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.