- Open Access
Flying through congested airspaces: imaging of chronic rhinosinusitis
© European Society of Radiology 2010
- Received: 23 May 2010
- Accepted: 2 June 2010
- Published: 27 July 2010
The complex regional anatomy of the nose and paranasal sinuses makes the interpretation of imaging studies of these structures intimidating to many radiologists. This paper aims to provide a key to interpretation by presenting a simplified approach to the functional anatomy of the paranasal sinuses and their most common (and most relevant) variants. This knowledge is basic for the full understanding of chronic rhinosinusitis and its computed tomography (CT) patterns. As fungal infections may be observed in the setting of chronic rhinosinusitis, these are also discussed. Chronic sinus inflammation produces bone changes, clearly depicted on CT images. Finally, clues to suspecting neoplastic lesions underlying inflammatory sinus conditions are provided.
- Paranasal sinuses
Chronic rhinosinusitis is a common disease, causing high social and economic costs , and heavily impacting on the patient’s quality of life . Imaging of chronic rhinosinusitis is often regarded as a difficult task, particularly by young residents, probably because of the complexity of the anatomy that makes the search for the key to unravelling such a labyrinth of air cells (and its pathological features) a tough task. However, first impressions often lie: the anatomy can be quite easily understood if described from the perspective of function; likewise, imaging findings in chronic rhinosinusitis can be quite clearly interpreted by radiologists when they have sufficient background on the functional anatomy. The purpose of this review is to offer a handful of basic concepts that may guide the inexperienced radiologist towards an easier approach to imaging of the paranasal sinuses in his or her daily routine.
Anatomy, stripped to the bone
A factor that cannot be neglected: the anatomical variants
Chronic rhinosinusitis and imaging: the survival kit
The rationale for imaging of patients affected with chronic rhinosinusitis, simplified to the extreme, consists in demonstrating any impairment of mucus clearance through the aforementioned pathways, along with any anatomical variant that may either incite rhinosinusitis or raise the risks of endoscopic surgery. Given its superb detail in depicting the thinnest bone structures, MSCT is the most suitable technique for the task ; the inherent contrast displayed on bone algorithm reconstructions among bone, air and soft tissues allows the radiation dose to be decreased to a minimum, thus allowing all information to be obtained with limited biological invasiveness.
MSCT reporting of the paranasal sinuses should be done in a centripetal fashion, thus moving from the centre, i.e. the nasal septum and the respective drainage pathways, to the periphery represented by all sinuses. Both soft-tissue and bone changes should be reported. Five patterns of rhinosinusitis, namely the infundibular, sphenoethmoidal, ostiomeatal and sporadic patterns as well as nasal polyposis are described [11, 12]. Sometimes several patterns are combined in the same patient.
The sporadic pattern is a large box in which several different conditions are stored, such as isolated mucosal thickenings, retention cysts, antrochoanal polyp, silent sinus syndrome, odontogenic sinusitis and mucocele.
Isolated mucosal thickenings are incidentally found in a large number of MSCT performed for non-sinonasal pathological conditions. As a general rule, such mucosal thickening is unrelated to clinical symptoms, and when seen in the maxillary sinus and ethmoid cells, it does not need to be reported unless the mucosal thickness exceeds 4 mm and 2 mm, respectively. On the other hand, the mucosal lining of the frontal and sphenoid sinuses should not be visible on MSCT under normal conditions .
Silent sinus syndrome refers to an obstructed maxillary sinus, occupied by inflamed mucosa and secretions, which is shrunken with depression of the orbital floor causing exophthalmia. It is called silent sinus syndrome when clinically there is no clue as to the maxillary sinus disease, only being discovered on an imaging study. Differentiation of this entity from a congenitally hypoplastic maxillary sinus may be difficult. Interestingly, antrochoanal polyp and silent sinus syndrome share the same pathophysiological mechanism, i.e. maxillary sinus blockage. In antrochoanal polyp, an increased intrasinusal pressure (due to partial ostial obstruction with unidirectional flow of air through the ostium into the sinus) is the trigger to force the expulsion of an intramural cyst through an accessory ostium . In silent sinus syndrome, conversely, complete sinus blockage and progressive resorption of air within the cavity leads to a pressure drop, orbital floor depression and sinus contraction  (Fig. 8).
The other side of the coin: bone changes
A common dilemma: how to differentiate polyps from neoplasms?
Fungal infection is rather common, given the ubiquitous presence of fungal agents in the environment and the high frequency of colonisation of sinonasal mucosa ; the role of fungi in promoting chronic rhinosinusitis has been suggested. Fungal infections may be classified as either non-invasive or invasive forms, according to the absence or presence of invasion of mucosa, submucosa, bone and vessels by hyphae .
The non-invasive form occurs in immunocompetent subjects and may further be classified as fungus ball and eosinophilic rhinosinusitis; these forms are seen in patients with chronic rhinosinusitis, and are non-responsive to medical and/or surgical treatment . Eosinophilic rhinosinusitis is also called allergic fungal rhinosinusitis. On MSCT, non-invasive fungal infection is suspected whenever spontaneously hyperdense material is detected within the sinus cavities.
Interpretation of the MSCT examinations of patients affected by chronic rhinosinusitis should always be aimed at answering the key questions asked by clinicians. Assessment of location and extent of the disease, along with drainage pathway impairment, contributes to selecting candidates for surgical treatment. Detailed reporting on anatomical variants allows the best surgical approach to be identified and the risk of iatrogenic complications to be decreased. Finally, accurate evaluation of any suspect MSCT findings helps to identify those patients in whom chronic rhinosinusitis is the secondary effect of a neoplasm obstructing the sinus drainage pathways.
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