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  • Etymology

    • Derived from “interstitial,” referring to the connective tissue framework of the lungs.

    AKA and abbreviation

    • No widely accepted abbreviations.

    What is it?

    • An interstitial pattern is a radiologic term describing abnormal findings in the lung interstitium on imaging studies.
    • The interstitium includes the connective tissue framework surrounding alveoli, interlobular septa, bronchovascular bundles, and subpleural areas.
    • Interstitial patterns indicate abnormalities in the lung parenchyma and are commonly identified on chest X-rays (CXR) and high-resolution CT (HRCT).
    • Interstitial lung abnormalities (ILAs): A subset of these patterns, representing incidental findings of interstitial changes that may be asymptomatic or precursors to progressive interstitial lung disease (ILD).

    Characterized by

    • Reticular abnormalities: Fine or coarse linear opacities forming a network, often basal and subpleural.
    • Ground-glass opacities: Hazy regions without obscuring vascular markings.
    • Nodular opacities: Discrete rounded opacities indicating granulomatous or neoplastic processes.
    • Honeycombing: Clustered cystic air spaces in the periphery, indicative of advanced fibrosis.
    • Traction bronchiectasis: Airway dilation due to fibrotic retraction.
    • Traction bronchiolectasis: Dilatation of smaller bronchioles caused by surrounding fibrotic retraction, commonly seen in fibrotic ILD.
    • Architectural distortion: Displacement of normal anatomic structures, often due to fibrosis.
    • Arcade fibrosis: Fibrosis along the bronchovascular bundles, frequently seen in early connective tissue disease-associated ILD.
    • Subpleural fibrosis: Fibrotic thickening of the lung’s periphery, often a precursor to honeycombing in advanced disease.

    Anatomical Classification of the Lung Interstitium

    • Axial (Peribronchovascular) Interstitium:
      • Surrounds the bronchovascular bundles, including the pulmonary arteries, veins, bronchi, and lymphatics.
      • Associated with lymphangitic carcinomatosis or sarcoidosis.
    • Parenchymal (Acinar) Interstitium:
      • Includes the walls of alveoli, alveolar ducts, and surrounding capillary networks.
      • Typical involvement in diseases like idiopathic pulmonary fibrosis (IPF) and hypersensitivity pneumonitis.
    • Peripheral (Subpleural) Interstitium:
      • Lies beneath the pleura and includes the interlobular septa.
      • Thickening frequently seen in pulmonary edema, asbestosis, or advanced fibrosis.
    • Interlobular Septal Interstitium:
      • Composed of connective tissue, small veins, and lymphatic channels along interlobular septa.
      • Thickening can appear as smooth, nodular, or irregular and is associated with pulmonary edema, sarcoidosis, or lymphangitic carcinomatosis.

    Caused by

    • Most common cause: Idiopathic pulmonary fibrosis (IPF). This encompasses a wide spectrum of diseases, including various idiopathic interstitial pneumonias (IIPs) such as usual interstitial pneumonia (UIP), non-specific interstitial pneumonia (NSIP), desquamative interstitial pneumonia (DIP), and acute interstitial pneumonia (AIP).
    • Other causes include:
      • Infections: Viral pneumonias, tuberculosis, Pneumocystis pneumonia.
      • Inflammation/Immune: Sarcoidosis, hypersensitivity pneumonitis, rheumatoid arthritis-associated ILD.
      • Neoplasm: Lymphangitic carcinomatosis.
      • Mechanical Trauma: Radiation-induced lung injury.
      • Metabolic: Pulmonary alveolar proteinosis, amyloidosis.
      • Circulatory: Pulmonary edema, venous congestion.
      • Inherited/Congenital: Hermansky-Pudlak syndrome, surfactant protein mutations.
      • Other: Drug toxicity (e.g., amiodarone, methotrexate), environmental exposures (e.g., asbestosis, silicosis).

    Resulting in

    • Impaired gas exchange due to thickening of the alveolar-capillary membrane.
    • Progressive dyspnea and reduced lung compliance.
    • Risk of respiratory failure in advanced cases.

    Structural changes

    • Thickening of interlobular septa, alveolar walls, peribronchovascular structures, and subpleural regions.
    • Inflammatory infiltration or fibrotic remodeling.
    • Development of:
      • Nodules
      • Arcade fibrosis
      • Reticular patterns
      • Traction bronchiectasis
      • Traction bronchiolectasis
      • Honeycombing

    Pathophysiology

    • Abnormalities arise due to inflammation, fluid accumulation, or fibrotic changes, disrupting normal alveolar and vascular functions.
    • Chronic processes lead to architectural distortion and irreversible lung scarring.

    Pathology

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

    Diagnosis

    • Requires correlation of clinical symptoms, radiologic findings, and occasionally histopathologic data.
    • HRCT is critical for identifying specific patterns and determining disease progression.

    Clinical

    • Symptoms: Dyspnea, dry cough, and fatigue.
    • Signs: Fine “Velcro-like” crackles and clubbing of fingers in advanced cases.

    Radiology Detail

    • CXR:
      • Reticular opacities, reduced lung volumes, and basal predominance.
      • Associated findings: Ground-glass changes or honeycombing in late stages.
    • CT:
      • Parts: Interlobular septa, alveolar walls, subpleural regions, peribronchovascular structures, and interlobular septal interstitium.
      • Size: Focal, segmental, or lobar.
      • Shape: Reticular (linear), nodular, or mixed reticulonodular.
      • Position: Basal and subpleural predominance in most fibrotic ILDs; peribronchovascular involvement in sarcoidosis or lymphangitic carcinomatosis.
      • Character: Chronic changes with architectural distortion, ground-glass opacity, traction bronchiectasis, traction bronchiolectasis, and honeycombing.
      • Time: Acute in infections or inflammatory conditions; chronic findings suggest fibrosis or scarring.
      • Associated Findings: Mosaic attenuation and volume loss.
    • Other relevant Imaging Modalities:
      • MRI: Rarely used but can demonstrate subtle inflammatory changes.
      • PET-CT: Useful for detecting metabolic activity in neoplastic or inflammatory conditions.
      • Ultrasound: Limited to pleural abnormalities.

    Pulmonary function tests (PFTs)

    • Restrictive pattern with reduced total lung capacity (TLC) and diffusing capacity for carbon monoxide (DLCO).
    • Oxygen desaturation during exercise.

    Recommendations

    • Early identification of patterns and multidisciplinary evaluation.
    • Serial imaging and PFTs to monitor disease progression.
    • Referral to pulmonology or radiology for advanced evaluation.

    Key Points and Pearls

    • Interstitial patterns on imaging suggest underlying ILD or interstitial involvement from other diseases.
    • Anatomical classification of the interstitium (axial, parenchymal, peripheral, interlobular septal interstitium) aids in narrowing differential diagnoses.
    • HRCT is crucial for distinguishing specific subtypes and assessing disease severity.
    • Identification of radiologic features such as honeycombing, arcade fibrosis, traction bronchiectasis, and traction bronchiolectasis aids in diagnosis.
    • Close clinical and radiologic monitoring is critical to prevent irreversible lung damage.
  • Structural Changes in Ground-Glass Opacity (GGO) that Reflect Fibrosis

    Ground-glass opacity (GGO) refers to a hazy area of increased attenuation in the lung on CT that does not obscure underlying bronchial and vascular structures. GGOs are often non-specific but can be a manifestation of both active inflammation or early fibrosis. When GGO reflects fibrosis, certain structural changes are present:

    Key Structural Changes in Fibrotic Ground-Glass Opacity

    1. Reticular Abnormalities Within GGO:
      • Thin or thickened interstitial lines superimposed on the hazy GGO pattern.
      • Represent fibrotic remodeling of the lung interstitium, particularly in the alveolar walls and interlobular septa.
    2. Traction Bronchiectasis and Bronchiolectasis:
      • Airway dilation caused by retraction forces from surrounding fibrotic tissue.
      • Presence of traction bronchiectasis or bronchiolectasis within GGO is a hallmark of fibrosis.
      • Reflects architectural distortion of lung parenchyma due to chronic scarring.
    3. Architectural Distortion:
      • Alteration of normal lung anatomy, including displacement of bronchi, vessels, and pleural surfaces.
      • Suggests chronic and irreversible fibrotic changes rather than reversible inflammation.
    4. Subpleural Sparing:
      • In early fibrotic disease, GGOs may spare the immediate subpleural regions, distinguishing them from active inflammatory changes.
    5. Volume Loss:
      • Associated loss of lung volume in the affected regions.
      • A consequence of fibrotic contraction and tissue remodeling.
    6. Ground-Glass Pattern Overlying Honeycombing:
      • In advanced fibrosis, GGO can be seen superimposed on honeycombing, particularly in diseases like usual interstitial pneumonia (UIP).
      • Indicates active fibrotic remodeling in areas of established structural damage.

    How to Differentiate Fibrotic GGO from Reversible GGO

    • Fibrotic GGO:
      • Associated with structural changes like reticulation, traction bronchiectasis, and architectural distortion.
      • Often stable or slowly progressive on serial imaging.
      • Indicates irreversible parenchymal damage.
    • Reversible GGO (e.g., due to inflammation or edema):
      • Typically lacks structural distortion.
      • Resolves or improves with treatment or over time.
      • Examples: Pneumonia, pulmonary edema, or hypersensitivity pneumonitis.

    Conditions Where GGO Reflects Fibrosis

    1. Idiopathic Pulmonary Fibrosis (IPF):
      • GGO in subpleural and basal regions with reticulation and traction bronchiectasis.
    2. Non-Specific Interstitial Pneumonia (NSIP):
      • Uniform GGOs with mild traction bronchiectasis; typically in a subpleural and basal distribution.
    3. Chronic Hypersensitivity Pneumonitis:
      • GGOs with centrilobular nodules, reticulation, and traction changes.
    4. Connective Tissue Disease-Associated ILD:
      • GGOs associated with fibrosis, often seen in systemic sclerosis or rheumatoid arthritis.

    Significance

    • Recognizing fibrotic changes in GGO is crucial for distinguishing reversible from irreversible disease.
    • It guides diagnosis, prognosis, and management, particularly in interstitial lung diseases where fibrosis indicates progression to advanced stages.