Interlobular septal thickening is a term often found in medical imaging reports. It describes a change in the fine structures within the lungs, visible on scans like CT. While it may sound alarming, it is not a disease itself but a sign of an underlying process. Understanding its meaning, causes, and significance helps clarify its implications.
The Lung’s Interlobular Septa Explained
Within the lung tissue, tiny functional units called secondary pulmonary lobules are organized. These lobules are separated by thin walls of connective tissue known as interlobular septa. These septa extend from the lung’s outer lining, the pleura, towards the central regions. They contain small pulmonary veins and lymphatic vessels, which drain fluid and waste products. Normally very thin and often not visible on standard imaging, they become apparent when thickened. This thickening indicates an accumulation of fluid, cells, or fibrous tissue within these partitions.
Reasons for Interlobular Septal Thickening
Interlobular septal thickening arises from various conditions affecting the lung’s interstitial framework. The thickening’s appearance—smooth, nodular, or irregular—often provides clues about the underlying cause.
Smooth thickening commonly occurs due to fluid accumulation, such as in pulmonary edema from congestive heart failure. Increased pressure in the pulmonary veins leads to fluid leaking into the septa. Other causes include pulmonary alveolar proteinosis, where fatty material builds up in the air sacs, and some infections like atypical pneumonia.
Nodular thickening, characterized by small, bead-like bumps along the septa, frequently points to cellular infiltration. Lymphangitic carcinomatosis, the spread of cancer cells through the lymphatic system in the lungs, is a common cause. Sarcoidosis, an inflammatory disease, and certain dust-related lung diseases like silicosis can also present with nodular thickening.
Irregular thickening is often a sign of fibrosis, indicating scarring within the lung tissue. This pattern is seen in fibrotic lung diseases such as idiopathic pulmonary fibrosis or chronic interstitial lung diseases. Drug-induced lung injury, from medications like methotrexate, can also lead to interlobular septal thickening.
Assessing the Significance of Thickening
The significance of interlobular septal thickening depends on its underlying cause and the patient’s clinical picture. It can be a temporary observation or indicate a serious, progressive disease. The specific pattern, distribution, and associated imaging findings are crucial for assessment.
Diffuse and bilateral smooth septal thickening, often with signs of fluid overload like an enlarged heart or fluid around the lungs, suggests pulmonary edema, commonly from heart failure. Unilateral or asymmetric smooth thickening might suggest lymphatic obstruction, such as from a tumor. “Kerley B lines” on imaging, short horizontal lines, are a specific sign of smooth septal thickening due to fluid accumulation.
Nodular or irregular thickening indicates more serious conditions. Nodular thickening, especially with other nodular lesions or enlarged lymph nodes, suggests infiltrative diseases like lymphangitic carcinomatosis or sarcoidosis. Irregular thickening suggests fibrosis, which can lead to permanent lung damage and reduced lung function. The “crazy paving” pattern, a combination of septal thickening and ground-glass opacity, requires careful evaluation, as it can be seen in various conditions from infection to proteinosis.
Identifying and Addressing Underlying Causes
Determining the specific cause of interlobular septal thickening involves a comprehensive diagnostic process. High-resolution computed tomography (HRCT) scans are the primary tool for visualizing these changes and characterizing their pattern. HRCT allows radiologists to assess if the thickening is smooth, nodular, or irregular, and to identify associated findings like ground-glass opacities, nodules, or architectural distortion.
Beyond imaging, a detailed patient history is crucial, including symptoms, medical conditions, medication use, and environmental exposures. Blood tests may look for markers of inflammation, infection, or autoimmune diseases. Pulmonary function tests assess lung function. In some instances, a lung biopsy may be necessary to confirm a diagnosis. Treatment targets the identified underlying condition, not the thickening itself. For example, diuretics treat fluid overload, antibiotics address infections, or specific therapies manage inflammatory or fibrotic lung diseases.