CT shows the bony anatomy very well; this provides a map for endoscopic sinus surgery. However, the mu-cosal changes that it can show rarely tell us about the pathology in the sinuses (Goldwyn et al., 1995; Cousin et al., 2000).
Fig. 7.1a-c The plain radiograph (a) does not show that the frontal sinus that is diseased drains into the left frontonasal in-fundibulum, as is shown on CT (b and c).
It is easy to fill out a CT request form, but this may be a mistake—although CT has good sensitivity for diagnosing paranasal sinus disease, it has poor specificity; for example, there are many false-positive changes. Before you request a CT scan, think why you are doing it and what information it will provide. Without a clinical picture based on the history, examination, and a trial of medical treatment, it is impossible to give any specific meaning to a finding of "mucosal changes" on a CT scan (Jianetto and Pratt, 1995).
Unfortunately, one in three asymptomatic people have an abnormal CT scan, which makes CT a poor diagnostic tool for rhinosinusitis. It is, therefore, important not to request a CT scan in the initial management of patients unless there are specific reasons to do so. These include:
• Suspected intracranial or intraorbital involvement as a complication of rhinosinusitis
• Suspected atypical infection or malignancy
• Specific pathology, e.g., mucoceles, benign tumors of the paranasal sinuses, where the extent of the lesion needs to be defined
• Prior to orbital or optic nerve decompression
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