CT anatomical grading of otosclerosis and its relationship with audiological examination

CT anatomical grading of otosclerosis and its relationship with audiological examination: A prospective comparative study

Abbreviations
ABG Air-bone gap
CHL Conductive hearing loss
FAF Fissula ante fenestrum
FAOW Focus anterior to the oval window
FPF Fissula post fenestrum
HRCT High resolution computed tomography
IAC Internal auditory canal
IAM Internal auditory meatus
KVP Peak kilovoltage
mAs Milliampere second
MDCT Multi-detector computed tomography
MHL Mixed hearing loss
MPR Multi planner reformatting
OW Oval window
RW Round window
SCC Semicircular Canals
SD Standard deviation
SNHL Sensorineural hearing loss

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Abstract
Background and purpose: The computed tomography (CT) grading system for otosclerosis was proposed by Symons and Fanning in 2005 and Vellion in 2006. The purpose of this study was to compare the two classifications, show the relationship with audiological examination and determine if one classification is preferred over the other according to the type of hearing loss.
Materials and methods: We prospectively studied 53 patients complaining of progressive hearing loss and diagnosed clinically and audiologicaly to had bilateral otosclerosis from March 2016 to May 2017. All patients were submitted to CT examination of the temporal bone. We graded the disease severity based on Symons/Fanning grading system and Veillion classification and showed the relationship between the radiological and audiological examinations of otosclerosis using Chi-square test.
Results: Six of 53 patients showed unilateral CT evidence of otosclerosis and 47 patients had bilateral CT evidence of otosclerosis. There is a statistically significant relationship between site of otospongiotic foci and type of hearing loss (p= 0.0003) (Table 3). There is a statistically significant relationship between the size of the focus at fissula ante fenestrum (FAF) and the increase of air conduction threshold resulting in the same association with air-bone gap (ABG) (p= 0.003 and 0.007, respectively). But, there is no significant relationship between the size of focus and bone conduction (p= 0.297)
Conclusions There is a statistically significant relationship between the type of hearing loss and the grades of otosclerosis according to Symons and Fanning grading and Veillion classification.

Introduction
Otosclerosis, an autosomal dominant condition involving the otic capsule, is histologically characterized by abnormal resorption and reformation of labyrinthine bone. Otosclerosis most commonly manifests clinically as a conductive hearing loss (CHL). However, because of variable penetrance, a mixed (conductive-sensorineural) hearing loss and purely sensorineural hearing loss (SNHL) can occur 1-5. Otosclerosis is classified into fenestral and retrofenestral types. The first is located in the lateral aspect of the oval window (OW), i.e. fissula ante fenestrum (FAF) (the commonest site). The second is located behind the OW, including the pericochlear region, the semicircular canals (SCC), internal auditory meatus (IAM), vestibule, and cochlea 6.
Computed tomography (CT) is the imaging modality of choice when imaging is performed 7, 8. CT used in the diagnosis of otosclerosis, in the assessment of disease extent including cochlear involvement, and in planning specific surgical treatment. Otosclerosis is characterized by lucent or hypodense foci within the otic capsule on CT. Other CT findings include a thickened footplate, narrowed OW or round window (RW) niche, and the double ring sign 9.
Previous literature showed controversial results regarding the relationship between the size of the foci and the degree of hearing loss. Some authors showed that the severity of cochlear disease on CT scanning correlates with the degree of SNHL 10, 11. However, others have not been able to show relationship between the disease severity and the severity of hearing loss, though sample sizes were small in some of these studies 12, 13. Various authors have used CT anatomical grading systems of otosclerosis in their studies. Symons and Fanning 14 have recently published a CT grading system for otosclerosis (Table 1A). Veillon classification 15 divides sites and extension of otosclerosis based on finding on high-resolution CT (HRCT) of the temporal bone into 6 types (Table 1B). The aim of this study was to compare the two classification systems, show the relationship with audiological examination and determine if one classification is preferred over the other according to the type of hearing loss.

Patients and methods
Ethical considerations
The present study was approved by the institutional review board. All patients were informed about the study and provided written informed consent. The study was performed in accordance with the ethical principles of the Declaration of Helsinki.
Study population
From March 2016 to May 2017, we prospectively studied 53 adult consecutive patients (23 men and 30 women; age range 17- 65 years; mean age 37.5 ± 7.3 years).
Inclusion criteria: All patients complained of progressive hearing loss and were diagnosed clinically and audiologicaly as otosclerosis.
Exclusion criteria
1. Patients with middle ear inflammations.
2. Pregnant female.
3. Obese patients more than 200Kg.
The patients were referred to the diagnostic radiology department at ”BLINDED” hospitals for doing CT examination to confirm the diagnosis and assess the preoperative anatomy of the petrous bone.
Patient assessment
All patients were subjected to full clinical history taking, clinical examination and audiological investigations including audiometry, tympanogram, and acoustic reflex morphology. According to the type of hearing loss, we divided our patients into three groups:
Group 1: Patients with CHL (76 patients).
Group 2: Patients with SNHL (4 patients).
Group 3: Patients with MHL (26 patients).
CT examination
All scans were obtained on a 128-slice spiral CT scanner (Phillips – ingenuity cor 128 T.M) with a 0.67 mm collimation, 0.67 mm slice thickness, 0.5 mm pitch, 15 cm field of view, 512 x 512 matrix size, zero-degree gantry tilt, 120 kVp, 200 mAs, 4000 window width, and 200 window level. All examinations were performed with the neck flexed such that the infra-orbito-meatal line was parallel to the scanning plane when obtaining images in the axial plane. No specific patient preparation was required apart of removal of any metallic accessories of the ear and hair, especially in female patients. Multiplanar reconstructions (MPR) used for doing coronal and sagittal images.
Image analysis
All CT data were transferred to the workstations and image analysis was performed on a dedicated platform Extended Brilliance Workstation (Philips Medical System, Best, The Netherlands) or the picture archiving and communication system (PACS) (PaxeraUltima- paxeramed). Two radiologists with 10 years of experience in temporal CT imaging independently reviewed all CT images. Any discrepancies in interpretation were resolved by a third radiologist with more than 15 years of experience in temporal imaging. The axial source and post-processing images were reviewed and classified as positive or negative cases. The positive cases showed either hypodense spongiotic foci affecting the otic capsule (fenestral or retro fenestral), or sclerotic lesions which appear as thickening of stapes footplate, obliteration of OW or RW, labyrinthine deformity or intralabyrinthine ossification. Negative cases showed normal otic capsule. Finally, we graded the otosclerosis in our patients based on Symons and Fanning grading system 14 (Table 1A) and Veillon classification 15 (Table 1B).
Statistical analysis
All data were collected, tabulated and statistically analyzed using MedCalc 13 (MedCalc Software bvba, Ostend, Belgium). Quantitative data were expressed as the mean ± standard deviation (SD). Qualitative data were expressed as absolute frequencies (number) and relative frequencies (percentage). Continuous data were checked for normality by using Shapiro Walk test. Independent Student t-test was used to compare two groups of normally distributed data while Mann-Whitney U was used for non-normally distributed data. One way ANOVA test was used to compare more than two groups of normally distributed data while Kraskall Wallis H test was used for non-normally distributed data. Percent of categorical variables were compared using Chi-square test or Fisher’s exact test when appropriate.

Results
Patients
This study included 53 patients (106 ears) with bilateral clinically and audiologically diagnosed otosclerosis. We reported positive CT findings in 100 ears and negative findings in six ears. FAF is affected in all positive ears (94.4%), whereas, the second most common affected site is the RW (30 ears, 28.3%) (Table 2).
The relationship between CT finding and type of hearing loss
There is a statistically significant relationship between site of otospongiotic foci and type of hearing loss (p= 0.0003) (Table 3). However, the internal auditory canal (IAC) involvement showed no significant relationship with the type of hearing loss (p= 0.203). There is a statistically significant relationship between the size of the focus at FAF and the increase of air conduction threshold resulting in the same association with ABG (p= 0.003 and 0.007, respectively), as when the focus increase in size, the ABG become high. But, there is no significant relationship between the size of focus and bone conduction (p= 0.297) (Fig. 1).
The relationship between classification/grading of otosclerosis and type of hearing loss
We found a statistically significant relationship between the grades of otosclerosis according to Symons and Fanning grading and the type of hearing loss (p ;0.0001) (Table 4). Grade 1 was detected in 76 ears, among whom 70 ears (92.1%) had CHL. Grade 3 was detected in only 4 ears with MHL. There is a statistically significant relationship between the type of hearing loss and Veillon classification (p ;0.001) (Table 4). Types Ib, type II, and type IVa were detected in 26, 38 and 22 ears, respectively, and showed a statistically significant relationship (p ;0.0001) with CHL with percentages of 92.4, 94.7, and 63.7, respectively. Types III and type IVb were detected in 2 and 12 ears, respectively, and showed a statistically significant relationship (p ;0.0001) with MHL with percentages of 100 and 83.3, respectively. There is a statistically significant relationship between the type of hearing loss and the RW otosclerosis (p ;0.0001) (Table 4).

Discussion
We proved that there is a statistically significant relationship between the size of the focus at FAF and the increase of air conduction threshold, resulting in the same association with ABG (p= 0.003 and 0.007, respectively), as when the focus increase in size, the ABG become high. But, there is no significant relationship between the size of focus and bone conduction threshold (p= 0.297). These results are in agreement with previous studies 13, 16. Naumann et al. 13 reported that there was a significant relationship (p= 0.005) between the size of the fenestral foci and the ABG, but no relationship (p; 0.1) between the extent of cochlear otosclerotic involvement and the bone or air conduction levels. Kutlar et al. 16 reported that there was no significant relationship between otosclerotic focus and hearing thresholds in early stages, but there was a significant relationship in advanced (diffuse) stages. In contrast, Abdel-Ghany et al. 17 were unable to find any significant relationship between the size of foci in any of the CT grades and hearing thresholds.
This study was done on the two last CT anatomical grading systems of otosclerosis published by Symons and Fanning 14, and Veillon 15. According to Symons and Fanning grading, we found a statistically significant relationship between the grades of otosclerosis and the type of hearing loss (p ;0.0001). This finding is consistent with previous studies 6, 18. Lee et al. 6 concluded that CT classification system of otosclerosis disease proposed by Symons and Fanning has high interobserver and intraobserver agreement with precise localization of otosclerotic foci around the cochlea with significant clinical relationship and association with the type of hearing loss. Swartz et al. 18 reported that cochlear otosclerosis presents with combined CHL and SNHL and successfully correlated the hearing loss frequency with the site of the cochlear capsule involvement.
We reported a statistically significant relationship between the Veillon classification and the type of hearing loss (p ;0.0001). Although type IVa was detected in 22 ears and included pericochlear, IAC, or RW affection, was associated with CHL in 14 ears (63.7 %) and with MHL in only 8 ears (36.3 %). This may be explained by the theory of Swartz et al. 18 who reported that otosclerotic foci have a local effect on the parts of the cochlea. However, against the theory of Parahy and Linthicum 19 who reported that the SNHL occurs in otosclerosis disease is a result of the release of cytotoxic enzymes which leads to hyalinization of the spiral ligament.
RW involvement is considered as a subtype of the type IVa of Veillon classification. We found a statistically significant relationship between the type of hearing loss and the Round window otosclerosis (p ;0.0001). In type B, there was hypodensity at the edge of the RW niche, but the RW membrane was normal. It was detected in only 6 ears and associated with 100% CHL. In type D, there was complete obliteration of the RW with basal turn extension. It was detected in about 14 ears and associated with SNHL in 2 ears (14.3%) and MHL in 12 ears (85.7%). This may be explained by the involvement of the basal turn of the cochlea in type D leading to decrease the vibration of the basilar membrane and increase the impedance of the inner ear fluid and then increase in bone conduction threshold as described by Mansour et al. 20.
Finally, we detected that Symons and Fanning grading is very good in localization of the foci which involve the cochlea, as grade 2 and 3 represent cochlear involvement either patchy or diffuse respectively, and grade 2 subdivided into more precise localization of foci in the different turns of the cochlea (apical, middle or basal). But, Veillon classification has a great advantage in the localization of RW foci which is the second commonest site of otosclerosis and may affect the decision of surgery. However, it is defective in the evaluation of foci around the cochlea as Symons and Fanning grading.
The present study has a strong advantage, as it is prospective study, thus avoiding the selection bias of a retrospective study. However, the present study has some

limitations: number of cases with SNHL due to otosclerosis was limited, so it was difficult to correlate between the degree of hearing loss with the CT anatomical grading systems, So additional study of whether the grade of otosclerosis correlates with frequency and degree of hearing loss would be interesting
Conclusion
The present study established two clinically important conclusions. First, there is a statistically significant relationship between the grades of otosclerosis according to Symons and fanning grading and Veillion classification with the type of hearing loss. Seconds, in cochlear otosclerosis, CT grading of Symon and Fanning is better in precise localization of foci around cochlea. But in cases of RW involvement, Veillon classification has a great advantage in the localization of RW foci which may affect the decision of surgery.

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