- Part 2. Challenges and limitations of implantable hearing devices (auditory implants) for sensorineural hearing loss
- 8.1 Auditory implants for moderate to severe sensorineural hearing loss
- 8.2 Capacity of VSB and MET with today’s sound processors
- 8.3 Interference direct sound and amplified sounds
- 8.4 References chapter 8
Part 2. Challenges and limitations of implantable hearing devices (auditory implants) for sensorineural hearing loss
8. Sensorineural hearing loss
8.1 Auditory implants for moderate to severe sensorineural hearing loss
To rehabilitate sensorineural hearing loss, conventional air-conduction hearing aids are the first choice (e.g. behind-the-ear devices or BTEs; Figure 2.1, chapter 2). For patient with severe to profound hearing loss (>80-90 dB HL), such devices might no longer be effective and cochlear implantation is considered to be the better treatment option (e.g. Verhaegen et al., 2008; Hoppe et al., 2015).
With BTEs, if the sensorineural hearing loss of a patient is not too severe, it is possible to make conversational speech audible again. However, owing to inevitably impaired analytical functioning of the cochlea, speech recognition will still be compromised; especially in noisy conditions despite appropriate amplification (Plomp, 1978). Today’s digital devices might deal with that to some extend. Algorithms have been developed that might enhance speech sounds relative to the noise e.g. noise reduction algorithms and adaptive directionality of the microphones (Dillon, 2012).
An alternative amplification option is implantable hearing aids or auditory implants. High sound fidelity, high output with less feedback problems and no occluding ear moulds were claimed advantages of such auditory implants over BTEs (e.g. Goode, 1995).
Since the late nineties, several different types of auditory implants have been introduced for patients with sensorineural hearing loss (Snik, 2011). Nowadays (summer 2016), two semi-implantable devices are on the market in Europe, viz. the Vibrant Soundbridge (VSB; Figure 2.6, chapter 2) and the Cochlear MET device (MET; Figure 8.1; Cochlear, Mechelen, Belgium). Initially, these devices were produced by newly established companies (named Symphonix and Otologics), which have been taken over by the CI companies Med-El and Cochlear, respectively.
Figure 8.1. The Cochlear MET middle ear implant, previously known as the Otologics MET. The processor (A) is worn externally. The implanted actuator (C) is directly coupled to the incus by means of a (vibrating) rod (D). Source: Cochlear Company, reproduced with permission.
Through the years, development of auditory implants was aimed at fully implantable, thus invisible devices. Nowadays, two fully implantable devices are commercially available, viz. the Cochlear Carina (Cochlear, Boulder, USA) and the Esteem (Envoy Medical, St. Paul, MN, USA).
The first audiological studies showed that the semi-implantable devices worked well but in terms of gain and speech recognition, however, they were at best comparable to conventional BTEs (e.g. Snik et al., 2003; Jenkins et al., 2004, Rameh et al., 2010). In 2005, after Round Table discussions, it was concluded that compared to BTEs, middle ear implants had no convincing advantages, except for patients who don’t tolerate ear moulds owing to e.g. external otitis (International consensus on middle ear implants; Magnan et al., 2005). Nowadays, this conclusion is widely accepted, also by the manufacturers of middle ear implants (e.g. VSB fact sheet, 2015). Edfelt et al. (2014) showed that for such patients (sensorineural hearing loss with comorbid external otitis) the application of a middle ear implant is even cost effective.
There is broad agreement on the use of today’s semi-implantable hearing devices in sensorineural hearing loss; such devices are advocated for patients with hearing loss < 70-80 dB HL who don’t tolerate ear moulds. So far, only for patients with therapy-resistant external otitis, a semi-implantable hearing device is proven effective and cost-effective.
Concerning the fully implantable devices, on the market since 2007-2008, candidacy might be the same. Klein et al. (2012) and Pulcherio et al. (2014) systematically reviewed the literature and concluded that when comparing the Carina and Esteem devices to BTEs, no structural audiological benefits were found. As these fully implantable devices are more expensive then semi-implantable devices, surgery is more complex and revision surgery is needed every 4 to 9 years (to replace the battery), the cost-utility ratio of fully-implantable devices is unfavorable. As Rameh et al showed in 2010, results with the Carina were poor in their (18) patients with sensorineural hearing loss of, on the average, 69 dB HL. The ‘gain in SRT’ (assesses gain at normal conversational levels, see Appendix 2) was only 13 dB. Indeed, only 20% of their patients was satisfied. However, nowadays, the Carina can be used with, additionally, an external processor. The advantage is that feedback, a serious complication of fully implantable devices, will be less of a problem, allowing a higher volume setting (still to be quantified).
To help patients with sensorineural hearing loss and severe external otitis, bone-conduction devices have been applied as well. Bone conduction works but it is not effective at all. Carlsson and Hakansson (1997) showed that in mixed hearing loss, bone conduction works but just to close (virtually) the air-bone gap. Studies with Baha applied in patients with pure sensorineural hearing loss indeed showed low benefit (Snik et al., 1995). Stenfelt et al. (2000) studied the use of Baha in patients with high-frequency sensorineural hearing loss, enabling an open ear fitting. It was concluded that BTEs were clearly the better option. With today’s powerful sound processors, results might be somewhat better but studies showing that are lacking.
To help patients with an obvious sensorineural hearing loss, bone-conduction devices have insufficient power
8.2 Capacity of VSB and MET with today’s sound processors
With more recent sound processors (Amade or Samba for VSB and BAP 2.2 for Cochlear MET), improvements in sound quality have been achieved. Concerning speech recognition, Figure 8.2 shows the %-correct score at normal conversational level (65 dB SPL) of 14 patients using today’s VSB processors (squares) and 10 patients using the MET BAP 2.2 processor (dots). Patients with ski-slope audiograms were excluded, following Verhaegen et al., 2008. The drawn lines are taken from that latter study, based on the speech performance of patients with previous generations of sound processors. The figure shows that the new data points are close to the lines or somewhat better (MET data); lines taken from Verhaegen et al. That was expected as these new processors have not been developed with the aim to improve gain and MPO, but to introduce new processing options. This means that the application region, previously defined by Verhaegen et al (2008), remains unchanged: the VSB is applicable up to a mean hearing loss (at 0.5, 1 and 2 kHz) of 65 dB HL and the MET up to 80 dB HL. Recently, Busch et al. (2016) performed a similar study in their patients using VSB and also found restrictions; they suggested to apply the VSB only in patients with sensorineural hearing loss up to 55-60 dB HL.
Figure 8.2. Speech recognition-in-quiet scores (presentation level 65 dB SPL; phoneme scores) as a function of the hearing loss of 14 VSB-Amade/Samba users (squares) and 10 MET BAP 2.2 users (dots). The drawn lines are best fits from a previous study using previous generations of speech processors (taken from Verhaegen et al., 2008).
8.3 Interference direct sound and amplified sounds
A problem of using auditory implants in sensorineural hearing loss is possible interference between the vibrations of the middle ear ossicles generated by the implant’s actuator and direct sounds that activate the ossicles in the normal way. It should be noted that owing to the digitally processing by the audio processor, the amplified sounds are delayed, typically by 4 to 10 ms. That implies that the direct and processed sounds will not be in phase for most frequencies. This also occurs when fitting digital BTEs with open ear molds, as shown by Groth & Sondergaard (2004) and Stone et al. (2008). The latter study showed that an annoying interference might occur that depends on the gain level of the device, on the delay of the amplified signal and on frequency (the higher the gain, the lower the delay and the higher the frequency, the lesser the annoyance). Studies looking into this issue using middle ear implants have not yet been published.
8.4 References chapter 8
Busch S, Lenarz T, Maier H. Comparison of alternative coupling methods of the Vibrant Soundbridge FMT. Presentation CI2016, Toronto, May 2016
Carlsson PU, Håkansson BE. The bone-anchored hearing aid: reference quantities and functional gain. Ear Hear. 1997;18:34-41
Dillon H. Hearing aids. 2012, Thieme Verlag, Stuttgart, Germany
Edfelt L, Stromback K, Grendin J, et al., Evaluation of cost-utility in middle ear implantation in the ‘Nordic School’; a multicenter study in Sweden and Norway. Acta Otolaryngol 2014;134:19-25
Goode RI. Current status and future of implantable electromagnetic hearing aids. Otolaryngol Clin N Am 1995;28:141-146
Groth J, Sondergaard MB. Disturbance caused by varying propagation delays in non-occluding hearing aid fittings. Int J Audiol 2004;43:594-599
Jenkins H, Niparko JK, Slattery WH, Neely JG, Frederickson JM. Otologics middle ear transducer ossicular stimulator. Performance results with varying degrees of sensorineural hearing loss. Acta Otolaryngol 2004;124:391-394
Klein K, Nardelli A, Stafinski T. A systematic review of the safety and effectiveness of fully implantable middle ear hearing devices: the Carina and Esteem systems. Otol Neurotol. 2012;33:916-921
Magnan J, Manrique M, Dillier N, Snik A, Hausler R. International consensus on middle ear implants. Acta Otolaryngol 2005;125:920-921
Maier H, Hinze AL, Gerdes T, Busch S, Salcher R, Schwab B, Lenarz T. Long-term results of incus vibroplasty in patients with moderate to severe sensorineural hearing loss. Audiol Neurotol 2015;20:136-146
Plomp R. Auditory handicap of hearing impairment and the limited benefit of hearing aids. J Acoust Soc Am. 1978;63:533-549
Pulcherio JO, Bittencourt AG, Burke PR, Monsanto Rda C, de Brito R, Tsuji RK, Bento RF. Carina® and Esteem®: a systematic review of fully implantable hearing devices. PLoS One. 2014;17;9(10)
Rameh C, Meller R, Lavielle JP, Deveze A, Magnan J. Long-term parient satisfaction with different middle ear hearing implants in sensorineural loss. Otol Neurotol 2010;31:883-892
Snik A, Noten J, Cremers C. Gain and maximum output of two electromagnetic middle ear implants: are real ear measurements helpful? J Am Acad Audiol. 2004;15:249-254
Snik A, Implantable hearing devices for conductive and sensorineural hearing impairment. In: F-G Zeng et al. (eds.) Auditory Prosthesis. New Horizons. Springer Handbook of Auditory Research 39, 2011:85-108
Snik AF, Mylanus EA, Cremers CW. Bone-anchored hearing aids in patients with sensorineural hearing loss and persistent otitis externa. Clin Otolaryngol Allied Sci. 1995;20:31-35
Stenfelt S, Håkansson B, Jönsson R, Granström G. A bone-anchored hearing aid for patients with pure sensorineural hearing impairment: a pilot study. Scand Audiol. 2000;29:175-185
Stone MA, Moore BCJ, Meisenbacher K, Derleth RP. To;erable hearing aid delays. V. Estimation of limits for open canal fittings. Ear Hear 2008;29:601-617
Verhaegen VJ, Mylanus EA, Cremers CW, Snik AF. Audiological application criteria for implantable hearing aid devices: a clinical experience at the Nijmegen ORL clinic. Laryngoscope. 2008;118(9):1645-1649
VSB fact sheet (VORP 503/Samba). Vibrant soundbridge System. Med-El website, downloads, 2015