Interfrontal Sinus Septal Cell: A Cause of Obstructing Inflammation and Mucoceles

BACKGROUND AND PURPOSE: Recent interest in the origin and complications associated with frontal intersinus septal cells prompted us to review our material looking for cases of the uncommon occurrence of a mucocele developing within such a cell. The purpose of this article was to present the imaging findings on 4 cases of mucoceles arising within surgically proven frontal intersinus septal cells. MATERIALS AND METHODS: A retrospective review of the case material in our department of otolaryngology since 2000 was made looking for cases of patients with surgically proven frontal intersinus septal cell mucoceles. Institutional review board approval for the review was obtained. Four cases were identified that also had CT and/or MR imaging studies. Examples of 4 additional classic frontal sinus mucoceles and 3 anterior ethmoid mucoceles were also identified for comparison. RESULTS: All 4 of the cases of frontal intersinus septal cell mucoceles had an expanded midline frontal sinus cell, which thinned the posterior and/or anterior frontal sinus tables. The classic frontal sinus mucoceles were either to the left or right side, and any table thinning was off midline. The ethmoid mucoceles were clearly centered below the frontal sinuses. CONCLUSIONS: The rare occurrence of a frontal intersinus septal mucocele can be diagnosed on CT and MR imaging studies, because its appearance in the midline is clearly distinct from the more common classic frontal sinus mucoceles that develop within the left or right frontal sinus proper. Distinction is also routinely made from large anterior ethmoid mucoceles.

T here has been some recent interest in the origin of, and complications associated with, frontal intersinus septal cells. [1][2][3][4][5][6] The complications are the result of disease within these septal cells obstructing the adjacent frontal sinuses. This is especially true in the uncommon occurrence of a mucocele developing within such a cell. The purpose of this article was to present the imaging findings on 4 cases of mucoceles arising within surgically proven frontal intersinus septal cells.

Materials and Methods
A retrospective review was made of the paranasal sinus cases operated on at our institution since 2000, which involved disease within a frontal intersinus septal cell. From this material, we found 4 cases of surgically proven mucoceles arising within a frontal intersinus sinus septal cell. These cases were imaged either with CT scans or MR images on a variety of scanners both within and outside our institution. The study was performed with approval of the internal review board No. 06-254. The patients ranged in age from 37 to 68 years, and there were 3 women and 1 man. The images were reviewed by a head and neck radiologist and the otolaryngologist who operated on the cases.

Results
All 4 of the cases showed an expanded central cell in the frontal sinus obstructing the more laterally positioned frontal sinus cavities (Fig 1-4). There was thinning of the midline anterior and/or posterior frontal sinus tables in all of the cases. In one case, a Pott puffy tumor developed from an expanding central frontal cell (Fig 4). For purposes of comparison, 4 cases of classic frontal sinus mucoceles were reviewed, and representative images are shown in Fig 5. Three cases of ethmoid muco-celes were also reviewed, and a representative image is shown in Fig 6. The classic frontal sinus mucoceles clearly arose in 1 side of the frontal sinuses, and if the frontal sinus tables were thinned, it was off of the midline. The large anterior ethmoid mucoceles all had their epicenter in the ethmoid complex, caudal to the frontal sinuses, and only impinged off of the midline on the lower frontal sinus. Thus, the frontal intersinus septal cell mucoceles were routinely distinguished from these other mucoceles.

Discussion
There are 3 principal groups of cells that impact the frontal sinuses. Of these, the supraorbital cells have received the greatest attention with regard to their role in the pathogenesis of frontal sinus disease and its management. 7 These cells pneumatize the orbital plate of the frontal bone posterior to the frontal recess. 8-10 They generally arise from anterior ethmoid cells, but they can also develop from a posterior cell, and they may be multiple. They drain into the frontal recess posterior to the frontal sinus ostium. Their incidence has been estimated at 5%-15% 11 in dissection studies, but imaging studies have detected a much greater prevalence (62%). 3 The intrafrontal (intrabullar, type III and IV) cells have the lowest incidence estimated at 1%-2%. On review of approximately 600 CT scans, Bent et al 12 detected them in 9 skulls (1.5%). In a later study of 50 patients examined with multiplanar CT, Lee et al 3 found type III cells in 8% of cases.
Finally, there is the intersinus septum cell (ISSC), which is common but has received the least attention in the literature. These cells are defined as a pneumatization within the midline or paramedian bony lamella (septum) between the frontal sinuses. Van Alyea 11 noted them to be the most frequent cell, present in 28 (11.6%) of his 242 dissection specimens. In a coronal CT scan study of 300 heads (200 patients with chronic sinusitis and 100 cadavers) by Merrit et al, 13 an ISSC was identified in 34%, 10 having multiple cells. The cells extended var-ious distances along the height of the septum. Paradoxically, a multiplanar 3D-CT scan study of 50 heads found a prevalence of 14%. 3 In a prospective CT scan study performed in the coronal and axial planes of 200 consecutive nonsinusitis patients, Som and Lawson 4 found ISSC in 30.5% of the cases, also having variable extension along the septum. However, this study revealed communications between most of these cells (85.5%) and the frontal sinus, suggesting that most of these cells may arise as diverticular extensions from the frontal sinuses rather than by the evagination of anterior ethmoid air cells from the frontal recess.
The literature contains few references regarding pathology of the ISSC. Van Alya 11 noted that the ISSC may pneumatize sufficiently to obstruct the frontal ostium. Merrit et al 13 reported 5 patients in their series of 200 patients with chronic sinusitis to have disease in an ISSC, of which 3 cases failed treatment. Chiu and Vaughan 1 reported enlarging a narrow frontal recess by removing an ISSC. Lawson also stated that, when a deviated intersinus septum is found to impinge on the frontal sinus ostium, an air cell in the lower part of the septum should be suspected.
The present study documents that an ISSC may become a mucocele sufficiently large to unilaterally block the frontal sinus outflow, or it may enlarge massively to form a giant mu-    cocele involving both frontal sinuses. Such a mucocele usually erodes the anterior and/or posterior sinus tables.
Current management of frontal and frontoethmoidal mucoceles is principally by endoscopic marsupialization into the nasal cavity, with external procedures reserved for inaccessible and recurrent lesions. Intersinus septal mucoceles present an algorithm shift. Although many small mucoceles may be successfully treated endonasally, large ones expand bilaterally, often sequestering the lateral recesses of the frontal sinus, and require external surgery (osteoplastic flap) primarily for management.

Conclusions
When a frontal sinus mucocele is seen to arise in the midline rather than primarily to one side of the frontal sinus, an uncommon mucocele arising within an interfrontal sinus septal cell should be considered. This may impact what the surgeon must do to correct the problem. A 53-year-old man presented with a mass in his upper medial right orbit. A coronal T2-weighted MR image shows a right frontal sinus mucocele that has expanded the sinus and depressed the superomedial orbital rim. Inflammatory mucosal disease is present in the left frontal sinus, both ethmoid sinuses, and the right maxillary sinus. B, A 46-year-old woman presented with a headache. An axial T1-weighted MR image shows a mucocele expanding the right frontal sinus and thinning the posterior sinus table to the right side, off of the midline.

Fig 6.
A 32-year-old man presented with a bulge in his medial right orbit and difficulty breathing through the right side of his nose. A coronal T2-weighted MR image shows a large right anterior ethmoid mucocele. It is clearly centered within the ethmoid complex, caudal to the frontal sinuses. Inflammatory mucosal disease is also present in the left ethmoid cells and the right antrum.