|Year : 2013 | Volume
| Issue : 2 | Page : 32-37
Approach to unequal hilum on chest X-ray
Supriya Sarkar, Debraj Jash, Arnab Maji, Anupam Patra
Department of Pulmonary Medicine, Nil Ratan Sircar Medical College and Hospital, Kolkata, West Bengal, India
|Date of Web Publication||18-Dec-2013|
Department of Pulmonary Medicine, Nil Ratan Sircar Medical College and Hospital, 19/8 Banerjee Para Road, P. O. Talpukur, District - 24PGS (N), 700 122, West Bengal
Source of Support: None, Conflict of Interest: None
Hilum is a tricky part in chest X-ray (CXR) for understanding. Anatomically, pulmonary arteries and veins, major bronchi, and lymph nodes with pulmonary arteries contributed most of the radiographic density of the hila with superior pulmonary veins making a smaller contribution. Though both hila should be indistinguishable in size and density, we do not get equal hila in majority of CXRs. Rotational malpositioning must be kept in mind, while interpreting CXRs of unequal hilum. For interpreting hilum, we should search for four factors shape, radiopacity, proportionate size, absolute size, hilar angle, contour of pulmonary artery. There are numerous causes of hilar enlargement. It is the clinical presentation which will help in tapering the differential diagnosis. Asymmetric hilum is a challenging subject for pulmonologists. Interpretation of unequal hilum depends on findings of detail history, meticulous clinical examination, and appropriate investigation.
Keywords: Approach, chest X-ray, hilum, interpretation, unequal
|How to cite this article:|
Sarkar S, Jash D, Maji A, Patra A. Approach to unequal hilum on chest X-ray. J Assoc Chest Physicians 2013;1:32-7
| Introduction|| |
Hilum is the most difficult part to interpret in a chest X-ray (CXR). Anatomically hila are composed of pulmonary arteries and veins, major bronchi, and lymph nodes. , Normally bronchi and lymph nodes do not cast any radiological shadow and pulmonary arteries make up most of the radiographic density of the hila with superior pulmonary veins make a smaller contribution [Figure 1]. Whereas, the inferior pulmonary veins enter the left atrium inferior to the hilum and make no contribution to hilar density.  Plain or digital CXR usually gives some clue about hila. Though both hila should be equal in size and density, we do not get identical hila in majority of CXRs. Unequal hilum may be falsely produced by patient rotation that may result in distortion of thoracic anatomy in CXR. Normally the distances between the medial ends of clavicles from the spinous process of the vertebral body are equal, and that become unequal in rotational malpositioning of patient.  When the pseudoinequality of hilum caused by malpositioning is excluded, we should proceed for further investigation.
|Figure 1: The pulmonary arteries (light gray) contributing most of the hilar density and superior pulmonary veins (dark gray) making a lesser contribution|
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| Anatomy of Hilum|| |
The hilar shadows in both projections are produced mainly by the right and left pulmonary arteries. The normal hilum has a branching vascular appearance with successively dividing blood vessels gradually tapering and diminishing in radiopacity [Figure 2]. The conical shaped lower part of each hilum that constitutes lower two-thirds of hilum has greater vascular density than the upper part.  This is because the lower halves of each lung are larger and receive more blood ﬂow. The left pulmonary artery is very characteristic in its appearance on the posteroanterior (PA) film, creating a density above and lateral to left main bronchus. In a normal subject, lateral border of the main pulmonary artery should be smooth. On the lateral view film, left pulmonary artery courses over the origin of left upper lobe bronchus which is seen as circle on edge. The left pulmonary artery descends behind the bronchus. A hilar region is a depression or fissure where vessels or nerves or ducts enter an organ. This region is not easily found because it is superimposed with other structures of the lung creating a complex structure which is difficult to define. We investigated how the radiologists find the region and what features they need about the region's size and location. A radiologist first searches from just below the aorta of the right lung, imagines a semicircle, with a radius size over one-third of the right lung width, from the bottom of the aorta, then checks the density of the region. If the dense region is wide enough around the semicircle or within the semicircle, it means the hilar region is considered big and vice versa. The radiologist then moves to the left lung and searches about 3 cm down from the right hilar region starting point and repeats b. and c. for the left lung. On the PA film, the right pulmonary artery casts a shadow that is mainly lateral to the bronchus intermedius and largely inferior to the right upper lobe bronchus. Like the left pulmonary artery, the major portion of the right descending pulmonary artery has a smooth contour. It must be emphasized that both pulmonary arteries are branching structures that result in multiple, superimposed densities.  In the lateral view, the right pulmonary artery is seen on end, with its superior portion superimposed over the origin of the left pulmonary artery and its inferior portion anterior and slightly inferior to the left upper lobe bronchus. Pulmonary veins can be best distinguished from arteries by tracing the course of vessels, keeping in mind that veins converge on the left atrium inferior to the hilum.
|Figure 2: Chest X-ray posteroanterior view of a normal individual highlighting hilum of both side|
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| Chest Radiology|| |
There is wide variation in the radiographic appearance of normal hilum and a small mass can be difficult to detect. Simply observing whether the hilum looks "big" is not a reliable means of determining whether it is normal or abnormal. Instead, accurate interpretation depends on an understanding of the normal anatomical features of the hilum and the changes that occur in various disease states. This requires detailed understanding of the anatomy of hilum in at least the PA and lateral projections. Although the absolute size of the hilum would seem to be a straightforward criterion of hilar enlargement, it is not reliable unless the enlargement is considerable. There is no deﬁnite measurement for hilar enlargement but hilar asymmetry should not be ignored.
The first problem in the evaluation of a hilar mass is its recognition. The radiological findings in the case of a hilar mass are increased density or alteration in the configuration of hilum. It is important to note whether the increased opacity blends imperceptibly with the normal pulmonary artery shadows or the pulmonary arteries are clearly distinguished from the suspected density. Once it has been established that hilum is abnormal, next step is to determine whether the increased opacity is an enlarged vascular structure or solid mass.
Lateral view can be helpful in revealing hilar abnormalities, especially when ﬁndings on the frontal view are equivocal. Correctly interpreting the lateral view requires knowledge of normal hilar anatomy. The trachea is an air-ﬁlled structure that tapers and ends at the level of the hilum. The right main pulmonary artery is located anterior to the distal trachea. The left main bronchus is visible as a round air-ﬁlled structure near the inferior end of the trachea. The left main pulmonary artery arches over the left main bronchus and parallels the inferior border of the aortic arch. The inferior pulmonary veins enter the left atrium just below the hilum. Normally, there is opacity anterior to the distal trachea (the right main pulmonary artery), but there should be aerated lung posterior and inferior to the distal trachea [Figure 3].
|Figure 3: Chest X-ray lateral view of (a) normal hilum (pulmonary arteries denoted by black arrows) (b) hilar mass|
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The lateral view is useful in predicting the presence of hilar masses or adenopathy. They appear as areas of increased opacity in locations where increased opacity is not normally seen like below the carina, posterior to the trachea, and anterior to the trachea above the hilum, etc.
There are few radiological signs that are helpful in assessing hilum.
There are four criteria for assessing hilum
- Dense Hilum' sign: Most common presentation of a hilar mass is increased density over hilum. The dense hilum sign suggests a pathological process at the hilum: Hilar malignancy or bronchogenic carcinoma should be suspected. On a well-centred chest PA radiograph the density of the hilum is comparable on both sides. In absence of calcification or adenopathy, the hila should appear of equal density and be symmetric. Identification of increased density is made by comparing with the opposite side
- 'Hilum overlay' sign: A silhouette sign of the hila is called the "hilum overlay sign". It is used to determine the localization of a lesion in the hilar region in CXRs. If hilar vessels can clearly be seen inside the lesion, the lesion is either anterior or posterior to the hilus. If the hilar vessels cannot be discriminated from the lesion, the lesion is at the hilus. An area of increased opacity either anterior or posterior to the hilum has no effect on the air adjacent pulmonary artery and therefore no effect on its visibility. Felson described this phenomenon as hilar overlay sign.  This is useful for separation of true hilar masses from superimposed pulmonary opacities. In such cases, lateral view will help in verifying that abnormality is not in the hilum
- 'Hilar convergence sign': The hilum convergence sign is a useful chest radiograph sign to help distinguish a bulky hilum due to pulmonary artery dilatation. Pulmonary vessels can be seen to converge and join a dilated pulmonary artery. The sign, described by Felson,  is useful to distinguish between a prominent hilum and an enlarged pulmonary artery. If branches of pulmonary artery converge towards central mass it is an enlarged pulmonary artery rather than mass or lymph node in the hilum
- Contour of the pulmonary artery: The contour of the pulmonary artery is visible because of adjacent aerated lung. When the aerated lung is either filled or displaced, the border of the pulmonary artery will no longer be detected. When the borders of pulmonary arteries are clearly identifiable, it must be assumed that the mass is discrete and either anterior or posterior to hilum
- Hilar angle: Hilar angle is the angle made between superior pulmonary vein and intralobar pulmonary artery. It is normally greater than 90° (concave). Loss of concavity will help in identifying hilar mass. Estimation of hilar angle will also help in identification of hilar mass.
Causes of hilar enlargement
- Shape - A branching vascular appearance is normal
- Radiopacity - Gradually diminishes toward the periphery
- Proportionate size - Two-third of the vascular density is in the lower portion of the hilum
- Absolute size - Not reliable unless enlargement is considerable; compare left and right for symmetry.
Unilateral or bilateral asymmetrical Lymphadenopathy
- Lymphadenopathy and tumors - rounded/lobular, nonbranching structures in which the radiopacity abruptly diminishes at the margin of the tumor or lymph node
- Pulmonary venous hypertension - enlargement of the superior pulmonary veins causes increased vascular density in the upper half of the hilum. Important causes of pulmonary venous hypertension are left ventricular failure, mitral stenosis, or mitral regurgitation
- Pulmonary arterial hypertension - central pulmonary arteries are dilated causing hilar enlargement with a branching appearance with peripheral pruning due to abrupt tapering of vessels. The causes of pulmonary arterial hypertension may be primary or secondary to lung diseases such as chronic obstructive pulmonary disease (COPD)
- Increased pulmonary blood ﬂow - increased central and peripheral pulmonary vascular markings (peripheral lung markings become visible in the peripheral 1-2 cm of the lung). Increased pulmonary blood ﬂow of two to three times greater than normal is required to make it radiologically visible. The mechanisms of increased pulmonary blood flow are left-to-right intracardiac shunts and hyperdynamic circulation.
Unilateral or bilateral asymmetric lymphadenopathy is the most important and challenging cause of unequal hilum that concern every pulmonologist. Important causes of hilar lymphadenopathy are:
Clinical approach to unequal hilum
- Tuberculosis - TB is the most important cause of asymmetric hilum particularly in our country. Hilar lymphadenopathy occurs as a major component of primary TB, usually occurs in children. Tubercular hilar lymphadenopathy may occur in adult particularly in immunocompromised persons (human immunodeficiency virus (HIV) infection)
- Bronchogenic carcinoma - most important cause of unequal hilum in adult. It may be due to spread to hilar lymph node or due to growth itself
- Lymphoma - usually causes bilateral hilar lymphadenopathy and may be unequal. Peripheral lymphadenopathy, a usual finding of lymphoma, may be absent and in that situation diagnosis becomes difficult
- Sarcoidosis - usually causes bilateral symmetrical hilar lymphadenopathy, but sometimes it may be asymmetric
- Other causes - infection caused by fungal, atypical mycobacteria, viral, tularemia, and anthrax may cause unilateral hilar enlargement. Silicosis, drug reaction, etc., are the other rare causes of unequal hilum.
A detail history, meticulous clinical examination, judicious use of diagnostic tools, and practical clinical analysis are necessary steps in our approach to unequal hilum.
| History|| |
A detail history from patients and sometimes from close relatives is an essential step. Particular emphasis has to be given on age (malignancy in elderly and TB in young patients), , sex, duration of illness, smoking history (bronchogenic carcinoma in smokers and sarcoidosis in nonsmokers), occupational history, travel to foreign country, presence of general symptoms (fever, weight loss, night sweeting for TB, and B symptoms for lymphoma), hemoptysis, shortness of breath, chest pain (persistent, progressive, and severe chest pain suggest malignancy), and symptoms of mediastinal involvement (hoarseness of voice dysphagia, dyspnea, neck swelling, etc.) suggestive of malignancy. On examination we should look for clubbing for bronchogenic carcinoma, generalized lymphadenopathy for lymphoma; supraclavicular lymphadenopathy for lung cancer; hepatospleenomegaly for lymphoma or sarcoidosis; superior vena caval syndrome and paraneoplastic syndrome for lung cancer; and systemic signs (arthralgia, skin rash, eye signs, etc.) for sarcoidosis. Accidental detection of hilar adenopathy in asymptomatic patients should suggest sarcoidois and sometimes TB. We should carefully search for palpable and loud pulmonary component of second heart sound for pulmonary arterial hypertension; murmurs and other signs of left ventricular disease; signs of congenital heart diseases, etc.
| Investigations|| |
Apart from routine investigation, we should look for HIV seropositivity, sputum for acid fast bacilli, sputum for malignant cells, and blood for serum angiotensin converting enzyme (SACE). SACE is elevated in 60-80% cases of sarcoidosis, , but increased levels are also seen in TB, histoplasmosis, coccidiodomycosis, mycobacterium avium intercellulare infection.  Mantoux test is usually positive in tuberculosis and negative in sarcoidosis. ,, It was found that 90.2% patients of sarcoidosis are Mantoux negative. , Other diseases that may cause negative mantoux in unequal hilum are lymphoma and malignancy.
| Special Investigation|| |
Contrast-enhanced computed tomography
CECT is helpful to differentiate lymph nodes from vascular structures as lymph nodes usually do not take contrast. However, nodes in Castleman's disease classically take contrast.  Hypervascular metastasis like that of melanoma, hypernephroma, thyroid carcinoma, Kaposi sarcoma can take contrast.  Mild enhancement may be seen in tuberculosis, fungal infection, lymphoma, metastatic lung cancer, and sarcoidosis. , Low attenuation areas in lymph node suggest necrosis and is classically seen in tuberculosis. , Low attenuation may also be seen in fungal infections, lymphoma, Whipple's disease, and metastasis from lung, testis, and ovary. Calcification within lymph nodes usually suggest granulomatous disease like TB, histoplasmosis, sarcoidosis (usually in 25% of cases), silicosis, nontuberculous mycobacterial infection; but may occur in metastasis in osteosarcoma, prostatic carcinoma, and lymphoma after radiotherapy.  Egg shell calcification although classically found in silicosis, but may also be seen in TB and sarcoidosis.  CT-thorax is useful to detect and evaluate associate lung parenchymal, airway, and pleural diseases.
FOB is a useful tool for assessing unequal hilum. Procedure that can be done through FOB are (i) transbronchial needle aspiration (TBNA) and subsequent cytology and staining for TB, fungus, etc.; and (ii) endobronchial ultrasound (EBUS) for identification and specification of lesions and for guided diagnostic procedure.
Includes (i) magnetic resonance imaging (rarely necessary except for diagnosis of mediastinal fibrosis, cardiac chamber enlargement); (ii) positron emission tomography (PET)-CT scan (to detect malignant lesion, but granulomatous diseases may be PET positive); (iii) thoracic USG (it is widely used in pleural diseases but now its scope in pulmonary medicine is increasing as it has the advantage of real time imaging); (iv) mediastinoscopy (useful for direct visualization and taking biopsy from the mediastinal lymphnodes); (v) mediastinotomy (particularly helpful from glands of aortopulmonary window); and (vi) thoracoscopy (useful for taking pleural and lung biopsy).
| Summary and Conclusion|| |
Primary TB is notorious for producing asymmetrical hilar lymphadenopathy.  Though primary TB can occur in adult it is considered primarily a childhood disease.  So, TB should be considered first in the differential diagnosis of unequal hilum in children. TB should be considered in differential diagnosis of unequal hilum in adult if there are constitutional symptoms like fever, weight loss, night sweat, etc. Malignancy should be considered in differential diagnosis of unequal hilum in adult particularly in elderly smoker. Histoplasma, though uncommon in India, is a well-known cause of asymmetrical hilar lymphadenopathy.  TB in India, histoplasmosis in North and South America, and melidiosis in Vietnam have similar clinical manifestations, so geographic location of patient's residence or history of travel to endemic country may be indicative of the etiology. Hilar lymphadenopathy with calcification is more common in histoplasmosis than tuberculosis. , Sarcoidosis patients may be asymptomatic or may have systemic symptoms like eye symptoms, arthralgia, cutaneous signs, cardiological symptoms along with hilar enlargement. Occupational history of quarrying, mining, drilling, tunneling, and abrasive blasting with quartz-containing materials favor the diagnosis of silicosis in individuals presenting with unequal hilum.  TB, histoplasmosis, other fungal diseases, nontuberculous mycobacteria, non-Hodgkin's lymphoma, bronchogenic carcinoma all can cause unequal hilum in a HIV positive individual. Pulmonary embolism particularly in high risk individuals should be considered in the differential diagnosis of unequal hilum as enlarged right descending pulmonary artery may produce unequal hilum. 
Asymmetric hilum is a challenging subject for pulmonologists. Interpretation of unequal hilum depends on findings of detail history, meticulous clinical examination, and appropriate investigation.
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[Figure 1], [Figure 2], [Figure 3]