The term chronic rhinosinusitis (CRS) is preferred because rhinitis almost always precedes sinusitis and sinusitis occurs concurrently with inflammation of the nasal passages. CRS diagnosis is symptom-based and requires persistence of patient complaints for more than 12 consecutive weeks.

Since many clinical symptoms of CRS are vague, subjective, and nonspecific, objective demonstration of mucosal inflammation is necessary to confirm the clinical diagnosis.

Anterior rhinoscopy may not always be able to confirm the presence of mucosal inflammation, and nasal endoscopy is required to visualize the middle meatus and ethmoid bulla.

CT, as the imaging standard for evaluation of the sinuses, is an excellent tool to confirm the presence and assess the extent of inflammation in the sinonasal cavity beyond what is permitted by endoscopy.

The plain radiographs lack the sensitivity, specificity, and anatomic precision needed for the management of these patients and are simply inadequate.

Limited sinus CT examinations which employ selected, noncontiguous axial slices may be adequate in some clinical scenarios when the patient had a thorough evaluation of the anatomy with a high quality CT previously.

One has to keep in mind, though, that the limited sinus study is limited not only in cost and radiation exposure, but in information as well.

Although CT provides excellent information about the extent and distribution of mucosal disease and status of the nasal air passages, it does not yield much information about the origin of the changes (e.g., infection, allergies, granulomatous inflammation, postsurgical scarring, etc.).

The CT signs suggestive of CRS include diffuse or focal mucosal thickening, partial or complete opacification, and bone remodeling and thickening caused by osteitis from adjacent chronic mucosal inflammation and polyposis.

The distribution of the inflammatory mucosal changes in the nasal cavity and sinuses may provide a clue as to the level of mechanical obstruction. Babble et al. defined five recurring patterns of inflammatory sinonasal disease including infundibular, OMU, sphenoethmoidal recess, sinonasal polyposis, and sporadic or unclassifiable disease.

In this study, the infundibular pattern (26% of patients) referred to the focal obstruction within the maxillary sinus ostium and ethmoid infundibulum that was associated with maxillary sinus disease. The OMU pattern (25% of patients) referred to ipsilateral maxillary, frontal, and anterior ethmoid sinus disease.

This pattern was caused by obstruction of the middle meatus. The frontal sinus is sometimes spared because of the variability in frontal sinus drainage pathway. The sphenoethmoidal recess pattern (6% of patients) resulted in sphenoid or posterior ethmoid sinus inflammation caused by sphenoethmoidal recess obstruction.

Diffuse nasal and paranasal sinus polyps occurred in 10% of the study population (sinonasal polyposis pattern). One-fourth of the patients in this study did not show a recognizable pattern.

Zinreich and others found middle-meatus opacification in 72% of patients with chronic sinusitis; 65% of these patients had mucosal thickening of the maxillary sinus. The patients with frontal sinus inflammatory disease had opacification of the frontal recess.

Frontal sinus opacification involving the OMU without maxillary or anterior ethmoid sinus inflammatory disease was rare. Yousem et al. found that when the middle meatus was opacified, associated inflammatory changes occurred in the ethmoid sinuses in 82% of patients and in the maxillary sinuses in 84%.

Bolger et al. found that when the ethmoid infundibulum was free of disease, the maxillary and frontal sinuses were clear in 77% of patients. Certain anatomic variants, as described, have been implicated as causative factors in the presence of chronic inflammatory disease.

Lidov and Som found that a large concha bullosa could produce signs and symptoms by narrowing the infundibulum. However, Yousem et al. found that the presence of a concha bullosa did not increase the risk of sinusitis.

This was corroborated by Bolger et al. who found that concha bullosa, paradoxic turbinates, Haller cells, and uncinate pneumatization were not significantly more common in patients with chronic sinusitis than in asymptomatic patients.

Yousem et al. found that nasal septal deviation and a horizontally oriented uncinate process were more common in patients with inflammatory sinusitis. Although these variants may not necessarily predispose to sinusitis, the size of a given anatomic variant and its relationship to adjacent structures are important in the development of sinusitis.

When sinus secretions are acute and of low viscosity, they are of intermediate attenuation on CT (10–25 Hounsfield units). In the more chronic state, sinus secretions become thickened and concentrated, and the CT attenuation increases with density measurements of 30 to 60 Hounsfield units.

Sinonasal polyposis has been recognized as a distinct form of CRS, both clinically and radiographically, although polyp formation is a nonspecific response to variety of inflammatory stimuli. There is an obvious association with asthma, aspirin-sensitivity, and eosinophilia.

The pathogenesis of sinonasal polyposis is very complex and not clearly understood. However, high recurrence rate of sinonasal polyposis is well documented.

Antrochoanal and sphenochoanal polyps appear as well-defined masses that arise from the maxillary or sphenoid sinus and extend to the choana through the middle meatus or sphenoethmoid recess, respectively.

They can present as nasopharyngeal masses. It is important to recognize their origin and relation to the maxillary or sphenoid ostium in treatment planning. Retention cysts are very common incidental findings in imaging studies and seen as very well-defined rounded masses, typically in the maxillary sinus floor.

Their clinical significance is not clear. They may become symptomatic if large enough to interfere with drainage pathways. Mucoceles, a complication of CRS, result from the obstruction of the sinus drainage and subsequent expansion of the sinus.

Mucoceles are more commonly seen in the ethmoid and frontal sinuses and present with symptoms secondary to compression of the adjacent structures in addition to usual symptoms of CRS.

Thickening and sclerosis of the bony walls of the sinuses have been traditionally attributed to the secondary reaction of the bone to a chronic mucosal inflammation.

More recent work suggests that the bone may actually play an active part in the disease process and that the inflammation associated with CRS may spread through the haversian system within the bone.

The combination of a surgical procedure and experimentally induced sinusitis creates an inflammatory process within bone with the classic histological features of osteomyelitis.

Furthermore, bone inflammation may induce chronic inflammatory changes in the overlying mucosa at a significant distance from the site of infection. Identification of bone thickening and sclerosis on CT exam is straightforward, due to CT’s exquisite ability to show the bone detail.

On MRI, the appearance of CRS varies because of the changing concentrations of protein and free water protons. Som and Curtin describe four patterns of MRI signal intensity that can be seen with chronic sinusitis:

1. hypointense on T1-weighted images and hyperintense on T2-weighted images with a protein concentration less than 9%;
2. hyperintense on T1-weighted images and hyperintense on T2-weighted images with total protein concentration increased by 20% to 25%;
3. hyperintense on T1-weighted images and hypointense on T2-weighted images with total protein concentration of 25% to 30%; and
4. hypointense on T1- weighted images and T2-weighted images with a protein concentration greater than 30% and inspissated secretions in an almost solid form.

MRI of inspissated secretions (i.e., those with protein concentrations greater than 30%) may have a pitfall in that the signal voids on T1- and T2-weighted images may look identical to normally aerated sinuses.

The correlation between patient symptoms and CT findings is difficult to determine partly due to the fact that chronic mucosal inflammation may be present without the findings identified on CT examinations such as mucosal hypertrophy and retained secretions and that a modest amount of inflammation diagnosed by CT may be present in asymptomatic persons.

Several studies failed to show a correlation between symptom severity and severity of CT findings. Particularly, symptoms such as headache and facial pain do not correlate with CT findings at all.

A positive correlation between the severity of symptoms and CT findings may be demonstrated when certain symptoms and negative CT exams are eliminated. The nasal endoscopy findings correlate with CT findings, though the correlation is less than perfect.

The positive predictive value of abnormal endoscopy for abnormal CT is greater than 90%, whereas the negative predictive value of normal endoscopy for normal CT is only 70%.

To better classify patients into diagnostic and prognostic categories, various symptom-, CT-, and endoscopy- scoring systems have been used. The Lund–MacKay scoring system is the most popular method applied to CT description of sinus disease because of its simplicity and reproducibility.

A score of 0, 1, or 2 is given to each of the five sites (anterior ethmoid, posterior ethmoid, frontal, maxillary, and sphenoid) on both sides of the sinonasal cavity for normal pneumatization, partial opacification, or complete opacification, respectively.

The ostiomeatal complex receives either 0 or 2. This yields a maximum score of 12 for one side. In a small study, the impact of CT on treatment decision was evaluated. CT changed the treatment in one-third of the patients and provided better agreement on treatment plan among ENT surgeons.