Single-sided deafness (SSD) causes a myriad of problems affecting an individual's ability to communicate. The most obvious impairment is difficulty in hearing sounds on the affected side due to the head shadow effect. This frequently results in an individual constantly adjusting their head position in an attempt to compensate for the handicap and in some cases rendering them oblivious to the presence of sound directed at the affected side . SSD also significantly impairs word discrimination, which varies as a function of residual hearing retained on the affected side. Those with profound SSD frequently report difficulty in understanding speech, particularly in noisy environments, even in the presence of normal hearing on the nonaffected side . Reducing binaural hearing to monaural hearing reduces one's ability to localize sound, and those with no functional hearing on the affected side find it difficult to determine the direction, distance, or movement of sound . The constant straining, postural adjustments, and apparent lack of awareness of sound on the affected side frequently result in socially awkward mannerisms, a sense of confusion and, in some cases, may lead to social isolation.
Rehabilitative options for SSD include contralateral routing of sound (CROS), osseointegrated implants, or bone-anchored hearing aids including those attached as dental appliances [4,5] and cochlear implants . Of these, cochlear implants have been a relatively late comer to the field due, at least in part, to concerns about the ability of the brain to sort out acoustic and electric stimuli and concern that the hearing from the cochlear implant would interfere with acoustic signaling processing from the good ear. All three modalities help overcome some of the deficits associated with SSD including the head shadow effect and perception of sound on the nonhearing side. They also improve hearing in noisy environments although when speech is at the better hearing ear, the addition of any noise received by the auxiliary microphone on the CROS or osseointegrated implant will degrade intelligibility (Dillon, 2001). Compared with CROS and osseointegrated implant devices, only cochlear implants offer the potential to restore binaural hearing, allowing the opportunity of sound localization . Although there are some data that compare outcomes in cochlear implant recipients with those provided by CROS or cochlear implant devices as detailed below, there are no large-scale studies to definitively demonstrate the superiority of cochlear implants over CROS or osseointegrated implants.
Binaural hearing is essential for sound localization. Sound localization requires correct calculation of three spatial coordinates: azimuth (the angle left or right of a neutrally positioned head), elevation (angle above or below the horizontal plane), and distance. In humans with two functioning cochleae, the auditory pathway uses interaural timing and intensity differences to calculate these coordinates , which can be further refined by monaural cues from structures of the pinna. For frequencies below 800 Hz, the auditory system relies mainly on phase delays caused by interaural time differences . For frequencies greater than 1600 Hz, the auditory system primarily relies on interaural level differences. Both phenomena are used in the transition zone from 800 Hz to 1600 Hz . In SSD, localization using interaural timing and intensity differences is not possible unless there is some way to replace the impaired acoustic sensor (deafened cochlea).
Although the cochlear implant is the only rehabilitative option available to provide direct stimulation to the compromised ear, it is not clear that use of a cochlear implant would necessarily restore interaural time detection differences in patients with SSD auditory system as sound processing with a cochlear implant occurs on a different time scale than acoustic processing.
In addition to differences in distance traveled by sound to reach the right and left ear, there will also be a difference in intensity between sound waves reaching the ears. The difference in sound intensity is a function of the distance traveled from the source to the ear (intensity and distance form an inverse square relationship). The head also interferes with the incoming sound wave forming an acoustic shadow or head shadow . The head shadow effect is more pronounced at higher frequencies than lower frequencies. Head shadow is a result of sound diffraction, which occurs when an object in the path of a sound wave has a dimension greater than 2/3 of the sound wavelength . For a human head with a diameter of 140 mm, this affects sounds with a frequency higher than ∼800 Hz to a moderate extent and significantly affects sounds with a frequency higher than ∼1600 Hz. Encoding of sound loudness by a cochlear implant differs from acoustic processing and, as with interaural timing differences, it is not clear whether cochlear implants restore a patient's ability to detect interaural intensity differences.
Monaural cues are limited to the outer ear. The structures of the pinna and the external auditory canal act as directional filters. As sound comes in contact with the pinna it is reflected or transmitted depending on frequency and the topography of the structure it encounters. The result is that different frequencies comprising the incoming sound overlap and cancel to varying extents thereby modifying the spectral composition of the sound as it enters the external auditory canal. If one has a baseline familiarity with an incoming sound, it may be possible to localize sound on the basis of spectral changes occurring at the level of the pinna [12,13]. These mechanisms persist following SSD.
SPEECH PERCEPTION IN QUIET AND NOISE
Binaural hearing is critical for speech processing. In addition to sound localization, redundant information received by two independent acoustic sensors allows for summation and squelch. Binaural summation occurs when the same acoustic stimulus is presented to both ears. The higher order auditory processing of the redundant information provides a 2–6 dB in signal threshold and is particularly beneficial in noisy environments. The squelch effect represents another form of higher order auditory processing in which noise from the ear perceiving a poorer signal to noise ratio is combined with noise from the ear with better signal to noise ratio [7,11,14–16]. This helps to separate out meaningful sound from background noise and imparts the ability to discriminate speech in noise at a 2–3 dB worse signal to noise ratio compared with purely monaural perception . As with sound localization, the head shadow effect can play a significant role in speech perception in that there may be a 5–6 dB decrease in threshold for sounds directly approaching the deaf ear .
Of the available rehabilitative options for SSD, CROS, osseointegrated implant devices, and cochlear implants are able to overcome deficits caused by the head shadow effect. All three devices effectively place an acoustic sensor on the side of the deaf ear. However, signals detected by CROS and osseointegrated implant devices are ultimately routed to the better hearing cochlea and so improvements in hearing would theoretically be limited to recovering the 5–6 dB loss caused solely as a result of the head shadow effect. As neither CROS nor osseointegrated implant devices use binaural signal processing, there is no expected improvement in speech perception from summation or squelch. Cochlear implants, however, allow for both an acoustic sensor and an electrical input to the deaf side. To the degree that the auditory system can effectively combine this electrical signal with acoustic hearing in the opposite ear, cochlear implant recipients will theoretically also benefit from summation and squelch.
EFFECTIVENESS OF COCHLEAR IMPLANTS TO IMPROVE SOUND LOCALIZATION AND SPEECH PERCEPTION
There are several recent studies examining the effectiveness of cochlear implants and other treatments in rehabilitating sound localization and speech discrimination in SSD [7,11,16–21,22▪▪,23▪▪,24,25▪–27▪].
Arndt et al.[7,19] compared sound localization using CROS, osseointegrated implant devices, or cochlear implant 6 months after implantation in a cohort of 11 patients. To determine effectiveness, seven loudspeakers were placed in a semicircle in front of the patients. Patients were then asked to identify which speaker was delivering the sound. Localization error, measured as the difference in azimuth angle between true and perceived sound source, was used to evaluate patients. Using this calculation, a lower score is interpreted as having better localization ability. Patients who received cochlear implants exhibited a significantly decreased localization error (15°) compared with controls (33.9°, P = 0.003) and patients with osseointegrated implant (30.4°, P = 0.002), and CROS hearing aid devices (39.9°, P = 0.001). Speech perception was also evaluated and compared with recipients of CROS and osseointegrated implant devices. Three conditions were used: first, sound and noise directed at the front of the patient's head, second, sound directed at the normal hearing and noise directed at the deaf side, and third, sound directed toward the deaf side and noise directed toward the normal hearing ear. When noise was directed head-on, there was no significant difference in improvement seen in those with cochlear implants versus CROS or osseointegrated implant devices implying all devices offer similar outcomes with this task. Patients who received cochlear implants demonstrated significant improvement in speech discrimination over those with CROS or osseointegrated implant devices in the second and third conditions implying a significant improvement because of utilization of binaural summation and squelch [7,19].
Hansen et al.[22▪▪] reported speech perception and sound localization outcomes in a cohort of 29 patients who underwent cochlear implantation with or without simultaneous labyrinthectomy. Postoperative data were available for 19 patients but 12-month postoperative data were available for only six patients. Of 19 patients with at least 3-month postoperative data, nine demonstrated improvement in sound localization on the basis of a multiple loudspeaker test. Of the six with 12-month postoperative data, four patients showed improvement in sound localization. Sound localization appeared to improve with experience, suggesting plasticity and adaptability in the auditory system with regards to processing of acoustic and electric stimuli for sound localization. There was also an improvement in speech discrimination. Of the 19 patients, 13 showed improvement in word score. The overall improvement on consonant-vowel nucleus-consonant (CNC) word scores was 28% +/−5.1 [mean +/− standard error (SE)] with individual improvement ranging from 64 to −26% (P < 0.05). Fourteen of 19 showed improvement in sentence scores as measured using AzBio sentences. Overall improvement on AzBio was 40% +/− 6.6 with individuals ranging from 92% improvement to −57% (P < 0.05). Although the small sample size limits the power of the study, there is an overall trend toward improvement in sound localization and speech discrimination at least at the 12-month postoperative mark. It should be noted, however, that many of the patients had a short duration of implant experience at the time this article was written [22▪▪].
In a study examining ten adults with unilateral hearing loss who underwent cochlear implantation, Firszt et al. reported that seven demonstrated an improvement in sound localization. Interestingly, these same seven had postlingual deafness in contrast to three who did not exhibit any improvement in sound localization and who had either prelingual or perilingual deafness. This study suggests that perhaps there are some limitations in improving localization in patients with prelingual or perilingual deafness . A more recent study by Cadieux et al. reported three of five adolescents with unilateral hearing loss had improved speech recognition and sound localization.
A recent study by Tavora-Vieira et al. examined the benefits of cochlear implantation in nine postlingually deafened patients with unilateral hearing loss and found that all nine patients exhibited an improvement in sound localization (P = 0.001) although a limited setup using a left, right, and central speaker were used. All nine patients also exhibited objective and subjective improvement in speech discrimination (P = 0.008) .
Hassepass et al.[23▪▪] evaluated three children with noncongenital unilateral hearing loss and reported that cochlear implantation imparts improved speech discrimination in noise as well as improved localization as determined by a multispeaker sound field test.
Jacob et al. in 2011 followed a cohort of 13 patients for 8 months and compared sound localization and speech discrimination in patients who received cochlear implant versus a control group with unaided SSD. Patients in the cohort exhibited improvement in sound localization as well as a subjective sense of greater ease in listening in noisy environments. They further noted that placing a cochlear implant did not have any impact on the normal hearing side .
A recent review by Vlastarakos et al. identified 27 studies evaluating the impact of cochlear implants single-sided deafness. After controlling for duplicate patients (a handful of patients were included in more than one study), six studies evaluated 63 patients for the ability to localize sound. All six studies reported a trend in improved sound localization, however only two of these studies, involving 25 patients, found the improvement to be statistically significant. Seven studies representing 85 patients evaluated speech perception in noise. Of these, six demonstrated improved speech discrimination with four of them (representing 50 patients) being statistically significant [31,32▪▪].
Tinnitus is a frequent sequela of hearing loss and ranges in severity from mild to severe with regard to impact on daily activities. Most individuals with SSD suffer from some degree of tinnitus. Several studies have suggested that cochlear implants reduce the severity of tinnitus  and indeed in early studies, potential tinnitus suppression was a motivation to offer cochlear implants to patients with SSD [16,18,20,22▪▪].
Several studies have sought to evaluate the effect of cochlear implants on tinnitus suppression. Tavora-Vieira had seven patients who reported severe tinnitus prior to having a cochlear implant placed. All seven reported a reduction in tinnitus when the processor was activated [25▪,29]. Ramos et al. in 2011 reported that in a cohort of ten patients, two experienced suppression of tinnitus, seven experienced reduction in tinnitus intensity, and one experienced no change after placement of a cochlear implant. In a study by Arndt et al. nine patients in the cohort had tinnitus of which six experienced improvement in tinnitus severity and three reported no change [7,19]. Van de Heyning et al. in 2008 studied a cohort of 22 patients with tinnitus and found that it was completely suppressed in three patients, improved in 18, and unchanged in one [16,18]. Hansen et al. in 2013 reported that while 12-month postoperative tinnitus questionnaires were not yet available, most patients in the 29 patient cohort reported improvement in tinnitus after activation of the cochlear implant. Biasco and Redleaf [26▪], in a recent meta-analysis drawing data from studies by Arndt, Ramos and Van de Heyning found that cochlear implants had a statistically significant improvement on the severity of tinnitus (P < 0.00001). Although the majority of the studies to date are promising, all of them have few patients and consequently lack statistical power.
Although there is increasing evidence that patients with SSD benefit from cochlear implants, there are several factors to consider in deciding whether a patient represents a suitable candidate. First, there are few cases of patients with congenital SSD who have received cochlear implants. Whether the results achieved in patients with postlingual SSD translate to this population, especially with regard to the processes that depend on binaural auditory processing, requires further study. Duration of deafness has been shown to be a significant determinant of cochlear implant performance in patients with bilateral deafness  and this likely holds true for patients with SSD. Thus, patients with long-standing SSD may not be suitable candidates for cochlear implant and would likely derive more benefit from other rehabilitative options (e.g., CROS or osseointegrated implant devices).
Patients with severe refractory Ménière's disease represent another population that warrants consideration for cochlear implants. Our group has offered simultaneous cochlear implants for those patients that are considered candidates for a surgical labyrinthectomy-based refractory vertigo or drop attacks and poor hearing [22▪▪]. Surgical ablation of the labyrinth, although addressing vertigo and drop spells, resulted in profound SSD. Cochlear implants effectively restored hearing to the deafened ear [22▪▪]. Thus, patients were provided relief from the vertigo attacks and remediation in hearing in the affected ear. In many patients, the cochlear implant also diminished the tinnitus that occurred in Ménière's disease,which can be debilitating.
Patients with a cochlear implant in one ear and good acoustic hearing in the opposite ear provide a unique opportunity to explore sound encoding and adaption with a cochlear implant. For example, assessing pitch mapping performance in the cochlear implant ear over time that can be compared with a normal hearing ear should help us understand the mechanisms and extent to which patients can utilize and adapt to different cochlear implant mapping strategies. Also, comparison to the normal hearing contralateral ear will allow identification and refinement of cochlear implant processing strategies that best mimic acoustic hearing.
Patients with SSD frequently suffer from communication problems that arise from significantly diminished sound localization and speech perception in the presence of background noise. As a result, many patients seek rehabilitative options. Of the currently available treatment options for SSD, CROS and osseointegrated implant devices are the most frequently used. Although these options provide a significant benefit by overcoming the head shadow effect to detect sound on the deafened side, they fail with regard to providing the benefits of binaural sound processing. Thus, they are unable to restore sound localization or improve speech discrimination through summation and squelch. Cochlear implants also offer the potential to diminish tinnitus. Careful attention to patient selection criteria will assist in identifying those patients most likely to be benefited by a cochlear implant or a different device. As the number of individuals with cochlear implants for SSD increases, there will be more opportunities to compare the outcomes of cochlear implants with those provided by CROS or osseointegrated implant devices.
The work was supported in part by research grant P50DC000242 from the National Institutes on Deafness and Other Communication Disorders, National Institutes of Health; the Lions Clubs International Foundation; and the Iowa Lions Foundation.
Conflicts of interest
None of the authors have conflicts of interest to disclose.
REFERENCES AND RECOMMENDED READING
Papers of particular interest, published within the annual period of review, have been highlighted as:
- ▪ of special interest
- ▪▪ of outstanding interest
1. Noble W, Gatehouse S. Interaural asymmetry of hearing loss, speech, spatial and qualities of hearing scale (SSQ) disabilities, and handicap. Int J Audiol 2004; 43:100–114.
2. Bess FH, Tharpe AM. An introduction to unilateral sensorineural hearing loss in children. Ear Hear 1986; 7:3–13.
3. Kamal SM, Robinson AD, Diaz RC. Cochlear implantation in single-sided deafness
for enhancement of sound localization
and speech perception. Curr Opin Otolaryngol Head Neck Surg 2012; 20:393–397.
4. Murray M, Miller R, Hujoel P, Popelka GR. Long-term safety and benefit of a new intraoral device for single-sided deafness
. Otol Neurotol 2011; 32:1262–1269.
5. Gurgel RK, Shelton C. The SoundBite hearing system: patient-assessed safety and benefit study. Laryngoscope 2013; 123:2807–2812.
6. Bishop CE, Eby TL. The current status of audiologic rehabilitation for profound unilateral sensorineural hearing loss. Laryngoscope 2010; 120:552–556.
7. Arndt S, Aschendorff A, Laszig R, et al. Comparison of pseudobinaural hearing to real binaural hearing rehabilitation after cochlear implantation in patients with unilateral deafness and tinnitus. Otol Neurotol 2011; 32:39–47.
8. Perrott DR, Musicant AD. Dynamic minimum audible angle: binaural spatial acuity with moving sound sources. J Aud Res 1981; 21:287–295.
9. Soeta Y, Nakagawa S. Effects of the frequency of interaural time difference in the human brain. Neuroreport 2006; 17:505–509.
10. Van Wanrooij MM, Van Opstal AJ. Contribution of head shadow and pinna cues to chronic monaural sound localization
. J Neurosci 2004; 24:4163–4171.
11. Gray L, Kesser B, Cole E. Understanding speech in noise after correction of congenital unilateral aural atresia: effects of age in the emergence of binaural squelch but not in use of head-shadow. Int J Pediatr Otorhinolaryngol 2009; 73:1281–1287.
12. Rogers ME, Butler RA. The linkage between stimulus frequency and covert peak areas as it relates to monaural localization. Percept Psychophys 1992; 52:536–546.
13. Wightman FL, Kistler DJ. Monaural sound localization
revisited. J Acoust Soc Am 1997; 101:1050–1063.
14. Tyler RS, Dunn CC, Witt SA, Preece JP. Update on bilateral cochlear implantation. Curr Opin Otolaryngol Head Neck Surg 2003; 11:388–393.
15. Schleich P, Nopp P, D’Haese P. Head shadow, squelch, and summation effects in bilateral users of the MED-EL COMBI 40/40+ cochlear implant
. Ear Hear 2004; 25:197–204.
16. Vermeire K, Van de Heyning P. Binaural hearing after cochlear implantation in subjects with unilateral sensorineural deafness and tinnitus. Audiology Neurootol 2009; 14:163–171.
17. Probst R. [Cochlear implantation for unilateral deafness?]. HNO 2008; 56:886–888.
18. Van de Heyning P, Vermeire K, Diebl M, et al. Incapacitating unilateral tinnitus in single-sided deafness
treated by cochlear implantation. Ann Otol Rhinol Laryngol 2008; 117:645–652.
19. Arndt S, Laszig R, Aschendorff A, et al. [Unilateral deafness and cochlear implantation: audiological diagnostic evaluation and outcomes]. HNO 2011; 59:437–446.
20. Punte AK, Vermeire K, Hofkens A, et al. Cochlear implantation as a durable tinnitus treatment in single-sided deafness
. Cochlear Implants Int 2011; 12 (Suppl 1):S26–29.
21. Firszt JB, Holden LK, Reeder RM, et al. Auditory abilities after cochlear implantation in adults with unilateral deafness: a pilot study. Otol Neurotol 2012; 33:1339–1346.
22▪▪. Hansen MR, Gantz BJ, Dunn C. Outcomes after cochlear implantation for patients with single-sided deafness
, including those with recalcitrant Meniere's disease. Otol Neurotol 2013; 34:1681–1687.
One of the largest series to date evaluating the impact of cochlear implantation in patients with single-sided deafness. Also addresses the potential for using cochlear implants with simultaneous labyrinthectomy in treatment of Ménière's disease.
23▪▪. Hassepass F, Aschendorff A, Wesarg T, et al. Unilateral deafness in children: audiologic and subjective assessment of hearing ability after cochlear implantation. Otol Neurotol 2013; 34:53–60.
Outcomes report of three children with SSD that received a cochlear implant demonstrating benefits of restoration of binaural hearing in pediatric patients.
24. Plontke SK, Heider C, Koesling S, et al. Cochlear implantation in a child with posttraumatic single-sided deafness
. Eur Arch Otorhinolaryngol 2013; 270:1757–1761.
25▪. Tavora-Vieira D, Marino R, Krishnaswamy J, et al. Cochlear implantation for unilateral deafness with and without tinnitus: a case series. Laryngoscope 2013; 123:1251–1255.
A recent study exploring the use of cochlear implantation in patients with single-sided deafness of variable cause presenting with and without tinnitus.
26▪. Blasco MA, Redleaf MI. Cochlear implantation in unilateral sudden deafness improves tinnitus and speech comprehension: meta-analysis and systematic review. Otol Neurotol 2014; 29: [Epub ahead of print].
A recent meta-analysis evaluating the impact of cochlear implantation on patients with single-sided deafness with regard to tinnitus control, speech perception, and sound localization.
27▪. Nawaz S, McNeill C, Greenberg SL. Improving sound localization
after cochlear implantation and auditory training for the management of single-sided deafness
. Otol Neurotol 2014; 35:271–276.
A case report describing a patient who was initially treated with contralateral routing of sound, then an osseointegrated implant and finally cochlear implantation.
28. Cadieux JH, Firszt JB, Reeder RM. Cochlear implantation in nontraditional candidates: preliminary results in adolescents with asymmetric hearing loss. Otol Neurotol 2013; 34:408–415.
29. Tavora-Vieira D, Boisvert I, McMahon CM, et al. Successful outcomes of cochlear implantation in long-term unilateral deafness: brain plasticity? Neuroreport 2013; 24:724–729.
30. Jacob R, Stelzig Y, Nopp P, Schleich P. [Audiological results with cochlear implants for single-sided deafness
]. HNO 2011; 59:453–460.
31. Vlastarakos PV, Nikolopoulos TP, Pappas S, et al. Cochlear implantation update: contemporary preoperative imaging and future prospects - the dual modality approach as a standard of care. Expert Rev Med Devices 2010; 7:555–567.
32▪▪. Vlastarakos PV, Nazos K, Tavoulari EF, Nikolopoulos TP. Cochlear implantation for single-sided deafness
: the outcomes. An evidence-based approach. Eur Arch Otorhinolaryngol 2013; 271:2119–2126.
Virtually all studies evaluating the impact of cochlear implantation in patients with single sided deafness lack statistical power. This meta-analysis draws conclusions based on data from 108 patients.
33. Pan T, Tyler RS, Ji H, et al. Changes in the tinnitus handicap questionnaire after cochlear implantation. Am J Audiol 2009; 18:144–151.
34. Ramos A, Polo R, Masgoret E, et al. Cochlear implant
in patients with sudden unilateral sensorineural hearing loss and associated tinnitus. Acta Otorrinolaringol Esp 2012; 63:15–20.
35. Rubinstein JT, Parkinson WS, Tyler RS, Gantz BJ. Residual speech recognition and cochlear implant
performance: effects of implantation criteria. Am J Otol 1999; 20:445–452.