Diagnosis

Right paratracheal air cyst/tracheal diverticulum

Findings

A digital CXR revealed a 2 * 1 cm ovoid lucency in the right paratracheal region at the level of the thoracic inlet (Figure 1).

The CT scan of the chest showed a 2 * 2 cm tubular air-filled structure on the right side of the thoracic inlet, posterior and lateral to the trachea (Figure 2A and B). No calcification was noted within this lesion and there was no evidence of wall thickening. No connection was noted between the lesion and the trachea on axial images. Coronal reformations demonstrate a 4-mm wide connection between the trachea and this air collection in a plane not readily visualized on axial images (Figure 3). The lungs were clear with no gross evidence of parenchymal destruction, infiltrate, or effusions.

Discussion

The term paratracheal air cyst is a relatively non-specific term for a paratracheal air collection. The differential diagnosis of such a collection includes tracheal diverticula, laryngocele, pharyngocele, Zenker's diverticulum, apical hernia of the lung, and apical paraseptal blebs/bullae.¹

There is scant published literature for this entity, and it is considered rare. One study by Goo et al¹ examined 65 patients with CT evidence of a paratracheal cyst without pathologic differentiation. Almost all of these cysts were located in the right paratracheal region with only one located in the left paratracheal region. The right-sided nature of the diverticula may be due to the fact that the esophagus generally lies to the left of the trachea at this level, leaving the right side unsupported. The majority of the air cysts were at the T2 level. The air collections ranged from 5 to 20 mm in the longest diameter in the transaxial plane and from 5 to 25 mm in the vertical plane. A right posterior and laterally oriented thread-like communication between the air cyst and trachea was found in 5 patients (8%). A total of 31% of the cysts had irregular wall thickening. On respiratory dynamic CT, the cysts expanded during forced expiration and shrank during inspiration. Change in the size of the paratracheal cyst during respiration suggests communication between the cyst and the airway. Although not pathologically proven, the authors felt that the cysts probably represented tracheal diverticula.¹

Other lesions in the differential diagnosis of a paratracheal air collection include a laryngocele, which can be seen on CT as an abnormal diverticula of the saccule of the laryngeal ventricle. Phayngeoceles and Zenker's diverticula can usually be identified by barium examination. An apical hernia has continuity with the remainder of the lung on CT scans. The identification of lung markings within the apical hernias is also helpful. Apical paraseptal blebs or bullae, which are air cysts within the lung, can easily be recognized on CT scans.¹

Our patient's "paratracheal air cyst" had no lung markings, and there was no connection to the lungs or esophagus. A short connection to the trachea was identified on coronal images. These findings are typical of tracheal diverticula.

Specifically, tracheal diverticulae are characterized by single or multiple outpouchings of the tracheal wall.^1,2 They are most frequently found incidentally in postmortem examinations, reported in 1% of patients at autopsy series.² Two types of tracheal diverticula have been described: congenital and acquired.

The congenital variety is thought to represent vestigial supernumerary lungs or aborted abnormally high divisions of the primary lung bud. They may arise 4 to 5 cm below the true vocal cords at the right tracheal side or a few cm above the carina. They are relatively small and narrow-mouthed and may occur in isolation or in association with other congenital anomalies within the tracheobronchial tree. The wall of a congenital tracheal diverticula is similar to the actual tracheal wall, containing smooth-muscle fibers, cartilage, and respiratory epithelium.

The acquired variety is thought to represent an outbulging at a weak spot in the posterior tracheal wall as a result of increased intraluminal pressure, as with a chronic cough.³ In the study by Goo et al,¹ patients with paratracheal air cysts had pulmonary function abnormalities indicative of an obstructive pattern more often than a control group, suggesting a possible association of tracheal diverticuli with emphysema. Acquired tracheal diverticula may arise at any level and are said to be typically wide-mouthed and larger in size than congenital diverticula. Histologically, they are lined by respiratory epithelium; however, no mucous glands, smooth muscle, or cartilage are found in the wall. They may be single or multiple.²

Mounier-Kuhn syndrome is a rare disorder characterized by multiple tracheal diverticula with associated marked dilatation of the trachea and main bronchi, bronchiectasis, and recurrent lower respiratory infections.⁴

The diagnosis of tracheal diverticula is relatively straightforward and can be made radiographically. It is an air-filled tubular structure, most often found posterior and slightly to the right of the trachea and communicating with the trachea. Cartilaginous rings within the wall of the diverticula strongly suggest a congenital form, while the absence of cartilaginous rings may suggest an acquired form. CT may also reveal whether the neck of the diverticula is small or large and whether the wall of the diverticula is thickened as a result of repeated inflammation.² Inflammatory changes around the diverticula may also be appreciated.⁵ When the neck of the diverticula is very small, communication with the trachea may not be well seen on CT.²

Tracheal diverticulua are usually asymptomatic⁶; however, they can act as a reservoir for secretions with secondary chronic infections of the tracheobronchial tree. Therefore, tracheal diverticula may present clinically with chronic cough, dyspnea, stridor, and repeated episodes of tracheobronchitis.²

Our patient denied any history of obstructive lung disease and only reported intermittent dyspnea. The tracheal diverticula may, therefore, be an incidental finding. It is, however, a possible etiology of or complicating factor in any future respiratory infections and may be a complicating factor in surgical intubation. One case in the literature reported a diverticula interfering with ventilation under general anesthesia; the diverticula was large enough to admit the endotracheal tube tip.⁷

Treatment options in tracheal diverticula include surgical resection in young patients and conservative symptomatic medical treatment with antibiotics, mucolytics, and physiotherapy in elderly and debilitated patients.^2,3

CONCLUSION

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A tracheal diverticulum is a paratracheal air cyst representing an outpouching of the tracheal wall.¹ A tracheal diverticulum is frequently an incidental finding in a postmortum examination, reported in 1% of patients in an autopsy series.² Usually asymptomatic, tracheal diverticuli can act as a reservoir for secretions, and patients can present with coughing, dyspnea, and stridor secondary to chronic infections of the tracheobronchial tree.² A straightforward radiographic diagnosis, paratracheal air cysts have also been found to be associated with obstructive pulmonary disease and may prove to be a radiographic marker.¹ They are resected or conservatively treated with antibiotics, mucolytics, and physiotherapy.^2,3

1. Goo JM, Im J, Ahn JM, et al. Right paratracheal air cysts in the thoracic inlet: Clinical and radiologic significance. AJR Am J Roentgenol. 1999;173:65-70.

2. Caversaccio M, Becker M, Zbaren, P. Imaging case study of the month: Tracheal diverticulum presenting with recurrent laryngeal nerve paralysis. Ann Otol Rhinol Laryngol. 1998;107:362-364.

3. Infante, M, Mattavelli F, Valente M, et al. Tracheal diverticulum: A rare cause and consequence of chronic cough. Eur J Surg. 1994;160:315-316.

4. Lazzarini-de-Oliveira LC, Franco C, Salles CLG, de Oliveira AC. Roentgenogram of the month: A 38 year old man with tracheomegaly, tracheal diverticulosis, and bronchiectasis. Chest. 2001;120:1018-1020.

5. Early EK, Bothwell MR. Case report: Congenital tracheal diverticulum. Otolaryngol Head Neck Surg. 2002;127:119-121.

6. Tanaka H, Mori Y, Kurokawa K, Abe S. Paratracheal air cysts communicating with the trachea: CT findings. J Thorac Imaging. 1997;12:38-40.

7. Dinner M, Ward R, Yun E. Case report: Ventilation difficulty secondary to a tracheal diverticulum. Anaesthesiology. 1992;77:586-587.

Prepared by Stephen Waite, MD, Senior Radiology Resident, Ashu Sharma, MD, Senior Radiology Resident, and Stephen Machnicki, MD, Chief of Magnetic Resonance Imaging in the Department of Radiology, Lenox Hill Hospital, New York, NY.