The horizontal fissure is found only in the right lung, where it separates the upper lobe from the middle lobe. It runs roughly horizontally forward from the oblique fissure to the front of the lung. Like the oblique fissure, it is lined by visceral pleura and is often incomplete.
| Location | Right lung only, along the line of the 4th rib and costal cartilage from the oblique fissure at about the midaxillary line to the sternum. |
|---|---|
| Parts |
|
Separates the right upper and middle lobes and allows them to move independently.
On a frontal chest X-ray the horizontal fissure is often visible as a thin line; displacement upwards suggests upper lobe collapse and downwards suggests middle lobe collapse. It may be absent or incomplete in a significant proportion of people. Fluid tracking into it is common in heart failure.
Fluid collecting within the horizontal fissure, typically from heart failure. It may appear as a lens-shaped shadow that clears with diuretics.
Collapse of the upper lobe that pulls the horizontal fissure upwards. It is often caused by a central tumour or mucus plug.
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