Bronchopulmonary Dysplasia Imaging - Stages and Techniques

Introduction:

Bronchopulmonary dysplasia is a serious respiratory disorder affecting infants who were born early (premature babies) or have low birth weight. It is caused by prolonged ventilation and the extent of the supplemental oxygen requirement. The most common symptoms of bronchopulmonary dysplasia are cough, rapid breathing, cyanosis (bluish skin), and shortness of breath. The risk factors of bronchopulmonary dysplasia include-

• Infection.

• Prolonged mechanical ventilation.

• Gender (male).

• Genetics.

• Maternal smoking.

• Degree of prematurity.

• Increased airway resistance.

• Pulmonary hypertension (a condition in which increased blood pressure affects the arteries of the lungs and the heart).

• Pulmonary interstitial emphysema (a lung condition in which air gets trapped outside the alveoli).

What Are the Different Stages of Bronchopulmonary Dysplasia?

Bronchopulmonary dysplasia is a chronic disease of scarring and repair that occurs in four stages.

• Stage 1 (days one to three) - In the earlier stage, the appearance of bronchopulmonary dysplasia is similar to hyaline membrane disease and also shows vascular hyperemia, the presence of hyaline membrane, and lymphatic dilatation.

• Stage 2 (days 4 to 10) - Bronchial obstruction, peribronchial fibrosis, squamous metaplasia, obliterative bronchiolitis, and bronchial necrosis are seen in stage 2. Stretching of the terminal bronchioles causes ischemic necrosis of the airways resulting in lung destruction. However, the hyaline membrane persists, and coalescence of the alveoli is seen.

• Stage 3 (11 to 20 days) - During this stage, the number of alveoli decreases, hypertrophy of the remaining alveoli, and hypertrophy of the muscles and glands of the bronchial wall. This is a progressive reparative stage in which there is a regeneration of clear cells and exudation of histiocytes and macrophages into the airways. Tracheomalacia (abnormal collapse of the tracheal cartilage), pulmonary hyperinflation, air trapping, tracheomegaly (abnormal dilatation of trachea), ciliary dysfunction, and interstitial edema (collection of fluid inside the lungs) is seen.

• Stage 4 (more than one month) - Emphysematous alveoli are seen during this stage. Chronic lung damage results in pulmonary hypertension (a condition in which increased blood pressure affects the arteries of the lungs and the heart) and cor pulmonale (right-side heart failure). Atelectasis, fibrosis, cobblestone appearance, and pleural pseudo fissures are also seen. Hypertrophy of the peribronchiolar muscle and thickening of the pulmonary arterioles are also seen.

What Are the Imaging Techniques Used in the Diagnosis of Bronchopulmonary Dysplasia?

Chest Radiography:

• In chest radiography, bronchopulmonary dysplasia shows various appearances based on their stages.

• In Stage 1 - The features of respiratory distress syndrome (RDS) are sometimes seen along with their complications, such as pulmonary interstitial emphysema (PIE) and pneumothorax (collection of air between the lung and the chest wall).

• In Stage 2 - In this stage, the chest x-ray shows generalized haziness, plethora, and fine interstitial opacities. The distribution is diffuse, and there is no lobar predilection. In severe cases, the distribution becomes coarse and widespread.

• Stage 3 - This stage is characterized by cystic changes, irregular opaque and dense areas, and bubble appearance.

• Stage 4 - In stage 4, there is an expansion of strands and bubbles and hyper-expansion of the lungs in this stage. Entrapment of air is more common in the lower lobes than in the upper lobes. Pulmonary hyperinflation has been seen in most cases. An enlarged heart is seen in the fourth week if a fluid overload or patent ductus arteriosus is present.

Computed Tomography (CT) Scan:

• In stage 1, high-resolution computed tomography (HRCT) shows the symptoms of respiratory distress syndrome, such as pulmonary interstitial emphysema and pneumothorax.

• In stage 2, HRCT shows interlobular septal thickening. Prominent pulmonary arteries, atelectasis, consolidation, hyperinflation, and cardiomegaly are often seen. Entrapment of air is seen as an area of abnormal lucencies on expiratory scans. In later stages, large cysts, pulmonary hypertension, fibrosis, and lung destruction are seen.

• In stage 3 fibrotic areas, atelectasis and clearing of the attenuated areas are seen.

• In stage 4 HRCT shows bronchiectasis, tracheomegaly, tracheomalacia, dilatation of pulmonary arteries, and air trapping. Air trapping is usually bilateral or asymmetric, which is clearly shown in expiratory scans. Right ventricular hypertrophy, pulmonary edema, triangular opacities by pleural fissuring, thickening of the bronchial wall, lung distortion, and fibrotic changes are seen. Fibrotic changes include reticulonodular or reticular opacity and pleural thickening. The diameter of the bronchi is decreased, and the ratio of the bronchial artery to the pulmonary artery diameters is also decreased.

Magnetic Resonance Imaging (MRI) Scan:

• MRI is the ideal cross-sectional imaging tool used in the pediatric population. However, conventional MRI has limited use in diagnosing lung disorders. As the lung parenchyma has low proton density and contains air tissue interfaces, it returns a very low decaying signal and produces low-resolution images of the lung tissues.

• Ultrashort echo-time (UET) MRI is the common MRI technique used in the diagnosis of pulmonary disorders.

• Hyperpolarized MRI techniques can measure the microstructural and functional biomarkers that are not visualized in other imaging techniques.

Ultrasound:

• Ultrasound shows the appearance of respiratory distress syndrome (RDS), also known as hyaline membrane disease, which develops into BPD.

• It appears as a hyperechogenic lung base, obscuring the diaphragm on transsplenic or transhepatic ultrasound with hyperechoic artifacts.

• The HMD pattern gradually transforms into a BPD pattern of hypoechogenic, irregular areas seen on day 18 of life with a negative predictive value of about 95 %.

What Are the Differential Diagnoses of Bronchopulmonary Dysplasia?

• Emphysema (a lung condition in which air sacs are damaged, resulting in breathing difficulties).

• Bronchiectasis (a lung condition that damages the airways and results in coughing up mucus).

• A bronchogenic cyst (abnormal fluid-filled lesion found in the mediastinum).

• Aspiration pneumonia (pneumonia due to breathing in liquid or food into the airways).

• Idiopathic pulmonary fibrosis (a lung condition that causes scarring of the lungs).

• Pneumonia (a lung infection that causes swelling of the air sacs of the lungs) in neonates.

• Viral pneumonia.

• Patent ductus arteriosus (an abnormal heart condition characterized by failure in the closure of the fetal ductus arteriosus).

• Esophageal atresia (congenital defect in which the esophagus fails to develop completely).

Conclusion:

Though numerous imaging technology advances, such as computed tomography (CT) and magnetic resonance imaging (MRI), the chest radiograph remains the effective imaging tool in diagnosing pediatric lung disorders. MRI is a lengthy procedure and usually requires heavy sedation or general anesthesia in pediatric imaging. However, ultrashort echo-time (UTE) MRI produces a 3D submillimeter image and tissue density visualization compared to the CT scan. In addition, there are recent advances such as low or ultra-low CT scan that shows lesser radiation exposure similar to that of chest radiography and also shows faster imaging that does not require sedation or general anesthesia in small children.