The air bronchogram refers to the presence of fluid or soft tissues in the bronchial area, specifically in the area in which it should be filled with air, making the bronchus more visible. They can be seen through a chest X-ray.
The rationale behind this finding is that the presence of fluid in the airway occurs because the cells that line the surface of this airway begin to secrete mucin, water and other proteins that have the function of stopping the progression of the microorganism towards the end of the airway. airway and eventually blood vessels.
Being in the presence of water, this liquid generates an air-water dissociation that significantly highlights the silhouette of the airway when seen on an X-ray. This is why this sign is highly suggestive of pneumonia; however, it is not entirely specific.
What the air bronchogram indicates is the presence of a foreign body in the airway; in the case of pneumonia, it is a bacteria. However, it could refer to any body, since it would generate the same reaction in the cells of the airway in order to prevent said object from migrating to the alveolus.
This is why the air bronchogram is generally associated with alveolar lesions, such as an inflammatory process, infarction, contusion, alveolar cell carcinoma or lymphoma. However, there will be other signs that will also guide the diagnosis towards any of these possibilities.
Generally, when there is a body in the airway that obstructs the flow of air and secretions, the latter accumulate in the distal portion of the airway, eventually dilating it in an abnormal and irreversible way, since their accumulation favors bacterial growth that destroys the cartilage and retractile elastic tissues that surround the airway.
This process is called bronchiectasis, and together with the air bronchogram, it forms a radiological pair highly suggestive of lung cancer of bronchial origin, one of the most frequent and lethal subtypes of this cancer, highly associated with cigarette smoke.
Symptoms
As we have already mentioned, the air bronchogram represents the hyperresponsiveness of the airway manifested as an increase in secretions in it; therefore, it tends to manifest with coughing, shortness of breath, and mucous sputum emission during coughing spells.
Radiography is still far from being replaced, as it is a quick, simple and inexpensive tool to make an initial anatomoclinical evaluation of the patient, especially the chest radiograph when evaluating the heart, lungs and other thoracic structures in the patient.
The lungs and airway are some of the regions that can best be evaluated in a simple postero-anterior radiograph, since this method allows to identify a wide range of radiological signs that guide in a highly sensitive way and specify the clinical diagnosis on many occasions.
The scientific basis of radiography lies in the density of the evaluated tissues, since this will determine how much radiation (X-rays in this case) the tissue will capture, and in this way it can be interpreted properly.
Densities
For example, we know that the density of water is greater than that of air. The heart is a tissue made up mostly of highly vascularized muscle (that is, with water), and also contains blood, so it is to be expected that this tissue is the one that captures the most X-rays in the study and, therefore, is look whiter (radio-opaque).
The terminations of the pulmonary airway where it occurs in gas exchange, also known as alveoli, are located in the region of the thorax, where there is more air; therefore, it is the area that captures the least X-rays and is observed almost completely black (radio-lucid).
Any pathological process that occurs within the thorax will change these characteristics. Depending on the changes, the doctor can orient his diagnosis towards the most likely disease process.
Among the most common diseases observed in the hospital environment are lower respiratory infections (or pneumonia) and, from the radiological point of view, these exhibit a very pathognomonic characteristic: the air bronchogram.
Causes
Apart from infectious processes, the air bronchogram can be caused by different reasons:
- Accumulation of water in the capillary bed of the lungs, due to heart failure or capillary injury (pulmonary edema).
- Collapse of the alveoli (atelectasis).
- Growth of a tumor within the airway (neoplasia).
- Pulmonary hemorrhage.
- Pulmonary infarction.
- Consolidated intrapulmonary, also of infectious origin but by bacteria or fungi -which go unnoticed through the airway as they are located in the alveoli-, which cause an organism response forming abscesses and intrapulmonary cavities that obstruct the airway.
Intrapulmonary consolidation could cause an air bronchogram, but in a very different way from the mechanism discussed in the case of typical bacterial pneumonias.
Treatments
Since most air bronchograms are of infectious origin, the initial management is usually with antibiotics; however, in the case of adult patients over 50 years of age, and in cases where antibiotics do not resolve the findings, the presence of a tumor should be ruled out with additional studies.
A simple chest tomography allows to evaluate in greater detail different planes of the lung and to verify or rule out the presence of a tumor or some anatomical abnormality.
A medical history of the patient with cigarette smoking or occupational exposure to toxic gases is necessary information that must be obtained to guide the diagnosis towards a possible tumor.
Regarding antibiotics, depending on the age of the patient and the symptoms, different causative agents of underlying pneumonia can be considered. One of the most common organisms is pneumococcus (Streptococcus pneumoniae), and it is treated with third-generation cephalosporins such as ceftriaxone or cefixime.
In young adults and children it is common to find pneumonias caused by intracellular bacteria, such as Mycoplasma pneumoniae and Chlamydia trachomatis. These pneumonias produce more indolent symptoms, and are treated with macrolides such as erythromycin and azithromycin.
References
- Aerial Brocograms: What are they? Pathologies, mechanisms and general characteristics, nd, Arriba Salud: arribasalud.com
- Air bronchogram - Definition, sf, CCM: salud.ccm.net
- Air bronchograms - CT, sf, Radiology St Vincent's University Hospital: svuhradiology.ie
- N Engl, (2015), Air Bronchogram, The new England Journal of medicine: nejm.org
- Dr Behrang Amini, sf, Air bronchogram, Radiopaedia: radiopaedia.org