Radiologists play a central role in detecting or excluding pneumonia in the community setting. They can also help clinicians narrow the etiologic differential diagnosis by means of pattern recognition and integration of epidemiologic, clinical, laboratory, and radiographic information, according to a leading European chest radiologist.
"In all existing definitions of pneumonia, the presence of a pulmonary abnormality compatible with pneumonia is an indispensable prerequisite," said Prof. Dr. Christian Herold, a professor of radiology at the University of Vienna. "Although some variation exists regarding the time frame between the onset of clinical symptoms and the development of a radiographically visible abnormality, the vast majority of infiltrates appear within 12 hours."
Caution is essential in cases of nosocomial pulmonary infections, which patients develop while hospitalized, Herold said at the Asian Oceanian Congress of Radiology in May. These patients may be seen in radiology departments within hours of the onset of clinical symptoms, but an abnormality may not be visible. The radiographic appearance of a pneumonic infiltrate can be delayed in neutropenic patients and in those with functional defects of granulocytes due to diabetes, alcoholism, or uremia.
Preexisting or concomitant pulmonary disorders and processes can hamper diagnosis of pneumonia, he said. Conditions that have a radiological appearance similar to pneumonia, such as atelectasis, edema, aspiration, hemorrhage, infarct, idiopathic interstitial pneumonia, pulmonary involvement in collagen-vascular disorders, and pleural effusion, may also interfere with diagnosis. These abnormalities sometimes mimic pneumonia or alter the radiographic appearance of an infiltrate.
Pattern recognition is based on categorization of the radiographic features of pneumonia into different morphologic categories and the correlation of these patterns with histopathologic changes caused by microbial agents. This process allows identification of different groups of potential underlying organisms.
In follow-up examinations, it is vital to consider the time period in which a pneumonic infiltrate may resolve.
"Most pneumonias regress within 10 to 21 days. After three months, two-thirds of patients with community-acquired pneumonia show clear lungs." Herold said. "In the rest, complete resolution may take up to six months, especially in patients with underlying lung disease (e.g., chronic obstructive disease), in immunocompromised individuals, and in the elderly."
Radiologists must differentiate patients who steadily improve from those who do not respond adequately to treatment. Some patients are not treated effectively, and a single drug strategy may not be sufficient for typical or atypical bacteria. The condition may represent noninfectious inflammation, he said.
CT is the method of choice to evaluate patients with recurrent pulmonary infiltrates, according to Herold. These recurrent infections usually indicate an underlying problem such as congenital or acquired immunologic disorders, cardiac abnormalities (congestive heart failure), or systemic conditions such as diabetes, alcoholism, and intravenous drug use. If they occur in the same anatomical location, the infiltrates may result from underlying structural defects, including chronic bronchitis, bronchiectasis, large cavities, and bronchogenic carcinoma. In these cases, CT is indicated to identify or exclude an underlying disorder and to plan further therapeutic measures.