4.1 Introduction
An attempt is made to develop a fitting model based on a ‘acceptable partial’ use of the
patient’s dynamic range of hearing. Audibility of normal conversational speech, with its 30
dB wide speech range, is an important factor in that model.
A new goal has been formulated: at least 35 dB of the patient’s ‘dynamic range of
hearing’ should be audible with a device; ensuring that, with proper amplification,
conversational speech is audible (Zwartenkot et al., 2014 and Rheinfeldt et al., 2015).
This criterion is indicated in Figure 4.1. The idea is that 35 dB HL is enough to hear
conversational speech adequately while the hearing device makes use of wide-dynamic
range compression as a kind of automatic volume control.
This criterion is referred to as the ‘DR>35 dB rule’. A second criterion is now introduced,
namely that at least 2/3 of the patient’s ‘dynamic range of hearing’ should be audible with
a minimum of 35 dB (referred to as the ‘DR 2/3 rule’). This criterion is based on the data
of Figure 3.2. Baha patients in that study were fitted with different types of linear devices
with volume wheel. Typically, their ‘dynamic range of hearing’ with the device equals
about 2/3 of the unaided range, up to 30 dB HL. This new criterion is illustrated by the
blue line in Figure 4.1.
Table 4.1 presents the MPO data taken from Table 2.1. The second and third columns of
this table indicate up to what mean SNHLc value the different devices fulfill the ‘DR>35
dB rule’ or ‘the DR 2/3’. These values deviate from those advocated by the
manufacturers; typically by 10 to 20 dB.
Figure 4.1. The minimal desired mean MPO versus mean SNHLc. Red line presents the
minimal desired MPO if the device-aided ‘dynamic range of hearing’ has to be 35 dB at
least (DR>35 dB rule). The blue line gives such target values if at least 2/3 of the
dynamic range should be audible with a minimum of 35 dB (DR 2/3 rule).
Amongst others, Table 4.1 suggests that the Sophono Alpha 1 device can only be used
in pure conductive hearing loss: indeed Sylvester et al. (2013) studying a group of
Sophono users came to a similar conclusion. The table further suggests that
percutaneous ‘BCDs‘ (with their most powerful processors) and the VSB device have
approximately the same capacity.
Table 4.1
| Device | Mean MPO* | ‘Max SNHLc’ if the ‘DR 2/3 rule’ is used | ‘Max SNHLc’ if the “DR>35 dB rule” is used |
| Sophono Alpha 1 | 53 dB HL | 5 dB HL | 20 dB HL |
| ‘Bonebridge’ | 64 dB HL | 20 dB HL | 30 dB HL |
| Standard Baha/ Ponto | 70 dB HL | 25 dB HL | 35 dB HL |
| Super power Baha/Ponto | 83-88 dB HL | 45-50 dB HL | 45-50 dB HL |
| ‘Vibrant Soundbridge’ | 81dB HL | 45 dB HL | 45 dB HL |
| Codacs ** | 100 dB HL | > 65 dB HL | > 65 dB HL |
| Baha Attract/BP110 | 64 dB HL | 20 dB HL | 30 dB HL |
* taken from Table 2.1
** no longer available
When using the milder criterion (DR > 35 dB rule), the application range is broader, see
third column. As said before, a ‘working range’ of 35 dB might be just sufficient for proper
amplification of speech, in combination with wide-dynamic range compression (Rheinfeldt
et al., 2015). ‘Wide-dynamic range compression’ increases the amplification of low-level
sounds while keeping the high-level sounds at loud but comfortable levels. ’Wide
dynamic range compression’ is the standard choice for patients with pure sensorineural
hearing loss using BTEs, who have a limited ‘dynamic range of hearing’ because of
physiological conditions. When applying ‘BCDs’ or MEIs, ‘wide-dynamic range
compression’ is also used, although the limited ‘dynamic range of hearing’ is not caused
by physiological conditions but technical restrictions of the used devices (low MPO).
Evidently, the starting point when choosing an amplification option should be selecting a
device with a high MPO. Especially in children, powerful devices should be used, see
Chapter 5.
Importantly; it should be noted that the ‘max SNHLc’ values per device type, as
presented in the table, are in agreement with maximum SNHLc values as obtained from
clinical studies, as presented in Figure 3.3.c. Using the trend-curves indicated in that
figure, the maximum SNHLc per device type could be assessed as well, e.g., approx. 45
dB HL for VSB and 15-20 dB HL for the transcutaneous ‘BCDs’. This agreement
(between Figure 3.3.c and Table 4.1 data) validates the MPO-based ‘max SNHLc-
values’ for application, as presented in the Table 4.1.
Concerning the use of hearing devices for conductive and mixed
hearing loss, the limited MPO has to be taken into account. Patients
‘compensate’ for a low MPO by lowering the amplification of the device,
leading to an unintended decrease in perception of speech. In short,
today’s passive transcutaneous ‘BCDs’ can be used in patients with
normal cochlear function or with a limited mean SNHLc, the active
‘Bonebridge’ in patients with mean SNHLc below 25 dB HL and the
percutaneous ‘BCDs’ and VSB in patients with mean SNHLc up to 50
dB HL. These values are more conservative (less optimistic) than those claimed by the manufacturers and might change when more powerful
processors are released.
4.2. Implanting elderly patients; the best solutions
Owing to aging or a progressive (hereditary) cochlear hearing loss, the SNHLc
deteriorates over time. Using data on the age-related hearing deterioration enables the
estimation of ‘years of effective use’ of a particular device. However, little is known
(reported) on age-related hearing deterioration in patients with mixed hearing loss. One
paper was identified; Figure 4.2 shows age-related deterioration in hearing of patients
with mixed hearing loss, studied by Iliadou et al. (2006). They reported both air- and
bone-conduction thresholds for patients suffering form OTSC7. The figure presents the
mean SNHLc (0.5, 1, 2 and 4 kHz averaged) as a function of age; see the first two rows.
Data up to 60 years were presented; the indicated mean SNHLc at 70 and 80 years are
(non linear) extrapolations. The thick red lines indicate whether or not a certain device
can (still) be used effectively. E.g., the figure shows that the Baha, with its ‘max SNHLc’
of 50 dB HL, can be used effectively up to approx. 80 years (the ‘max SNHLc’ refers to
the maximum sensorineural hearing loss component for successful application see Table
4.1). The Ponto device and the VSB might be applied successfully up to an age of
approx. 75 years.
Probably, the mildest scenario is that of a patient with a stable conductive hearing loss
component and, additionally, presbycusis. Using the median age-related hearing
deterioration for men (ISO 7029; 2017), Figure 4.3 is obtained. It should be noted that if
the patient suffers from mixed hearing loss caused by chronic otitis media, they might
have an additional sensorineural hearing loss caused by the infections and/or treatments,
which is approx. 20 dB (mean value 0.5 to 4 kHz; see ee Table A9.1, Chapter 9, and
related text). Taken this 20 dB into account, the effective use decreases to 50 years for
the ‘Bonebridge’ and 75 years for VSB and the percutaneous ‘BCDs’ while for such
patients with an age of 50+, the passive transcutaneous ‘BCD’ with magnetic coupling is
not an effective option.
Figure 4.2. Effective device use for a patient with progressive hearing loss owing to otosclerosis (OTSC7), based on the ‘DR 2/3 rule’. SNHLc stands for the cochlear hearing loss of patients
Figure 4.3. Effective device use for a patient with stable conductive hearing loss component and a sensorineural component that deteriorates over time owing to presbycusis. The ‘DR 2/3 rule’ was applied
Again, the percutaneous ‘BCD’ and the VSB seem to be the better choice.
When choosing for a particular treatment, effectiveness in the long run is one of the
factors that should be considered. The expected degree of hearing deterioration over
time should be assessed from the patient’s history.
It should be noted that the conclusion drawn from Table 4.1 and Figures 4.2 and 4.3 are
valid for the specified device types. If more powerful audioprocessor are released,
conclusions might change.
Effectiveness in the long run is an often ignored but important factor
when counselling specifically elderly patients (if implant surgery is
involved).
4.3 Attempt to formulate a prescription procedure
So far, the evaluation of the capacity of various amplification options aimed at the
MPO, as a low MPO restricts the ‘aided dynamic range of hearing’ of the patient. As said
before, to deal with that limitation, compression is often used. Another technical limitation might be device noise as patients with predominant conductive hearing loss might hear it,
owing to normal cochlear sensitivity. In that case ‘expansion’ (reduced amplification for
soft sounds) can be used to make the noise inaudible (Dillon, 2012). Information on the
noise level of a device should be clearly indicated on datasheets, as is available for
percutaneous ‘BCDs’. Problems with the noise floor have been reported with some VSB
processors (e.g. Linder et al., 2009), which, according to the manufacturer, have been
solved in newer sound processors. For the Cochlear Codacs device, the noise floor is
approximately 40 dB HL (Cochlear’s data sheets), which can be lowered to 25 dB HL.
This should explicitly be taken into account when counselling this device for a given
patient.
To choose the best device for a given patient is of utmost important. Note that when
considering the application of a MEI, well-masked bone-conduction thresholds are
essential to ensure that the cochlea of the to-be-implanted ear is sensitive enough for
successful application of the implant. On the other hand, for the application of
‘BCDs’, unmasked bone-conduction thresholds of the to-be-treated ear should be
considered. Owing to the limited transcranial attenuation of bone vibrations, the ‘BCD’ will
stimulate the cochlea with the best sensitivity, which might be either the ipsilateral or
contralateral cochlea (Stenfelt, 2012). Consequently, the fitting should be based on the
unmasked bone-conduction thresholds.
Regarding device fitting, it should be realized that both a ‘BCD’ and a MEI, with its
actuator coupled to one of the cochlear windows, directly stimulate the cochlea. This
simply implies that we can use our knowledge on fitting conventional hearing devices
(e.g., BTEs) in pure sensorineural hearing loss; thus we can make use of the well-known
validated prescription rules, like the NAL and DSL rules (Dillon, 2012).
To develop a practice-based prescription procedure, we used published data. The
selected studies, used in Chapter 3 were used again. For each of the included studies,
the ‘amplification at cochlear level’ was calculated as a function of frequency. That
amplification, referred to as ‘effective gain’, is, by definition, the frequency specific bone-
conduction threshold (cochlear threshold) minus the device-aided threshold (for more
information, see Appendix 2.3). To obtain a relative amplification value, corrected for the
degree of hearing loss, the ‘effective gain’ was divided by the bone-conduction threshold,
referred to as the ‘gain ratio’. According to the NAL-RP, that ratio should be approx. 0.45
(with some minor corrections; Dillon, 2012).
Twenty papers were included, which provided all the data needed for the calculations.
Figure 4.4 presents the ‘gain ratios’, presented per device type and the degree of
cochlear loss (mean SHNLc). To deal with the discontinuities that we found when
evaluating Figures 3.2 and 3.3, the 20 studies were divided into 3 subgroups, according
to the mean SNHLc of the participants :group 1 comprised the studies in patients with a
mean SNHLc below 25 dB HL, group 2 those with SNHLc between 25 and 40 dB HL and
group 3 those with a SNHLc exceeding 40 dB HL; see Table 4.2. Using Table 4.1, for
each subgroup, the amplification options are added, third row.
Table 4.2. Subgroups to deal with the discontinuities observed in Figure 3.2
| Group 1 | Group 2 | Group 3 | |
| Mean SNHLc | < 25 dB HL | 25 to 40 dB HL | > 40 dB HL |
| Figure 3.2 shows: | Negative gain | Intermediate | Positive gain |
| Amplification options based on Table 4.1 and noise floor data | Percutaneous ‘BCD’/VSB/’Bonebridge’ | Percutaneous ‘BCD’/VSB | Percutaneous ‘BCD’/VSB |
| Hypothetical target for gain | Predominant conductive loss: compensate the air-bone gap! | Intermediate | ‘Compensate’ the SNHLc as if it was a pure sensorineural loss |
Furthermore, using Table 4.1 and noise level data, for each subgroup, the amplification options are listed.
Lines in Figure 4.4 are labeled according to device type.
Figure 4.4. The gain ratio (gain divided by threshold) as a function of frequency. Figure 4.4A presents the data of group 1 (BAHS stands for Baha and Ponto together: 1 study, n=20; VSB: 3 studies, n=34 and Bonebridge: 1 study, n=12), Figure 4.4B for group 2 (BAHS: 6 studies, n=113; VSB: 7 studies, n=97) and Figure 4.4C for group 3 (BAHS: 1 study, n=12; VSB: 3 studies, n=47).
Obviously, Figure 4.4 shows that all the ‘gain ratios’ are below 0.45, the value taken from the NAL-RP rule (valid at 1, 2, 4 kHz; Dillon, 2012). The figure also shows that the ‘gain ratio’ is the highest at 2 kHz, irrespective of device type and subgroup. Furthermore, it is evident that inter-group differences are large (compare subfigures A, B and C). Differences between hearing devices within groups are less outspoken.
These data have been used to develop the practice-based prescription procedure: for mixed hearing loss, it is assumed that the cochlear loss (SNHLc) should be ’compensated’, equally as prescribed for sensorineural hearing loss. For group 1 with predominant conductive hearing loss, thus without a SNHLc (close to) 0 , the amplification (and thus the ‘gain ratio’) should be (close to) 0. However, negative ‘gain ratios’ are seen (Figure 4.4.A), as we also saw in Figures 3.2 and 3.3. Related to this, Dillon argued that in patients with a conductive hearing loss, not the whole AB gap should be compensated, but only partially (Dillon, 2012, Chapter 10.4). Following that reasoning, it is suggested that, based on our data (Figs 3.2 and 3.3), the device-aided thresholds might be 25 dB HL instead of the ideal value of (closer to) 0, for patients with a (sub) normal cochlear function. More details are found in Snik et al. (2019).
Table 4.3 presents the desired device-aided thresholds and ‘effective gain’, based on the data presented in Figure 4.4.
Table 4.3. Target values as a function of SNHLc for 1, 2 and 4 kHz
|
SNHLc (dB HL) |
0 |
10 |
20 |
30 |
40 |
50 |
60 |
|
Target aided threshold (dB HL) |
<25 |
<25 |
<25 |
<25 |
<25 |
<27 |
<33 |
|
Effective gain (aided minus cochlear thresholds dB) |
-25 |
-15 |
-5 |
5 |
15 |
23 |
27 |
Following the NAL rule, amplification at 0.5 kHz might be set 8 dB lower than that at the higher frequencies (to deal with upward spread of masking; Dillon, 2012).
What are the limitations of this proposed procedure? It should be noted that this evaluation is based on today’s hearing implants. Furthermore, the procedure has not (yet) been validated.
This practice-based prescription procedure can be used irrespective of the type of device used. Prerequisite is that a device is chosen with sufficiently high MPO, as listed in Table 4.1, preferably following the ‘DR 2/3 rule’.
Importantly, note that for percutaneous ‘BCDs’, Hodgetts and Scollie (2017) developed a dedicated prescription rule, based on the DSL fitting rule. That procedure is the preferred option, taking, amongst others, the limited MPO of these ‘BCDs’ explicitly into account.