000 Interstitial Lung Disease

Etymology

Derived from “interstitial,” referring to the connective tissue framework of the lungs, and “disease,” indicating a pathological condition.

AKA and abbreviation

Interstitial lung disease (ILD). Also referred to as diffuse parenchymal lung disease (DPLD).

What is it?

Interstitial lung disease (ILD) encompasses a diverse group of over 200 disorders that primarily affect the lung interstitium—the connective tissue that supports the alveoli and surrounding structures. ILDs are characterized by varying degrees of inflammation, scarring (fibrosis), and architectural distortion, leading to progressive impairment of gas exchange. ILDs can result from identifiable causes, such as autoimmune diseases or environmental exposures, but many cases are idiopathic. Early detection is critical, as ILD can progress to irreversible fibrosis, significantly impacting quality of life and survival.

Characterized by

Radiologic and histopathologic features, including:

  1. Reticular abnormalities: Network of fine linear opacities, often in a basal and subpleural distribution.
  2. Ground-glass opacities: Hazy areas that do not obscure underlying vascular structures.
  3. Traction bronchiectasis: Irregular dilation of airways due to fibrotic retraction.
  4. Honeycombing: Clustered cystic air spaces in the periphery, indicating advanced fibrosis.
  5. Architectural distortion: Displacement of normal lung anatomy due to fibrosis.
  6. Nodules: May indicate granulomatous inflammation or neoplastic infiltration.

Caused by

1. Idiopathic Interstitial Pneumonias (IIPs):

Although termed “idiopathic,” some IIPs have known associations or predisposing factors. These subtypes include:

  • Usual Interstitial Pneumonia (UIP): The hallmark pattern of idiopathic pulmonary fibrosis (IPF), characterized by patchy fibrosis and honeycombing. Does not have a clear association with a specific cause.
  • Non-Specific Interstitial Pneumonia (NSIP): Can occur as an idiopathic process but is often associated with collagen vascular diseases, such as rheumatoid arthritis, systemic sclerosis, and polymyositis/dermatomyositis. Features uniform fibrosis and inflammation.
  • Desquamative Interstitial Pneumonia (DIP): Strongly linked to smoking, with characteristic macrophage accumulation in alveoli.
  • Respiratory Bronchiolitis-Associated Interstitial Lung Disease (RB-ILD): Exclusively associated with smoking, marked by pigmented macrophages in the small airways.
  • Acute Interstitial Pneumonia (AIP): A rapidly progressive and severe form of ILD with no identified external trigger, associated with diffuse alveolar damage.
  • Cryptogenic Organizing Pneumonia (COP): Previously termed bronchiolitis obliterans organizing pneumonia (BOOP). Granulation tissue plugs within small airways and alveolar ducts, typically with no identified cause, although post-infectious cases are recognized.
  • Lymphoid Interstitial Pneumonia (LIP): Can be idiopathic but frequently associated with autoimmune diseases (e.g., Sjögren’s syndrome, systemic lupus erythematosus) or immunodeficiency (e.g., HIV, common variable immunodeficiency).

2. Connective Tissue and Autoimmune Diseases:

ILD can arise secondary to systemic autoimmune conditions, including:

  • Rheumatoid Arthritis
  • Systemic Lupus Erythematosus
  • Systemic Sclerosis (Scleroderma)
  • Polymyositis/Dermatomyositis
  • Sarcoidosis

3. Occupational and Environmental Exposures (Pneumoconioses):

Inhalation of various dust particles can lead to ILD:

  • Asbestosis: Due to asbestos exposure.
  • Silicosis: Resulting from inhalation of silica dust.
  • Coal Worker’s Pneumoconiosis: Also known as “black lung disease,” from coal dust exposure.

4. Drug-Induced ILD:

Certain medications are known to cause interstitial lung changes, such as:

  • Amiodarone
  • Methotrexate
  • Bleomycin

5. Infections:

Chronic infections can lead to interstitial changes, including:

  • Tuberculosis
  • Viral Pneumonias
  • Pneumocystis Pneumonia

6. Other Causes:

Additional factors contributing to ILD include:

  • Radiation-Induced Lung Injury: Following therapeutic radiation.
  • Hypersensitivity Pneumonitis: Immune response to inhaled organic antigens.
  • Lymphangitic Carcinomatosis: Spread of malignancy through lymphatic vessels in the lungs.

Resulting in:

  • Progressive dyspnea and exercise intolerance.
  • Dry, nonproductive cough.
  • Hypoxemia leading to respiratory failure in advanced cases.

Structural changes:

  • Fibrosis of alveolar walls and interstitial spaces.
  • Inflammatory infiltration in early stages.
  • Remodeling and destruction of lung architecture with honeycombing in advanced stages.

Pathophysiology:

  • Early stage: Inflammation and immune activation lead to alveolar epithelial injury.
  • Chronic stage: Dysregulated repair mechanisms result in fibroblast proliferation, extracellular matrix deposition, and progressive fibrosis.

Pathology:

  • Non-Specific Interstitial Pneumonia (NSIP): Uniform inflammation and fibrosis.
  • Usual Interstitial Pneumonia (UIP): Patchy fibrosis and honeycombing.
  • Granulomatous Inflammation: Seen in sarcoidosis and hypersensitivity pneumonitis.

Diagnosis:

  • Clinical, radiologic, and pathologic correlation required.
  • Exclusion of known causes, such as infections, drugs, and exposures.

Management:

  • Supportive care: Oxygen therapy, pulmonary rehabilitation, and vaccinations.
  • Lifestyle changes: Smoking cessation and avoidance of triggers.
  • Medications: Antifibrotic agents (e.g., pirfenidone, nintedanib) or immunosuppressants (e.g., corticosteroids, azathioprine) based on underlying pathology.
  • Advanced therapy: Lung transplantation in severe cases.

Closing Note:

Some of these diseases have characteristic appearances on radiologic studies. These findings are elaborated on in dedicated sections for individual diseases within this common framework, providing detailed descriptions specific to each condition.

Egashira , R  CT Findings of Thoracic Manifestations of Primary Sjögren Syndrome: Radiologic-Pathologic Correlation  RadioGraphicsVol. 33, No. 7

Gotway, M et al, Challenges in pulmonary fibrosis · 1Use of high resolution CT scanning of the lung for the evaluation of patients with idiopathic interstitial pneumonias BMJ

 Mueller-Mang C, et al  What Every Radiologist Should Know about Idiopathic Interstitial Pneumonias RadioGraphicsVol. 27, No. 3 2007

Radiology Key – Excellent article

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