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Idiopathic azygos vein aneurysm: a rare cause of mediastinal mass

Abstract

Venous aneurysm of the azygos arch is a very rare cause of mediastinal mass and is usually an incidental finding on chest radiography. Nowadays the diagnosis is made by non-invasive tests such as thoracic CT scanning and/or magnetic resonance imaging. The case is described of an asymptomatic woman in whom a mediastinal mass due to an azygos vein aneurysm was diagnosed by non-invasive procedures, the aetiology of which, in all probability, was idiopathic.

  • mediastinal mass
  • azygos vein
  • aneurysm

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Enlargement of the azygos arch vein can be due to an increase in central venous pressure, portal vein hypertension, azygos continuation of the inferior vena cava (IVC), a tumour, or local thrombosis located in the inferior vena cava.1 2 High central venous pressure is by far the most common cause and mainly results from cardiac decompensation.3 True aneurysmal dilatation is a highly uncommon entity which has been reported on very few occasions.1 2 4-9

We report a case of azygos vein aneurysm diagnosed by computed tomographic (CT) scanning and magnetic resonance imaging (MRI).

Case report

A non-smoking 64 year old woman was referred to our hospital with coughing, wheezing, and scanty expectoration. Except for mild hypertension, her past medical history was unremarkable. No significant trauma was recorded. Physical examination was normal except for the presence of bilateral wheezing; no signs of cardiac failure were detected. Blood analysis (including a haemogram, differential cell count, hepatic and renal function, ionogram, and proteinogram) and arterial blood gas tensions in room air were normal.

The posteroanterior chest radiograph was normal (fig 1a) but the lateral view showed a possible retrotracheal mediastinal mass (fig 2). A second chest radiograph on expiration showed a mediastinal mass (fig1b). A CT scan before and after intravenous contrast enhancement confirmed the presence of a slowly enhancing mass in the pathway of the azygos arch measuring 3.5 cm (fig 3). Axial T1 weighted and T2 weighted magnetic resonance imaging showed a heterogeneous mass and, after injection of gadolinium-DTPA, the lesion became homogeneously hyperintense and isointense with respect to the rest of the vascular structures (aorta, vena cava). However, contrast uptake occurred later due to upward venous flow in the azygos vein (fig 4). The findings were compatible with an aneurysm of the azygos vein.

Figure 1

(a) Full inspiration. Normal appearance of the mediastinum. (b) The lesions become larger with full expiration, providing that the mass is not solid in character (arrow).

Figure 2

Lateral view showing a mass localised in the retrotracheal area (arrow).

Figure 3

CT section at the level of the azygos arch confirming the presence of a mass located in the azygos path (arrow). No anomalies were seen in higher and lower slices.

Figure 4

T1 scan after injection of gadolinium showing the mass with similar enhancement to the aorta.

Discussion

Most patients with venous aneurysms are asymptomatic and the lesion is detected as an incidental finding on the chest radiograph (table 1). The aetiology of the azygos aneurysm was unknown, but since no history of high pressure in the azygos system or recent trauma were recorded despite extensive anamnesis, clinical and radiographic examination, it was assumed to be congenital.

Table 1

Characteristics of venous aneurysms reported in the literature

During the third and fourth weeks of gestation the cardinal vein system develops. This system consists of paired anterior and posterior cardinal veins which unite to form a short common cardinal vein. The anterior cardinal vein gives rise to the subclavian, the internal jugular, the brachiocephalic vein, and the superior vena cava. The posterior cardinal veins are replaced by two additional pairs of veins, the subcardinal and the supracardinal veins (which gives rise to a portion of the inferior vena cava and the azygos system). A segment of the right supracardinal vein anastomoses with a part of the superior vena cava (derived from the anterior cardinal vein) so that the azygos drains into the right atrium via the superior vena cava.10 11 It has been postulated that an aplasia or hypoplasia of the superior vena cava could affect azygos drainage,6 but in our case no cava affection was observed. Moreover, the aneurysm was localised at the junction of the supracardinal and the anterior cardinal veins which, anatomically, is a weak point. This lends support to the idea of a congenital aetiology in our case.

Veins are composed of three layers: the intima, the media, and the adventitia. During vasculogenesis the first layer to appear is the intima, and the tunica media develops once a stable vascular pattern has been formed (by interaction of the epithelium and the mesenchyma). In cases of congenital dilatation the venous layer to be affected is the media, as opposed to acquired dilatation (i.e. varicose veins) where there is a fibrosis beneath the endothelium.12 To confirm this hypothesis it is necessary for a histological study to be performed to detect alterations in the venous endothelium.

In patients with an azygos venous aneurysm a frontal chest radiograph may demonstrate a prominent azygos vein or an abnormal mediastinal density. On a frontal chest radiograph, characteristically, the size of the aneurysm changes with the respiratory movements (especially with Valsalva manoeuvre), as happened in our case.1 In the past, venography used to be performed to suggest the diagnosis.2 6 Nowadays, dynamic enhanced CT scanning and MRI not only provide non-invasive methods for the evaluation of vascular abnormalities,5 13 but can also rule out other entities which may produce enlargement of the azygos vein. However, the blood flow in the aneurysmal vein is sometimes so slow that MRI may provide an image similar to a solid mass.5 The signal

intensity of the lesion, in our case, varied depending on whether upflow or downflow were saturated, thus confirming its vascular nature.

Although this entity is very rare, we believe that it should be borne in mind for the differential diagnosis of a mediastinal mass. Moreover, it should be stressed that, for an accurate diagnosis, invasive tests are not necessary. Follow up of this lesion is important as an azygos vein aneurysm may grow.2

References

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