Asthma

Typical findings in an asthmatic chest radiograph include hyperexpanded lungs evident best with flattened diaphragms and narrowed cardiac shadow and mediastinum. On a lateral projection the chest may appear "barrel" shaped.

The radiograph may be normal or show hyperinflation. Areas of segmental and subsegmental atelectasis may occur due to mucus plugging or smooth muscle hypertrophy. Central bronchial wall thickening is common. Radiographs are helpful to exclude an acute abnormality such as pneumonia or pneumothorax.

Preferred Examination: CXR remains the initial imaging evaluation in most individuals with symptoms of asthma. The value of CXR is in revealing complications or alternative causes of wheezing and the minor importance of wheezing in the diagnosis of asthma and its exacerbations. CXR usually is more useful in the initial diagnosis of bronchial asthma than in the detection of exacerbations, although it is valuable in excluding complications such as pneumonia and asthma mimics, even during exacerbations.
High-resolution CT (HRCT) is a second-line examination. It is useful in patients with chronic or recurring symptoms and in those with possible complications such as allergic bronchopulmonary aspergillosis and bronchiectasis.
Limitations of Techniques: CXR is limited by frequent and sometimes subjective findings that are not specific for asthma; these nonspecific findings include airway thickening and hyperinflation. Nevertheless, in the appropriate clinical setting, CXR findings can support the diagnosis of asthma.
HRCT is more costly than CXR and exposes the patient to more radiation. Nevertheless, CT scans can demonstrate a number of findings that support the diagnosis of asthma; examples of such findings include bronchial wall thickening, bronchiectasis, mucoid impaction, and airtrapping, among others.

 

Pathophysiology:
Signs of asthma

Primarily, asthma is manifested by a sudden or prolonged onset of airway narrowing, which accounts for the varying degrees of airway obstruction and accompanying sensation of an inability to breathe in and, more importantly, to breathe out; these symptoms herald hyperinflation. The total lung capacity (TLC), functional residual capacity (FRC), and residual volume (RV) increase.

The hallmark of airway obstruction is a reduction in ratio of the forced expiratory volume in 1 second (FEV1) and the FEV1 to the forced vital capacity (FVC). However, the earliest effects of airway disease are believed to occur in the small airways (<2 mm in diameter), and they are more difficult to measure reliably with standard pulmonary function tests (PFTs), such as tests of the following: forced expiratory flow after 50% of vital capacity has been expelled (FEF50), forced expiratory flow after 25-75% of vital capacity has been expelled (FEF25-FEF75) and maximum midexpiratory flow rate (MMEFR). Less common evaluations include tests of airway resistance (RAW) and single-breath carbon monoxide diffusion capacity (DLCO/VA). With the former, resistance is markedly increased, although the findings are highly variable; with the latter, results are normal or slightly elevated in uncomplicated asthma (Collard, 1994; Boulet, 1995).

Whether the more severe and permanent features of distal airway obstruction and coexisting emphysema supervene in the later and more complicated stages of asthma is controversial. However, in one study, patients with partially reversible airway obstruction that persisted after optimal corticosteroid treatment had a normal diffusion capacity that was comparable to that of patients with completely reversible asthma (Hudon, 1997). The functional abnormalities reflect airway narrowing that results from multiple causes, including bronchial smooth muscle contraction; mucous plugging from mucous gland hypersecretion; submucosal, peribronchial, and interstitial edema from loss of capillary and arteriolar cellular interconnections; and cellular infiltrative changes involving plasma cells, lymphocytes, macrophages, and leukocytes.

Chronic or incompletely reversible asthma is characterized by variable obstruction; a more fixed degree of airway narrowing may also be present in some patients. With repeated episodes of clinically important airway narrowing, generalized thickening of the airways occurs as a result of smooth muscle hyperplasia, postinflammatory thickening of the bronchial basement membranes, and mucous gland hypertrophy.

Complications
Complications of asthma often are more apparent than the direct airway imaging findings that are the sequelae of asthma.

Spontaneous pneumothorax is an uncommon but well-recognized phenomenon.

The chest radiograph (CXR) is an important tool in the examination of patients with an exacerbation of asthma, but patients should not be left waiting in the treatment room for CXR before treatment (Swain, 1984).

Pneumothorax may be evident radiographically before it is identified clinically (Gay, 1978).

Pneumothorax often occurs during recurrent episodes of bronchospasm, as well as in other conditions. The presence of an air-fluid level in a hydropneumothorax can be confused with pneumatocele, infected cysts, and cavitary lung disease.

An unusual condition of diffuse pulmonary ossification associated with pneumothorax is described in a patient with bronchial asthma (Ikeda, 1998).

Pneumomediastinum can occur, particularly in young adults (see Images 1-2). The pathophysiologic nature of the process, as previously determined from animal studies, is reported (Jamadar, 1996). Using perfluorocarbon liquid ventilation in a hypoxic patient with status asthmaticus and tension pneumothorax, the authors traced the heavily radiopaque substance through ruptured alveoli, the interlobular spaces, and the axial interstitium to the mediastinum. The liquid agent remained in the interstitial space for 30 days without complications. Pneumomediastinum generally is self-limited, and it requires no additional therapeutic measures beyond those for asthma exacerbation (Ba-Ssalamah, 1999).

Emphysema can occur in the subdural space, especially in children. Air moves posteriorly from a pneumomediastinum into the intervertebral foramina and further into the most nondependent potential spaces adjacent to the brain coverings (Caramella, 1997). Spinal epidural emphysema is also described in asthmatics, tracking along the great vessels bound by the mediastinal pleural layers (Tsuji, 1989). As reported, these events typically have no neurologic sequelae.

Pneumopericardium is uncommon and more likely to occur in younger persons because their pericardial layers are more loosely apposed than those in adults (van der Klooster, 1998). As with pneumomediastinum, pneumopericardium generally has a benign course.

Pneumothorax, pneumomediastinum, pneumoretroperitoneum, pneumorrhachis (ie, spinal epidural air), and extensive subcutaneous emphysema are more likely to occur with episodes of coughing or increased intrathoracic pressure that accompany the recruitment of expiratory muscles when normal airflow is substantially reduced (van der Klooster, 1998).

More serious is subarachnoid hemorrhage, which developed in a patient with status asthmaticus who received ventilation with permissive hypercapnia. This technique minimizes the overall delivered minute ventilation and airway pressures to maintain adequate oxygenation. However, because of elevated CO2 partial pressures, it also led to cerebral vasodilatation and increased intracranial pressure; when combined with coughing spells and other forms of transmitted intrathoracic pressure, the ventilation was believed to promote cerebral edema and limit cerebral venous drainage (Rodrigo, 1999).

Noisy breathing may mask potential physical findings in pneumothorax, pneumomediastinum, and pneumonia; therefore, CXR should be performed prior to the use of mechanical ventilation.
 

Taken From http://www.bbc.co.uk/health/asthma/