The frontal sinus surgery is regarded as one of the most challenging operation because of the narrow of the region, the complicated anatomy and the critical structure of orbit and skull base. The frontal recess is described as a complex space within an inverted funnel-like space whose apex is at the frontal ostium.1,2 It is frequently pneumatized with a group of ethmoid cells including agger nasi cells (ANC), frontal cells (FC) types 1–4, supraorbital ethmoid cells (SOEC), suprabullar cells (SBC), frontal bullar cells (FBC), and intersinus septal cells (ISSC). Knowledge of the structure and cells in frontal recess is the key for the surgeon to complete dissect all the ethmoid cells and achieve an unblocked frontal drainage pathway.2–4 The frontal recess cells can be classified into 3 groups based upon their locations in the frontal recess.2 The anterior group consists of the ANC and FC, whose anterior part attached to the frontal process of maxilla and posterior boundary is a free partition in the frontal recess. The posterior group includes FBC, SBC, and SOEC; whose superior/posterior boundary is the skull base, whereas their anterior/inferior boundary is a free partition within the frontal recess. The third group is consists of ISSC, which located in the medial frontal recess. During endoscopic frontal sinusotomy, complete removal of all these cells is essential to ensure frontal sinus patency for adequate drainage and ventilation.5,6
The intersinus septal cell is also termed as “interfrontal sinus septal cell” or “frontal wishbone”.2,7 It is a cell between two frontal sinuses and occurs in about one third of the population. Although the prevalence of ISSC is not very uncommon, only a very few literature addressed the anatomy and pathology within the frontal ISSC.7–11 The clinical significance of this anatomic variant still remains unclear. The purpose of this study was to clarify the anatomy, classification of ISSC by reviewing the frequency of these cells in patients with and without frontal sinusitis. Furthermore, frontal sinusotomy was performed to verify its presence and elucidate its clinical significance.
This study was performed at the Department of Otolaryngology and the Department of Radiology, Peking University People's Hospital. We prospectively identified 200 consecutive subjects who had undergone computed tomography (CT) scans of the nasal cavities and paranasal sinuses between November 2010 and May 2012. These subjects were divided into group 1 (frontal sinus free group, 120 subjects) and group 2 (frontal sinusitis group, 80 subjects). The subjects of group 1 were all patients with clinical symptom but free from frontal sinus disease. The patients of group 2 were patients undergone functional endoscopic sinus surgery (FESS) because of frontal sinusitis. They had all consented to the examination.
Protocol of CT scan
Spiral CT scans of the nasal cavities and paranasal sinuses were performed on a GE LightSpeed CT scanner (GE Medical Systems, Milwaukee, Wisc, USA) with overlapping 0.625-mm thick axial cuts, 0.625-mm interval. The following scanning parameters were used: 120 kV, 400 mA, and the intermediate window and level settings were 2000 and 200 Hounsfield units, respectively. The CT data were then reconstructed into coronal and parasagittal images on a computer workstation.
Criteria and classification of ISSC
The criteria of ISSC were defined as an area of pneumatization in the septum forming the median or paramedian separation between frontal sinuses.10 Furthermore, we have classified this cell into two types: Type I ISSC (Figures 1 and 2): the ISSC communicated with either one or both frontal sinuses. In other word, the ostium of the type I ISSC was within the frontal sinus. In these cases, the interseptal cells were considered to arise from a frontal sinus and not directly as a result of migration from the frontal recess. Type II ISSC (Figures 2 and 3), the ISSC had no demonstrable connection to either frontal sinus. In other word, the ostium of the type II ISSC was within the frontal recess. For these patients, these cells were considered to be displaced cells of ethmoid origin. Each CT scan was studied jointly by one ear, nose, and throat surgeon and one radiologist for minimizing variability of interpretation. The patients in group 2 had undergone FESS and the ISSC were all further proved by surgery. The surgery was performed with Image 1 Camera, 4 mm rigid endoscopes with deflection angles of 0 and 30 and 70 degree and a set of instruments (Karl Storz GmbH & Co., Germany). A “Xomed XPS 3000” unit (Medtronics/Xomed, Jacksonville, FL, USA) was used for microdebrider.
Statistical analysis was performed with SPSS package (version 13.0 for Windows, SPSS Inc., USA). The chi-square test was applied for comparing the prevalence of total ISSC and 2 types of ISSC. P values below or equal to 0.05 were defined as statistically significant.
The ISSC was obvious when reviewing the coronal and axial CT scans. The sample consisted of 200 patients, 120 in group 1, 80 in group 2. In group 1, the age ranged from 20 to 81 years (mean 43 years), 76 men (63.3%) and 44 women (36.7%). In group 2, the age ranged from 14 to 80 years (mean 47 years), 52 men (65%) and 28 women (35%). Of the 200 CT scans reviewed, ISSC were present in 90 (45%) (Figures 1–3). Seven scans demonstrated bilateral ISSC (Figure 2), bringing the total count to 97 cells. Of the 120 scans in group 1, ISSC were present in 49 (41%), among which type I ISSC was in 22 (18%) and type II was in 27 (23%). Of the 80 scans in group 2, ISSC were present in 41 (51%), among which type I ISSC (Figures 2 and 3) was in 16 (20%) and type II was (Figures 1 and 2) in 25 (31%). There were no statistically significant differences about the frequency distribution of total ISSC, type I and II ISSC between group 1 and group 2 (Table 1).
There are 3 principal groups of cells that impact the frontal sinuses from anterior, posterior and medial aspect. The anterior group cells such as agger nasi cell and frontal cell have received much attention. But it is lucky to see that only the type III and IV frontal cells have entered into the frontal sinus and the prevalence rate of these cells is not very high.2,12,13 The posterior group cells such as frontal bullar cell and supraorbital cells arise from bulla and pneumatize along skull base and orbit into frontal sinus from posterior frontal recess. They drain into the frontal recess posterior to the frontal sinus ostium. Usually the frontal sinus drainage pathway was obstructed posteriorly by these cells. There are some literature addressed the anatomy and clinical significance of these cells.14–16 The incidence of ISSC has been reported change from 8% to 77%.7,13 But most of the papers reported it accounted about one third of the cases.2,9,10 It can impact the frontal sinus drainage pathway medially. Its potential role in pathogenesis of frontal sinusitis had been underestimated in the past. Recently some authors began to recognize its role in the pathogenesis of frontal disease.7–9,11 The purpose of this study was to clarify the anatomy, classification of ISSC and its clinical significance in Chinese subjects.
The definition of ISSC was made by several authors with somewhat similar content. The ISSC was defined by Van Alyea in 1941 as a cell confined to the thin septum separating the frontal sinuses.17 Lee et al2 made a definition of ISSC as: pneumatization of the frontal sinus septum, drains into one frontal recess. Wormald4 defined ISSC as: the cell is associated with the frontal sinus septum and compromises the frontal ostium by occupying part of the frontal ostium. It is always opened into the frontal recess. All these authors defined that ISSC should drain into one frontal recess. But contrary to the present convention that frontal intersinus septal cells originate as displaced ethmoid cells from the frontal recess. Som et al9 found that most such cells are actually diverticula from the frontal sinuses themselves, which means that some ISSC drain into frontal recess, the others drain directly into frontal sinus. Som suggest that the genesis of intersinus septal air cells may primarily be from the frontal sinuses themselves or originated from ethmoid extensions. So according to this hypothesis, we classified the ISSC into 2 types. Type I ISSC: the cell communicated with either one or both frontal sinuses. Type II ISSC: the ISSC had no demonstrable connection to either frontal sinus but communicated with the frontal recess. There is no author but Merritt classified the ISSC so far. But that classification can not provide more information for guiding the surgery for opening the frontal ostium. Consequently it is not very popular and always be neglected. According our classification, the type I ISSC originates from the frontal sinus. This kind of ISSC always drain into the frontal sinus and can not be easily be touched and removed by surgeon, so usually the ostium of the ISSC is patent and the surgeon need not to touch the ISSC. But for type II ISSC, it is the extension of ethmoid, so it is relatively easily to be removed from the frontal recess. Usually the upper part of the uncinate process formed the lateral wall of Type II ISSC (Figure 1). By means of removing the upper part of unicate process, the ISSC can be opened. The frontal ostium becomes patent like an ostium after a Draf II frontal sinustomy.18 The differences are that the Draf II frontal sinusotomy widen the anterior and posterior diameter of the frontal recess, while removal of the ISSC can enlarge the medial and lateral diameter. By removing Type 2 ISSC, no drill is needed and the mucosa can be preserved. The frontal ostium can be rapidly recovered. As showed in Figure 1, the patients' frontal ostium becomes patent only after 2 week of the FESS.
The prevalence of ISSC is 45% in this study group. It appears to be more prevalent than previous report. Without the help of CT scan, Van Alyea reported only 12%. Merritt found 38% subjects with ISSC in 300 CT scan. But only coronal CT scan was analyzed in this study. Som reported 30.5% ISSC in 200 cases without sinusitis. However, the protocol of the CT has not been mentioned in the study. In our study a multislice CT helical scanner requires scans to be performed in the axial plane at 0.625 mm interval with coronal and parasagittal reconstruction. The scan interval is very thin so very small cells will not be missed. All the 80 patients in group 2 were patients of sinusitis with frontal disease. Because of the lack of previously published data on the classification of ISSC in Chinese patients, we cannot compare our results directly with others. In Som's study, 52 of 61 ISSC (85.3%) were seen to communicate with the frontal sinuses and 9 of 61 (14.7%) had no demonstrable connection to either frontal sinus. This result is somewhat different from our result. Our data shows that 49 of 90 (54%) was type I ISSC and 41 of 49 (46%) was type II ISSC. The difference might be that these studies were undertaken in different population of different race. And the result of Som was only the analysis of CT scans, our study proved the CT results by surgery. The ISSC may pneumatize the full length of the frontal sinus or only the lower part of the intersinus septum. The lower ISSC usually be ignored. By means of operation these lower ISSC can be confirmed. Although there were no statistically significant difference of ISSC in both frontal sinusitis group and disease free group, we can see that there is a trend of more ISSC in frontal sinusitis group, especially the type II ISSC. So by means of remove the ISSC the surgeon can widen the frontal sinus ostium.
In conclusion, this report found the prevalence of ISSC was very high in Chinese patients. The classification of ISSC was helpful for surgeon to operate according whether it communicate with frontal sinus or frontal recess. The type II ISSC could be relatively easily be removed from recess. For patients with ISSC, removing the type II ISSC was a good way to open the frontal ostium without damaging the mucosa.
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