The first seven chapters of this website deal with amplification options for hearing impaired patients with conductive or mixed hearing loss. If reconstructive surgery is not an option, several amplification options (hearing devices) are available: besides the conventional behind-the-ear device (BTE), bone-conduction devices (‘BCDs’) and middle-ear-implants (MEIs). However, patients with conductive or mixed hearing loss cannot use a BTE if the hearing loss is caused by atresia of the ear canal or a chronic infected ear. Noteworthy, in contrast to BTEs, ‘BCDs’ and MEIs have limited capacity. Therefore, it is a challenge to find the best device for a given patient, especially as the brochures, provided by the manufacturers of these devices, are often quite optimistic. An objective comparison between the capacities and limitations of all the amplification options is needed, and that is one of the aims of this website. Second aim is to give an overview of basic knowledge in this field.

To choose a device, the output capacity and the amplification (gain) is of high importance, however, also other issues play a significant role like safety and stability, MRI compatibility, aesthetics, user’s satisfaction, costs, etc. Focus is on those parameters that can be assessed (rather) objectively and can be compared mutually, e.g., output/amplification of the devices, stability and MRI compatibility.

Chapters 2, 3 and 4 are the main chapters, addressing basal audiological/technical issues like device characteristics, categorisation of the available devices, device-fitting procedures with an overview of stability figures and complications. Special attention is paid to decreasing effectiveness of these devices in the elderly, owing to aging. Chapters 5 to 8 deal with clinical application in specific patient groups. Chapter 9 discusses borderline areas for the application of all the hearing devices, including cochlear implants (CIs).

This overview might not be fully accurate owing to on-going developments in this field, although it is updated regularly. Last partial update: early 2024. Information on relatively new active transcutaneous bone-conduction devices (the Osia device; Cochlear, Mechelen, Belgium and the de Sentio device; Oticon, Askim, Sweden) will be added later this year.

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

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

A conductive hearing loss is caused by some medical or anatomical middle ear problem
or atresia of the ear canal. Surgical intervention is the preferred treatment. However,
surgery doesn’t always solve the problems or, in other words, doesn’t always result in
closure of the gap between the air- and bone-conduction thresholds (referred to as the
AB-gap) (e.g. Nadaraja et al., 2013). In search of what is an acceptable remaining
hearing loss after surgery, it has been argued by surgeons that thresholds of 30 dB HL
are sufficient (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); as it may affect 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 lower
(see also Chapter 7 of this website).

If an effective surgical hearing solution cannot be guaranteed or if the patient declines
from 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). Besides BTEs, essentially, these devices directly stimulate the cochlea
(‘BCDs’ and MEIs 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 BTEs, see paragraphs 2.2 and 2.3, Chapter 2.

Prescription of the desired amplification and output. What do we know about optimal
fitting (any type of) hearing devices in conductive and mixed hearing loss? What are the
desired values for amplification and the maximum output? For patients with pure
sensorineural hearing loss, several well-validated prescription rules are available, like the
NAL-NL rule and the DSL i/o rule (Dillon, 2012). In such patients, hearing thresholds are
elevated while the loudness discomfort levels (or the highest sound pressure levels which
are tolerated by the patient or the LDLs) are not, leading to a reduced ‘dynamic range of
hearing’. Consequently, the output of the BTE used has to be limited to prevent loud
sounds to exceed the patient’s LDLs.

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.
Theoretically, in case of a pure conductive hearing loss, the desired gain (or
amplification) should be equal to the conductive hearing loss (full compensation),
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 and DSL rules. However, the problem of relatively low capacity or maximum output
(MPO) of ‘BCDs” and MEIs is a limiting, interfering factor as discussed in the next
chapters. MPO refers to the loudest sound that can be produced by a device.

A univocal fitting procedure taking the limited capacity of the devices into account is
missing. In order to suggest a dedicated 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 amplification setting of the devices (Chapter 3).