory nerve itself.

The variety of etiologies in AN/D results in a heterogeneous population. Starr¹² reported the diagnoses from 70 patients with AN/D as fitting into the following categories: 40% hereditary, often associated with Charcot-Marie-Tooth disease; 20% with a mix of etiologies including toxic-metabolic (i.e., anoxia, hyperbilirubinemia), immunologic, and infectious; and 40% idiopathic. Transient AN/D has also been documented with temperature fluctuation in cases of severe illness.^13,14 In addition, AN/D may coexist with peripheral neuropathy, which has been observed as a loss of myelin and neural fibers in the cross section of sural nerves from some AN/D patients.¹²

Fitting AN/D patients with hearing aids may not provide sufficient benefit for communication because amplification provides increased sound intensity but does not have the capability to contribute to improved neural synchrony. After careful evaluation and when recommended, electrical stimulation with cochlear implants holds promise as a treatment option for individuals who are severely hearing impaired from AN/D,^15,16 and has been successful in our clinical experience. However, cochlear implantation may be contraindicated in cases in which neural function is significantly compromised or the auditory nerve is deficient or absent.¹⁷

ACQUIRED DEAFNESS

In young children, many acquired forms of deafness cannot be easily differentiated from genetic deafness. Prenatal infection with TORCH microorganisms (toxoplasmosis, syphilis, rubella, cytomegalovirus [CMV], and herpes) is commonly associated with deafness. This spectrum of infections can result in reduced ganglion cell counts, cognitive dysfunction, and abnormal position of the facial nerve, all issues limiting the effectiveness of cochlear implants or increasing the risk of cochlear implant surgery. Prematurity and low birth weight, low Apgar scores, and hyperbilirubinemia can all be associated with deafness and, because of the central auditory processing abnormalities associated with these conditions, expectations for performance outcome following cochlear implantation should be tempered. Similarly, there can be rehabilitation needs and problems with these multiply disabled children.

Autoimmune inner ear disease (see Chapter 153) is typically rapidly progressive and often associated with highly favorable outcomes in postlingual patients receiving cochlear implants, likely due to the well-preserved primary afferent neuron population and the short duration of deafness. Although it is unusual for patients with bilateral Meniere disease to lose enough hearing to require cochlear implantation, such patients typically perform at excellent levels with an implant, likely due to significant residual hearing and auditory memory.

Many inherited or acquired diseases affect the temporal bone, which can produce severe to profound hearing loss requiring cochlear implantation. Otosclerosis, Paget disease, Camurati-Engelmann disease,¹⁸ and meningitis with secondary labyrinthitis ossificans are a few examples of diseases that can present management challenges with cochlear implantation. Aside from the potential difficulty in electrode insertion, the reduction in bone density often leads to unwanted sequelae such as facial nerve stimulation due to current spread outside of the cochlea, affecting the postoperative programming of the device.

Although rare, bilateral temporal bone fractures resulting in deafness can be rehabilitated with cochlear implants. Early implantation should be performed to avoid cochlear fibrosis. If imaging studies suggest trauma to the auditory nerve, it is important to determine if the auditory nerve is functional using promontory electrical stimulation while recording electrically evoked auditory brainstem potentials (ABR). The use of auditory brainstem implantation in this clinical setting has been reported in Europe¹⁹; however, the first such application of this technology in the United States occurred only recently.

PHYSICAL EXAMINATION

All adults or children with profound hearing loss should have a complete physical examination. Clinical correlates of specific etiologies of sensorineural hearing loss are addressed in Chapters 147, 148, and 149 of this text.

CHRONIC SUPPURATIVE OTITIS MEDIA

Cochlear implantation was initially viewed as contraindicated in young children with chronic suppurative otitis media (CSOM) because of the potential risk of infection.²⁰ However, selective retrospective studies have shown that the prevalence and severity of OM does not increase following implantation,^21,22 leading surgeons to advocate cochlear implantation if the ear is dry at the time of implantation.

Some surgeons advocate a two-stage operative approach; the first operation involves a radical mastoidectomy (if not already performed), eustachian tube obliteration, and mastoid cavity obliteration with oversewing of the ear canal. Cochlear implantation is performed at a later time, usually 2 to 6 months postobliteration.²³ The major risk of mastoid obliteration is the formation of cholesteatoma, which must be carefully monitored on a long-term basis. Other authors advocate an individualized management strategy: (1) patients with a dry tympanic membrane perforation receive a first-stage myringoplasty followed by implantation in 3 months; (2) patients with cholesteatoma or an unstable mastoid cavity receive a radical mastoidectomy and obliteration followed months later by a second stage cochlear implantation; (3) patients with a stable cavity receive one-stage cavity obliteration and electrode implantation.²⁴ Finally, some practitioners advocate treating radical mastoid cavities with a one-stage operative approach that includes oversewing the external auditory canal and cochlear implantation without obliteration or reduction of the cavity.^25,26 Luntz and colleagues²⁷ have described a treatment algorithm with multiple steps aimed at resolving the draining ear. Using this strategy, cochlear implantation is performed at the completion of any step if the ear is dry. The existence of multiple protocols for managing cochlear implant candidates with CSOM reflects the problematic nature of this disease process. Regardless of the management protocol, all patients currently receive selected antimicrobial prophylaxis immediately before implantation.

Microbial biofilms are a common if not normal phenomena in nature; however, in the clinical arena biofilm formation is associated with increased morbidity and mortality.^28,29 Biofilms are characterized by a complex three-dimensional architecture with a network of adherent cells connected by water channels and encapsulated within an extracellular matrix.³⁰ Biofilms are recalcitrant to antibiotics, antiseptics, and industrial biocides. Possible mechanisms include (1) restricted penetration of drugs through the matrix; (2) phenotypic changes resulting from a decreased growth rate or nutrient limitation; and (3) surface-induced expression of resistance genes. Although there is an extensive literature on bacterial biofilms, little attention has been paid to medically relevant fungal biofilms, despite the fact that yeasts are the third leading cause of catheter-related infections, with the second highest colonization to infection rate and the highest crude mortality. Transplantation procedures, immunosuppression, use of chronic indwelling catheters, and prolonged intensive care unit stays are salient risk factors for fungal disease.³¹ Biomedical devices including stents, shunts, and prostheses (voice, heart valve, knee, etc.); implants (breast, lens, dentures, etc.); endotracheal tubes; pacemakers; and various types of catheters have been shown to support colonization and biofilm formation by Candida.³² Antifungal therapy alone has been shown to be insufficient for cure, requiring removal of the biomedical device.³³

Since the advent of antibiotics in the 1940s, many authors have reported fungal overgrowth following antibacterial therapy.³⁴ Although bacteria usually cause CSOM, fungal infection or overgrowth is surprisingly common. One prospective study in CSOM patients reported growth of Candida species in 10% of ears with purulent otorrhea and in 35% of ears treated for purulence with topical ciprofloxacin for 3 weeks.³⁵ Furthermore, another study in which cultures were obtained from ears with ventilation tubes and otorrhea before and after a 10-day course of topical ofloxacin ear drops (Floxin) or oral amoxicillin/clavulanate demonstrated a 5% incidence of Candida superinfections in the amoxicillin/clavulanate group but negligible incidence in the ofloxacin group.³⁶ These results confirm the findings of an earlier study in which the incidence of Candida was significantly greater for patients treated with amoxicillin/clavulanate compared with a group treated with topical ofloxacin. We have had one case of fungal colonization of a cochlear implant with Candida albicans.³⁷ No guidelines have been proposed for dealing with episodes of otitis media in the early postoperative period. During this time interval, the physical barrier created by the fibrous sealing of the cochleostomy is in the process of being laid down, and thus the electrode array provides direct access to the inner ear. An infection of the middle ear during this period may easily extend along the electrode array, induce damage to the auditory nerve, and possibly lead to biofilm formation, requiring immediate removal of the implant. In addition, this puts the patient at higher risk for further spread of infection into the intracranial space and subsequent meningitis.³⁸ The high prevalence of bacterial biofilms in chronic otitis media have been recently demonstrated,³⁹ and bacterial biofilms have also been demonstrated in cases of infected cochlear implants that required explantation.⁴⁰

IMAGING

Preoperative high-resolution temporal bone computed tomography (CT) scans, without contrast, should be performed on all cochlear implant candidates. Determination of intact internal auditory canals, normalcy of the cochlea, primary or secondary bone diseases affecting the cochlea, and presence of a wide vestibular aqueduct represent an important data set to complete. Many children with congenital deafness will be found to have an associated cochlear malformation, typically dilated vestibule, wide vestibular aqueduct, cochlear hypoplasia, or a common cavity.⁴¹ When congenital or acquired narrow internal auditory canals are identified on preoperative CT scanning, primary afferent innervation may be lacking and cochlear implantation is therefore contraindicated.^18,42 Another important finding to identify is a wide vestibular aqueduct, which, when present, is an indication to perform preoperative magnetic resonance imaging (MRI). This is an important preoperative consideration because this abnormality is associated with an abnormal communication between the cerebrospinal fluid (CSF) space and the cochlea. Clinically, this is often associated with a “perilymph gusher,” and sealing of the cochleostomy with pericranium or fascia is particularly important to avoid meningitis following acute suppurative otitis media. Hypoplastic cochleas are associated with shorter length and range in character from a common cavity to an incomplete partition at the apical region of the cochlea. In children with auditory neuropathy/auditory dys-synchrony, MRI should be performed because up to 18% of these children can have small or absent cochlear nerves despite normal internal auditory canal size as demonstrated with CT.¹⁷ Other anomalies such as the abnormal course or position of the facial nerve and round window niche occur. With the exception of an absent auditory nerve or internal auditory canal, most malformed cochleas can be implanted with a sufficient number of electrodes to provide open-set (unlimited word or sentence possibilities) speech perception.

Labyrinthitis ossificans can occur after meningitis, particularly when Streptococcus pneumoniae is the infecting mocroorganism.³⁸ Whereas temporal bone CT can show complete ossification well, MRI can provide complementary information when partial ossification has occurred. T2-weighted MRI sequences are particularly useful in determining whether a scala tympani with partial ossification or fibrosis contains perilymph.

High-resolution temporal bone CT scanning can also be helpful in sorting out issues related to device dysfunction or when unexpectedly poor outcome occurs. Figure 158-1 shows the temporal bone containing a Med-El C40+ device in a patient who has otosclerosis and the new onset of facial nerve stimulation. The reduced bone density and lucency seen around the otic capsule resulting from her advanced cochlear otosclerosis allowed current to spread from the cochlea to the facial nerve. Figure 158-2 shows the temporal bone of a young child whose performance was unexpectedly low and who was also noted to have facial nerve stimulation with cochlear implant use. After his referral to our center for evaluation, a high-resolution temporal bone CT was obtained. The CT scan showed a malformed cochlea with an incomplete partition at the apex and the electrode tip leaving the cochlea and extending into the internal auditory canal. This type of cochlear malformation is typically associated with a thin partition between the modiolus and the internal auditory canal.

Figure 158-1. High-resolution temporal bone axial computed tomography scan using a bone window algorithm shows a right ear with a well-positioned electrode array (Med-El C40+) within a cochlea affected by otosclerosis. Note the reduced density of the otic capsule bone and the lucency surrounding the otic capsule (center, top right). This patient was experiencing facial nerve stimulation with cochlear implant use due to current spread through the otic capsule to the facial nerve

Figure 158-2. High-resolution temporal bone axial computed tomography (CT) using a bone window algorithm shows a right ear with a cochlear implant electrode array that has broken through the cochlea/modiolar wall to enter the internal auditory canal (CII Bionic Ear with HiFocus 1j electrode). The implanting surgeon, at another institution, did not recognize that the child had an incomplete partition at the apex of the cochlea. This malformation can be associated with a thin bony wall between the cochlea and the modiolus. This child was performing at a level lower than expected with his cochlear implant and had facial nerve activation with implant use, which led to the CT scan to evaluate the electrode position. The child’s second side was implanted with a HiRes 90K device, and the initial device was removed and replaced with a HiRes 90K device, resolving his facial nerve stimulation problem.

ADULT AUDIOLOGIC PROTOCOL

For adults, sound detection and speech perception abilities are assessed to determine candidacy. Preoperatively, patients are evaluated with a battery of measures while using hearing aids, and the results are compared with the most recent average and range of cochlear implant performance. Preoperative measures are also repeated postimplant for longitudinal monitoring of patient performance. Single-subject research designs are often implemented in clinical trials in which each patient serves as his or her control, primarily due to the large variability within the population of cochlear implant candidates and users. The selection of preoperative test measures, therefore, should also take into consideration comparative postimplant measures.

Preimplant audiologic tests include unaided and aided detection thresholds for pure-tone and warble-tone stimuli, respectively. Unaided thresholds are obtained in each ear individually, and aided detection thresholds may be obtained monaurally and binaurally. Although there are no aided detection level criteria, it is helpful to determine aided levels as one aspect of appropriate hearing aid fitting and for comparison with expected postimplant sound field detection thresholds. For some patients, aided testing can also reveal recruitment (i.e., unusual sensitivity to loud sounds), which may limit benefit from amplification due to the inability to incorporate needed hearing aid gain.

Aided speech perception abilities are often assessed in both monaural and binaural conditions, depending on the use of amplification in each ear. Speech perception measures are conducted in the sound field, typically at a presentation level of 60 dB SPL and include open-set recorded presentations of words and sentences in quiet, and if appropriate, in noise. In the best-aided condition, the assessment of individual ears provides critical information for determining which ear to implant for unilateral implantation. In addition, the best-aided condition, whether it be either ear alone or both ears together, provides information about the candidate’s maximum performance for comparison with cochlear implant performance.

Word and sentence recognition tests included in the Minimum Speech Test Battery for Adult Cochlear Implant Users (MSTB) are used at many cochlear implant centers to assess performance. The MSTB is a set of compact disc recordings designed to provide word and sentence tests for the preimplant and postimplant evaluation of speech recognition, regardless of implant device. The Consonant-Nucleus-Consonant (CNC) Monosyllable Word Test⁴³ assesses single-syllable word recognition. One CNC list contains 50 monosyllabic words presented in an open-set format. The CNC Words were among the original set of words from which the Northwestern University Auditory Test 6 (NU6) Monosyllabic Word Test⁴⁴ were taken.

The presentation of auditory-only sentence lists from the Hearing in Noise Test (HINT)⁴⁵ evaluates each patient’s ability to understand sentence material in quiet or in the presence of background noise. Each HINT list is phonemically balanced and contains 10 sentences recorded by a male speaker that are equivalent in the features of length, intelligibility, and naturalness. When the sentences are presented in noise, the signal-to-noise ratio typically used is +10 dB, although this ratio can be adjusted to make the test condition more or less difficult. For patients with some open-set speech recognition for words and/or sentences preoperatively, the BKB (Bamford-Koval-Bench) Sentences⁴⁶ may be administered in an adaptive procedure (i.e., BKB SIN; sentences-in-noise). In this condition the sentences are presented at a fixed presentation level (e.g., 65 dB SPL), and the noise is varied for signal-to-noise (S/N) ratios between +20 and −5. Clinical observations suggest that, when testing adults, scores on open-set word and sentence measures, particularly in the presence of noise, are more reflective of patient satisfaction with hearing aids and more useful for determining cochlear implant candidacy than unaided and/or aided detection thresholds.

As performance with cochlear implants continues to improve because of advances in technology and broadening candidacy criteria, it is recommended that speech perception performance be assessed with measures that approximate everyday listening. Study findings by Skinner and colleagues⁴⁷ and our cochlear implant team⁴⁸ suggest that speech recognition tests be presented at 60 rather than 70 dB SPL to determine implant candidacy. These findings have resulted in the use of 60 dB SPL in U.S. clinical trials and general clinical practice across centers. Additional steps toward the use of measures that approximate everyday listening include the presence of background noise, variation in speaker gender and rate, and variation in the location of the speaker.

Following the evaluation of sound detection and speech perception, other areas of assessment may include vestibular testing, tinnitus assessment, and patient satisfaction or quality-of-life questionnaires.

OUTCOME EXPECTATIONS FOR ADULTS

Almost all patients demonstrate improved sound detection with their cochlear implants compared with their preoperative performance with hearing aids, and this is especially evident in the high-frequency range. Average postoperative sound-field detection thresholds for warble-tone stimuli are approximately 25 to 30 dB HL for frequencies 250 through 4000 Hz.⁴⁸

When determining patient’s expectations for cochlear implant performance and when counseling patients preimplant, it is important to stay abreast of both the average speech perception performance of cochlear implant recipients and the range of performance. In a recent study of 78 of adult cochlear implant users, 26 each with the Clarion, Nucleus, and Med-El device, the average CNC word scores at 70, 60, and 50 dB SPL were 42%, 39%, and 24%, respectively.⁴⁸ In this same group of subjects the mean HINT scores at 70, 60, and 50 dB SPL were 72%, 73%, and 57%, respectively. When the HINT was presented at 60 dB SPL in the presence of speech spectrum noise at an S/N ratio of +10, the average score for this subject sample was 48%. These results represent average performance; however, there was a great deal of variation in scores for individuals, ranging from 0% to 100% for most measures. In general, patients perform less well on single syllable word tests compared with sentence tests, and less well in the presence of noise than in quiet. Many cochlear implant users can understand sentences without lipreading cues and can therefore converse on the telephone. Although the primary objective of speech coding strategies is the perception of speech, some patients also enjoy music.

The majority of postlingually deafened adults demonstrate significant preoperative to postoperative improvements on open-set speech perception measures, often as early as 1 month postimplant. Compared with postlingual affected adults, some prelingually affected adults, defined as having onset of profound or severe-to-profound hearing loss at younger than 3 to 6 years of age (depending on the respective study), demonstrate open-set speech recognition, although the percentage is smaller and often the length of device use needed to achieve this is longer. Although the average postoperative scores for individuals with prelingual hearing loss are generally lower compared with those with postlingual hearing loss, there have been significant preoperative to postoperative improvements in speech perception reported for this group.⁴⁹ Therefore adults with prelingual onset of severe-to-profound hearing loss may be appropriate candidates for cochlear implantation.

Providing that older patients are enjoying relatively good health, there is presently no upper age limit for cochlear implantation. Audiologic results for cochlear implant users ages 65 to 80 years indicate significant improvements for both preoperative to postoperative comparisons^50,51 and for varied speech stimulus presentation levels.⁴⁸

COMBINED ELECTRIC AND ACOUSTIC STIMULATION

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