Interlobular Septal Thickening

Smooth Septal Thickening

Definition

Interlobular septa are sheetlike structures 10–20-mm long that form the border of the secondary pulmonary lobules. The septa are usually perpendicular to the pleura in the lung periphery. They are composed of connective tissue and contain lymphatics and pulmonary venules. On CT scans, diseases affecting one of the components of the septa are responsible for thickening and thus cause the septa visible [1] (Figs. 16.1 and 16.2).

Diseases Causing the Pattern

The diseases causing smooth interlobular septal thickening include pulmonary edema (Fig. 16.2), pulmonary lymphangitic carcinomatosis (PLC), lung involvement of lymphoma or leukemia (Fig. 16.1), diffuse alveolar hemorrhage (DAH), pneumonias, and lipid storage disease (Niemann–Pick disease) [2] (Fig. 16.3).

The septa may show irregular thickening in pulmonary fibrosis.

Distribution

In most diseases, lung abnormalities show random and patchy and extensive distribution. In PLC, distribution may

be unilateral or bilateral, focal or diffuse, and symmetric or asymmetric. In Niemann–Pick disease, lesions may initially involve only the base and progress to the entire lung [2]. Also in pulmonary fibrosis, the lesions usually start in the lower lung zones to spread to the middle and upper lung zones.

Clinical Considerations

Impaired cardiac function (heart failure or atrial fibrillation) and fluid overload (e.g., for chemotherapy) in the condition of hampered cardiac function are the most common cause of interstitial pulmonary edema [3]. The most common primary cancer sites related to PLC are the breasts, lungs, colon, and the stomach [4]. Niemann–Pick disease is caused by an inherited defect in the production of sphingomyelinase, resulting in the deposition of sphingomyelin in the liver, spleen, lungs, bone marrow, and brain. Pulmonary involvement can be asymptomatic or severe enough to cause respiratory failure. The disease is characterized histopathologically by the aggregates of large foamy cells (NP cells) in the alveolar septa, bronchial wall and the pleura, thus causing ground-glass opacity (GGO) in the upper lung zones and interlobular septal thickening in the lower lung zones [2].