ALLERGIC BRONCHOPULMONARY ASPERGILLOSIS.
Pulmonary aspergillosis is a spectrum of mycotic diseases caused by Aspergillus species, usually Aspergillus fumigatus. This intensely antigenic and ubiquitous soil fungus is commonly found in the sputum of healthy individuals. However, in susceptible hosts, its ability to invade the arteries and veins facilitates its hematogenous spread.
Pulmonary aspergillosis may take any of 4 forms:
- Allergic bronchopulmonary aspergillosis (ABPA) is caused by a hypersensitivity reaction to the fungus and most commonly occurs in those with asthma.
- Saprophytic aspergillosis, or aspergilloma, is the most common form. This form is noninvasive and involves colonization of preexisting cavities.
- Chronic necrotizing aspergillosis, also called airway-invasive or semi-invasive aspergillosis, is a chronic cavitary pneumonic illness that often affect patients with preexisting chronic lung disease.
- Angioinvasive aspergillosis affects immunocompromised patients and is often fatal.
Pathophysiology : ABPA represents a hypersensitivity reaction to A fumigatus in patients with long-standing asthma or cystic fibrosis. Excessive mucus production in association with impaired ciliary function leads to mucoid impaction of the airways. The plugs of inspissated mucus contain A fumigatus and eosinophils, but the organisms remain within the bronchial lumen; this feature differentiates ABPA from invasive aspergillosis.
The primary diagnostic criteria for ABPA include the following:
- Asthma, 84-96%
- Blood eosinophilia, 8-40%
- Elevated serum IgE levels
- Positive skin test results for A fumigatus
- Elevated serum levels of IgE and IgG specific for A fumigatus
- Presence of precipitating antibodies to A fumigatus
The primary radiologic criteria include fixed or transitory pulmonary infiltrates and central bronchiectasis as a late manifestation. A set of secondary criteria can sometimes be applied; these include the presence of A fumigatus mycelia in the sputum, the expectoration of brown sputum plugs, and a delayed cutaneous reaction to A fumigatus antigen.
ABPA can be staged by using the following clinical and radiologic criteria:
Stage I - Acute presentation with 6 of the 8 primary diagnostic criteria listed aboveStage II - Resolving pulmonary infiltrates with decreasing IgE levels leading to remissionStage III - Recurrence of acute symptoms after a period of remissionStage IV - Steroid dependency
- Stage V - Irreversible lung damage leading to fibrosis.
CT SCAN FINDINGS :
Bronchiectasis and peribronchial thickening are the most common CT findings in ABPA. Appearances tend to be more severe than in those of chronic uncomplicated asthma. ABPA typically involves the segmental and subsegmental bronchi, particularly those in the upper lobes. However, recent studies have shown that central bronchiectasis simply indicates long-standing severe inflammation; as a marker, it is not as specific for ABPA as was once thought.
High-attenuating mucoid impaction, present in as many as 30% of patients, is a characteristic finding. Occasionally, lobar or segmental atelectasis may be a feature. Mucus plugging of the small airways can be observed on high-resolution CT scans, with resultant centrilobular nodularity and the tree-in-bud sign. Abnormalities of lung attenuation due to either mosaic perfusion or air trapping may also be identified. Scans obtained during expiration are useful in differentiating the findings in this instance.
The CT and chest radiographic appearances of an aspergilloma are similar. The fungal ball is seen as a mass of soft-tissue attenuation within a pulmonary cavity. An anterior air crescent is visible if the patient is supine. The mobile nature of the mass can be demonstrated by scanning the patient in the prone position; the fungal ball falls to the dependent portion of the cavity. The cavity wall and adjacent pleura are frequently thickened, although these findings have been shown to resolve with successful treatment or with the spontaneous resolution of the infection.
CT findings in chronic necrotizing aspergillosis include areas of chronic progressive peripheral consolidation, multiple nodular opacities, and low-attenuating masslike lesions. Abnormalities may be unilateral or bilateral, with an upper-lobe predilection. Cavitation is a common feature, and this often leads to the development of an intracavitary segment of sequestrated lung, which may mimic a mycetoma. Extension into the chest wall and mediastinum are also described.
CT findings of angioinvasive aspergillosis include multiple nodules associated with a halo of ground-glass attenuation, which represents adjacent hemorrhage, and pleural-based wedge-shaped areas of consolidation, which correspond to hemorrhagic infarcts. The air-crescent sign may be observed in the recovery phase.