Bronchiolitis is a non-specific inflammatory injury of the small airways of the respiratory system of less than 2mm in diameter that connects the bronchi to the alveoli. The inflammatory process can be due to various causes like viral infection, connective tissue disorders, or inhalation of dust particles.

What Are the Types of Bronchial Diseases?

It can be classified into the following types:

Constrictive bronchiolitis.
Diffuse panbronchiolitis.
Respiratory bronchiolitis.
Mineral dust bronchiolitis.
Follicular bronchiolitis.

What Is Constrictive Bronchiolitis?

Also known as obliterative bronchiolitis or bronchiolitis obliterans. It is constrictive because of the submucosal and peribronchiolar fibrosis that leads to narrowing and complete obliteration of the lumen of the bronchioles.

It is caused due to:

Connective tissue disorders.
Inhalational injury.
Chronic hypersensitivity pneumonitis.
Organ transplantation.

Histopathological Findings: The fibroblast proliferates, and the stromal edema is seen along with thick collagen deposition, resulting in the constriction of the lumen. Patchy areas of fibrosis make it difficult to diagnose if not appropriately sampled.

Radiographic Features:

Chest Radiography:

Hyperinflation.
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Bronchial wall thickening may be seen.
Peripheral attenuation of vascular markings.
Reticulonodular opacities.

HRCT(High-Resolution Computed Tomography):

Mosaic pattern of decreased attenuation and vascularity.
Evidence of air trapping.
Peripheral cylindrical bronchiectasis.

What Is Acute Bronchiolitis?

It is the most common respiratory tract disease in infants and children during winter and is characterized by increased heart and respiratory rates and prolonged expiration. Wheezing and crackles may also be heard.

Causes: It is mostly caused by the respiratory syncytial virus(RSV).

Acute and chronic inflammation of small bronchioles with epithelial necrosis and sloughing and an intraluminal exudate filled with neutrophils are observed in the histopathological findings of the condition.

Radiographic Features:

Chest Radiograph:

Hyperinflammation.
Tiny nodules.
Patchy ground glass opacities.
Linear opacities.

HRCT:

Ill-defined centrilobular nodules represent bronchioles impacted with inflammatory cells and peribronchial inflammation.
Branching linear opacities represent the inflamed airway walls.
Focal areas of consolidation due to bronchopneumonia.

Treatment: The symptoms are usually mild, so they can generally be managed at home.

Swyer-James Syndrome is one of the complications of acute bronchitis. It, also known as Mac Leod’s syndrome, is an early childhood infection, most commonly viral pneumonia, characterized by a reduced pulmonary vasculature with hyperlucency and decreased volume of the involved lung or lobe.

What Is Diffuse Panbronchiolitis?

It is the rarest form of bronchiolitis, particularly in Asia, and it is characterized by bronchiolar inflammation and chronic sinusitis. The patient will have a cough with purulent sputum, dyspnea, airflow obstruction, decreased breathing sounds with coarse crackles, and wheezing will be seen.

Lab Diagnosis: The most characteristic feature is elevated levels of cold agglutinins, but the test for Mycoplasma pneumoniae is negative. Initially, the sputum only contains Haemophilus influenzae, but later on, colonies of Pseudomonas aeruginosa also appear, accelerating the destruction, often leading to chronic respiratory failure and cor pulmonale.

Pulmonary Function Test: Shows marked obstructive impairment.

Histopathological Findings: Bronchiolocentric infiltration of lymphocytes, plasma cells, and foamy macrophages is seen. Neutrophils and T-lymphocytes have an important role to play. Accumulation of these neutrophils is associated with injury to the airways. Intraluminal exudates form polyploid plugs. Also, there is an increased number of dendritic cells in the epithelium and submucosa.

Radiographic Features:

Chest Radiograph:

Small, diffused, ill-defined nodular opacities with a symmetrical distribution.
Mild to moderate hyperinflation is seen.

HRCT:

Centrilobular nodules with branched tubular opacities give a “tree-in-bud” appearance.
Thickened and ecstatic bronchioles.
Peripheral air trapping.

Treatment: Start with Erythromycin 400-600mg daily. It is effective in inhibiting mucus production from the epithelium.

What Is Respiratory Bronchiolitis?

It is commonly associated with cigarette smoking and is rarely seen in non-smokers. The most characteristic feature is the intraluminal accumulation of pigmented macrophages. It is usually asymptomatic except for the “smoker’s cough,” In some cases, symptomatic diffuse parenchymal lung infiltrates are known as respiratory bronchiolitis-associated interstitial lung disease.

Radiographic Features:

Chest Radiograph: Normal with no lung infiltrates.

HRCT:

Diffused, poorly circumscribed centrilobular nodular opacities.
Acute respiratory bronchiolitis: poorly circumscribed centrilobular opacities.
Chronic respiratory bronchiolitis: better circumscribed centrilobular nodular opacities and branched tubular opacities.

What Is Mineral Dust Airway Disease?

It is associated with restrictive lung disease due to parenchymal fibrosis. It is due to the deposition of dust particles such as silica, asbestos, mica, iron, coal, and aluminum oxide in and around the small airways and alveoli. In response to the inhaled deposits, chronic inflammation and fibrosis of the bronchioles with luminal narrowing occurs.

Radiographic Features: Chest radiograph and HRCT both show small ill-defined punctate opacities.

What Is Follicular Bronchiolitis?

The most characteristic feature of follicular bronchiolitis is the presence of hyperplastic lymphoid follicles with reactive germinal centers along with broncho vascular bundles. The lymphoid hyperplasia is in response to an external stimulus or altered systemic immune response, as seen in rheumatoid arthritis patients.

Radiographic Features:

Chest Radiograph: Normal.
HRCT: Diffuse bilateral centrilobular nodules, peribronchial nodules, and patchy areas of ground glass opacities. Mild bronchial dilatation with bronchial thickening is also seen sometimes.

Treatment: Corticosteroids and bronchodilators are the drugs of choice. Erythromycin is also considered to be beneficial.

What Is Hypersensitivity Pneumonitis?

It is one of the most common interstitial lung diseases. It is an immune-mediated inflammatory disease involving the lung parenchyma and terminal airways secondary to repeated inhalation of various dust particles in a sensitized host. Bronchiolitis in hypersensitivity pneumonitis is characterized by a patchy peribronchial infiltrate of lymphocytes with isolated multinucleated histiocytes.

Radiographic Features:

HRCT:

Areas of decreased attenuation and mosaic pattern suggest air trapping - a sign of bronchial obstruction.
Ground glass opacities and the reticular pattern result from parenchymal inflammation and fibrosis.
Poorly circumscribed centrilobular nodules represent intraluminal granulation tissue.

Treatment:

Exposure to such triggering agents in a sensitized host should be avoided. In an unknown cause, Corticosteroid remains the mainstay of therapy.

What Is Cryptogenic Organizing Pneumonia?

It is also known as bronchiolitis obliterans without organizing pneumonia (BOOP), a pure bronchiolitis obliterans disease. It clinically presents as a non-productive, persistent cough with fever and fatigue. The most characteristic physical findings are inspiratory “Velcro” crackles and increased respiration rate. There can be increased levels of C-reactive protein and erythrocyte sedimentation rate.

Histopathological Findings: It can be characterized by polypoid intraluminal plugs of proliferating fibroblasts and myofibroblasts within the alveolar spaces. The fibroblastic plugs form casts that outline the branching configuration of alveolar ducts and distal airways. The lumen appears to be impeded from within. The intraluminal fibro myxoid tissue shows increased blood vessel formation, which is mediated by vascular endothelial growth factors and a few other growth factors.

Radiographic Features:

Chest Radiograph: Patchy bilateral alveolar infiltrates give ground glass appearance and may appear as multifocal mass regions of consolidations. Hyperinflation is not seen.
HRCT: Shows heterogeneous consolidation regions
Bilateral ground glass opacities with reticular changes.

Treatment: Corticosteroids such as Prednisone is the choice of drug with gradual tapering of dosage.

Conclusion:

Bronchial disorders are prevalent and can occur in conditions affecting the larger airways or the distal parenchyma. Cough and dyspnea are frequently seen. HRCT is an important diagnostic tool. It is usually missed in a standard lung biopsy as the areas of involvement are patchy. One can reach the diagnosis only with good clinical knowledge, HRCT, and pulmonary function testing.