Discussion:
This study, to our knowledge, is the first describing the relationship
between size, site and activity of perforations in ATSI children. In
keeping with other studies21, active mucosal COM
created a greater HL than inactive disease. Multiple factors such as
otorrhoea, mucosal hypertrophy, sclerotic change of the ossicles and
generalised inflammation have been hypothesised to explain this
finding21.
Increasing TM perforation size also corresponded with an increase in HL.
With active perforations, there was less hearing difference with
increasing size compared to the inactive group, possibly due to oedema
and fluid within the middle ear. We believe the absence of a significant
increase in HL in both active and inactive perforations involving the
50-75% is due to the small number in the 50-75% group, making this a
likely Type 1 error. An increase in HL with increased TM perforation
size is consistent with the consensus of both theoretical and clinical
studies in the literature4, 6, 8, 10-12, 16. In a
study of 78 patients (107 ears) with inactive mucosal COM, perforations
were classified as a percentage of total surface area using image
processing software8. A strong correlation was found
between HL and size of perforation8. A study of 300
ears divided perforations into small, medium and large using digital
measurements11. The mean air-bone gap (ABG) increased
with size11. This has been attributed to a decreased
surface area for the amplification of sound, and a reduction in the area
ratio between the TM and the stapes footplate14. The
accuracy of perforation size estimated by 6 physicians was compared to
objective software7. Between the doctors, agreement
was high with kappa measures above 0.81 which compared favourably to the
kappa of between 0.62 and 0.93 utilising the
computer7.
Our study found no difference in HL between frequencies across all
perforation sizes and locations. This contradicts others that showed HL
to be greater in the lower frequencies4, 6, 8, 11, 21.
We hypothesise this may be due to the unique disease processes and lack
of treatment in our patient group rather than the now discredited
hypothesis of different anatomical factors between ATSI and Caucasian
ears.
The literature regarding perforation location and hearing levels is
conflicting. We found no statistically significant difference in hearing
levels between perforation location and all size/activity groups. This
is consistent with the findings of several studies using a variety of
methods6, 7, 12. One study used audiometric data
from 56 subjects (62 perforations) and found no statistically
significant difference in the ABG between anterior and posterior
perforations6. Another studied 38 patients (44
perforations), again with no statistically significant difference found
between anterior and posterior perforations, both in terms of mean air
conduction threshold, mean ABG, and frequency
comparisons12. In 156 adult patients (172
perforations) undergoing myringoplasty for TM repair, there was no
significant difference between pre-operative ABG between all quadrants
for each frequency7. In 35 adult Nigerian patients (42
perforations), no significant difference was found between TM
perforation location and HL in patients with pure conductive
HL22. However, in patients with a mixed HL, the loss
was greater for posterosuperior based perforations22.
Several other studies also demonstrate increased HL with posterior
perforations. Studies consistently utilise the manubrium mallei as the
angled division between anterior and posterior6-8, 10,
12, 15, 23. However, the exact classification of location varies
somewhat and is not often illustrated, making comparison difficult.
Statistically significant findings from a number of studies found that
posterosuperior11, 15,
posteroinferior14 and posterior8, 9,
13 perforations were associated with greater HL. In 70 young adult
males (60 perforations) posteroinferior perforations had greater HL than
anteroinferior perforations, with a greater difference apparent at low
frequencies14. For perforations less than 10%, this
difference was variable and inconsistent14. More
recently, in the largest study thus far, 700 patients (1400
perforations) with inactive mucosal COM were
studied15. Maximum HL was seen with posterosuperior
perforations (48.6dB) and anterosuperior perforations had the least HL
(24.0dB)15. When perforations involved 2 quadrants,
involvement of a posterior quadrant resulted in higher HL compared to
anterior quadrants but there was no significant difference between
anteroinferior and posteroinferior quadrants15. One
recent study found exclusively posterior perforations had a 12% greater
ABG when compared with exclusively anterior perforations, however this
was only significant at 500Hz8. Another looked at 90
ears and found the mean HL in anterior perforations was 29.9dB compared
to the posterior perforation group with 44.9dB13.
Many of these studies cite the “phase-cancellation effect” as a factor
influencing HL for posterior perforations8, 9, 13-15.
This hypothesises that a posterior perforation exposes the round and
oval windows simultaneously to sound causing phase-cancellation and
further increasing the level of HL7, 21. Contrary to
this belief, experimental and theoretical studies by
others4, 6, 24, 25 found that perforation site did not
affect pressure differences between the oval and round windows at all.
Pressures were measured at the stapes and round windows in 11 temporal
bones with no otologic disease4. When anteroinferior
and posteroinferior perforations were then created, no pressure
differences between the two sites were found whereas the main
contributing factor to HL in TM perforations was the sound pressure
difference across the TM4. The same group hypothesised
that wavelengths of sound <4kHz are actually larger than the
middle ear depth and therefore should not cause phase
cancellation6. Additionally, one other compounding
paper found that posteroinferior perforations had lower mean ABG levels
than anteroinferior and posterosuperior locations by 12-14dB, thereby
further contradicting the phase cancellation theory11.
It is clear from the conflicting literature that there are likely to be
other factors contributing to the variation in hearing levels with
perforations at different locations. Involvement of the manubrium mallei
has been discussed in many studies12, 14, 21 with some
finding perforations with malleolar involvement showed greater
HL15. Additionally, in patients with disease lasting
longer than 10 years, the average HL was greater at 52 dB compared to
36dB with disease durations of <1 year15.
The volume of air in the middle ear and mastoid cell complex also
appears to affect sound transmission. Using computed
tomography10 and tympanometry6, 11,
12, mean ABGs correlated inversely with middle ear and mastoid
volume6, 10-12.
Perforation shape may also influence hearing levels with one study
suggesting spindle-shaped perforations created greater conduction
disturbance compared to circular perforations21.
In a multivariate logistic regression analysis on 67 patients (86
perforations) aged 10 and over size of perforation proved to be the only
significant predictor of HL severity23.
The logistical challenges of data collection in our study population due
to age, remoteness and resources mean that the variables measured in the
literature cited above could unfortunately not be measured in this
study.
This study has again demonstrated that children in the APY Lands are
burdened with significant HL associated with potentially preventable and
treatable disease. These findings
assist with the qualitative estimation of HL from otoscopy based on size
and site of the perforation. In resource poor areas such as the APY
Lands, simple qualitative measures hold great value. Hearing losses
greater than expected based on this data may indicate damage to other
structures other than the TM.