Imaging Disease Processes of the Paranasal Sinuses
Sinusitis is among the most common ailments in the United States, with over 30 million Americans affected by sinusitis each year.27 Over 15 million visits to family physicians each year are related to sinus inflammation. Most cases of acute sinusitis are related to an antecedent viral upper respiratory tract infection. There is resultant swelling that results in apposition of the mucosal surfaces within the paranasal sinuses leading to obstruction of the normal drainage pathways. Inadequate drainage of secretions results in bacterial overgrowth and sinus infection. Although literature suggests that use of CT in uncomplicated community-acquired bacterial sinusitis is not cost-effective,28 unenhanced coronal CT imaging may be obtained in patients with suspected acute sinusitis to assess mucosal disease and/or blockage of the drainage passageways that support this clinically suspected diagnosis ( Fig. 3.3A,B ).29–31 Although the empiric antibiotic therapy or treatment according to clinical guidelines are considered the cost- effective pathways,28,32 the imaging still could be used for those patients refractory to medications and antibiotics. If imaging is contemplated to follow-up a patient treated for sinusitis, it is best to obtain imaging 4 to 6 weeks following therapy as the resolution of the radio-logical findings may lag behind the clinical response.
In patients being assessed with CT imaging for chronic sinusitis, it is important for the radiologist to report the areas of mucosal thickening within the paranasal sinuses as well as the drainage passageways of the ostiomeatal complex and sphenoethmoidal recess ( Fig. 3.4A,B ).33,34 The location of sinusitis is as important as the extent of disease in producing symptoms. An evaluation of the nasal cavity and osseous confines (medial orbital walls, cribriform plate, sphenoid sinus roof), and identification of anatomic variants is essential in the assessment of sinonasal pathology. The presence of air-fluid levels or bubbly mucoid material should be reported. Hyperdense sinus contents may reflect the presence of inspissated secretions and/or fungal elements, and is an important finding in diagnosing allergic fungal sinusitis ( Fig. 3.5A,B ). Hemorrhage usually in the setting of trauma or instrumentation is also hyperdense.
When evaluating patients with chronic sinusitis for potential FESS, it is important to evaluate certain anatomic landmarks on high quality, thin section unenhanced CT images. Direct coronal images may be obtained, or direct thin section axial images may be obtained and coronal reformations created from these. The medial orbital walls, the cribriform plate, and the roof and lateral walls of the sphenoid sinus should be evaluated for osseous deficiencies. An unrecognized defect in the lamina papyracea may result in orbital penetration and hematoma formation, whereas a dehiscence in the cribriform plate or sphenoid sinus could result in a cerebrospinal (CSF) leak ( Fig. 3.6A,B ), intracranial complications (meningitis, encephalocele) ( Fig. 3.7 ), or carotid artery complications (perforation, acute subarachnoid hemorrhage; pseudoaneurysm formation).35–39 The radiologist and clinician must also assess for anatomic variants or secondary changes of the drainage passageways that may impact surgery. For instance, if the uncinate process and/or middle turbinate are opposed to the orbital floor ( Fig. 3.8A,B ) and the endoscopist is unaware, vigorous removal may result in orbital penetration ( Fig. 3.9 ).
Post-FESS scanning is not accurate in distinguishing inflammation, granulation, and fibrous tissue. The absence of disease on a postoperative study is reliable, but the converse is not true. False-positive studies are common. In cases of suspected complications following FESS, CT scan is the study of choice. Many of these complications are evident within 24 to 48 hours following instrumentation. CT is accurate in identifying orbital hematomas, optic nerve injury in the orbit, as well as other orbital injuries.36,39 A CSF leak due to inadvertent injury to the cribriform plate or overly vigorous removal of the attachment of the middle turbinate to the ethmoid fovealis may be immediately evident in the operating room, or may present days to weeks after surgery with rhinorrhea or symptoms of meningitis ( Fig. 3.6A,B ).35,40
Complications of acute sinusitis include periorbital cellulitis and abscess formation, meningitis, thrombophlebitis (including cavernous sinus thrombosis), subdural empyema, brain abscess, and perineural and perivascular spread of infection (especially in invasive fungal disease).41–44 These acute intracranial complications are most accurately assessed with a combined brain and orbital MRI, including contrast-enhanced imaging ( Fig. 3.10A,B ). Contrast- enhanced CT is reliable in assessing orbital and periorbital infection as long as there is not concern for extension to the orbital apex or intracranial compartment (in which case MRI should be obtained).7,14,45
Mucoceles may complicate chronic sinusitis, facial trauma, or sinus surgical instrumentation. Mucoceles develop from obstruction of sinus ostia or septated compartments of a sinus and represent mucoid secretions encased by mucus- secreting epithelium (sinus mucosa). In over 90% of cases, mucoceles occur in the frontal sinuses or the ethmoid air cells.46–48 Signs and symptoms of mucoceles are related to a mass effect such as frontal swelling, headache, and orbital pain. Orbital extension may result in proptosis or diplopia.49 A secondary infection (mucopyocele) or a direct extension into the anterior cranial fossa is not infrequent.50–54 Advances in endoscopic sinus surgery have led to simple drainage procedures, even for some seemingly very complicated mucoceles.
In the radiologic evaluation of mucoceles, CT best shows the osseous changes of the sinus walls, which may be expanded and thinned. With large mucoceles, the adjacent bones may have frank defects ( Fig. 3.11A ). MRI may better assess the interface of a mucocele with intraorbital and intracranial structures. Enhanced MRI is useful in distinguishing a mucocele, which demonstrates peripheral enhancement, from a neoplasm, which typically demonstrates solid enhancement ( Fig. 3.11B ).14,55 Mucoceles have a spectrum of signal characteristics on MRI depending on their protein content relative to free water.56,57