Chapter 1. General introduction; outdated fitting protocols?

If reconstructive surgery is not an option, several amplification options are available for patients with conductive or mixed hearing loss. These options (hearing devices, implantable or not) are not equivalents. To find the best device for a patient, the manufacturer’s brochures are often of limited help. A comparison between devices is needed and that is the aim of this blog. Second aim is to give an overview of basic knowledge in this field.

To choose for a device, the output capacity is of high importance, however, other issues play a significant role like safety and stability, MRI compatibility, aesthetics, user’s satisfaction, costs, etc. Some of these parameters are assessed objectively and compared, namely output of the devices, stability and MRI compatibility. Special attention is paid to longevity. Chapter 1 presents the introduction; chapters 2, 3 and 4 describe a fitting model based on the output of the devices. Chapter 5, 6 and 7 deal with clinical application.

This overview will never been complete owing to ongoing developments in this field. Therefore, it will be updated twice a year.

Part 1. Challenges and limitations of implantable hearing devices (auditory implants) for conductive and mixed hearing loss 

1. General introduction; outdated fitting protocols?

Scope of this blog is to give an up-to-date overview of the effectiveness of amplification options for hearing impaired subjects, written for professionals and students. For further background details, see Appendix 1.

A conductive hearing loss component (or an air-bone gap) is caused by some medical or anatomical middle ear problem or atresia of the ear canal. Surgical reconstruction is the preferred treatment. However, surgical treatment doesn’t always result in closure of the air-bone gap (e.g. Nadaraja et al., 2013). In search of what is an acceptable remaining hearing loss after surgery, it has been argued that thresholds of 30 dB HL or better are socially acceptable (Browning, 1993). 

However, based on a review of the literature, it has been stated that already 15 dB HL is a handicapping hearing loss for children (see Chapter 1 in Northern & Downs, 1991); affecting the spontaneous development of speech and language. This means that at least for children with conductive hearing loss, the aim of an intervention should be post-intervention hearing thresholds of 15 dB HL or better (see also Chapter 7 of this website).

If an effective surgical reconstruction cannot be guaranteed or if the patient declines from such surgery, then amplification is the next option. Nowadays, several different types of hearing devices are on the market for the rehabilitation of conductive and mixed hearing loss (Snik, 2011). These devices all directly stimulate the cochlea (bone-conduction devices, middle ear implants with their actuator directly coupled to one of the windows of the cochlea), bypassing the impaired middle ear. Under certain conditions such devices might be a better option than conventional air-conduction devices as described in paragraphs 2.2 and 2.3. In Chapter 5 the application of these devices in specific groups of patients with conductive or mixed hearing loss is discussed, namely in patients with congenital malformations of the middle ear/ aural atresia, chronic otitis media with effusion or suppurative otitis media and in patients with otosclerosis.

Prescription of gain and output. What do we know about fitting (any type of) hearing devices in conductive and mixed hearing loss? What are the desired values for gain and output? For patients with pure sensorineural hearing loss, several well-validated fitting rules are available, like the NAL-NL rule and the DSL i/o rule (see Chapter 10 in Dillon, 2012). In patients with such hearing loss, hearing thresholds are elevated while the loudness discomfort levels (or the highest sound pressure levels which are tolerated by the patient; LDLs) are not, leading to a reduced dynamic range of hearing. Consequently, the gain (amplification) of hearing devices has to be limited to prevent loud sounds to exceed the patient’s LDLs. To deal with this issue, adaptive gain can be applied, also called compression, as prescribed by the two fitting rules.

In case of pure conductive hearing loss, not only the (air-conduction) hearing thresholds are elevated but also the LDLs, thus the dynamic range of hearing is not affected. In principle, in case of conductive hearing loss, the desired gain (or amplification) should equal to that conductive hearing loss, resulting in normal input to the cochlea. In mixed hearing loss, additionally, about half of the sensorineural hearing loss component should be ‘compensated’, according to the NAL or DSL rule. However, following the textbooks, praxis showed that measured gain in patients with conductive or mixed hearing loss is not that high (see Chapter 10.5 in Dillon, 2012). Dillon advocates the use of the prescription rules developed for pure sensorineural hearing loss (NAL/DSL) and add extra gain equal to one quarter of the width of the air-bone gap. In case of a pure conductive hearing loss this means that approximately 75% of the conductive hearing loss is compensated, not 100%. According to Dillon, this 75% is enough, firstly, because full compensation ‘with current technology’ is not a good option owing to limitations in available gain and the maximum output of the hearing devices. Second argument was that in normally functioning ears the acoustic (stapedius) reflex attenuates loud sounds (above 80-90 dB HL). This attenuation will be absent in non-functional middle ears. Therefore, the maximum output and the gain should be limited (however, the role of the acoustic reflex as an effective attenuator of loud sounds is still under debate; Coletti & Sitoni, 1986; Moller, 2012). Thirdly, it was argued that normal might not be the best; maybe even normal hearing subjects would appreciate some attenuation of everyday sounds. It was suggested that normal hearing subjects could miss the first 20 dB of our dynamic range of hearing.

To summarize, there is limited knowledge about optimal fitting of hearing devices in conductive and mixed hearing loss. The problem of relatively low maximum output (MPO) of several hearing devices seems to be a valid argument; see next chapters. A univocal fitting procedure is missing. In order to suggest a fitting protocol (Chapter 4), we will start with discussing in detail the MPO of several amplification options (Chapter 2) and how limited MPO affects the gain setting of the devices (Chapter 3). Such information is important, at least in case of children, in whom every decibel counts (Northern & Downs, 1991).

Next chapter >>>

Appendices >>>

References and Abbreviations >>>