Hilar Enlargement

Pulmonary Arterial Hypertension

Pulmonary arterial hypertension is a major cause of bilateral hilar enlargement ( Fig 11.5, A-D ). The loss of normal arterial tapering results in an abrupt change in caliber between the proximal pulmonary arteries and peripheral pulmonary arteries. There is marked enlargement of the proximal vessels ⁷² and abrupt tapering or loss of the peripheral vessels. Measurements of the width of the right descending pulmonary artery (>16 mm) and of the left descending pulmonary artery (>18 mm) correlate well with an elevated pulmonary arterial pressure (>20 mm Hg). ³⁶⁸ Other radiologic clues to the diagnosis of pulmonary artery hypertension include enlargement of the main pulmonary artery and enlargement of the right ventricle. The main pulmonary artery produces a bulging rounded shadow that projects over the left pulmonary artery and may appear to enlarge the left hilum on the PA view (see Fig 11.5, A ). On the lateral view, enlargement of the right ventricle and main pulmonary artery has the effect of producing an opacity over the ascending aorta or sometimes filling in the retrosternal clear space. Main pulmonary artery enlargement is easily confirmed by CT showing that the main pulmonary artery is larger than the diameter of the aorta (see Fig 11.5, D ). ⁶⁶

Postcapillary pulmonary arterial hypertension is usually the result of congestive heart failure. Identification of the anterior segmental bronchus leading to either upper lobe may also be helpful in evaluating suspected pulmonary arterial enlargement. Left-sided heart failure is the most common cause of right heart failure and therefore is a common cause of pulmonary arterial hypertension. This is all preceded by pulmonary venous hypertension; as a result, left heart failure differs significantly from most of the other causes of pulmonary artery hypertension described in this chapter. Congestive heart failure and mitral stenosis both result in enlargement of the upper lobe vessels and constriction of the lower lobe vessels, or cephalization. Identification of the anterior segmental bronchus leading to either upper lobe may also be helpful in evaluating suspected cephalization of flow to the upper lobes. When the bronchus is seen on end, it appears as a ring shadow. The segmental pulmonary artery is adjacent to the bronchus. In the normal patient, the size of this artery is slightly larger than that of the bronchus when seen on end. When the anterior segmental artery is more than twice the caliber of the anterior segmental bronchus, pulmonary arterial enlargement can be inferred. A change in the size of a segmental artery over a short period may provide a helpful clue to the diagnosis of pulmonary arterial hypertension secondary to congestive heart failure ( Fig 11.6, A and B ). Cephalization ⁴⁵⁴ is rapidly followed by interstitial edema as the intravascular pressure exceeds the osmotic and interstitial pressures, which normally maintain fluid in the vascular compartment. The radiologic result is a fine reticular pattern and increased vascular markings with blurring of the margins of the pulmonary vessels. This may be accompanied by enlargement of the hila, main pulmonary artery, and right ventricle.

Because of the increased markings, this radiologic appearance of postcapillary pulmonary arterial hypertension is in sharp contrast to the appearance of the causes of precapillary pulmonary artery hypertension. Precapillary pulmonary artery hypertension results in enlargement of the hilar vessels with a decrease in the peripheral vessels. In contrast with postcapillary pulmonary artery hypertension, it requires consideration of a larger number of possible causes.

Chronic lung diseases are a major cause of precapillary pulmonary artery hypertension; therefore identification of other signs of lung disease may be key to the correct diagnosis of pulmonary arterial hypertension. In the case of emphysema, the presence of large avascular areas surrounded by thin lines that indicate bullous lesions is helpful in making the diagnosis. The other radiologic signs of emphysema are discussed in Chapter 22 . In addition to emphysema, chronic restrictive interstitial diseases from a variety of causes, including idiopathic pulmonary fibrosis, scleroderma, sarcoidosis, and cystic fibrosis, may cause pulmonary arterial hypertension (see Chapter 19 ).

Pulmonary embolism is another major cause of pulmonary arterial hypertension related to lung disease. ⁶⁵ An acute, massive pulmonary embolism may produce radiologic signs of pulmonary arterial hypertension with marked prominence of the hilar vessels, obliteration of the peripheral vessels, and right-sided heart enlargement. The diagnosis of chronic pulmonary emboli may be suggested by a clinical history of recurrent episodes of pleuritic chest pain and hemoptysis associated with thrombophlebitis, but frequently the history is less dramatic. The diagnosis of pulmonary artery hypertension resulting from a pulmonary embolism is usually confirmed with CT ( Fig 11.7, A-D ).

Metastatic tumor emboli are rarely diagnosed premortem. Increasing size of the proximal pulmonary arteries may be the only chest radiographic finding. Radionuclide lung scans are reported to show multiple, small peripheral defects. Because these emboli are usually very small, they do not produce the segmental and lobar defects expected with thromboemboli. In addition, pulmonary arteriograms typically reveal small tortuous vessels typical of pulmonary arterial hypertension and fail to demonstrate the very small tumor emboli. Because of the severe pulmonary arterial hypertension, pulmonary arteriography is a high-risk procedure in patients with tumor emboli. The diagnosis is sometimes confirmed by lung biopsy or at the postmortem examination.

Congenital heart disease is another important cause of pulmonary arterial hypertension ( Fig 11.8, A-C ). It is most commonly secondary to severe left-to-right shunts that remain undiagnosed and untreated for a long time. These include VSDs, ASDs, patent ductus arteriosus, and, less frequently, the admixture lesions—transposition of the great vessels and truncus arteriosus. These heart lesions are usually suspected from clinical signs, particularly when characteristic murmurs are detected. The VSD, ASD, and patent ductus arteriosus are acyanotic lesions until the pulmonary arterial hypertension becomes severe enough to reverse the shunts. This is in contrast to the admixture lesions, which are a cause of early cyanosis. Definitive diagnosis of the cardiac causes of pulmonary hypertension may be made by echocardiography, CT, magnetic resonance imaging (MRI), or catheterization.

The remaining cause of pulmonary arterial hypertension is referred to as idiopathic or primary. ⁴⁷⁶ Although this is a diagnosis reached by exclusion, it is not a rare condition, and it has an unfavorable prognosis. Of the causes of pulmonary arterial hypertension considered in this chapter, chronic pulmonary emboli are the most difficult to distinguish from primary idiopathic pulmonary hypertension. Only the characteristic plexiform arterial changes seen in histologic sections of the lung permit the definitive diagnosis of idiopathic pulmonary arterial hypertension ⁶⁰⁷ —plexogenic pulmonary arteriopathy.