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Chest Imaging

Asthma

disease characterized by bronchial hyperreactivity with variable degrees of airway obstruction. Its incidence is increasing worldwide and now affects more than 5% of the population in the United States with a 50% increase in prevalence in the past decade. Asthma is now considered primarily an inflammatory disease characterized pathologically by bronchial goblet cell hyperplasia with infiltration of mast cells and eosinophils in the respiratory epithelium and vascular permeability changes. In severe disease, airway narrowing is due to mucus stasis and plugging and mucus gland and smooth muscle hypertrophy. Between asthmatic attacks, the above changes may be muted or lacking. Some patients with chronic asthma have bronchiolar and peribronchiolar scarring as well as bronchiectasis.

The clinical diagnosis of asthma is most commonly made by documenting physiological airway obstruction that responds to bronchodilators. In more occult cases, asthma may be diagnosed by the presence of airway hyperreactivity to an inhaled substance, most commonly metacholine. Although many asthmatics have no airway obstruction much of the time, a significant subgroup has chronic, partly reversible or irreversible obstruction that may be indistinguishable from that seen in chronic obstructive pulmonary disease (COPD).

On the chest radiographs of patients with asthma, bronchial wall thickening is the most common abnormality. Overinflation may also be identified radiographically but it is rarely marked in asthmatic patients who do not also have emphysema. Indeed many patients with asthma have normal or reduced lung volume even during acute exacerbation of their condition. The utility of chest radiographs in patients with acute exacerbation of asthma remains controversial. In patients with chronic asthma, a chest radiograph is usually indicated when the symptoms are refractory to conventional treatment.

The clinical indications for performing CT scans in patients with asthma include detecting bronchiectasis in patients with suspected allergic bronchopulmonary aspergillosis, documenting the presence and extent of emphysema in smokers with asthma, and identifying conditions that may be confused with asthma, such as hypersensitivity pneumonitis. In uncomplicated asthma, high resolution computed tomography (HRCT) scans may show bronchial dilatation, bronchial wall thickening,mucoid impaction, decreased lung attenuation, air trapping and small centrilobular nodules. Expiratory CT can show air trapping even in patients who have normal inspiratory scans (Fig.1). These abnormalities may or may not be reversible with treatment.

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Fig.1

Expiratory HRCT scan in an asthmatic patient showing patchy areas of air trapping. The inspiratory HRCT scan was normal, except for bronchial wall thickening.
Asthma, Fig.1