Etymology

• Derived from Latin: “inter-” meaning “between,” “lobular” referring to the secondary pulmonary lobule, and “septal” indicating the connective tissue walls separating lobules.

AKA

• None.

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What is it?

• Interlobular septal thickening refers to the abnormal thickening of the connective tissue walls (septa) that separate adjacent secondary pulmonary lobules.
• It is a radiologic finding indicative of underlying disease affecting the lung interstitium.
• Kerley B lines are a specific radiographic sign of smooth interlobular septal thickening, most commonly identified on chest X-ray, but the term may also be applied in CT imaging for descriptive purposes.

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Characterized by

• Thickened or irregular septa that become visible on imaging modalities, particularly CT and CXR.
• Can appear:
  • Smooth: Due to fluid accumulation (e.g., pulmonary edema).
  • Nodular: Resulting from lymphatic or neoplastic infiltration.
  • Irregular: Associated with fibrosis or chronic interstitial diseases.
• Nodules along thickened septa are most commonly of soft tissue density but may occasionally be calcified, especially in granulomatous diseases such as sarcoidosis or silicosis.
• Kerley B lines describe short, horizontal linear opacities caused by smooth thickening of interlobular septa, most prominent in the lung periphery.

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Anatomically affecting

• The interlobular septa within the secondary pulmonary lobule, which contain lymphatics and pulmonary veins.

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Pathophysiology

• Thickening occurs due to the accumulation of fluid, cells, or fibrosis within the septal connective tissue.
• Causes include fluid overload, inflammation, neoplastic infiltration, or fibrosis.

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How does it appear on each relevant imaging modality?

Principles

• Parts: Linear or septal structures within the secondary pulmonary lobule.
• Size: Normally too thin to be visible; thickening makes them radiologically apparent.
• Shape: Can be smooth, irregular, or nodular.
• Position: Prominent in the interlobular regions, commonly near the periphery of the lungs.
• Character:
  • Smooth: Often seen in conditions like pulmonary edema or pulmonary venous hypertension.
  • Nodular: Nodules may be calcified, as seen in granulomatous conditions.
  • Irregular: Found in chronic fibrotic interstitial lung diseases.
• Time: Can vary in progression, appearing transiently (e.g., fluid overload) or permanently (e.g., fibrosis).

CXR

• Appears as fine linear opacities forming a reticular pattern, typically in the lung periphery or lower lobes.
• Kerley B lines are seen as short, horizontal linear opacities at the lung periphery, typically near the costophrenic angles.

CT

• Smooth septal thickening: Suggests pulmonary edema or lymphatic congestion (e.g., heart failure).
• Nodular septal thickening: Associated with lymphangitic carcinomatosis or granulomatous diseases like sarcoidosis.
• Irregular septal thickening: Seen in chronic interstitial lung diseases, such as idiopathic pulmonary fibrosis (IPF).
• On CT, the concept of Kerley B lines is less commonly used, as interlobular septa are directly visualized and described as smooth thickening or nodular thickening.

MRI

• Rarely used but can demonstrate septal thickening as high-signal lines in the lung interstitium on specific sequences.

PET-CT

• Septal thickening with increased metabolic activity can indicate malignancy (e.g., lymphangitic carcinomatosis).

Other Modalities

• Ultrasound: Rarely visualized, but pleural thickening near septal thickening may be inferred in associated diseases.

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Differential Diagnosis

• Smooth septal thickening:
  • Pulmonary edema (cardiogenic or non-cardiogenic).
  • Pulmonary venous hypertension.
• Nodular septal thickening:
  • Lymphangitic carcinomatosis.
  • Sarcoidosis (perilymphatic nodules).
• Irregular septal thickening:
  • Interstitial lung diseases (e.g., UIP, NSIP).
  • Chronic hypersensitivity pneumonitis.

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Recommendations

• Further imaging:
  • Chest CT for detailed assessment and characterization of the thickening.
  • Consider contrast-enhanced CT for vascular or lymphatic involvement.
• Clinical correlation:
  • Assess for symptoms such as dyspnea or systemic signs (e.g., malignancy, heart failure).
• Additional tests:
  • Echocardiography to evaluate for pulmonary venous hypertension.
  • Biopsy in cases of suspected malignancy or idiopathic fibrosis.

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Key Points and Pearls

• Interlobular septal thickening is a sign, not a diagnosis, requiring correlation with clinical, imaging, and laboratory findings.
• Kerley B lines aka Smooth Thickening  is a radiographic sign of smooth interlobular septal thickening, most commonly identified on CXR but may also describe similar findings on CT.
• Smooth thickening (used interchangeably with Kerley B lines)  is commonly seen in pulmonary edema, often reversible.
• Nodular thickening (e.g., beaded septum sign): Suggests lymphangitic carcinomatosis or granulomatous diseases like sarcoidosis.
• Irregular thickening is often associated with chronic fibrotic diseases.

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Parallels with Human Endeavors

• The interlobular septa act like partitioned compartments in a building, where thickening can be compared to damage or overloading of structural walls.
• Smooth thickening resembles fluid accumulation in pipes during overuse, nodular thickening mimics clogged ducts with debris, and irregular thickening reflects long-term wear and fibrosis in an aging system.
• The concept of Kerley B lines on CXR reflects how small but consistent structural changes, when visible, can reveal larger systemic dysfunctions, much like early warning signs in engineered systems (e.g., pressure buildup in pipelines).