Radiographic findings:This radiograph is taken from another asthmatic patient at a time when he had symptoms. It does not show hyperinflation and shows minimal, if any, bronchial thickening. In the frontal view (A), note the paired, anterior segmental arteries and bronchi (in cross section). The lungs demonstrate bilateral, band-like densities with a flattened, triangular appearance. On both views, one angle of each triangle points toward its hilum (arrows on frontal view). This feature identifies the atelectasis as segmental and sub-segmental because the atelectasis originates from a segmental bronchus. This pattern differs from band-like, discoid (also known as plate-like or platter) atelectasis, which crosses segments and lobes, and is often oriented in a completely horizontal plane.

A

B

Radiographic findings: Although hyperinflation with a normal heart size is thought to be a usual finding in stable asthmatics, it is found in only 24% of cases. Many patients have normal or reduced volumes. Bronchial wall thickening, which is nonspecific, occurs in 50-70%. Complications of asthma include pneumothorax [1].

Indications for a chest radiograph: In an asthmatic, indications include fever, evidence of heart disease or other lung disease, history of illicit drug use, seizures, or immunosuppression. A radiograph in these situations helps to diagnose pneumonia or heart failure.

Indications for CT: CT is done to detect bronchiectasis in suspected ABPA, to diagnose emphysema in smokers, and to identify entities that may be mistaken for asthma, like hypersensitivity pneumonia. Scans may show bronchial wall thickening, mucoid impaction, decreased lung attenuation due to air trapping or hypoxic oligemia, and centrilobular thickening (caused by bronchiolar inflammation). CT scans of nasal sinuses may also be abnormal in asthmatic patients [1].

CT in allergic vs nonallergic asthma: A CT study of 70 persons with allergic (median age 30 y) and 56 persons with nonallergic asthma (median age 54 y), all nonsmokers, indicated that those with nonallergic asthma had more bronchiectasis, bronchial wall thickening, emphysema, and linear shadows than those with allergic asthma. They also found that the abnormalities on CT increased with severity and duration of asthma in both groups [2].

Correlations of CT with function: CT studies of stable asthmatic patients have also addressed correlations with pulmonary function tests. One study found that CT was of limited value in distinguishing mild asthmatics from normal subjects, but in asthmatics with FEV1 of less than 60% predicted, the ratio of bronchial lumen diameter to arterial diameter (mean 0.48) on inspiratory scan was significantly lower than in the normal controls [3].

Diagnosis of air trapping: A method to measure air trapping on end expiratory CT scans showed significantly more air trapping in 18 nonsmoking asthmatics than in controls. The percentage of pixels (pixel index) of low attenuation ( -900 Hounsfeld units) was measured at end expiration on images made just above the diaphragm. The mean pixel index was 4.45 in asthmatics and 0.16 in control subjects. Further, the pixel index showed significant correlations with RV, FRC, TLC, and FEV1 [4].

Differential diagnosis: Angina; cardiac asthma related to heart problems or hypertension; acute glomerulonephritis with hypertension and heart failure; vocal cord dysfunction; airway obstruction by tumor, Wegener's granulomatosis, amyloidosis, sarcoidosis, tracheobronchopathia osteochondroplastica, polychondritis, fungal tracheobronchitis, or foreign body; bronchiolitis; and COPD may mimic bronchial asthma [1,5].

References

1. Lynch D. Imaging of asthma and allergic bronchopulmonary mycosis. Radiol Clin N Am 1998; 36:129-142.

2. Paganin F, Séneterre E, Chanez P, Daurés J, Bruel J, Michel F, Bousquet J. Computed tomography of the lungs in asthma: influence of disease severity and etiology. Am J Respir Crit Care Med 1996; 153:110-114.

3. Park C, Müller N, Worthy S, Kim J, Awadh N, Fitzgerald M. Airway obstruction in asthmatic and healthy individuals: inspiratory and expiratory thin-section CT findings. Radiology 1997; 203:361-367.

4. Newman K, Lynch D, Newman L, Ellegood D, Newell Jr J. Quantitative computed tomography detects air trapping due to asthma. Chest 1994; 106:105-109.

5. Woolcock A. Asthma. Textbook of Respiratory Medicine, 2nd ed. J Murray & J Nadel (eds) WB Saunders, Philadelphia,1994, 1288-1330.

Image 2

Table of Contents