Respiratory Pathology: An Overview

• Respiratory pathology focuses on diseases affecting the respiratory system including the lungs, airways, pleura, and related structures.
• It involves the examination and diagnosis of tissue samples obtained through biopsies, resections, and cytology.

Key Areas in Respiratory Pathology

• Inflammatory Lung Diseases: Characterized by inflammation of the lung tissue.
• Diffuse Alveolar Damage (DAD): Acute lung injury pattern seen in conditions like ARDS.
  • Presents histologically with hyaline membranes.
  • Can lead to fibrosis in the long term.
• Pneumonia: Infection of the lungs, classified by causative organism and pattern of involvement.
  • Bacterial pneumonia shows alveolar consolidation with neutrophils.
  • Viral pneumonia often exhibits interstitial inflammation with lymphocytes.
• Chronic Obstructive Pulmonary Disease (COPD): Group of lung diseases characterized by airflow obstruction.
  • Emphysema: Destruction of alveolar walls leading to enlarged airspaces.
  • Chronic Bronchitis: Excessive mucus production with chronic inflammation of the bronchi.
• Asthma: Chronic inflammatory disorder of the airways with reversible airflow obstruction.
  • Characterized by bronchial hyperresponsiveness, inflammation, and mucus plugging.
• Interstitial Lung Diseases (ILD): Heterogeneous group of disorders characterized by fibrosis of the lung interstitium.
  • Idiopathic Pulmonary Fibrosis (IPF): Specific type of ILD with a usual interstitial pneumonia (UIP) pattern.
    • Histologically shows honeycombing, fibroblast foci, and patchy fibrosis.
  • Non-Specific Interstitial Pneumonia (NSIP): Another pattern of ILD, often associated with connective tissue diseases.
    • Uniform fibrosis and inflammation are typical findings.
  • Sarcoidosis: Systemic disease with non-caseating granulomas in the lungs and other organs.
• Lung Tumors: Neoplasms arising from the lung tissue.
  • Small Cell Lung Carcinoma (SCLC): Aggressive tumor with neuroendocrine differentiation.
    • Characterized by small, dark cells with scant cytoplasm and high mitotic rate.
  • Non-Small Cell Lung Carcinoma (NSCLC): Includes adenocarcinoma, squamous cell carcinoma, and large cell carcinoma.
    • Adenocarcinoma: Most common type, often arising in the periphery of the lung.
      • Can show lepidic, acinar, papillary, or solid patterns.
    • Squamous Cell Carcinoma: Typically arises centrally and may show keratinization and intercellular bridges.
• Pleural Diseases: Conditions affecting the pleura, the lining of the lungs.
  • Mesothelioma: Malignant tumor arising from mesothelial cells.
    • Associated with asbestos exposure.
    • Can have epithelioid, sarcomatoid, or biphasic patterns.
  • Pleural Effusion: Accumulation of fluid in the pleural space.
    • Can be transudative (due to hydrostatic or oncotic pressure imbalances) or exudative (due to inflammation or malignancy).
• Vascular Diseases: Affecting the pulmonary vasculature.
  • Pulmonary Embolism (PE): Blockage of pulmonary arteries by thrombi.
  • Pulmonary Hypertension: Elevated blood pressure in the pulmonary arteries.

Diagnostic Techniques

• Histopathology: Microscopic examination of lung tissue.
  • Hematoxylin and Eosin (H&E) staining is commonly used.
  • Special stains like Masson's trichrome and Elastic stains highlight fibrosis and elastic fibers.
• Cytology: Examination of cells from respiratory samples.
  • Sputum cytology: Analysis of cells coughed up from the respiratory tract.
  • Bronchial washings/brushings: Samples collected during bronchoscopy.
  • Fine needle aspiration (FNA): Used to sample lung masses or lymph nodes.
• Immunohistochemistry (IHC): Use of antibodies to detect specific proteins in tissue samples.
  • Helps in differentiating tumor types and identifying infectious agents, for example TTF-1 in lung adenocarcinomas.
• Molecular Testing: Analysis of DNA or RNA to identify genetic mutations or infectious organisms.
  • Used for targeted therapy selection in lung cancer (e.g., EGFR, ALK).
  • Detection of viral or bacterial pathogens.
• Microbiology: Culture and identification of microorganisms from respiratory samples.
  • Used to diagnose bacterial, fungal, and viral infections.

Specific Diseases and Conditions

• Cystic Fibrosis: Genetic disorder causing abnormal mucus production.
  • Leads to chronic lung infections and bronchiectasis.
• Bronchiectasis: Irreversible dilation of the bronchi.
  • Caused by chronic infections, cystic fibrosis, or immune deficiencies.
• Pneumoconiosis: Lung diseases caused by inhalation of mineral dusts.
  • Asbestosis: Caused by asbestos exposure, leading to fibrosis and increased risk of mesothelioma.
  • Silicosis: Caused by silica dust, leading to nodular fibrosis.
  • Coal Worker's Pneumoconiosis: Caused by coal dust, leading to coal macules and fibrosis.
• Hypersensitivity Pneumonitis: Immunologically mediated inflammation of the lung parenchyma.
  • Caused by inhalation of organic antigens.
• Eosinophilic Pneumonia: Accumulation of eosinophils in the lung.
  • Can be idiopathic or secondary to infections, drugs, or parasites.
• Lung Abscess: Localized collection of pus in the lung.
  • Usually caused by bacterial infection.

Common Histopathological Findings

• Hyaline Membranes: Pink, glassy membranes lining alveolar ducts and alveoli in DAD.
• Fibroblast Foci: Collections of fibroblasts and myofibroblasts in IPF.
• Honeycombing: Enlarged airspaces with thick fibrous walls in advanced fibrosis.
• Granulomas: Collections of immune cells, often seen in sarcoidosis and infections.
  • Caseating granulomas: Contain central necrosis, typical of tuberculosis.
  • Non-caseating granulomas: Lack central necrosis, seen in sarcoidosis.
• Interstitial Fibrosis: Thickening and scarring of the lung interstitium.
• Squamous Metaplasia: Transformation of columnar epithelium to squamous epithelium.
  • Often seen in response to chronic irritation or inflammation.
• Mucus Plugging: Obstruction of airways by mucus.
  • Common in asthma and chronic bronchitis.
• Emphysema: Destruction of alveolar walls leading to enlarged airspaces.

Importance of Clinical Correlation

• Accurate diagnosis requires integration of clinical history, radiological findings, and pathological findings.
• Communication between pathologists, pulmonologists, and other clinicians is essential for optimal patient care.

Explore respiratory system diseases, including lungs and airways. Learn about inflammatory lung diseases, pneumonia, and COPD. Understand the pathology of conditions like Diffuse Alveolar Damage and Emphysema.