Horizontal fissure

Last revised by Ciléin Kearns on 10 Oct 2023

The horizontal fissure, also called the minor fissure or transverse fissure, is a unilateral structure in the right lung that separates the right middle lobe from the right upper lobe.

The horizontal fissure arises from the right oblique fissure and follows the 4th intercostal space from the sternum until it meets the oblique fissure as it crosses the right 5th rib 1.

The horizontal fissure is highly variable and can be incomplete or absent in some patients. According to different postmortem dissection studies, the prevalence of incomplete horizontal fissures can vary between 8-35% and may be absent in 3-50% of right lungs 2-4.

A minority of individuals have a horizontal fissure in the left lung, independent of the right horizontal fissure.

The horizontal fissure can be visualized on both conventional radiography and computed tomography (CT) scans.

It is seen in ~65% (range 5-80%) of normal frontal chest radiographs as a thin line running horizontally from the edge of right lung towards the right hilum, at approximately the level of the anterior 4th rib 5.

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