Published online Dec 15, 2020.
https://doi.org/10.5624/isd.2020.50.4.365
Cone-beam computed tomographic imaging of silent sinus syndrome: A case series and a literature review
Abstract
While silent sinus syndrome (SSS) is familiar to otolaryngologists and ophthalmologists, it is a rare clinical entity in dentistry and is likely to be underdiagnosed due to dentists' lack of awareness of this condition. SSS presents a diagnostic challenge to dentists, as patients typically have no history of trauma or sinusitis. The characteristic feature of SSS is a gradual retreat of the maxillary sinus walls, resulting in enophthalmos and hypoglobus. Multidetector (multislice) computed tomography is the imaging modality of choice for SSS and other paranasal sinus diseases. Cone-beam computed tomography promises to be an alternative low-dose imaging modality. This report describes 3 cases of SSS in adults, who had no identified clinical symptoms except diminutive and opacified maxillary sinuses, as well as the inward bowing of the sinus walls as noted on cone-beam computed tomographic imaging.
Silent sinus syndrome (SSS) is a well-known entity in otolaryngology, originally named by Soparker et al. in 1994.1 In that study, the authors presented a group of 14 patients with spontaneous enophthalmos and hypoglobus with maxillary sinus opacification and collapse, along with a lack of history of related trauma or surgery. Earlier, in 1964, comparable observations were published by Montgomery.2 Also referred to as imploding antrum and chronic maxillary sinus atelectasis, SSS is characterized by painless enophthalmos and inward retraction of the maxillary sinus walls as detected on imaging studies.1 Many such cases have since been identified and reported in the otolaryngology and ophthalmology literature. However, this condition is generally unfamiliar to dentists, and it is thus seldom reported in the dental literature despite being observable on advanced imaging studies acquired for other oral and maxillofacial diagnostic purposes.
SSS is a relatively rare clinical condition that is likely under-reported due to dentists' lack of awareness and its unique radiographic appearance.1 This is particularly important when surgery is planned in the area of interest. SSS generally presents in the third through fifth decades of life.3 The term SSS is often used interchangeably with chronic maxillary atelectasis; however, SSS is characterized in particular by the absence of chronic sinusitis.4 Although SSS can be initially identified using clinical features, the final diagnosis must be confirmed radiographically.
The most common tool used for 3-dimensional imaging is multidetector computed tomography (MDCT).5 Two fundamental problems with this imaging modality are the relatively high dose of radiation administered and the relatively high cost.5 In contrast, cone-beam computed tomography (CBCT) produces images with high spatial resolution (approaching 76 µm) with a much lower radiation burden.5 Additionally, artifacts from metallic dental restorations are less apparent with this technique. Veldhoen et al.6 compared MDCT and CBCT and indicated that, at a lower radiation dose, CBCT provided better subjective image quality and resolution. Bacher et al.5 showed that, independent of imaging technique, the salivary glands, brain, and thyroid received the highest equivalent dose during paranasal sinus imaging; the effective dose with CBCT was 30 µSv, whereas low-dose and standard MDCT protocols resulted in effective doses of 200 µSv and 1200 µSv, respectively. The comparison between MDCT and CBCT remains controversial in many settings due to the latter's lack of capacity to adequately image soft-tissue entities. However, justification for using a higher-dose examination in the absence of associated soft tissue pathoses is difficult. The judicious use of radiation is advocated by the American Dental Association and the U.S. Food and Drug Administration, as well as all dental specialty organizations through their support of the Image Wisely campaign. It is important to keep doses low while maintaining image quality.7 The purpose of the present report was to describe relatively less well-known imaging features of SSS and to advocate for the use of CBCT for diagnostic imaging purposes.
Case Reports
A retrospective review of clinical and imaging data from between March 2019 and October 2019 was performed using the patient database at our institution. Patients' demographic data, clinical presentation, and radiographic findings were reviewed. The studies were analyzed with regard to the overall appearance of the maxillary sinuses, including the overall volume, the configuration of cortical contours, and the appearance of the sinus infundibulum and the uncinate process. Adjacent structures including the orbits, middle meatus, and middle turbinates were also evaluated. No history of trauma, sinus surgery, or space-occupying masses of any kind was reported. CBCT studies were performed using a dedicated maxillofacial CBCT unit (iCAT FLX; Imaging Sciences International, Hatfield, PA, USA), at 120 kVp, 5 mA, and a 20-second exposure time. The size of the field of view was 16 cm×13 cm for case 1 and 23 cm×17 cm for cases 2 and 3. All images were acquired and viewed as Digital Imaging and Communications in Medicine files by board-certified oral and maxillofacial radiologists with several years of experience reading such studies. These images were saved in JPG format with a resolution of 300 dots per inch.
For all cases, measurements were made on the images in which the smallest dimensions were observed in all planes of interest for the purpose of comparison with the normal side. These measurements illustrate the diminutive size of the affected sinus in views orthogonal to the x-, y- and z-axes.
Case 1
A 32-year-old man was referred for evaluation of an unerupted left maxillary tooth. Physical examinations of the head and neck region revealed no abnormal findings. All of the patient's vitals were within normal limits, and no other abnormal extraoral findings were detected. A CBCT study revealed a relatively small left sinus with its superior third positioned lower than its anatomic counterpart along the x-axis. The left orbital floor appeared to be retreating into the maxillary sinus, with its cortical integrity maintained. Concha bullosa was observed on the right, with mild deviation of the cartilaginous portion of the nasal septum. The ostiomeatal complex appeared to be well-aerated and patent with no evidence of occlusion (Fig. 1).
Fig. 1
Case 1. Axial (A) and coronal (B) images demonstrate left maxillary sinus volume loss relative to the right side.
Case 2
A 19-year-old man was referred for evaluation of the temporomandibular joints as part of orthodontic diagnosis and treatment planning. Physical examination of the head and neck region revealed no abnormal findings. All of the patient's vitals were within normal limits, and no other abnormal extraoral findings were detected. A CBCT study showed a hypoplastic left maxillary sinus, with a soft-tissue density filling the sinus. The anterolateral wall of the left sinus appeared to be retracted posteriorly. In addition, the floor of the left orbit (and the roof of the left maxillary sinus) was displaced inferiorly. On the affected side, the patency of the ostiomeatal complex could not be established (Fig. 2).
Fig. 2
Case 2. Axial (A) and coronal (B) images demonstrate a smaller and completely opacified left maxillary sinus.
Case 3
A 69-year-old patient (sex not stated) was referred for the evaluation of suspected pathosis involving a cloudy left maxillary sinus noted on a panoramic study. Physical examination of the head and neck region revealed no abnormal findings. All of the patient's vitals were within normal limits, and no other abnormal extraoral findings were detected. CBCT was performed to evaluate a potential underlying lesion and showed a relatively small left maxillary sinus with inward bowing of the anterolateral wall. Retraction of the medial wall into the sinus cavity was also noted. The lateral border of the sinus appeared sclerotic. The anterior floor of the left bony orbit appeared to be positioned inferior to its anatomic counterpart on the right. The left middle meatus was wider than the right, and the uncinate process was thin and appeared to be retracted into the orbital wall, obstructing the left maxillary sinus infundibulum. Mild mucosal thickening was noted within the right sinus. The patient had no symptoms that could be attributed to clinical sinusitis. No fluid accumulation was noted (Fig. 3).
Fig. 3
Case 3. Axial (A) and coronal (B) images demonstrate opacification and inward retraction of the walls of the left maxillary sinus.
Discussion
SSS has several unique imaging characteristics. Typical radiographic features include a marked unilateral decrease in the overall volume of the sinus due to inward retraction of the sinus walls, which in turn augments the ipsilateral orbital volume and results in the concomitant expansion of the middle meatus on the affected side.3 The affected maxillary sinus also demonstrates opacification, while occlusion of the ostiomeatal complex and flattening of the malar eminence may be seen.8 As demonstrated in Figures 1, 2, 3, maxillary sinus structures were measured.9 Analysis of the axial and coronal CBCT sections revealed characteristics including unilateral volume loss in the maxillary sinus, retracted sinus walls, partial to complete soft-tissue opacification of the maxillary sinus, and inferior bowing of the orbital floor. A distinct lack of history of trauma or sinusitis was noted for these cases. The radiological differential diagnosis of SSS includes congenital maxillary sinus hypoplasia, chronic sinusitis, and mucocele, as well as changes seen after maxillary sinus and orbital trauma. Some of the distinguishing features of congenital maxillary sinus hypoplasia include the arresting of sinus development due to infection or trauma, as well as congenital first arch syndrome and developmental anomalies.10 Congenital anomalies such as Treacher Collins syndrome are associated with unilateral maxillary sinus hypoplasia.11 All features of maxillary hypoplasia were absent in the cases in the present study.
Based on a study of patients at the Massachusetts Eye and Ear Infirmary, chronic maxillary atelectasis has been classified into 3 stages. This classification is based on anatomical changes. Stage 1 (membranous deformity) is marked by lateralization of the maxillary fontanelle; stage 2 (bone deformity) involves the inward bowing of 1 or more walls of the maxillary sinus, and in stage 3 (clinical deformity), enophthalmos, hypoglobus, and/or deformity of the midface are noted.12, 13 Based on this classification, the cases in this study fell within stage 2.
As the use of CBCT in diagnostic maxillofacial imaging has become more frequent, its range of applications has also expanded. Paranasal sinus imaging is one such CBCT application outside of dentistry.7 The present report also provides a summary of relevant studies that have investigated the routine use of complex imaging in medicine for sinus pathoses, in an effort to provide some perspective regarding the potential applications of computed tomography for which low-dose CBCT techniques could be used. A narrative literature search of the PubMed database was performed with the following Boolean queries: 1) paranasal sinus AND imaging, 2) paranasal sinus imaging AND MDCT OR CBCT, 3) comparison of radiation dose CBCT AND MDCT, and 4) the above searches combined as follows: 2 AND (3 OR 1).
The search results were limited to papers in English published in the past 10 years. This time frame was chosen for 2 reasons. First, even though the widespread implementation of CBCT in dentistry started in the mid-2000s, relatively few studies are available on its use in paranasal sinus imaging. Second, the technology itself has rapidly developed over time, with improved spatial resolution, better signal-to-noise ratios, and significantly lower radiation doses. Therefore, findings from older studies may not be as relevant as more recent research. The 10-year cut-off reflects our need to include as many papers as possible while attempting to stay relevant in a changing technology landscape. The titles and abstracts of the retrieved papers were manually screened. Papers meeting the following inclusion criteria were selected for the final analysis: empirical investigations (quantitative and qualitative), comparison of the radiation dose associated with CBCT and MDCT, and paranasal sinus imaging. Snowballing from the results of this literature search, retrieved citations were used as the basis for new searches using the citation's bibliography, authors' names, and “related articles” as search options, as appropriate. A total of 14 papers met the inclusion criteria and repeatedly recommended CBCT as the preferred imaging modality over MDCT for the imaging of SSS and/or paranasal sinus disease or anomalies, due to the lower radiation dose associated with CBCT (Table 1).
Table 1
Literature review of imaging of paranasal sinus pathoses
In several studies, a significantly lower overall radiation dose has been demonstrated to be associated with CBCT compared to MDCT/multislice computed tomography, suggesting that CBCT should likely be the imaging modality of choice when appropriate demographic and clinical features are noted with an absence of history of trauma, surgery, or developmental anomalies.14 A study by Almashraqi et al. compared a low-dose MDCT protocol with CBCT, and CBCT was found to be associated with approximately a 40% reduction in radiation dose and comparable image quality relative to MDCT.15 Furthermore, a technical valuation report on imaging performance and radiation dose for a commercially available CBCT scanner illustrated some merits of CBCT imaging, including significantly better spatial and contrast resolution and reduced radiation dose.16
The exact pathophysiology of SSS is still a subject of debate, and 3 hypotheses have been proposed.8 According to the first hypothesis, a prolonged subatmospheric (negative) pressure is created in the sinus due to complete obstruction of the maxillary ostium, hypoventilation, and secretion/mucus retention. The cause of obstruction of the maxillary sinus may be inspissated secretions, hypermobility of the infundibular wall, mucocele or nasal polyp, and/or narrowing of the ostium by inflamed mucosa or adjacent Haller cells.17 The negative pressure creates a vacuum that may induce osteopenia, bone remodeling, and sinus wall retraction. As a result, the orbital floor becomes thin, and support for the orbital contents is lost, leading to enophthalmos.8 The second hypothesis suggests that inflammatory processes lead to osteopenia with subsequent erosion of the bony orbital floor. Stashenko et al. and Amano et al. have shown that inflammatory cells can produce cytokines that have the potential to inhibit osteoblast and collagen synthesis, leading to osteopenia.8 The third hypothesis states that SSS develops in individuals with congenitally hypoplastic maxillary sinuses that become infected. However, studies have shown that SSS occurs in normal and well-developed maxillary sinuses.8 Functional endoscopic sinus surgery is the preferred treatment for SSS, with expected results of improved drainage of the maxillary sinus and restoration of its anatomy.18
In summary, this report was written to highlight the incidental finding of SSS, a relatively rarely reported and often underdiagnosed sinus pathosis, on CBCT images. Furthermore, it underlines features that were observed using a relatively high spatial resolution modality with a significant reduction in radiation dose. Dental professionals should recognize the existence of SSS in otherwise asymptomatic patients and institute management where needed.
Conflicts of Interest:None
References
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Güldner C, Ningo A, Voigt J, Diogo I, Heinrichs J, Weber R, et al. Potential of dosage reduction in cone-beam-computed tomography (CBCT) for radiological diagnostics of the paranasal sinuses. Eur Arch Otorhinolaryngol 2013;270:1307–1315.
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