• Etymology:
  • The term “reticular” derives from the Latin word “reticulum,” meaning “net,” describing the net-like or lattice appearance seen in imaging studies of the lungs.
• AKA:
  • Reticulonodular pattern (if nodules are also present).

• What is it?

  A reticular pattern refers to a network of interlacing linear opacities in the lungs, typically caused by thickening of the interstitial structures, such as septa or connective tissue.

• Characterized by:
  • A fine, coarse, or irregular network of lines creating a web-like appearance.
  • May indicate interstitial lung disease or other pathologies involving the lung interstitium.
• Anatomically affecting:
  • Pulmonary interstitium, specifically:
• Interstitium of the Secondary Lobule
  • The secondary pulmonary lobule is a functional unit of the lung that is typically defined by the structures supplied by a single small bronchiole. In fibrotic lung diseases with a reticular pattern on imaging, the involvement is often related to changes in specific structures within the secondary lobule. The primary structures affected include:
    •  Interlobular Septum: In the interlobular septa within the secondary pulmonary lobule.
      • Involvement in Reticular Disease:
        • Fibrosis or scarring of the interlobular septa can lead to
          • thickening  which can be irregular and incomplete
          • increased opacity,
      • Intralobular Septa:
        • Fibrosis of the intralobular septa (interalveolar septa) can also contribute to the reticular pattern.
  • Axial Interstitium: Peribronchovascular Interstitium
    • Fibrosis in the peribronchovascular interstitium can lead to a reticular pattern,
    • especially if it affects the connective tissue around small bronchioles.
  • Subpleural Connective Tissue:
    • Fibrosis in the subpleural connective tissue
• Pathophysiology:
  • The reticular pattern arises due to fibrosis, inflammation, or fluid accumulation within the interstitial compartments of the lungs.
  • Chronic conditions may lead to distortion of normal lung architecture and impaired gas exchange.
• Causes include:
  • • Most Common Causes:
      • Circulatory:
        • Pulmonary Edema (cardiogenic or non-cardiogenic).
      • Infection:
        • Viral pneumonias.
        • Fungal infections.
      • Inflammation:
        • Idiopathic Pulmonary Fibrosis (IPF).
        • Non-Specific Interstitial Pneumonia (NSIP).
        • Sarcoidosis.
        • Hypersensitivity Pneumonitis.
      • Other Causes include:
        • Neoplasm:
          • Lymphangitic carcinomatosis.
        • Inhalational Diseases:
          • Asbestosis.
          • Silicosis.
        • Autoimmune Diseases:
          • Rheumatoid arthritis.
          • Systemic sclerosis.
        • Infiltrative Diseases:
          • Amyloidosis.
        • Inherited and Congenital:
          • Hermansky-Pudlak syndrome.
        • Iatrogenic:
          • Radiation-induced fibrosis.
        • Idiopathic:
          • Cryptogenic organizing pneumonia (COP).
• Histopathology:
  • The reticular pattern corresponds to interstitial thickening due to fibrosis, edema, inflammation, or cellular infiltration.
• Imaging
  • Applied Anatomy:
    • Parts:
      • Interstitial structures, including septal interstitium intralobular interstitium, axial (bronchovascular), and peripheral interstitium
    • Size:
      • Fine, medium, or coarse reticulations depending on disease severity.
    • Shape
      • Linear or branching lines forming a net-like structure.
    • Position:
      • Varies based on etiology (e.g., diffuse or localized, upper lower lung zones).
    • Character:
      • Lines may be sharply defined or hazy, depending on associated changes (e.g., fibrosis, edema).
    • Time:
      • Acute (e.g., pulmonary edema) or chronic (e.g., pulmonary fibrosis).
• CXR:
  • Appears as a network of linear opacities that can be fine (e.g., early interstitial edema) or coarse (e.g., advanced fibrosis).
  • Varies based on etiology (e.g., diffuse or localized, upper lower lung zones). ILD usually predominantly lower lobes
  • May co-exist with nodular patterns or ground-glass opacities.
• CT:
  • Provides detailed visualization of interstitial thickening.
  • Fine reticulations are better seen in high-resolution CT (HRCT).
  • Findings include:
    • Interlobular septal thickening (e.g., in pulmonary edema).
    • Subpleural reticulations (e.g., in idiopathic pulmonary fibrosis).
    • Honeycombing in advanced fibrosis.
  • Distribution helps narrow differential diagnosis
    • e.g.,
      • lower lobe in ILD,
      • upper lobe in sarcoidosis).
• MRI:
  • Limited role but can identify gross interstitial changes.
  • Best used for evaluating concurrent cardiac or mediastinal pathologies.
• PET-CT:
  • May show increased metabolic activity in active inflammatory or fibrotic processes.
  • Helps differentiate between active disease and stable fibrosis.
• Other:
  • Ultrasound can identify pleural thickening or subpleural abnormalities but has limited utility in detecting reticular patterns.
• Differential Diagnosis:
  • Interstitial Lung Disease (ILD):
  • Idiopathic Pulmonary Fibrosis (IPF).
  • Non-Specific Interstitial Pneumonia (NSIP).
  • Sarcoidosis.
  • Hypersensitivity Pneumonitis.
  • Pulmonary Edema:
  • Cardiogenic or non-cardiogenic.
  • Infections:
  • Viral pneumonias.
  • Fungal infections.
  • Autoimmune Diseases:
  • Rheumatoid arthritis.
  • Systemic sclerosis.
  • Inhalational Diseases:
  • Asbestosis.
  • Silicosis.
• Recommendations:
  • Correlate imaging findings with clinical presentation and laboratory tests (e.g., autoimmune markers, infection workup).
  • High-resolution CT (HRCT) for detailed assessment.
  • Pulmonary function tests (PFTs) to evaluate restrictive lung disease.
  • Biopsy in cases of unclear etiology or to confirm specific interstitial lung diseases.
• Key Points and Pearls:
  • A reticular pattern is a hallmark of interstitial involvement and may indicate a wide range of diseases.
  • Lower lobe-predominant reticulations are characteristic of ILD , while upper lobe involvement suggests sarcoidosis or chronic hypersensitivity pneumonitis.
  • Cardiomegaly on imaging would suggest interstitial edema, while elevated inflammatory markers could point to active infection or inflammation.
  • Honeycombing on CT signifies advanced fibrosis and often poor prognosis.
  • Early recognition and differentiation of causes are essential for targeted management.

Reticular Pattern on CXR

In this patient the reticular pattern is superimposed on centrilobular emphysema and is associated with new multifocal GGO’s as well as new small basilar cysts raising the possibility of LIP (HIV history)

Hemorrhage

CHF

Aspiration

HIV ? Infection

Acute Eosinophilic Pneumonia vs COP

NSIP

Alveolar Septal Amyloidosis

Alveolar Proteinosis

Acute Moderate CHF with Interstitial Edema

Irregular

Nodular

Small Airway Disease and Nodular Thickening of the Interlobular Septum -Right Upper Lobe
Active TB

Sarcoidosis Interlobular and Intralobular Nodules

 

Sarcoidosis End Stage Fibrotic

Calcified Nodular

Lymphangitis Carcinomatosis

Adenocarcinoma of Left Lung  with
Coarsened Septal Thickening LUL

Primary Head and Neck Cancer and Lymphangitis

LIP

Post Ablation

 

• Subpleural reticulation
  •  typically in a peripheral subpleural distribution
  • located ≤1 cm from the pleura
    • normal aging
      • can be accompanied by subpleural cysts,
      • independent of smoking history
    • UIP
      •  bilateral basilar subpleural reticulation,
      • traction bronchiectasis,
      • architectural distortion and
      • honeycombing
• UIP reticulation

  

  UIP and Reticular Pattern
  70 year old male with UIP and CHF showing extensive dominantly bibasilar and peripheral reticulation

  

  

  

  

  

  

  

  

   

  Reticular Changes Caused by Fibrosis of the Interlobular Septa

Combined Pulmonary Fibrosis and Emphysema

• Post COVID 19

• 

• Diffuse Reticular Lung Disease
  • PCP HIV

  • 

    

    Reticular Changes Post XRT 

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