A Retrospective Study to Identify the Relationship Between the Dimension of Osseous External Auditory Canal and Chronic Otitis Media
Whether the dimension of the osseous external auditory canal (OEAC) is related to chronic otitis media (COM) remains an important but unresolved issue. In the literature, routine canaloplasty has been suggested to help elucidate this problem. In this study, we intended to investigate the relationship between the diameter or shape of OEAC and COM. We retrospectively reviewed the high-resolution computed tomography (HRCT) images of the temporal bones of 62 patients with unilateral COM who underwent tympanoplasty from January 1, 2011, to December 31, 2013. For comparison, the HRCT images of another 62 patients with normal ears were collected to be the control group. The shape of OEAC was categorized into 5 groups according to Mahboubi’s classification, and the dimension of each patient’s OEAC was measured at 2 defined sections (annular section and isthmus/midcanal section). The most prevalent shape of OEAC for the lesion sides of the ears was cylindrical, followed by conical, and hourglass in both the COM and the control groups. As to the dimension of OEAC, there were no significant differences between lesion ears and normal ears among patients in the COM group as well as between lesion ears in the COM group and normal ears in the control group. We concluded that the bony dimension of the external ear canal was not related to COM. This suggested that chronic inflammation or infection of middle ear may not cause bony thickening of OEAC, despite the kind of OEAC shape involved.
Canal stenosis is defined as narrowing of the dimension of the external auditory canal (EAC), with a cause that can be either congenital or acquired. Multiple etiological factors of acquired canal stenosis have been published to date, including persistent otitis externa,1 trauma (surgical or nonsurgical), malignancy2, or irradiation.3 In 1999, Kimberley et al suggested that chronic otitis media (COM) could cause acquired canal stenosis; therefore, routine canaloplasty is necessary when performing tympanoplasty.4 Based on our clinical observations and daily practice, there seems to be a tendency for most patients with COM to have a smaller dimension of the EAC. However, the relationship between COM and acquired canal stenosis has rarely been mentioned in the literature in the last 15 years.1,5
The human EAC is composed of 2 portions—the cartilaginous and the osseous part. In most otolaryngology texts, the osseous external auditory canal (OEAC) is described as located in the medial two-thirds of EAC, and its anatomic feature can be evaluated clearly by high-resolution computed tomography (HRCT).6-7 In 2012, Mahboubi introduced a standard and objective method to determine the anatomic dimension of OEAC.5 In this study, we try to figure out whether the dimension of OEAC or the shape of OEAC was related to COM.
Patients
We conducted a retrospective analysis of high-resolution temporal bone computed tomography (CT) images of 2 cohorts of adult patients. The first cohort (COM group) consisted of all patients with unilateral COM who underwent tympanoplasty from January 2011 to December 2013 by the same surgeon. Ears with exostoses, canal atresia, congenital canal stenosis, history of temporal bone fracture, otologic surgery, bilateral COM, cholesteatoma, or tumor were excluded from this study. In addition, HRCT images of temporal bone taken from other hospitals with different protocols or with low resolution quality were also excluded.
As to the second cohort (the control group), patients who also received HRCT studies of temporal bone between 2011 and 2013 due to certain indication other than COM and the exclusion criteria mentioned earlier were included. They all had normal anatomic structure of bilateral ears without any active disease proven by HRCT of temporal bone.
Imaging of the OEAC
All the CT scans were obtained using Philips Brilliance 40-slice CT scanners. Axial and coronal plane CT images of the temporal bones were evaluated, with the axial images taken parallel to the ground. The slice thickness was 1 mm, and the interval thickness was 0 mm.
Classification of OEAC Shape
The shape of OEAC was evaluated by axial and coronal images of HRCT. We classified all the patients into 5 groups by their OEAC shape (Figure 1) based on Mahboubi’s anatomic study in 20125—conical, ovoid, reverse conical, hourglass, and cylindrical shapes.
Figure 1. Categorization of shapes of OEAC based on Mahboubi’s classification in 2012: (A) hourglass, (B) conical, (C) cylindrical, (D) reverse conical (A ∼ D: images are from axial sections); (E) hourglass shape (E: image is from coronal section). OEAC indicates osseous external auditory canal.
Measurements of OEAC Dimension
We separately measured the OEAC dimension at 2 sections of the axial and the coronal view. In the hourglass group, the first section (which we defined as the isthmus section) was used to measure the distance from bone to bone at the isthmus section on axial (Figure 2) and coronal views. The second section (which we defined as the annular or membranous section) was used to measure the maximal distance from bone to bone at the tympanic membrane section. However, in the nonhourglass group (including the groups of conical, ovoid, reverse conical, and cylindrical shapes), the first section was standardized and replaced by measuring the distance from bone to bone at the midcanal section (Figure 3). That is because there was no obvious isthmus that could be identified in the nonhourglass group.
Figure 2. Defined measurement of OEAC in hourglass group. OEAC indicates osseous external auditory canal.
Figure 3. Defined measurement of OEAC in non-hourglass group-conical shape as example. OEAC indicates osseous external auditory canal.
Statistical Analysis
The paired t test was used to analyze the differences in OEAC dimension between the 2 sides of ears of the same people within the COM and normal groups; the Student t test was used to analyze the differences in OEAC dimension between different groups. A P value <.05 indicated statistical significance. The software used for statistical analysis was SPSS for Windows, version 17.0.1 (SPSS Inc, Chicago, Illinois).
There were 62 patients with unilateral COM who underwent tympanoplasty from 2011 to 2013 who were enrolled into the COM group. The mean age of this group was 53.03 years (range 18-84). Overall, there were slightly more female patients than male patients (M/F: 22/40), and there was no statistical difference between the sides of lesion ears (right/left side: 28/34). Additionally, 53 patients received tympanoplasty type I, 1 patient received classic type III tympanoplasty, and 8 patients received modified type III tympanoplasty.
Shape of the OEAC
The prevalence of different shapes of OEAC among total patients, the COM group, and normal group was analyzed. We found that cylindrical shape was the most common type of OEAC shape of COM (36%), followed by conical (31%), hourglass shape (27%), and reverse-conical shape (6%). A similar result was found in the control group and among total patients. Neither the COM group nor the normal group had ovoid shape of OEAC.
Dimensions of the OEAC
The dimensions of the OEAC at the first (isthmus/midcanal) and the second (annular/membranous) section showed no statistical significance between lesion ears and normal ears within the COM group (Table 1). In the normal group, the dimensions of the OEAC also showed no significant difference between bilateral ears (Table 2). We compared the 62 lesion ears in the COM group with the 124 normal ears in the control group, and there was no significant difference as to the dimensions of the OEAC on axial and coronal images. We further divided patients into hourglass and non-hourglass subgroups within the COM and control cohorts. In the hourglass subgroup, there was no difference between lesion ears in the COM group and normal ears in the control group. The same result could be seen in the nonhourglass subgroups (Table 3).
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Table 1. Dimensions of OEAC at Annulus and Isthmus/Mid-Canal Section (COM Group).
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Table 2. Dimensions of OEAC at Annulus and Isthmus/Mid-Canal Section (Normal Group).
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Table 3. Comparison of Dimensions of OEAC at Annulus and Isthmus/Mid-Canal Section Between COM (62 lesion ears) and Normal Group (124 Normal Ears).
In several earlier studies8,9 the term “canal stenosis” was often used to describe cases of “canal atresia.” However, these are 2 quite different entities. A canal atresia consists of a soft tissue plug in the proximal portion of the EAC, attached to the lateral surface of the tympanic membrane.1 The etiology may be traumatic,10 postoperative,11 neoplastic, or postinflammatory.12-13 A canal stenosis, on the other hand, is a narrowing of the dimension of the EAC that is either congenital or acquired. An acquired canal stenosis (previously mentioned in the introduction section) can be secondary to persistent otitis externa,1,14 trauma (surgical or nonsurgical), malignancy,2 or irradiation.3
Although uncommon, COM has also been suggested to be one of the proceeding conditions of acquired canal stenosis.1,4 The presence of canal stenosis secondary to COM causes several problems. First, the chronic discharging ear may become difficult to treat locally, which results in higher failure rates when only medical treatment is applied. Second, the narrowed canal may cause hearing loss, and in some circumstances can make hearing aid use difficult. Finally, the stenotic ear canal may trap the keratin debris and, over time, can give rise to cholesteatoma formation. Kimberley et al suggested that COM is related to acquired canal stenosis. Typically, routine canaloplasty was required while performing tympanoplasty.4 The aim of canaloplasty here was to produce a dry, patent ear canal and to break the vicious cycle formed by COM and canal stenosis.
In the past decades, imaging of the temporal bone, particularly by HRCT, has played an increasingly important role for diagnosis, surgical decision, and patient follow-up. High-resolution computed tomography can undoubtedly provide reliable details of temporal bone anatomy.15 Therefore, we conducted this study to evaluate the EAC by HRCT. We focused on the osseous portion of the EAC because it consisted of approximately the medial two-third of EAC and could be evaluated clearly on HRCT. In addition, the osseous portion of EAC was not as elastic as the cartilaginous portion and was the main portion to enlarge while performing canaloplasty.
In the literature and based on clinical observations, the shape of the OEAC has been suggested to be an etiologic factor in chronic otitis externa (COE).14 But, does the shape of OEAC also play a role in the pathogenesis of COM? There were studies that attempted to classify the shape of OEAC. In 1980, Eckerdal and Ahlqvist reported 3 different shapes of the OEAC: cone-shaped (64.1% of the normal patients), hourglass-shaped (32.1%), and ovoid (3.8%).16 Another study7 used axial CT images with slice thicknesses of 1.5 and 2 mm to distinguish 8 different shapes with the uniform shape (same as the cylindrical shape), which was most prevalent among normal people. In a more recent study by Mahboubi in 2012, the shape of OEAC was divided into 5 groups, with the most common type to be conical (64%). In this study, we followed the most comprehensive classification developed by Mahboubi. Similarly, we found the most prevalent shape of OEAC in both the COM and the control group were cylindrical and conical shape, followed by hourglass shape. Regarding the prevalence of OEAC shape, there was no difference between the lesion ears in the COM group and normal ears in the control group. In other words, the shape of OEAC does not seem to play as important a role as COE in the pathogenesis of COM.
As we mentioned earlier, those patients with COM appeared to have smaller EAC dimensions, and routine canalplasty was suggested while performing tympanoplasty.4 However, in this study, we found that there was no difference in the OEAC measurements between lesion ears and normal ears in the COM group (Table 1). In addition, there was no difference between lesion ears in the COM group and normal ears in the control group (Table 3). This finding suggested that chronic inflammation or infection status may not cause bone thickening of the OEAC. Interestingly, COM may be related to canal stenosis by thickening of the skin and subcutaneous tissue. However, this was not within the scope of this study.
In the current study, we have provided objective information about the OEAC of patients by HRCT of the temporal bone. In the future, there may be more advanced technologies that can measure the dimension and the shapes of OEAC with greater precision. Then, we can investigate the relationship between the external ear canal and COM in both bony and soft tissue aspects.
In this study, we found that the bony dimension of the external ear canal was not related to COM. This suggested that chronic inflammation or infection of the middle ear may not cause bony thickening of OEAC, despite the shape of the OEAC.
Declaration of Conflicting Interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) received no financial support for the research, authorship, and/or publication of this article.
The author(s) received no financial support for the research, authorship, and/or publication of this article.
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