INTERSTITIAL LUNG DISEASES: Radiological Appearances and Pathological correlation

 

Interstitial lung diseases (ILDs) are a diverse group of disorders characterized by inflammation and fibrosis of the lung interstitium. This article will explore the radiological appearances of ILDs, primarily using high-resolution computed tomography (HRCT), and will also examine the associated pathology.

Anatomy of the Secondary Pulmonary Lobule

The secondary pulmonary lobule is the smallest unit visible on HRCT imaging, measuring approximately one centimeter in size. It is supplied by the lobular bronchus and lobular artery, which divide into terminal bronchioles and respiratory bronchioles. Each bronchiole is accompanied by an associated acinar artery. 

The interlobular septa, which are thin connective tissue structures about 0.1 millimeters thick, contain pulmonary veins, lymphatics, and connective tissue, forming the lung's interstitium.

Patterns of ILD on Imaging

Interstitial lung diseases can present with various radiological patterns, which can be categorized into lesions with increased and decreased lung attenuation:

1. Increased Lung Attenuation:

• Ground Glass Opacities (GGOs): These allow for the visualization of vascular markings beneath the opacities.
• Consolidations: Characterized by obscured vascular markings, indicating more dense lung involvement.

1. Decreased Lung Attenuation:

• Honeycombing: Appears as peripheral multi-layered cystic changes.
• Bronchiectasis: Represents dilated bronchioles and bronchi.
• Emphysematous Changes and Pneumatoceles: Indicate areas of lung destruction.

Additionally, ILDs may present with nodules or linear/reticular opacities due to interlobular septal thickening.

Classification of Interstitial Lung Diseases

Interstitial lung diseases can be classified into idiopathic and known causes.

Idiopathic Interstitial Lung Diseases (IIDs)

Usual Interstitial Pneumonia (UIP): Characterized by an apical-basal gradient, more prominent in the basal regions, often presenting with honeycombing, traction bronchiectasis, and reticular opacities.

Non-Specific Interstitial Pneumonia (NSIP): Features a central sparing pattern with peripheral opacities. Commonly shows ground glass opacities, reticular opacities, and micro-nodules. A distinctive feature is the sparing of immediate subpleural regions.

Respiratory Bronchiolitis and Desquamative Interstitial Pneumonia (DIP): Associated with smoking, presenting with pigmented macrophage deposition.

Cryptogenic Organizing Pneumonia (COP): Commonly occurs in non-smokers, showing peripheral and peri-bronchial opacities with migrating patterns and no response to antibiotics.

Acute Interstitial Pneumonia (AIP): Presents with diffuse alveolar damage, often showing bilateral symmetrical opacities and progressing to consolidation.

Known Causes of Interstitial Lung Diseases

1. Sarcoidosis
2. Connective Tissue Disorders (e.g., rheumatoid arthritis, systemic sclerosis, systemic lupus erythematosus)
3. Hypersensitivity Pneumonitis
4. Silicosis
5. Lymphangioliomyomatosis and Langerhans Cell Histiocytosis
6. Pulmonary Alveolar Proteinosis

Miscellaneous Causes

• Infective: Viral pneumonias, Pneumocystis jirovecii pneumonia, Mycoplasma pneumonia.
• Non-infective: Eosinophilic pneumonia, Acute Respiratory Distress Syndrome (ARDS), Lipoid pneumonia, Diffuse Alveolar Hemorrhage.

Treatment Approaches

The treatment for ILDs varies based on the specific disease. For UIP, corticosteroids and immunosuppressive agents (like cyclosporine) are often employed to provide symptomatic relief and slow disease progression. In NSIP, corticosteroids or cytotoxic drugs can be effective, particularly in the cellular subtype.

Key Points for Reporting ILDs on HRCT

• Rule out known causes before diagnosing idiopathic interstitial lung diseases.
• Classification of ILDs is based on histological findings rather than imaging alone.
• Follow-up scans are necessary due to the dynamic nature of ILDs.
• Surgical lung biopsy may be required to confirm the diagnosis and should involve multiple lobes.

Conclusion

Understanding the radiological appearances and classifications of interstitial lung diseases is crucial for accurate diagnosis and management. For further educational content, consider following platforms dedicated to radiological education.

For more videos and updates, follow us on YouTube and Instagram at Radiology Noodles. Thank you!