Учебники / Rhinosinusitis - A Guide for Diagnosis and Management 2008

Chapter 10

Radiology: Its Diagnostic Usefulness

in Rhinosinusitis

Laurie A. Loevner

This chapter addresses imaging modalities available to assess disease processes of the sinonasal cavity and provides some direction on when and how to use them. To understand the pathogenesis and imaging appearances of rhinosinusitis and other pathological processes that may affect the paranasal sinuses, a brief review of sinus development and anatomy as it pertains to mucociliary clearance is essential. Subsequently, focused imaging assessment of disease processes, including rhinosinusitis and neoplasms, is covered.

Development

The maxillary sinuses are the first of the paranasal sinuses to develop, beginning in the first trimester of gestation and usually completed by adolescence [1]. The ethmoid air cells arise from numerous evaginations from the nasal cavity, beginning with the anterior air cells, and progressing to the posterior air cells. The ethmoid air cells start to develop between the end of the first trimester and the mid-second trimester of gestation, and their final adult proportions are usually attained during puberty. The sphenoid sinus is present by the second trimester of pregnancy, and usually finishes its growth by the time a child reaches 10 years of age. The frontal sinuses are the only sinuses that are consistently absent at birth. Their development is variable, beginning during the first few years of life, and is completed in early adolescence [1].

Anatomy

To understand the disease processes that may affect the paranasal sinuses and the nasal cavity, it is important to understand the anatomy as well as the normal drainage patterns of the sinonasal cavity [1,2]. The paranasal sinuses and nasal

L.A. Loevner

Department of Otorhinolaryngology – Head and Neck Surgery, University of Pennsylvania, Philadelphia, PA, USA

e-mail: laurie.loevner@uphs.upenn.edu

cavity are lined by ciliated columnar epithelium, which contains both mucinous and serous glands. The common drainage pathway for the frontal sinuses, maxillary sinuses, and anterior ethmoid air cells is through the ostiomeatal complex [2]. The ostiomeatal unit comprises a drainage pathway that consists of the maxillary sinus ostium, the infundibulum, the hiatus semilunaris, and the middle meatus (Fig. 10.1a,b). This drainage conduit is centered about the uncinate process (an osseous extension of the lateral nasal wall). Secretions that accumulate within the maxillary sinuses circulate toward the maxillary sinus ostium propelled by cilia within this sinus. From the maxillary ostium, mucus circulates through the infundibulum located lateral to the uncinate process. Secretions progress through the hiatus semilunaris, an air-filled channel above the tip of the uncinate process and anterior and inferior to the ethmoidal bulla (the largest ethmoid air cell), and then pass into the middle meatus, the nasal cavity, and ultimately into the nasopharynx. They are then swallowed.

The frontal sinuses drain inferiorly via the frontal ethmoidal recess/nasofrontal duct into the middle meatus, the common drainage site also for the anterior ethmoid air cells, which have ostia in contact with the infundibulum of the ostiomeatal complex. The nasofrontal duct is the channel between the inferomedial frontal sinus and the anterior part of the middle meatus. The anteriormost ethmoid air cells, the agger nasi cells, are located in front of the middle turbinates, which are in turn located anterior, lateral, and inferior to the frontal ethmoidal recess. Inconstant ethmoid air cells located along the anterosuperior maxillary surface just inferior to the orbital floor, referred to as maxilloethmoidal or Haller cells, are present in less than one-half of imaged patients. These cells are important because if they are opacified they may obstruct the infundibulum of the ostiomeatal unit.

The posterior ethmoid air cells are located behind the middle turbinate and secretions drain through the superior meatus, the supreme meatus, and/or other tiny ostia under the superior turbinate into the sphenoethmoidal recess, the nasal cavity, and finally into the nasopharynx (Fig. 10.1c,d). Cilia are necessary for the drainage of the sphenoid sinuses as secretions must be propelled to the ostia that is located above the sinus floor.

The three sets of turbinates in the nasal cavity include the superior, middle, and inferior turbinates. Occasionally, there may be a supreme turbinate located above the superior turbinate. When the middle turbinate is aerated, it is termed a concha bullosa, present in up to 30% to 50% of patients. Large or opacified concha bullosa may obstruct the ostiomeatal complex, the common drainage passageway of the frontal sinus, maxillary sinus, and anterior ethmoid air cells.

The nasal septum separates the right and left nasal turbinates, dividing the nasal cavity in half. The anterior and inferior nasal septum is made up of cartilage. The posterior portion of the nasal septum is osseous. The superior posterior osseous portion is the perpendicular plate of the ethmoid bone, and the inferoposterior osseous portion is the vomer. The septum within the nasal cavity is lined by squamous epithelium; e remainder of the nasal cavity and the paranasal sinuses are lined by columnar epithelium.

Fig. 10.1 Normal anatomy of the mucociliary drainage of the paranasal sinuses. (a, b) Coronal computed tomography (CT) images in bone algorithm show the normal drainage pathway of the maxillary sinus, anterior ethmoid air cells, and frontal sinus via the ostiomeatal complex (OMC). *, uncinate process; O, maxillary sinus ostium; i, infundibulum; white line, middle meatus; m, middle turbinate; IT, inferior turbinate. Coronal (c) and axial (d) CT images in bone algorithm show the normal drainage of the posterior ethmoid air cells and the sphenoid sinus via the sphenoethmoidal recess into the nasal cavity (R)

The nasolacrimal duct runs from the lacrimal sac at the medial canthus, along the anterior and lateral nasal wall, and drains into the inferior meatus. There is normal cyclical passive congestion and decongestion of each side of the nasal cavity and ethmoid air cells, which includes temporary mucosal thickening in these structures.

Blood supply to the sinonasal structures comes from the internal and external carotid arteries. The arterial supply to the frontal sinuses is from supraorbital and supratrochlear branches of the ophthalmic artery, while venous drainage is

through the superior ophthalmic veins. The ethmoid air cells and sphenoid sinus also receive blood supply from branches of the sphenopalatine artery (arising from the external carotid circulation) as well as ethmoidal branches of the ophthalmic artery (arising from the internal carotid circulation). Venous drainage is via nasal veins into the nasal cavity, and/or ethmoidal veins that drain into the ophthalmic veins, which then subsequently drain into the cavernous sinus. Branches of the maxillary artery that arise from the external carotid circulation supply the maxillary sinuses predominantly. These sinuses drain through facial and maxillary veins, the latter communicating with the pterygoid venous plexus. The venous drainage pattern of the paranasal sinuses (ultimately communicating with the cavernous sinus and pterygoid venous plexus) is responsible for the potential intracranial complications of rhinosinusitis including meningitis, subdural empyema, and venous thrombosis.

Imaging Sinonasal Disease: The Radiologist’s Arsenal

Plain Film Radiographs

Several years ago, computed tomography (CT) replaced plain film radiographs as the mainstay in evaluating sinonasal disease. In the 1980s, functional endoscopic sinonasal surgery (FESS) supplanted external procedures such as the Caldwell-Luc and maxillary antrostomy for the treatment of rhinosinusitis, which has required much greater anatomic precision than is provided by plain film radiographs. Overlapping structures on plain film radiographs limit evaluation of the ostiomeatal complex, as well as the individual paranasal sinuses. There is also insufficient detail regarding the osseous confines of the sinonasal cavity.

Plain film radiographs are sometimes obtained in intensive care unit settings when rhinosinusitis is suspected or needs to be excluded in fevers of unknown origin and the patient is too sick to come to the radiology department for CT imaging. However, portable CT units are rapidly multiplying, making access to portable CT scans in hospitalized patients the new reality.

Computed Tomography

As functional endoscopic sinonasal surgery (FESS) has replaced external surgical procedures for treating rhinosinusitis, CT imaging has become necessary to provide the surgeon with precise anatomic information. Functional endoscopic sinonasal surgery is performed through an intranasal endoscope, and CT provides the precise anatomic information as seen by the endoscopist. Surgery is directed toward removing blockages to mucociliary clearance at the ostiomeatal complex. For the surgeon performing FESS, coronal CT is ideal as it simulates the appearance of the sinonasal cavity from an endoscopic view.

Direct coronal thin section imaging (1.5 to 3 mm) is frequently obtained through the paranasal sinuses. Using the newer helical CT scanners, high-quality axial reformatted images may be created from these coronal images. Alternatively, direct axial CT imaging is performed with subsequent creation of coronal reformatted images. Many computer software programs allow instant three-plane reconstructions (for instance, coronal and sagittal reconstructions from axial images). Intravenous contrast material is usually not necessary in sinonasal CT imaging for inflammatory disease. If CT imaging shows findings like bone destruction or extension of disease outside the sinonasal cavity concerning a more aggressive process such as a neoplasm or invasive rhinosinusitis [3,4], magnetic resonance (MR) imaging should be obtained without and with intravenous contrast administration, which is a more sensitive study. If the patient has a contraindication to MR imaging (i.e., pacemaker) and an enhanced study is indicated, then contrast-enhanced CT is the appropriate alternative study.

Magnetic Resonance Imaging

Because of its excellent soft tissue resolution and multiplanar capabilities, MR imaging has become an increasingly important technology in assessing patients with both benign and malignant neoplasms of the sinonasal cavity, as well as meningoencephaloceles and aggressive infections such as invasive fungal rhinosinusitis. A combination of sagittal, axial, and coronal imaging provides excellent anatomic information regarding the extent of sinonasal tumors. Multiple different image acquisitions (sequences) are obtained, including T1-weighted and T2-weighted as well as contrast-enhanced multiplanar imaging. In most instances, excellent anatomic resolution may be acquired from an MR examination performed in a standard head coil. On occasion, imaging of the sinonasal cavity may be performed with a surface coil positioned over the face (the sinus of interest). Imaging of sinonasal malignancies and aggressive infections must include high-resolution views not only of the sinonasal cavity but also of the orbits, skull base, and the intracranial compartment. Magnetic resonance imaging allows discrimination of inflammation and inspissated secretions from neoplasm and other nonneoplastic masses (i.e., encephalocele) and is valuable in assessing for extension of disease (neoplasms and aggressive infections such as invasive fungus) outside the sinonasal cavity into the intracranial compartment, the eye, and the base of skull [3].

Sinonasal secretions have variable signal intensity patterns on MR related to multiple factors including the protein concentration relative to mobile water protons, viscosity, and cross-linking of glycoproteins [5]. As the protein concentration in secretions increases relative to free water, the signal intensity on T1-weighted imaging changes from hypointense (dark) to hyperintense (bright) to hypointense again. On T2-weighted images, secretions with low protein content are initially bright; however, as the protein content and viscosity increase, the signal intensity decreases.

Fig. 10.3 CT imaging of chronic rhinosinusitis before functional endoscopic sinonasal surgery (FESS). (a) Coronal images [note: (b) is anterior to (a)] show mucosal disease in the bilateral maxillary sinuses, opacification of the ethmoid air cells (E), opacification of the nasal cavity (NC), and opacification of the frontal sinuses (F). The ostiomeatal complexes (OMC) are obstructed

levels should be noted. Hyperdense sinus contents on CT may reflect the presence of inspissated secretions, fungal elements, or hemorrhage in the setting of trauma or instrumentation.

When evaluating patients for chronic rhinosinusitis and potential functional endoscopic sinus surgery (FESS), it is important to evaluate certain anatomical landmarks on high quality, thin section unenhanced CT images of the sinonasal cavity. 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, cribriform plate, and the roof and lateral walls of the sphenoid sinus should be evaluated for osseous defects or deficiencies. A defect in the lamina papyracea may result in orbital penetration and subsequent hematoma formation, whereas a dehiscence in the cribriform plate or sphenoid sinus could result in a cerebrospinal fluid (CSF) leak, intracranial complications (meningitis, encephalocele), or carotid artery complications (perforation with acute subarachnoid hemorrhage; pseudoaneurysm formation). The radiologist and clinician must also assess for anatomic variants or secondary changes of the drainage passageways that may impact on surgery. For instance, is there an atelectatic infundibulum/ostiomeatal complex (OMC) in which the uncinate process or middle turbinate is opposed to the orbital floor (Fig. 10.4a,b)? If so, and the endoscopist is unaware, vigorous removal may result in orbital penetration (Fig. 10.5).

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

Fig. 10.4 Atelectatic right uncinate process/OMC identified in a patient before FESS for chronic rhinosinusitis. (a, b) Contiguous direct coronal CT images show the right OMC (uncinate process and middle turbinate) are apposed to the orbital wall. The cribriform plate (C) and lamina papyracea (L) are intact. The right ethmoid air cells are opacified. m, middle turbinate

Fig. 10.5 Defect in the orbital floor following FESS. The * shows a defect in the floor of the right orbit with herniation of orbital fat into the defect. The patient had intermittent diplopia

complications following FESS, CT scan is the study of choice. Many of these complications are evident within 24 to 48 h following instrumentation. Computed tomography is accurate in identifying orbital hematomas and optic nerve injury in the orbit, as well as other orbital injuries (medial rectus contusion). A CSF leak caused by inadvertent injury to the cribriform plate or overly vigorous removal of the attachment of the middle turbinate to the fovea ethmoidalis may be immediately evident in the operating room, or it may present days to weeks after surgery with nasal drainage (CSF leak) or symptoms of meningitis (Fig. 10.6a,b).

Complications of acute rhinosinusitis include periorbital cellulitis and abscess formation (Fig. 10.7a,b), meningitis, thrombophlebitis (including cavernous sinus

Fig. 10.6 Cerebrospinal fluid (CSF) leak and meningitis presenting approximately 2 weeks following FESS. Coronal (a) and axial (b) CT scans show intracranial air (*), seen as hypodense or dark areas. There is a surgically created defect in the left fovea ethmoidalis (ˆ)

Fig. 10.7 Periorbital cellulitis and abscess formation complicating acute rhinosinusitis. (a) Unenhanced axial T1-weighted MR image shows periorbital soft tissue (*) in the left eyelid and preseptal tissues. (b) Enhanced axial T1-weighted MR image at the same level as (a) shows areas of fluid with rim enhancement consistent with abscess formation (a)

thrombosis), subdural empyema, brain abscess, and perineural and perivascular spread of infection (especially in invasive fungal disease). These acute complications are most accurately assessed with combined brain and orbital MR imaging including contrast-enhanced imaging; however, contrast enhanced CT is reliable in assessing orbital and periorbital infection so long as there is not concern for extension to the orbital apex or intracranial compartment (in which case MR should be obtained).