What Is the Hilum of the Lung?

Definition and Anatomy of the Hilum in the Lungs

diagram illustrating the location of the hilum in the lung
Where is the hilum of the lungs, what structures occur here, and what medical conditions affect this region?. istockphoto.com

Definition: Hilum of the Lung

The hilum of the lung is the wedge-shaped area on the central portion of each lung, located on the medial (middle) aspect of the lung. The hilum is where the bronchi, the arteries, veins, and nerves enter and exit the lungs.

Anatomy of the Hilum

Both the right and the left lung have a hilum which lies roughly midway down the lungs, and slightly towards the back (closer to the vertebrae than to the front of the chest.) Each lung may be visualized as having an apex (the top) a base (the bottom) a root and a hilus.

The major bronchi, pulmonary arteries, pulmonary veins, and nerves are the structures which enter and exit the lungs in this region. Lymph nodes, called hilar lymph nodes, are also present in this region. Both hilum are similar in size, with the left hilum usually found slightly higher in the chest than the right hilum.

Imaging of the Hilum

On a chest x-ray, the hilar region reveals a shadow which consists of the combination of lymph nodes, the pulmonary arteries, and the pulmonary veins. Due to the overlap of these structures, it can sometimes be difficult to detect enlargement of these lymph nodes or the presence of a mass in this region. This is one of the reasons why ordinary chest x-rays can miss lung cancer.

Imaging tests such as CT scans (especially with contrast) can lead to better visualization of these structures.

Sometimes, further testing such as a PET scan, bronchoscopy with endobronchial ultrasound, or mediastinoscopy may be needed to better visualize the region or to obtain a biopsy sample.

Abnormalities of the Hilum

This hilar region of the lung may be affected by tumors in the region (including metastatic tumors), enlargement of hilar lymph nodes, as well as abnormalities of the pulmonary arteries or veins.  

Hilar Enlargement/Hilar Masses

There are four main reasons why the hilum of one or both lungs may appear enlarged on an x-ray.

 These include:

  • Tumors and lymphadenopathy - Cancers such as lung cancer and lymphomas, as well as cancer that has spread to this region from other parts of the body (metastatic cancer) can cause masses in this region )as well as enlargement of the lymph nodes due to metastases.)
  • Pulmonary venous hypertension (elevated pressure in the pulmonary veins) - Pulmonary venous hypertension may occur due to medical conditions such as heart failure and heart valve problems such as mitral stenosis and mitral regurgitation.
  • Pulmonary arterial hypertension (PAH - elevated pressure in the pulmonary arteries) - PAH may occur as a primary disease (not secondary to another problem) or as a secondary problem which in turn is caused by chronic obstructive pulmonary disease (COPD.)
  • Increased pulmonary blood flow - Conditions such as cyanotic congenital heart disease (heart defects present at birth which cause a blue tinge to the skin due to a reduced oxygen content) can result in increased pulmonary blood flow.

Hilar Lymphadenopathy (Enlarged Hilar Lymph Nodes)

Enlarged lymph nodes in the hilum may occur in both the right and left hilum (bilateral lymphadenopathy) or on side alone (asymmetric lymphadenopathy.) Causes may include:

  • Other infections such as mycobacteria, viral infections such as infectious mononucleosis, berylliosis, tularemia, histoplasmosis, and coccidiomycosis can lead to enlarged lymph nodes in this region.

Example: John was told that the cancer in his right lung had spread to his hilar lymph nodes, lymph nodes near the area where the bronchi enter the lungs.

Sources:

Nin,C., de Souza, V., do Amaral, R. et al. Thoracic Lymphadenopathy in Benign Diseases: A State of the Art Review. Respiratory Medicine. 2016. 112:10-7.

Sarkar, S., Jash, D., Maji, A., and P. Anupam. Approach to unequal hilum on chest X-ray. The Journal of Association of Chest Physicians. 2013. 1(2):32-37.

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