Incomplete fissure sign | Radiology Signs

Answer:

Incomplete fissure sign refers to loculated pleural fluid that collects within the recess of an anatomically incomplete interlobar fissure (a fissure that does not fully separate two lobes, leaving a zone of parenchymal fusion). On CT and chest radiograph this produces a wedge- or triangular-shaped opacity along the course of the fissure, with a lucent gap medially where the fissure is fused and the fluid cannot extend — a finding that can mimic a mass or consolidation if the fissural anatomy is not recognized.

Why is it called so?

It is named for the underlying anatomic variant — an incomplete interlobar fissure — that permits fluid to loculate only where the fissure is patent, producing a shape and distribution distinct from free-flowing pleural effusion. Recognizing this appearance as a consequence of fissural incompleteness (rather than a true parenchymal or pleural mass) is the point of the sign.

Pathophysiology

Interlobar fissures are frequently anatomically incomplete, with variable degrees of parenchymal fusion between adjacent lobes. When pleural fluid accumulates, it preferentially layers into the patent portion of an incomplete fissure. This creates a curvilinear or wedge-shaped opacity that stops abruptly where the fissure is fused, with lucency medial to it. The appearance can be mistaken for a pulmonary nodule, mass, or focal consolidation, so correlating with the known fissural anatomy (and follow-up or multiplanar CT) is important to avoid misdiagnosis.

Alternative names: Incomplete pulmonary interlobar fissure sign

Other associated named signs: None

References

1. Lubner MG. The incomplete fissure sign. Radiology. 2008;247(2):589-590.