Paratracheal Air Cysts: Sonographic Findings in Two Cases

Young-Jin Kim; Eun-Kyung Kim; Jinna Kim; Cheong Soo Park

doi:10.3348/kjr.2003.4.2.136

Abstract

We report two cases of paratracheal air cyst discovered incidentally at neck ultrasonography and confirmed at CT. The cysts were located at the inferoposterior aspect of the right lobe of the thyroid. Ultrasonography revealed an ill-defined hypoechoic mass containing hyperechoic foci which mimicked calcifications. Neck CT confirmed the presence of an ill-defined air pocket which communicated with the trachea through a slit.

Paratracheal air cyst is a rarely described lesion, and its pathologic nature is not clearly proven (1-4). Previous reports state that the cyst was identified at routine chest radiography or chest CT (1, 2); the sonographic findings have not, however, been described. In this report, we present two cases of paratracheal air cyst with peculiar sonographic findings.

CASE REPORTS

Case 1

A 43-year-old woman underwent neck sonography as part of a routine medical checkup. Six years earlier she had received a renal transplant, and the kidney involved had been functioning well up to the time of the checkup.

Neck ultrasonography revealed a 2.9×1.4-cm hypoechoic mass at the inferoposterior aspect of the right lobe of the thyroid (Figs. 1A, B). The lesion, located outside the thyroid capsule, had an ill-demarcated margin and contained multiple hyperechoic foci which appeared to be small calcifications. The thyroid gland was normal in echogenicity and size, and no focal lesion was observed. Cervical lymphadenopathy was absent.

Subsequently, on the basis of the sonographic findings, a tumor of the right parathyroid gland was suspected, and needle aspiration biopsy (NAB) was planned. Further sonography suggested that the hyperechogenic foci were mobile. To rule out Zenker's diverticulum, esophagography was also performed, and the findings were negative.

For further evaluation, neck CT scans were obtained, and an unusual air pocket was found on the right side of the trachea. It had an irregular wall (Fig. 1C), and multiplanar reformatted imaging showed that a slit-like structure linked it with the trachea (Fig. 1D). The findings of chest radiography were normal, however. After reviewing all the above findings, a paratracheal air cyst was diagnosed.

Case 2

A 65-year-old man presented with anterior neck pain. Sonography indicated that the thyroid gland was normal, but at the inferoposterior aspect of its right lobe, an ill-defined hypoechoic mass was discovered (Figs. 2A, B). The lesion contained multiple echogenic foci which appeared to be mobile. Esophagography revealed no evidence of esophageal diverticulum, but neck CT demonstrated an ill-defined air pocket, which communicated with the trachea (Figs. 2C, D). Chest radiographs revealed mild emphysematous change but did not depict the air pocket in the right lower neck region.

DISCUSSION

A paratracheal air cyst is histopathologically diverse. In surgically confirmed cases, pathologic diagnoses of these cysts vary (tracheal diverticulum, or lymphoepithelial or bronchogenic cyst), but all are lined by ciliated columnar epithelium and communicate with the trachea (1-4).

The differential diagnosis of a paratracheal air collection includes pharyngocele, esophageal diverticulum, laryngocele, apical hernia of the lung, and apical paraseptal blebs or bullae (1, 2). Because of the reverberation artifact at ultrasonography, Zenker's diverticulum may be seen as a mass with hyperechoic foci, mimicking a thyroid nodule, but the shape of the mass changes as the patient swallows, and the hyperechoic foci are mobile (5, 6). A barium swallow study is used to confirm Zenker's diverticulum and pharyngocele. Laryngocele can be identified at CT as abnormal dilatation of the saccule of the laryngeal ventricle (7), and CT also shows that apical hernia of the lung is contiguous with the remainder of the lung. At chest radiography, the hernia tends to enlarge on forced expiration and shrink on inspiration (8). Apical paraseptal blebs or bullae, which are air cysts within the lung, can be recognized on CT scans.

In most previously reported cases, paratracheal air cysts were identified at routine chest radiography and CT, and were located at the right posterolateral aspect of the trachea at the level of the T1-T2 vertebral body (1-4). Goo et al. (4) reported that if a right paratracheal air cyst was observed at CT, this could be a sign of obstructive lung disease, clinically, and of the presence of emphysema, radiologically.

At initial ultrasonography, we misdiagnosed the paratracheal air cyst as a calcified parathyroid mass. Surprisingly, the cyst appeared as a hypoechoic mass-like lesion at ultrasonography, though CT revealed no fluid density within it.

We constructed a paratracheal air cyst model by trapping a balloon between slices of pork (Figs. 3A, B), and ultrasonography of the model demonstrated a hypoechoic mass with multiple echogenic foci (Fig. 3C). This outcome was probably due to the strong reflection of the ultrasound beam from the anterior wall of the balloon, and the multiple echogenic foci detected within the mass may be due to reverberation artifact. This phantom model explains why the paratracheal air cyst was seen as a hypoechoic mass containing hyperechoic foci. Moreover, compression by the sonographic transducer distorts the wall, making it more irregular, and this invokes artifacts from many different directions. As a result, the shape of the hyperechoic foci is more irregular.

In conclusion, when sonography depicts a mass-like lesion containing hyperechoic foci at the right inferoposterior aspect of the thyroid gland and to the right of the trachea, the possibility of a paratracheal air cyst should be considered. CT may help disclose the nature of the lesion before performing an invasive procedure such as NAB.