horizontal fissure

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

Gross anatomy

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

Variant anatomy

The horizontal fissure is highly variable and can be found to 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 the patients' right-sided lungs .

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

Radiographic features

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

Plain radiograph

seen on ~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 4 rib