Hilum of the lung
Each lung (the right and left lungs) can be divided into four main sections: the apex, base, root, and the hilus, or hilum of the lung. Hila, or lung roots, are relatively complicated structures that consist mainly of the major bronchi and the pulmonary arteries and veins. The hilum of the lung is found on the medial aspect of each lung, and it is the only site of entrance or exit of structures associated with the lungs. That is to say, both lungs have a region called the hilum, which serves as the point of attachment between the lung root and the lung.
Structurally, the hilum is a large triangular depressed area on the lung that is located just superior to the center of the mediastinal surface and behind the cardiac impression of each lung, and is found nearer to the back border than to the front. The rib cage is separated from the lung by a two-layered membranous coating called the pleura. The hilum is where the connection between the parietal pleura (covering the rib cage) and the visceral pleura (covering the lung) connect, which denotes the meeting point between the mediastinum and the pleural cavities.
Lung roots are enclosed in a short tubular sheet of pleura that joins the pulmonary and mediastinal parts of pleura. The lung root extends inferiorly as a narrow fold known as the pulmonary ligament. Additionally, it lies opposite to the bodies of the fifth, sixth, and seventh thoracic vertebrae. Structures that form the root of the lung enter and exit at the hilum, and allow the root to be connected to the heart and to the trachea. Functionally, this means that the hilum aids the lung roots by anchoring the lungs to the heart, trachea, and surrounding structures.
The lung root is formed by: the bronchus, the pulmonary artery and veins, the bronchial arteries and veins, the pulmonary plexuses of nerves, lymphatic vessels, bronchial lymph glands, and areolar tissue, all of which are enclosed by a layer of the pleura, which is a thin smooth layer of protective tissue.
More specifically, a lung root has:
- a principal bronchus on one side
- the eparterial and hyparterial bronchus on the other side
- one pulmonary artery
- two pulmonary veins (superior and inferior)
- bronchial arteries (one on one side, and two on the other side)
- bronchial veins
- anterior and posterior pulmonary nerve plexuses
- bronchopulmonay lymph nodes
- areolar tissue
Anatomically, the root of the right lung lies behind the superior vena cava and part of the right atrium, and beneath the azygos vein. The root of the left lung passes under the aortic arch and in front of the descending aorta. Other structures include the phrenic nerve, pericardiacophrenic artery and vein, and the anterior pulmonary plexus, which lie in front of each lung root, whereas the vagus nerve and posterior pulmonary plexus lies behind each lung root. Finally, the area surrounding the hilum of the lung is called the perihilar region.
Hilar abnormalities can present both unilaterally and bilaterally. Since the hila consist of vessels, bronchi, and lymph nodes, changes in a chest X-ray will present as a change in position, size, and/or density of the hilar region. Common causes of these changes will include various types of lung cancers.
Changes in size/density
Bilateral, symmetrical hilar enlargement should raise the suspicion of sarcoidosis, especially in the presence of paratracheal enlargement, or lung parachymal shadowing. Clinically, the patient often presents with joint pain and erythema nodosum. Differentials include pulmonary arterial hypertension, which may also cause bilateral and symmetrical hilar englargement, or lymphoma, metastatic disease, or infection.
Asymmetric hilar enlargement is often the result of breast cancer and/or metastatic disease, with a clinical presentation that may include shortness of breath. In the case of metastatic disease, chest X-ray should reveal multiple small lung nodules.
Changes in hilar position
Abnormal hilar position should be approached by looking at whether the structure has been pushed or pulled. Is there a lung abnormality that has reduced or increased the volume of one lung (such as by hemothorax or pneumothrorax)? Does the patient have a past medical history that would have caused a chance in position? Such as having a hilar malignancy that is treated with radiotherapy, and has resulted in a deviation from normal hilar position.