Hilum Anatomy and Abnormalities

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

The hilum of the lung is the wedge-shaped area on the central portion of each lung, located on the medial (middle) aspect of each 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

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 

This hilar region of the lung may be affected by tumors in the region (including both primary tumors and 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 cancers 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. Possible causes of enlarged hilar lymph nodes (lymphadenopathy) are discussed below.
  • 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), an 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 most often 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:

    • Lung cancer. Lung cancer is the most common cause of unequal hilar regions in adults, both due to the presence of tumor and to the presence of involved lymph nodes.
    • Other cancers. Metastatic breast cancer can lead to hilar lymphadenopathy both due to the spread of the cancer to this region and due to involved lymph nodes. Lymphoma and other mediastinal tumors may also lead to enlarged hilar lymph nodes as well.
    • Tuberculosis. Worldwide, tuberculosis is the most common cause of hilar adenopathy in children.
    • Inflammation. Conditions such as sarcoidosis, amyloidosis, and silicosis can cause hilar lymphadenopathy. Sarcoidosis is the most common cause of bilateral hilar lymph node enlargement, especially in young adults. The hilar lymph node enlargement with sarcoidosis is usually symmetric in contrast to other common causes. Drug reactions are also a relatively common cause of hilar adenopathy.
    • Other infections. Infections such as mycobacteria, viral infections such as infectious mononucleosis, berylliosis, tularemia, histoplasmosis, and coccidiomycosis can lead to enlarged lymph nodes in this region. In the United States, histoplasmosis is common in the Ohio and Mississippi River Valleys, with coccidiomycosis more common in the southwestern states.
    • Other causes. Castleman's disease is a condition characterized by abnormal lymph tissue. Heart failure may also lead to hilar adenopathy.

    Bottom Line

    There are a number of conditions which can cause an abnormal appearance of the hilum on imaging studies, many of which are serious. The first step, however, is to make sure that any findings are not due simply to malposition of the body when taking these films. With the number of structures that pass through this area, even mild rotation may give the appearance of an abnormality when none is present.

    Tumors, both primary and metastatic, are a far too common cause of both hilar masses and lymphadenopathy. The most common causes overall include tuberculosis worldwide, and conditions such as histoplasmosis, coccidiomycosis, and sarcoidosis in the United States.

    If your doctor notes an abnormality on your exam, further testing will be indicated. She will also ask a careful history looking for any other symptoms suggestive of a tumor, infection, or inflammatory process. Most of the time a biopsy will be needed to obtain a definitive diagnosis.

    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.

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