Chapter 3. Basic considerations; the effect of low MPO on gain

What is the effect of a relatively low MPO? Figure 3.1A (upper, right) shows the
audiogram of a patient with conductive hearing loss. Cochlear thresholds and the
loudness-discomfort-levels (LDLs), taken from Dillon and Storey (1998), are indicated in
Figure 3.1B (upper, left).

figure 3.1.1    figure 3.1.2

figure 3.1.3

Figure 3.1. Figure A (upper left) shows the audiogram; figure B (upper, right) present the
cochlear thresholds and the LDLs. Figure C (lower figure) shows the MPO values
(labelled with ‘S’) of the applied transcutaneous ‘BCD’.

The patient’s dynamic range of hearing is, by definition, the difference between the
cochlear threshold and LDLs. Evidently, Figure 3.1C shows that the MPO values,
indicated by symbol S, split the patient’s dynamic range of hearing into two parts; an
audible part (45 dB wide, see vertical red arrows) and a second unused part (40 dB
wide). The speech area of the patient’s own voice is schematically indicated, with the
yellow part being ‘inaudible’. It should be noted that it is assumed that the amplification of
the device was 0 (which means that with the device, the AB-gap was ‘functionally’
‘closed’).. With that gain of 0 , the MPO values indicate that the ‘BCD’ saturates at 50-60
dB HL. Saturation causes annoying distortions. Patients can prevent distortions, by
turning down the volume (thus reducing the amplification). For instance, in this case, the
chosen reduction in amplification might be 20 dB. In that way, the louder part of the
patient’s own voice (yellow area), that range from 50 to 70 dB HL, will be perceived
undistorted, ranging from 30 to 50 dB HL. Now, the patient’s own speech fits nicely in the
patient’s ‘BCD’-aided dynamic range of hearing. However, that 20 dB attenuation
(negative amplification) negatively affects speech perception of someone else, talking at
a normal conversational level. Note that in the audiogram such a ‘negative gain’ of 20 dB
shows up as a ‘remaining AB-gap’ of 20 dB. This example underlines that it is better to
use a ‘BCD’ with a higher MPO, even in patients with a (sub)normal sensorineural
hearing loss component.

The choice for a ‘negative gain’ to deal with a relatively low MPO, proved to be a general
observation amongst patients using ‘BCDs’.

The next question is: in daily practise, what is the volume setting (chosen amplifications)
by ‘BCD’ users with varying degrees of cochlear hearing loss ? This question was
answered with the help of our Baha database by re-examined the data of 89 patients
using Baha devices (Snik et al., 2004). At that time, Baha devices with (predominantly)
linear amplification were used (Baha Compact, Classic, HC220, Cordelle) and with a
volume wheel that enabled the patients to choose their preferred amplification (Snik et
al., 2004).


Figure 3.2 The ‘Baha’-aided thresholds (at 0.5, 1 and 2 kHz averaged) as a function of
the mean SNHLc (sensorineural hearing loss component; same frequencies). For clarity,
individual data of the 89 Baha users were grouped into 12 classes of each 5 dB wide.
The diagonal line represents the (cochlear) hearing thresholds, thus the thresholds
without the Baha

Figure 3.2 shows that patients with (sub)normal cochlear function (low mean SNHLc) set
the volume of their Baha such that the gain is negative: the ‘Baha’-aided thresholds are
worse (higher) than the threshold without the ‘Baha’. This results in a remaining AB-gap
of around 10 dB, which is seen for SNHLc values up to approx. 25 dB HL.

Patients with more severe SNHLc need ‘positive’ amplification to be able to properly hear
other subjects talking, which, according to Figure 3.2, is found for SNHLc > 30 dB HL.

It should be noted that with increasing mean SNHLc, more and more patients used the
more powerful Baha sound processors (Snik, 2014). Per patient, the type of Baha device
used was documented and, consequently, its MPO was known. The MPO values were
used to obtain the ‘input level at saturation’ or the sound level at which the MPO was
reached. This ‘input level’ is simply the MPO value minus the amplification of the Baha (output = input + amplification). The amplification itself is easily obtained by subtracting
the Baha-aided thresholds from the cochlear thresholds (thresholds without the Baha).
These calculated (individual) input levels at which the Baha devices saturate, (again)
grouped in the 12 classes, are presented in Figure 3.2 as the blue line (with label
‘saturation’). The figure clearly shows that the input level at saturation restricts the
dynamic range of hearing with the Baha significantly. Before discussing the
consequences, Figure 3.2 was redrawn, see Figure 3.2bis.

 3.2_bis  3.2_bis_2

Figure 3.2bis Redrawn of Figure 3.2 with more details, see text

The left graph of Figure 3.2 bis  shows the remaining capacity of ear of patients with SNHLc
between 0 and 60 dB HL. As said before, the diagonal line presents the cochlear
thresholds while the dotted line presents the patient’s LDL. The right graph shows
the aided situation after Baha fitting, taken from Figure 3.2. The difference between aided
thresholds and input level at saturation is the patient’s aided dynamic range of hearing or
his ‘auditory window’ to the world.

The Baha-aided dynamic range of hearing is restricted; as an example, at a SNHLc of 45
dB HL, a patient’s unaided dynamic range of hearing is 50 dB HL (95-45 dB, left-hand graph, red arrow) and with the Baha, the dynamic range is only 35 dB HL (40 to 75 dB
HL, right-hand graph, red arrow).

The subfigure on the right side presents schematically sounds from the real auditory
world. Let us consider normal conversational speech. That level fluctuates between 25
dB HL and 55 dB HL (indicated in the subfigure, right-hand side; according to Killion &
Christensen, 1998). Combining the right-side subfigure and the right-hand graph shows
that in the present patient group, the 30 dB speech range is only fully audible when the
mean SNHLc is 15 dB HL or less, while at a mean SNHLc of 60 dB HL, the audible
proportion of speech range has reduced to just 5 dB HL.

Furthermore, we learn from Figure 3.2 that the patients, irrespective of their mean
SNHLc, set the volume of their device such that the device saturates at input sound
levels of 70-75 dB HL. Indeed, this choice does enable undistorted perception of even
the loudest parts of normal conversational speech. So, the figure suggests that the
patients in this study don’t compromise with regard to the input level at saturation, but,
consequently, they do compromise with regard to the softest speech sounds that they
can hear.

As indicated before, the basic rule for desired amplificatiion in pure sensorineural hearing
loss equals approximately half the hearing loss(e.g., Dillon, 2012). As the figure shows,
that is not accomplished at all. E.g. at a mean SNHLc of 45 dB HL, the amplification is
only 7 dB (see Figure 3.2). The limited MPO plays an important, probably decisive role in
the low amplification setting, as chosen by the patients.s.

So far, the analysed data concerned the Baha device. Figure 3.3 presents similar data of
all implantable systems available (early 2017), including besides the Baha the
percutaneous Ponto (Oticon Medical, Askim, Sweden), the ‘Bonebridge’ (Med-El,
Innsbruck, Austria), Sophono (Medtronic, Boulder, Co, USA), the transcutaneous Baha
Attract (Cochlear BAS, Gothenburg, Sweden) and the Codacs device (Cochlear,
Mechelen, Belgium). A systematic review of the literature was carried out.

To find the most relevant studies it was decided, first of all, to use studies that had been
selected before by authors who published systematic reviews. This concerned reviews by
Colquitt et al., 2011; Verhaert et al., 2013; Nadaraja et al., 2013; Ernst et al., 2016;
Sprinzl and Wolf-Mangele, 2016 and Dimitriades et al., 2016. Only those studies were re-
considered that presented audiometric data; including air- and bone-conduction
thresholds as well as device-aided thresholds. Duplicates were removed as well as
studies with five patients or less and studies published before 2006. Unpublished
presentations were also excluded.

Using the reviews by Colquitt et al., Nadaraja et al. and Verhaert et al. Concerning the percutaneous Baha and Ponto mechanisms, nine publications met the inclusion criteria (Kompis et al., 2007; Priwin et al., 2007; Flynn et al., 2009; Fuchsman et al., 2010; Ricci et al., 2011; Marsella et al., 2011; Bouhabel et al., 2012; Concerning VSB using the reviews by Verhaert et al. and Ernst et al., 14 publications were included (Beltram et al. 2009; Streitberger et al. 2009; Colletti et al. 2011; Baumgartner et al., 2010; Frenzel et al., 2010; Beleites et al. 2011; Bernardeschi et al., 2011; ., 2013). Sprinzl andWolf-Mangele (2016) reviewed the outcomes of ‘Bonebridge’ application. Only two studies met the criteria (Sprinzl et al., 2013; Ihler et al., 2016). Dimitriades et al. (2016) reviewed the transcutaneous Baha Attract; three studies were included (Iseri et al., 2014; 2015; Briggs et al., 2015). Since the number of included publications regarding the Baha Attract and the ‘Bonebridge’ were low and the publications with the percutaneous Baha/Ponto outcomes were rather dated (<2012), we performed an additional literature search, using Pubmed. For Bonebridge and Baha Attract, search terms were the respective device names together with '2016'; concerning ‘Bonebridge’, four studies were added (Gerdes et al., 2016; Baumgartner et al., 2016; Smerber et al., 2016; Zernotti et al., 2016), for Baha Attract, no further study with relevant audiometric data was found. Concerning Baha/Ponto, the sound-processor's names were used and the search period was 2011-2016. In contrast to the other devices, number of patients studied had to be ten or more. Seven studies were identified (Pfiffner et al., 2011; Flynn et al., 2012; Bosman et al., 2013; Kurz et al., 2013; Desmet et al., 2013; Kompis et al. 2014; Gerdes et al. , 2016). Concerning the Sophono device, a systematic review has not yet been published. Therefore, we searched for studies using only 'Sophono' as search term and applying the introduced inclusion criteria. Six studies were included (Siegert and Kanderke, 2013; Sylvester et al., 2013; Hol et al., 2013; Denoyelle et al., 2015; Shin et al., 2016; Magliulo et al., 2015). Finally, data obtained with the Codacs device were included, which is the most powerful implantable device (Zwartenkot et al., 2014). Searching PubMed, three papers were identified comprising audiological data obtained in more than five patients (Busch et al., 2013; Lenarz et al., 2013; 2014).

Altogether, 48 studies were included in this analysis. Figure 3.3A  presents per study the
mean device-aided threshold at 0.5, 1, 2 and 4 kHz as a function of the mean cochlear
threshold, SNHLc, averaged over the same frequencies. Again, this figure shows that,
generally, when the (mean) SNHLc is below 25-30 dB HL, a remaining AB gap (data
points above the diagonal) is found, in agreement with Figure 3.2. Remarkably, Figure
3.3.b shows a rather large variability between studies.

Figure 3.3B presents the same data as Figure 3.3A; this time, trend-curves are
presented, drawn by eye through the data points for a each device. Considering the bleu
line, representing the trend for VSB, the VSB-aided thresholds are at a rather stable level
(25-30 dB HL) up to a SNHLc of approx. 45 dB HL. Above that value, the VSB-aided
thresholds seem to deteriorate exponentially, indicating that the device is no longer
powerful enough for such degrees of the SNHLc. For the Baha Attract and Sophono
devices, similar trends are seen, however, with lower cut-off SNHLc values, in between
15 dB HL and 20 dB HL. Obviously, these different types of hearing devices are not
equivalents, which conclusion is in line with the variance in objective MPO data, as
presented in Table 2.1.

For the Baha/Ponto devices, no clear cut-off value for SNHLc is found. This is caused by
the availability of several processors with increasing power, which have been applied in
the patients with more severe SNHLc.

Even taking this device-related limitation into account, an obvious variation between
studies is seen. This suggests that well-defined device-fitting procedures are needed.
This issue is the subject of the next chapter (chapter 4).


Figure 3.3.a.  Mean device-aided threshold (0.5, 1, 2, 4 kHz) as a function of the mean
SNHLc as obtained from 48 studies using the percutaneous Baha/Ponto devices (black
diamonds), the VSB (big blue dots), the ‘Bonebridge’ (red squares), the Baha Attract
(green triangles), the Sophono (small black dots) or the Codacs (green circles)


Figure 3.3.b. The same figure as Figure 3.3A, however, this time trend lines are indicated
for the VSBand the passive transcutaneous ‘BCDs’