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Thoracic Imaging |
1 From the Department of Radiology, University of British Columbia, Vancouver Hospital and Health Sciences Centre, 855 W 12th Ave, Vancouver, British Columbia, Canada V5Z 1M9 (T.J., N.L.M.); the Department of Respiratory Medicine, Tosei General Hospital, Aichi, Japan (H.T., Y.K.); the Department of Radiology, National Kinki Chuo Hospital for Chest Disease, Osaka, Japan (M. Akira); the First Department of Internal Medicine, Kumamoto University School of Medicine, Japan (K.I., M. Ando); and the Department of Radiology, Osaka University Medical School, Japan (O.H., N.T., H.N.). Received June 29, 1998; revision requested August 13; revision received September 4; accepted November 20. Address reprint requests to N.L.M.
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Abstract |
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MATERIALS AND METHODS: The study included 36 patients (20 men, 16 women; age range, 22–83 years; mean age, 61 years) with histopathologically proved acute interstitial pneumonia who were identified retrospectively. The time between symptom onset and CT was 2–90 days (mean, 22 days; median, 17 days). The presence, extent, and distribution of various CT findings were evaluated. Disease duration and extent of each finding were compared by using the Spearman rank correlation coefficient.
RESULTS: Areas with ground-glass attenuation, traction bronchiectasis, and architectural distortion were present in all 36 patients. Airspace consolidation was present in 33 patients (92%). The extent of areas of ground-glass attenuation (r = 0.45, P < .01) and the extent of traction bronchiectasis (r = 0.35, P < .05) correlated with disease duration. No other significant correlation was found between the CT findings and disease duration.
CONCLUSION: A combination of ground-glass attenuation, airspace consolidation, traction bronchiectasis, and architectural distortion is seen in the majority of patients with acute interstitial pneumonia. The extent of ground-glass attenuation and traction bronchiectasis increases with disease duration.
Index terms: Bronchiectasis, 60.26 • Lung, acute interstitial pneumonia, 60.213 • Lung, CT, 60.12111, 60.12118 • Lung, interstitial disease, 60.213
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Introduction |
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The histologic hallmark of acute interstitial pneumonia consists of diffuse alveolar damage (1,3–7). Diffuse alveolar damage manifests as injury to the alveolar lining and endothelial cells, pulmonary edema, hyaline membrane formation, and, later, proliferative changes involving alveolar- and bronchiolar-lining cells, as well as interstitial cells. The histopathologic appearance of diffuse alveolar damage is temporal and can be conveniently divided into three interrelated and overlapping phases that correlate with the clinical evolution of the disease: acute exudative phase, subacute proliferative phase, and chronic fibrotic phase (4,5,8).
Description of the computed tomographic (CT) findings of acute interstitial pneumonia has been limited to a small number of cases (9–11). The abnormalities consist of bilateral areas of ground-glass attenuation, airspace consolidation, traction bronchiectasis, and, occasionally, focal areas of honeycombing.
The aims of this study were to characterize the CT findings in a large number of patients with acute interstitial pneumonia and to correlate the extent of the various CT findings with the time between the onset of symptoms and the date of CT.
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MATERIALS AND METHODS |
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TOPAbstractIntroductionMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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All patients underwent thin-section CT of the chest. The CT scans consisted of 1.0–1.5-mm-collimation sections reconstructed by using a high-spatial-frequency algorithm. The protocols consisted of thin sections obtained at 1-cm (28 patients) or 2-cm (seven patients) intervals and of five thin sections that complemented a complete set of conventional CT scans of the chest (one patient). The CT scans were obtained with a variety of scanners. None of the patients received an intravenous administration of contrast medium.
The CT images were reviewed by two observers (T.J., N.L.M.) who looked at all of the images together, and final decisions on the findings were reached by consensus. The lungs were divided into three zones (upper, middle, and lower); each zone was evaluated separately. Each of the three zones corresponded to approximately one-third of the images from the lung apex to 1 cm below the domes of the diaphragm.
The observers assessed the presence, extent, and distribution of areas of ground-glass attenuation, airspace consolidation, nodulular opacities, intralobular reticular opacities, traction bronchiectasis, interlobular septal thickening, and thickening of bronchovascular bundles. The presence, extent, and distribution of associated findings, such as honeycombing, emphysema, architectural distortion, lymph node enlargement, and pleural effusion, were also assessed.
Areas of ground-glass attenuation were defined as areas of hazy increased attenuation without obscuration of the underlying vascular markings. Airspace consolidation was considered present when the opacities obscured the underlying vessels. Traction bronchiectasis was defined as irregular bronchial dilatation within areas with parenchymal abnormality. Architectural distortion was considered present when interlobar fissures and hila were displaced or when secondary pulmonary lobules, bronchi, or vessels were distorted. Lymphadenopathy was considered present when the short-axis diameter of the nodes was greater than 1 cm.
The anatomic distribution was noted to be central if there was a predominance of abnormalities in the inner third of the lung, peripheral if there was a predominance of abnormalities in the outer third of the lung, dorsal if there was predilection in the dependent portion, and random if there was no predominance. Zonal predominance was assessed as being upper, lower, or random. Upper lung zonal predominance was considered present when most of the abnormalities were above the level of the tracheal carina, and lower lung zonal predominance was considered present when most of the abnormalities were below this level.
The extent of involvement of each abnormality was assessed independently for each of the three zones of each lung. The CT score in the upper, middle, and lower lung zones was determined by visually estimating the extent of disease in each zone. The score was based on the percentage of lung parenchyma that showed evidence of abnormality and was estimated to the nearest 5% of parenchymal involvement. The overall percentage of involvement was calculated by averaging the scores from each of the six lung zones. The extent of traction bronchiectasis was evaluated by counting the number of segments or subsegments that showed traction bronchiectasis.
The following 18 segments or subsegments were evaluated: right apical upper, right anterior upper, right posterior upper, right lateral middle, right medial middle, right superior lower, right medial basal, right anterior basal, right lateral basal, right posterior basal, left apicoposterior upper, left anterior upper, left superior lingular, left inferior lingular, left superior lower, left anteromedial basal, left lateral basal, and left posterior basal.
The extent of traction bronchiectasis was also quantified by assessing the generations of bronchial divisions involved. Traction bronchiectasis was scored as follows: 0 for no bronchial dilatation; 1 for dilatation limited to the bronchi distal to the sixth-generation bronchi, with the main bronchi being considered the first generation; 2 for bronchial dilatation involving the fifth-generation bronchi; 3 for bronchial dilatation involving the fourth-generation bronchi; 4 for bronchial dilatation involving the third-generation bronchi; and 5 for bronchial dilatation involving the bronchi proximal to the second-generation bronchi.
The date of disease onset was defined as the date of onset of respiratory symptoms and was obtained from a retrospective review of the patients' records. The mean disease duration was 22 days (range, 2–90 days; median, 17 days). Patients were assigned to the following three groups according to the duration of their disease: A, less than 1 week; B, 1–3 weeks; and C, more than 3 weeks.
Disease duration, extent of each finding, and generations of bronchi showing traction bronchiectasis were compared by using the Spearman rank correlation coefficient (12). The differences in the extent of each abnormality in each subgroup were evaluated by using the Kruskal-Wallis test, and the difference in prevalence was analyzed by using the 
2 test (12). Follow-up CT scans were obtained in five patients 16 days to 18 months (median, 53 days) after the initial scans were available for review.
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RESULTS |
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TOPAbstractIntroductionMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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Less common CT findings included pleural effusion (n = 11 [31%]), emphysema (n = 9 [25%]), honeycombing (n = 5 [14%]), and lymphadenopathy (n = 3 [8%]). Lower lung zonal predominance was found in 14 patients (39%) and upper lung zonal predominance in five (14%). Dependent distribution was found in nine patients (25%). Peripheral distribution was seen in three patients (8%), and central distribution was seen in two patients (6%).
The correlations between disease duration and extent of each CT finding are summarized in Table 1. Extent of areas with ground-glass attenuation (r = 0.45, P = .008), numbers of segments showing traction bronchiectasis (r = 0.361, P = .033), and generations of bronchial divisions involved in traction bronchiectasis (r = 0.347, P = .04) correlated significantly with disease duration. No other correlation between CT findings and the interval between the onset of symptoms and the date of CT was found.
The extent and prevalence of each abnormality found at CT in each subgroup are summarized in Table 2. Except for the absence of honeycombing in the 1st week, there was no significant difference in the extent and prevalence of each abnormality between the three groups (A, <1 week; B, 1–3 weeks; and C, >3 weeks between the onset of symptoms and the date of CT).
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DISCUSSION |
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TOPAbstractIntroductionMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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In patients with acute interstitial pneumonia who have prolonged disease, typically for more than 1 month, dilated, restructured airspaces may develop, resembling the honeycomb changes seen in usual interstitial pneumonia (6). The histologic features of fibrosis in acute interstitial pneumonia, however, are distinct from those in usual interstitial pneumonia (1,6). The fibrosis in acute interstitial pneumonia is characterized histologically by numerous fibroblasts but relatively little collagen deposition and a more or less uniform appearance from field to field at low-power magnification. The fibrosis in usual interstitial pneumonia, on the other hand, is characterized by the presence of relatively few fibroblasts but extensive collagen deposition. Histologically, usual interstitial pneumonia typically has a heterogeneous appearance with focal areas of normal lung, chronic inflammation, fibrosis, and honeycombing (1,6). The honeycomb cysts in acute interstitial pneumonia are lined with an alveolar epithelium, while those in usual interstitial pneumonia are lined with a bronchiolar epithelium (6).
In the current study, all patients had areas of ground-glass attenuation, and the extent of areas of ground-glass attenuation was correlated with the duration of disease. Ground-glass attenuation at thin-section CT usually indicates the presence of potentially treatable or reversible disease (13,14). In acute interstitial pneumonia, however, areas of ground-glass attenuation are seen in all three histologic phases and therefore reflect different histologic findings (10,11). In the exudative phase, ground-glass attenuation reflects the presence of alveolar septal edema and hyaline membranes along the alveolar walls, while in the proliferative phase, ground-glass attenuation is due to intraalveolar and interstitial organization. In the fibrotic phase, ground-glass attenuation results from alveolar septal fibrosis. Consequently, in acute interstitial pneumonia, ground-glass attenuation is not helpful in predicting the underlying histology.
In the present study, the number of segments that demonstrated traction bronchiectasis and the number of generations of bronchial divisions involved by traction bronchiectasis correlated significantly with disease duration. Remy-Jardin et al (15), in a study of interstitial fibrosis in systemic sclerosis, demonstrated that airway dilatation within areas of ground-glass attenuation is an indirect sign of lung fibrosis. Furthermore, it has been suggested that the extent of traction bronchiectasis may reflect the severity of fibrosis (16). Ichikado et al (11), in a study in which they correlated CT with histologic findings of acute interstitial pneumonia, showed that traction bronchiectasis is seen in the proliferative and fibrotic stages of the disease.
Recently, Howling et al (17) also reported that dilatation of the airways within areas of ground-glass attenuation was a frequent observation in the acute phase of adult respiratory distress syndrome that tended to persist at follow-up, usually accompanied by the CT finding of supervening pulmonary fibrosis. Consequently, it seems reasonable to conclude that traction bronchiectasis indicates progression of disease in acute interstitial pneumonia. In contrast, honeycombing is relatively uncommon in patients with acute interstitial pneumonia. Honeycombing was not seen in the 1st week after the onset of symptoms and, with the exception of one patient, when present thereafter, acute interstitial pneumonia involved only a small portion of the lungs.
In one patient, a follow-up CT scan obtained 53 days after the initial scan, when the patient was still receiving assisted ventilation, showed progression of disease with the development of extensive reticular opacities, traction bronchiectasis, and honeycombing. In the four other patients in whom follow-up CT scans were obtained 16 days to 18 months after the initial scan, there was considerable improvement in the areas of ground-glass attenuation and consolidation; all four patients had mild to moderately extensive reticular opacities consistent with residual fibrosis.
In the present study, although airspace consolidation was seen in 33 patients (92%), its extent did not correlate with disease duration. Intraalveolar edema and hemorrhage in the exudative phase, intraluminal organization in the proliferative phase, and intraalveolar fibrosis in the fibrotic phase can result in airspace consolidation at thin-section CT (10,11). Therefore, airspace consolidation is not helpful in predicting the underlying histopathologic findings.
Our study has several limitations. It did not include patients with 48 hours or less between the onset of symptoms and CT scanning (early exudative phase). Moreover, it was very difficult to determine the onset of disease in each patient, even though clinical records were carefully evaluated. In conclusion, a combination of ground-glass attenuation, airspace consolidation, traction bronchiectasis, and architectural distortion is seen in the majority of patients with acute interstitial pneumonia. To evaluate the progress of fibrosis in acute interstitial pneumonia, it is essential to assess the extent of areas of ground-glass attenuation and traction bronchiectasis. These areas increase in extent with increased duration of disease. Honeycombing is not seen during the 1st week after the onset of symptoms.
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Footnotes |
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References |
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