Imaging Rhinosinusitis

Acute Rhinosinusitis Air–fluid levels and complete opacification of a sinus are the imaging hallmarks of acute rhinosinusitis (ARS). More than one sinus are usually involved, typically the maxillary and the ethmoid sinuses. CT is much more sensitive than plain radiographs.

Air–fluid level is a very specific sign for ARS in the appropriate clinical setting. Other causes of altered air–fluid level in a sinus include trauma, prolonged supine position, and intubation (typical ICU patient), recent nasal irrigation, cerebrospinal fluid (CSF) leak, and chronic rhinosinusitis (CRS).

ARS is diagnosed on the basis of history, clinical presentation, and physical examination. Imaging is not recommended for diagnosing ARS because of the cost-containment concerns and radiation exposure, unless the patient is not responding to initial treatment or there is a complicating factor.

A limited sinus CT consisting of five to six axial noncontiguous images is utilized in many practices for the initial diagnosis of ARS. The charge for limited sinus CT is only slightly higher than the charge for plain radiographs and the radiation dose is reduced, while the diagnostic accuracy remains reasonably high.

The cost-effectiveness of limited sinus CT exam, which changes with evolving technology and its impact on treatment plan, has not been well studied. When the clinical question is whether there is sinusitis or not, a limited sinus CT would be adequate.

The reduced cost and radiation dose of limited sinus CT cannot be used as a justification for imaging of uncomplicated ARS. Authors believe that the diagnosis of ARS should be made clinically, and that when imaging is necessary, a ‘‘high quality’’ CT exam should be performed.

ARS is often a self-limited disease and symptoms subside with or even without treatment; however, in rare instances, catastrophic complications occur.

Because of the close proximity of the sinuses to the brain and orbit, and the naturally present dehiscences and vascular channels in the sinus walls, infection can spread to the orbit and intracranial compartment.

Sub-periosteal phlegmon and abscess result when the infection is limited by the intact periorbita. Penetration of the periorbita allows infection to spread to the orbital soft tissues causing orbital cellulitis, which may lead to permanent loss of vision.

Intracranial complications such as meningitis, epidural and subdural empyema, brain abscess, and cavernous sinus thrombophlebitis typically occur in the setting of frontal and sphenoid sinusitis.

Early identification and treatment are essential in preventing catastrophic results. Contrast-enhanced CT detects most orbital complications. MRI is the study of choice for intracranial extension.

Subacute Rhinosinusitis

Subacute rhinosinusitis is clinically defined as persistence of symptoms for more than four weeks and up to 12 weeks. There is no specific radiological sign for subacute rhinosinusitis and a typical CT study shows some opacification of one or more sinuses.

Fungal Rhinosinusitis

Fungal rhinosinusitis (FRS) differs from bacterial and other types of sinusitis not only in etiology but also in demographics of the effected population, clinical approach, diagnosis, treatment, and prognosis. There are two main forms of FRS: invasive and noninvasive.

Within these categories, five clinicopathologically distinct entities are defined:

  1. acute invasive,
  2. chronic invasive granulomatous form,
  3. chronic invasive nongranulomatous form,
  4. fungus ball and
  5. allergic fungal sinusitis.

It must be emphasized that FRS is a spectrum of disease and the differences in clinical presentation are largely determined by the host defense system. Therefore, it is not uncommon to see overlapping clinical and imaging features.

Acute Invasive FRS

Acute invasive FRS is seen primarily in immunocompromised patients and is fatal if untreated. A high index of clinical suspicion and biopsy of the middle turbinate are necessary for early diagnosis, which may be life-saving.

CT study obtained early in the disease course may be normal or show nonspecific mucosal thickening indistinguishable from the appearance of bacterial/ viral disease. Bone destruction and swelling of the soft tissues adjacent to the paranasal sinuses occur in advanced disease.

Chronic Invasive FRS

Chronic invasive FRS has been associated primarily with immunocompromised patients; however, it does occur in the non-immunocompromised as well and has a more protracted course with relatively slow progression of disease, sometimes despite treatment, and high recurrence rate.

There is no apparent difference in clinical and radiological features of the granulomatous and nongranulomatous forms. The radiological hallmark of chronic invasive FRS is bone destruction, which is better depicted with CT, whereas MRI better defines the soft-tissue extent of disease and brain involvement.

Foci of increased attenuation (on CT) in the sinus mucosal thickening may indicate fungal colonization as found in 74% of our patient population. The radiological differential diagnosis of chronic invasive FRS is broad and includes benign and malignant neoplasms, infectious and idiopathic granulomatous diseases, and allergic fungal sinusitis.

Fungus Ball

Fungus ball refers to a sinus mass that consists of packed hyphae. Patients with fungus ball are typically immunocompetent and present with varying nonspecific sinus-related complaints. Serendipitous identification of fungus balls is not uncommon.

Diffuse opacification of a single sinus is the most common radiographic feature. Foci of hyperattenuation in the center of the sinus mass is seen in approximately 50 to 74% of the patients. Large calcified concretions are characteristic of the disease but uncommonly found. Thickening of the sinus walls is common. Bone erosion may occasionally be seen.

Allergic FRS

Allergic FRS, an immunologically mediated hypersensitivity reaction to fungi, is the most common fungal disease of the sinuses. A central area of hyperattenuation on sinus CT is almost always present and corresponds to markedly decreased T2 signal on MRI.

This appearance is due to the metabolized ferromagnetic elements (primarily iron) and calcium within the concretion. Expansion of the involved sinuses with bone remodeling or destruction is common.

Saprophytic Colonization

Saprophytic colonization of the sinonasal mucosa is very common, particularly in patients who had undergone sinus surgery, and mere presence of fungi on the mucosa does not necessarily constitute the disease.