Assessment of the distribution and volume of air chambers around the inner auditory canal on high-resolution computed tomography scans of the temporal bone.

To the Editor: Acoustic neuroma is the most common tumor in the cerebellopontine angle (CPA) area. The surgical approaches used for acoustic neuroma include the labyrinth, middle cranial fossa, posterior sigmoid sinus, and posterior labyrinth routes. However, regardless of the approach used to expose the tumor in the internal auditory canal (IAC), the bone around the IAC must be abraded, which inevitably exposes the surrounding air cells to damage and increases the risk of cerebrospinal fluid (CSF) leak after surgery. CSF leak is the most common postoperative complication after the removal of a tumor from the CPA area and has a reported incidence of 2% to 10% after acoustic neuroma surgery.[1-3] Clinical treatment of CSF leak is challenging; and continuous lumbar puncture and drainage or reoperation are sometimes required to avoid meningitis. Therefore, prevention of postoperative CSF leak is a key issue to consider when surgery is performed to remove a tumor in the CPA area. Risk factors for CSF leak after tumor removal in the CPA area include a high body mass index, cranial hypertension, hyper pneumatization of the air chambers in the temporal bone, and a long operating time.[4-6] Although several studies have shown that CSF leak is related to the air chamber in the temporal bone, their conclusions are mostly based on the air chamber in the mastoid portion.[7-9] Moreover, some researchers have proposed that hyper pneumatization of the petrous apex is an independent risk factor for CSF leak,[4] but have not provided quantitative assessment data. Therefore, a comprehensive assessment of pneumatization of the air cells in the petrous apex is needed before surgery to prevent postoperative CSF leak. Air cells in the petrous apex around the IAC reach from the superior margin to the inferior margin of the petrous bone and from the labyrinth to the IAC. There is a positive relationship between the volume of the air chamber around the IAC and the actual surgical cavity that needs to be treated, that is, the larger the volume of the air chamber around the IAC, the larger the cavity that needs to be treated during surgery. However, there have been no reports on the three-dimensional distribution or volume of air cells around the IAC. Therefore, the aims of this study were to (1) perform a multi-planar reconstruction and analysis of the air chambers around the IAC using standardized temporal high-resolution computed tomography (HRCT) data and (2) observe the distribution of air chambers in the different quadrants centered on the IAC and calculate their volumes to provide an anatomic foundation for predicting potential sites of postoperative CSF leak before surgery. This study was approved by the Institutional Review Board in Beijing Tongren Hospital, Capital Medical University (No. HXP201611ST). All the patients signed an informed written consent form. A total of 110 temporal HRCT scans obtained for 73 patients with acquired sensorineural hearing loss (35 males, 38 females) of mean age 41 ± 16 (range, 18–76) years at Beijing Tongren Hospital between January 2019 and December 2019 were retrospectively analyzed. The inclusion criteria were that the structures of temporal bones were normal on HRCT. The exclusion criteria were as follows: (1) patients with external, middle, and inner ear malformation or inflammation; (2) patients with a history of ear trauma; (3) patients with a history of ear surgery; and (4) patients with a high jugular bulb or an anterior sigmoid sinus. The HRCT images were obtained using a SOMATOM Definition Flash scanner (Siemens, Erlangen, Germany) in a spiral mode. All scans were obtained with the patient in the supine position, and the scanning range was from the superior margin of the petrous bone to the inferior margin of the external auditory meatus. The scanning parameters were as follows: voltage, 140 kV; effective tube current, 150 mAs/slice; collimator width, 64 mm × 0.6 mm; pitch, 0.5; tube rotation time, 1 s per revolution; field of view, 180.00 mm; matrix, 512 × 512; slice thickness 0.5 mm; and inter-slice gap, 0.25 mm. All source images were reconstructed using the bone algorithm. The window width was kept at 4000 HU and the window level at 700 HU. The source HRCT images were sent to an image post-processing workstation (Extended Brilliance Workspace 2, Phillips Medical Systems, Best, The Netherlands) and then standardized for multi-planar reconstruction. The reformatted baseline of the oblique plane was parallel to the long axis of the superior semicircular canal. To standardize the volume, the following temporal bone structures were marked: the middle cranial fossa meninges (superior margin of the petrous bone), inferior margin of the petrous bone, lateral semicircular canal, posterior semicircular canal, common crus, the bottom of the IAC, and porus of the IAC. Axial images were used to completely expose the horizontal semicircular canal and show the posterior semicircular canal simultaneously. A straight line was drawn along the middle line of the longitudinal axis of the IAC, after which a vertical line was drawn through the common crus and used as the starting point to cut the IAC into several 1-mm thick slices. The boundaries of the air chamber around the IAC were defined as follows: superior boundary, the middle cranial fossa meninges (superior margin of the petrous bone); inferior boundary, the inferior margin of the petrous bone; anterior boundary, the bottom of the IAC, and posterior boundary, the porus of the IAC. On an axial view, the air chamber around the IAC was divided into four quadrants, labeled counterclockwise as I (anterior–superior), II (posterior–superior), III (posterior–inferior), and IV (anterior–inferior). We calculated the area of the air chambers in the petrous bone in four quadrants for each layer. Next, the volume of each layer was obtained by multiplying the area of the air chamber in each layer by 1 mm. Finally, the total volume of the air chamber around the IAC was calculated. In this study, we found that the volume of each quadrant around the IAC and the total volume of the four quadrants varied widely from patient to patient. The average volumes of quadrants I, II, III, and IV were 66.87 ± 132.16 mm3, 67.99 ± 111.47 mm3, 236.45 ± 372.47 mm3, and 302.37 ± 560.17 mm3, respectively. And the average total air chamber volume was 673.68 ± 737.47 mm3, which is consistent with the range of 0.93–3.56 cm3 previously reported for the anterior portion of the petrous apex.[10] This wide range of values means that the relevant anatomic variables in the air chambers around the IAC should be assessed in each individual patient before surgery. In quadrant I, the air chamber was not developed (air volume, 0) in 37 temporal bones, accounting for 33.6% of the entire study cohort; for quadrants II, III, and IV, 56, 2, and 9 temporal bones were not developed, accounting for 50.9%, 1.8%, and 8.2%, respectively. Furthermore, the air chambers were not developed in quadrants I and II in 30 cases, which represented 27.3% of the total study cohort. Analysis of variance and Tamhane's T2 multiple comparisons test revealed a statistically significant difference in air chamber volume between quadrants I, II, III, and IV (P < 0.05) but no significant difference between quadrants I and II or between quadrants III and IV (both P > 0.05) [Figure 1]. The air volumes were significantly greater in quadrants III and IV than in quadrants I and II (P < 0.05). This finding indicates that the volumes of the air chambers in the posterior–inferior and anterior–inferior quadrants should be investigated carefully while planning CPA surgery.Figure 1: (A–C) The axial view of the IAC and the four quadrants. (D) Comparison of mean volume of four quadrants. Green line: middle line of a longitudinal axis of the IAC on HRCT; blue line: the vertical line perpendicular to the middle line through the common crus; red line: middle line of the IAC perpendicular to the green line in the axial view of the IAC on HRCT. ∗ P > 0.05. HRCT: High-resolution computed tomography; IAC: Internal auditory canal.This cohort contained HRCT data for 52 male ears and 58 female ears. There was no significant sex-related difference in air volume for quadrants I, II, and III (P > 0.05, Wilcoxon rank-sum test). However, there was a significant difference between the air volume in quadrant IV and total air volume (P < 0.05). The air volume was significantly greater in men than in women [Supplementary Table 1, https://links.lww.com/CM9/B331]. In this study, we found no significant correlation between patient age and the volume of the air chambers around the IAC (P > 0.05). The patients in this cohort were all at least 18 years of age, indicating that the air chambers around the IAC do not change with age. Our findings in this regard are consistent with a previous suggestion that the development of the mastoid air chambers is independent of age in persons over the age of 20 years.[11] The Wilcoxon rank-sum test revealed a significant difference in air volume in quadrants III and IV (P < 0.05) but not in quadrants I and II (P > 0.05) between the left and right ears. There was no significant difference in total air volume between the two sides (P > 0.05) [Supplementary Table 2, https://links.lww.com/CM9/B331]. This study investigated the three-dimensional distribution and volume of the air chambers around the IAC. Its findings provide a new perspective on these chambers. Unlike in previous studies of the air chambers at the petrous apex, the present study shows that the distribution and volume of these air chambers is an important consideration while predicting the risk of postoperative CSF leak before surgery and during surgical planning. Moreover, its findings show that standardized post-processing of HRCT images is useful for determining the distribution and volume of the air chambers around the IAC. Acoustic neuroma is one of the most common tumors found in the posterior cranial fossa. Complete removal of a tumor in the IAC requires the removal of an extensive amount of bone around the IAC. During surgery, the air chambers around the IAC may be opened, leading to exposure of the subarachnoid space and a risk of CSF leak in the temporal bone area or directly into the nasopharynx via the Eustachian tube. CSF leak occurs nearly twice as often in patients with a well-pneumatized petrous apex compared with those in whom the petrous apex is poorly pneumatized. Furthermore, larger acoustic neuromas have less risk of CSF leak, possibly because they extend into and obstruct the air chambers in the petrous apex surrounding the IAC, which decreases the communication channels for CSF.[12] Therefore, accurate evaluation of the distribution and volume of air chambers around the IAC is one of the key issues while planning surgery to remove a CPA tumor. Our finding indicates that the volumes of the air chambers in the posterior–inferior and anterior–inferior quadrants may play an important role. Taken together, we found that the distribution and volume of the air chambers in the different quadrants around the IAC may help to prevent CSF leak after surgery in the CPA area. Acknowledgments The authors thank the patients, as well as the global network of investigators, nurses, study coordinators, and operations staff. Funding None. Conflicts of interest None.