The horizontal fissure separates the right upper lobe from the right middle lobe. It can be seen on normal chest X-rays as a thin line running roughly horizontally from the edge of the lung towards the right hilum.
The most common accessory fissure you will see on a chest X-ray is an azygos fissure. This occurs in approximately 1-2% of individuals.
The azygos vein usually runs horizontally along the right side of the mediastinum. It hooks forwards over the right main bronchus, draining the azygos system into the superior vena cava.
If there is an azygos fissure, the vein appears to run within the lung, but is actually surrounded by both parietal and visceral pleura. The azygos fissure therefore consists of four layers of pleura, two parietal layers and two visceral layers, which wrap around the vein giving the appearance of a tadpole.
Assessing the fissures
Occasionally lung disease is limited in extent by a fissure. This can help locate a disease process more specifically to a lobe. For most cases this degree of accuracy is not clinically important, unless further action such as biopsy or surgery is required, in which case other imaging such as CT would probably be performed. In most cases it is still best to refer to the location of lung abnormalities seen on a chest X-ray in terms of lung zones rather than lobes.