Acute infectious sinusitis usually results from extension of organisms from the nasal cavity as a complication of an upper respiratory tract infection. Occasionally, infectious agents enter the maxillary antrum from infected dental roots. In the majority of these infections, pathogens are gram-positive bacteria. The maxillary sinus is most commonly involved, followed in frequency by the frontal, ethmoid, and sphenoid sinuses. Sinus infection results in swelling of the mucous membranes and copious mucoid secretions. In a poorly draining sinus, these secretions are good culture media and can rapidly become purulent.
Acute sinusitis has radiographic features that distinguish it from allergic sinusitis. In acute sinusitis, the thickened mucosal margin is smooth rather than scalloped and follows the contour of the sinus walls. Air-fluid levels are the hallmark of infectious sinusitis. Infections are often confined to a single sinus but may also involve the ipsilateral maxillary, frontal, and ethmoid sinuses owing to some obstructive process in the middle meatus. A pansinusitis usually represents an infectious complication of an allergic sinusitis. Polyp formation is not a feature of infectious sinusitis. In general, bone changes are not seen in acute sinusitis. The delicate ethmoid septa, however, may become indistinct owing to demineralization caused by hyperemia.
Chronic sinusitis is usually of infectious origin and is due either to multiple recurrent infections or to a single prolonged low-grade infection. Long-standing allergic sinusitis can also result in identical radiographic changes. On CT scan the mucosa becomes thickened and appears very dense; the bony walls become sclerotic and thickened and eventually may encroach upon the cavity of the sinus. Chronic sinusitis during childhood may result in hypoplasia of the involved sinus.
Retention cysts are associated with sinusitis, usually becoming apparent after the sinusitis has resolved. These lesions are lined with epithelium and result from obstruction of a mucous gland. Retention cysts occur most commonly in the maxillary sinus and are usually asymptomatic. On both CT and plain films, retention cysts are smooth and broad-based and appear as dome-shaped densities, most often arising from the floor of the maxillary sinus antrum.
When acute infectious sinusitis is properly diagnosed and treated, it resolves without complication. When infection persists, it may extend to adjacent compartments of the face. Ethmoid sinusitis is a frequent cause of orbital cellulitis. An infection may enter the orbit directly through either foramina, osseous dehiscences, or thin portions of the lamina papyracea. It may also push the periorbita laterally and thereby bulge into the orbit, forming a subperiosteal abscess. Infectious involvement of orbital structures results in lid swelling, proptosis, and scleral thickening with enhancement on CT scans. Occasionally, orbital emphysema is also present. Edema and thickening of the extraocular muscles are common. The intraconal extension of an infectious process can result in retrobulbar neuritis and swelling of the optic nerve. The superior orbital fissure syndrome sometimes occurs when such lesions involve the orbital apex. Usually the sixth cranial nerve is the first to be affected, followed by the third and fourth cranial nerves.