Allergic rhinitis and sinusitis are general reactions of the upper respiratory tract to a variety of foreign proteins. The inflammatory response results in edema and thickening of the mucous membrane. Usually the margins of the edematous mucosa have a scalloped appearance, but in severe cases mucus may completely fill a sinus, making it difficult to distinguish an allergic process from infectious sinusitis. Characteristically, all of the paranasal sinuses are affected and the adjacent nasal turbinates are swollen. Air-fluid levels and bone erosion are not features of uncomplicated allergic sinusitis, however, swollen mucosa in a poorly draining sinus is more susceptible to secondary bacterial infection.
Anosmia is a common symptom in patients with chronic allergic rhinitis. Membrane thickening and excess secretions in the olfactory recesses (superior nasal cavities) prevents air from reaching the olfactory mucosa.
The osteomeatal complex (OMC) is an important functional unit for drainage of the maxillary sinuses, and the OMC must be carefully evaluated in patients with chronic sinusitis. A number of developmental and acquired conditions can obstruct the OMC. Persistant membrane thickening from allergic rhinitis interferes with normal sinus drainage. Bullous enlargement of an adjacent ethmoid air cell or a Haller cell extending laterally below the orbit can encroach on the infundibulum. Functional endoscopic sinus surgery (FESS) selectively removes some of the bone and soft tissues to open up the OMC.
Chronic allergic sinusitis is often associated with nasal polyposis. Frequently, the polyps arise from the turbinates and bulge into the nasal cavities, where they may completely obstruct the nasal airways. A choanal polyp is a large inflammatory polyp that protrudes posteriorly into the nasopharynx. In more severe cases, involvement of the sinuses can occur, particularly the ethmoid and maxillary sinuses. The ethmoid sinuses may be filled with polypoid tissue, causing destruction of the delicate ethmoid septa and expansion of the ethmoid sinuses into the adjacent orbits.
CT relies on the high contrast between air, soft tissue and bone to evaluate the paranasal sinuses. Membrane, polyps, and mucous have similar density, but the polypoid appearance helps distinguish the inflammatory polyps. On T2-weighted MR images, the edematous membrane and mucous are distinctly hyperintense, whereas nasal polyps are of more intermediate signal intensity.