Nodules on the Interlobular Septa

• What is it:
  • Nodules on the interlobular septa are small, localized opacities along the thin connective tissue walls that separate secondary pulmonary lobules.
  • These nodules are associated with perilymphatic or vascular processes affecting the septal lymphatics or vessels.
  • This finding is also known as the beaded septum sign, describing the appearance of beads along the septal lines on imaging.
• Etymology:
  • “Nodule” derives from the Latin word nodulus, meaning “small knot.”
  • “Interlobular septa” refers to the thin connective tissue boundaries of the secondary pulmonary lobule.
• AKA:
  • Septal nodules.
  • Beaded septum sign.
• Abbreviation:
  • INS (Interlobular Septal Nodules).
• How does it appear on each relevant imaging modality:
  • Chest CT (preferred):
    • Parts: Discrete nodules located along the interlobular septa.
    • Size: Typically small (<1 cm).
    • Shape: Round or oval with smooth or slightly irregular margins.
    • Position: Distributed along the interlobular septa, often in a perilymphatic pattern.
    • Character:
      • May exhibit ground-glass attenuation or calcifications, depending on the cause.
      • Associated thickening of the interlobular septa is often seen in certain conditions.
  • Chest X-ray:
    • Usually undetectable due to the thinness of interlobular septa and low resolution.
    • May appear as subtle reticular patterns if associated with septal thickening.
• Differential diagnosis (starting with the most likely causes):
  • Infection:
    • Granulomatous diseases (e.g., tuberculosis, fungal infections) involving lymphatic channels.
  • Inflammation:
    • Sarcoidosis: Perilymphatic nodules along interlobular septa are a hallmark finding.
    • Rheumatoid nodules: Occasionally associated with pleural and interlobular septal involvement.
  • Neoplasm:
    • Lymphangitic carcinomatosis: Tumor spread along lymphatics commonly involves interlobular septa.
  • Idiopathic:
    • Idiopathic interstitial pneumonias (e.g., nonspecific interstitial pneumonia, NSIP).
• Recommendations:
  • Further evaluation:
    • High-resolution CT (HRCT) to confirm nodular distribution and evaluate associated findings like septal thickening or lymphadenopathy.
    • PET-CT for metabolic activity assessment in cases of suspected malignancy or systemic inflammation.
    • Biopsy (e.g., transbronchial or surgical) for indeterminate nodules or suspected granulomatous or neoplastic causes.
  • Laboratory workup:
    • Autoimmune panels (e.g., ANA, ANCA) for inflammatory diseases.
    • Infectious serologies or cultures for microbial causes.
• Key considerations and pearls:
  • The beaded septum sign strongly suggests a perilymphatic distribution, most commonly seen in sarcoidosis or lymphangitic spread of malignancy.
  • Concomitant lymphadenopathy strengthens the suspicion of sarcoidosis or malignancy.
  • Septal nodules with calcifications often indicate healed granulomatous disease (e.g., prior tuberculosis or histoplasmosis).
  • A comprehensive approach, incorporating imaging findings, clinical history, and laboratory results, is essential for accurate diagnosis and management.