Left horizontal fissure

Last revised by Ciléin Kearns on 9 Mar 2023

The left horizontal (or minor) fissure is an accessory fissure found in 1.6-8% of individuals 1,3.

The left horizontal fissure separates the lingula from the rest of the left upper lobe (anterior-apicoposterior segments). The segmental anatomy of the left lung is preserved, despite the accessory fissure, with the lingula retaining its superior and inferior segments rather than assuming the lateral and medial arrangement of the right middle lobe segments 2.

On frontal radiographs, the fissure resembles the right horizontal fissure, except that it may be higher (more common) or lower. The fissure usually slopes caudally from its medial to its lateral margin 2. It tends to be dome-shaped on at least one projection.The fissure infrequently has a horizontal course 1.

Like the right horizontal fissure, it can become conspicuous on plain film by air space opacification on either side or pleural effusion involving it.

Consider:

  • pleural-parenchymal scarring

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