Pneumonia-A Matter of Degree?
Pneumonia-A Matter of Degree?
ABSTRACT & COMMENTARY
Synopsis: High-resolution computed tomography detected 44% more cases of pneumonia than did routine chest radiography.
Source: Syrjala H, et al. High-resolution computed tomography for the diagnosis of community-acquired pneumonia. Clin Infect Dis 1998;27:358-363.
Syrjala and colleagues in finland compared the results of routine chest radiography (CR) and high-resolution computerized tomography (HRCT) of the chest in 47 adults presenting with signs and symptoms suggestive of the presence of community-acquired pneumonia. Films were reviewed independently by two masked radiologists. A CT diagnosis of pneumonia required the presence of a "segmental, peribronchovascular or scattered ground-glass, or reticular opacity, or consolidation compatible with acute-phase lung involvement." A final diagnosis of pneumonia, however, required follow-up CR resolution of infiltrates or clinical recovery. The kappa values for agreement between the radiologists were 0.87 for CR and 0.79 for HRCT; initial disagreements were settled by consensus.
Nineteen patients were admitted to the hospital, and 28 were managed as outpatients. The patients ranged in age from 17-85 years, and 30 (64%) were male.
Eighteen (38%) of the patients had CR evidence of pneumonia; HRCT identified all 18 as well as an additional eight with pneumonia for a total of 226 (53%) (P = 0.004). HCRT identified pulmonary infiltrates not seen on CR in five patients who required hospitalization. Bilateral lung involvement was identified in only six patients by CR but in 16 by HRCT (P = 0.001). CR identified upper lobe, lower lobe, and lingular infiltrates less often than did HRCT.
COMMENT BY STAN DERESINSKI, MD, FACP
The major drawbacks of using HRCT in the diagnosis of pneumonia are cost and radiation exposure. Both could be reduced by extending the scan distance intervals beyond the 10 mm recommended for patients with interstitial lung disease, an adjustment which, based on the size of the lesions observed by Syrjala et al, is feasible in the detection of pneumonia. They found that no infiltrate was seen on only a single "slice." An optimally adjusted HCRT, according to Syrjala et al, results in a radiation dose equivalent to approximately five conventional CRs.
In the frequently distorted system of reimbursement for medical services in the United States, detection of pneumonia by HCRT not seen on CR may increase payment, especially for inpatients, well beyond the cost of the procedure. This, of course, is not a reason to perform the procedure, but it will certainly tempt some.
Routine CR cannot distinguish bacterial from "atypical" pneumonia. Unfortunately, it appears that this is also true for HCRT. For instance, pneumonia due to S. pneumoniae cannot be distinguished from that due to C. pneumoniae by CR.1 Another study of community-acquired pneumonia reported differences in HRCT appearance between 18 patients with bacterial and 14 with "atypical" pneumonia (12 due to Mycoplasma pneumoniae).2 However, the overlap in findings between the two was too great to make the test diagnostically useful.
On the other hand, HRCT may be of some use in the distinction between active and inactive pulmonary tuberculosis, with centrilobular densities in and around the small airways and "tree-in-bud" appearance being more characteristic of the former, along with macronodular (5-8 mm) lesions.3 Patients with inactive tuberculosis all had fibrotic lesions; distortion of bronchovascular structures, emphysema, and bronchiectasis were also frequent.
The study by Syrjala et al also demonstrates that the presence or absence of pneumonia may often not be a clear-cut distinction. Indeed, it may be more a matter of degree of involvement of lung parenchyma, than the simple presence or absence of pneumonia in a variety of respiratory tract infections. Perhaps with sensitive enough techniques, pulmonary parenchymal abnormalities will prove nearly universally present in respiratory tract infection.
References
1. Kauppinen MT, Lahde S, Syrjala H. Roentgenographic findings of pneumonia caused by Chlamydia pneumoniae. A comparison with Streptococcus pneumoniae. Arch Intern Med 1996;156:1851-1856.
2. Tanaka N, et al. High resolution CT findings in community-acquired pneumonia. J Comput Assist Tomogr 1996;20:600-608.
3. Hatipoglu ON, et al. High resolution computed tomographic findings in pulmonary tuberculosis. Thorax 1996;51:397-402.
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