Assessment of sphenoid sinus related anatomic variations with computed tomography

Introduction Frequent and broad application of endoscopic sinus surgery (ESS) in parallel with experience makes it imperative to know the anatomy and the existing pathology very well before surgery. This study examines the association between anomalies in the sphenoid sinus area in paranasal sinuses computed tomography (PNS-CT) and pathological findings and determines variations of sphenoid sinus. Methods A total of 200 cases (100 women, 100 men) who had PNS-CT in the emergency and radiology polyclinics within the period of one year were included in this study. Bone tissue anomalies and soft tissue pathologies were assessed in the CT. Results Pterygoid process was found in 36.75% of our cases, anterior clinoid pneumatization was found in 21.25%; vidian canal in 34.25%, foramen rotundum in 17.5% and ICA in 12.75% had protrusion into the sphenoid sinus; 8.25% were found to have onodi cell, 11.25% were found to have multiple septation, 16.75% were found to have mucosal thickening and 2.5% were found to have retention cyst. Conclusion The importance of PNS-CT in terms of determining anatomic variations before ESC and predicting possible complications during surgery has been emphasized once more. In our study, as sphenoid sinus pneumatization increased, the projection of neighbouring vein and nerve structures into the sinus was found to increase as well.


Introduction
Endoscopic sinus surgery (ESS), which gains importance each passing day, is a method safely used in the treatment of paranasal sinus diseases. Frequent and broad application of ESS in parallel with experience makes it imperative to know the anatomy and the existing pathology very well before surgery. Pathology of sphenoid sinus and the neighbouring structures is an endoscopic surgical method frequently used by otolaryngologists and skull base surgeons. Thus, anatomic assessment of each patient before surgery is important in preventing intraoperative complications [1].
Since the progress of optical nerve and internal carotid artery is important, preoperative imaging has become a standard procedure.
Axial and coronal tomography imaging is required for the correct presentation of anterior skull base, lateral wall and sinuses [2].
Today, computed tomography is a golden standard procedure in the imaging of sinuses [3]. As of its localization, sphenoid sinus acts as a corridor for surgeons in reaching neighbouring structures [4].
However, during surgery, serious complications may occur as a result of injuries of vital neighbouring structures such as internal carotid artery and optical nerves [5,6].
Thus, an extensive knowledge of anatomic variables will certainly decrease surgical risks [7]. In transsphenoidal and skull base surgery, presenting sphenoid sinus ostium is an important step and guide [8,9]. The spheonid sinüs is deriving from cephalic mesoderm (orbito-sphenoid part) and neural crest (basis-post sphenoid part) which placed in the mid part of the skull [10]. Understanding the sphenoid sinus anatomy as a whole is a must for successful sinus surgery [11]. This sinüs can be detected earlier ages (<2 years) on PNS-CT scan and it reaches its mature form within 14 years [12].
Sphenoid sinus has a changeable anatomic structure [11]. Despite the normal direction of pneumatization is inferoposterolateral way, the direction of pneumatization can shows quite variances in each human subject [12]. Patient specific pneumatization, different numbers and positions of septa affect the attitudes of surgeons towards skull base and paracellular space [11]. Although isolated sphenoid sinus pathologies are rate entities, they are still significant since delays in their diagnosis and treatment may cause serious problems. This significance results from the fact that sphenoid sinus is adjacent to vital structures such as optical nerve, internal carotid artery, cavernous sinus and skull base and from the fact that its anatomy is quite varied. As well as experience, defining the anatomic reference points in the surgical technique used and understanding the variations are also indispensable for highest success and lowest complication. With pre-operative imaging, important information specific for each patient is obtained; besides a road map is determined for safe surgery and approaches towards bones and soft tissues are determined [13]. This study examines the association between anomalies in the sphenoid sinus area in paranasal sinus tomographies (PNS-CT) and pathological findings and determines variations of sphenoid sinus. Bone tissue anomalies (pterygoid process and anterior clinoid process pneumatization, vidian canal and foramen rotundum bone canal protrusion, onodi cell, multiple septation) and soft tissue pathologies (mucosal thickening, polyp, cyst) were examined.

Statistical analysis
The comparisons of multiple variances were applied with using oneway ANOVA test and significant parameters were analysed with post hoc test. Mann-Whitney U-test was used to determine the differences between two groups. SPSS 15.0 software statistical analyse program was used to analysing of each statistical test. The p value <0.05 was expressed as significant.

Discussion
Although conventional radiographic examinations are used frequently and as the first step examination in practical application, they do not allow for the obvious imaging of ethmoid and sphenoid sinuses [14,15]. When these are compared by CT, rates of pseudo positive and pseudo negative findings are known to be very high [15,16]. In tomographic sections, bone anomalies and mucosal pathologies can be assessed together. It is known that although conventional graphies do not give enough information about osteomeatal complex, frontal recess, posterior ethmoid sinuses, ethmoid bulla, sphenoid sinus and sphenoethmoidal recess and anatomical variations, these can be shown in detail with coronal CT [14][15][16][17]. Today, in a great number of centers, coronal PNS-CT is used routinely for the diagnosis of chronic sinus diseases.
Since the levels of sphenoid sinus pneumatization vary to a great extent, they are classified as three types; conchal, presellar and sellar [18]. With surgical techniques that developed in addition to these, this classification was enlarged into four types; conchal, presellar, sellar and postsellar [12].
Sphenoid sinuses expand into the sphenoid bone back from posterior ethmoid cells and expand into the pterygoid laminas with the greater wings of the sphenoid bone [19]. As sphenoid sinus expands, carotid artery and optical nerve are reported to form a bulge on the sinus lateral wall in parallel with the amount of expansion [15]. In our study, in parallel with this view, the vein and nerve structures neighbouring the sinus were found to remain within the sinus as the sinus pneumatization increased. When this expansion continues to anterior clinoid processes, there occurs a risk of injury for the optical nerve during surgery [20,21]. It is extremely important to have information before surgery on an area which shows so much variation. For the same reason, showing maximum attention to the sphenoid sinus walls during ESC is necessary to protect from possible complications.
While a statistically significant difference was found between the control group and the patient group in CT analysis in terms of mucosal pathologies, no significant difference was found for the bone structure [15,17].
In literature, the presence of anterior clinoid and pterygoid process pneumatizations and onodi cells was reported to have no effect on the pathophysiology of sinus disease [15] and 4-39% of mucosal pathology was found in sphenoid sinuses in CT [15][16][17]23].
In our study, 16.75% mucosal thickening was found in sphenoid sinus and no polyp was found. In line with the literature, bony anomaly and mucosal pathology were not found to progress together to a large extent.