orbital fat density and fluid collections on the medial or superior orbital wall. Subperiosteal abscesses are rim-enhancing collections between the periorbita and orbital wall; these lesions are well demonstrated by contrast CT or MRI.
• Intracranial complications include epidural, subdural, and intracerebral abscesses; these are usually secondary to frontal sinusitis, and most commonly occur in children. MRI is more accurate in assessing dural reaction and cavernous sinus thrombosis; CT findings of subdural and epidural abscess include a convex rim-enhancing collection; however, the appearance of intracranial abscesses varies depending on stage; surrounding cerebral edema can be impressive on both imaging modalities.
• Vascular complications, such as thrombophlebitis and thrombosis of the sagittal and cavernous sinuses, can be associated with intracranial abscesses and are also better detected on MRI; flow sensitive imaging techniques such as contrast-enhanced venography can be utilized.
• Patients with acute sinusitis resolve completely, so their CT scans return to normal; in patients with chronic sinusitis, there are always inflammatory changes on the CT scan even if the patient has few symptoms at the time of scanning; within each affected sinus there may be a combination of mucosal inflammation, polyps, and secretions.
• The aim of radiologic imaging in CRS is to secure the diagnosis, assess the extent of mucosal edema, and determine the sinus anatomy (the latter two are in preparation for surgical intervention).
• CT is the preferred imaging modality due to its superior bony detail; on CT scans, mucosal inflammation is demonstrated by soft tissue thickening, polyps are lobulated masses with soft tissue attenuation, and secretions may have different radiodensities, depending on their protein composition (Figure 9–1).
• Affected areas may include ethmoid infundibulum obstruction causing maxillary sinus disease; ostiomeatal complex obstruction causing maxillary, frontal, and ethmoid sinus disease; and sphenoethmoid recess obstruction causing sphenoid sinus disease. In severe cases, all of the sinuses are involved (pansinusitis) (Figure 9–2).
• Anatomical risks factors include infraorbital cells, uncinate process variants, maxillary sinus hypoplasia, nasal septal deviation, concha bullosa, and previous trauma (Figure 9–3). Most of these predispose to CRS by causing narrowing of the sinus drainage pathways.
FIGURE 9–1. Bone window CT scans reveal the highly variable pattern of sinus pneumatization. Identification of the frontal sinus drainage pathway requires reconstruction of the pathway in three dimensions, and so studying the axial and sagittal scans is very important. In this case the frontal sinus drainage pathway, which is obstructed by mucosal thickening, can be identified posterior to the agger nasi cell and anterior to the front face of the bulla ethmoidalis.
FIGURE 9–2. CT scans reveal the extent of mucosal involvement in cases of CRS, which facilitates surgical planning. In this case of pansinusitis, incidental note of a small osteoma arising from the bulla ethmoidalis is made (arrow).
FIGURE 9–3. Careful preoperative reading of the CT scan allows for identification of potential intraoperative hazards. In this case there is an old burst fracture of the right medial orbital wall, which was endoscopically obscured by mucosal thickening of the ethmoids. Failure to recognize this abnormality preoperatively would increase the risk of breaching the displaced lamina intraoperatively. The proximity of the superior oblique and medial rectus muscles to the lamina, and the ease of their injury once a limit is breached, is demonstrated by the arrows.
• Inflammatory mediators (particularly after sinus surgery) can cause neo-osteogenesis leading to sclerosis of the bony walls and septations.
• Fungal sinusitis is a relatively common cause of CRS.
• Most cases are non-invasive, in which fungal colonization of the sinus cavity incites an immune response in an immunocompetent patient.
• Invasive fungal sinusitis is a life-threatening condition that is fortunately a rare complication of immunocompromising conditions.
• Non-invasive fungal sinusitis: includes two subtypes—allergic fungal sinusitis and fungus ball. In allergic fungal sinusitis, CT scans show involvement of multiple sinuses with opacification, expansion, and relatively radiodense allergic mucin (the difference in radiodensity between the mucin within the sinus and the edematous mucosal lining creates the double density sign, Figure 9–4). Untreated disease results in bony erosion with orbital and intracranial extension by pressure expansion; fungal balls usually involve a single sinus (typically the maxillary or sphenoid). CT findings include sinus expansion, calcification (from the fungal hyphae), and bony erosion (although sometimes hyperostosis can be seen).
• Invasive fungal sinusitis: this condition usually occurs as an acute, devastating consequence of severe immunocompromising conditions; however, in certain parts of the world chronic forms of invasive fungal sinusitis are seen. CT findings of acute invasive fungal sinusitis include bony destruction and soft tissue extension outside of the sinuses; vascular invasion leads to necrotic mucosa that does not enhance with gadolinium on MRI; MRI is the best modality to detect cavernous sinus involvement and orbital, dural, and intracerebral extension.
• In this condition, chloride transport defects cause high mucus viscosity and impaired ciliary motility; this causes sinus ostial obstruction and results in chronic sinusitis, often with associated sinonasal polyposis.
FIGURE 9–4. A bone window axial CT of a case of fungal sinusitis in which the difference in radiodensity between the fungal debris within the sinus and the polypoid mucosal lining is highlighted by the arrows.
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