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Thứ Hai, 16 tháng 9, 2013



Bronchogenic cyst
Dr Yuranga Weerakkody and Dr Jeremy Jones et al.     http://radiopaedia.org/articles/bronchogenic-cyst



Bronchogenic cyst result from an anomalous super numerary budding of the ventral or tracheal diverticulum of the foregut during the sixth week of gestation and is thus part of the spectrum of broncho pulmonary foregut malformations [1].

The most frequent location is mediastinal and subcarinal
[1].They usually contact the carina or main bronchi, but may be seen anywhere along the course of trachea and larger airways. They frequently project into the middle or posterior mediastinum and rarely into anterior mediastinum [2],[3].

Extra mediastinal bronchogenic cysts may be located in the lung parenchyma, diaphragm or pleura
[3]. Intra pulmonary cysts are usually found in the perihilar areas or rarely peripherally in the lower lobe [1]. Bronchogenic cysts have also been reported near the midline in the upper thoracic or lower cervical chest wall [1].

Lesions may be classified as anterior mediastinal if located anterior to heart or great vessels (prevascular space), posterior mediastinal if located in either para spinal regions and middle mediastinal if located in the para tracheal or subcarinal regions or along the course of the esophagus

Bronchogenic cysts vary in size and maybe quite large
[2]. They are usually discrete and unilocular [4]. The cyst contents usually consist of thick mucoid material. The cysts can grow very large without causing symptoms, but may compress surrounding structures, particularly the airway and give rise to symptoms .In rare cases, they become infected or hemorrhage occurs into the cyst. These complications may be life threatening, particularly in infants and young children [2].

In the work by Mc Adams et al, most affected patients presented in the first few decades of life. Presentation beyond 50 years is distinctly unusual. Most of the patients were symptomatic at presentation and chest pain was the most common complaint. Symptoms were likely related to mass effect on adjacent structures. Maximal cyst diameter ranged from 1.3 to 11 cms with a mean of 4.8 cms

In young children, they tend to compress or distort the trachea and bronchi resulting in clinically obvious respiratory compromise. Compression of a single bronchus may result in focal pulmonary hyperinflation, mimicking congenital lobar emphysema on x rays

Bronchogenic cysts usually do not present in the neonatal period, although they can do so when located near major airways and undergoing rapid expansion. More commonly, they present with recurring episodes of infection or wheezing or can be asymptomatic and discovered serendipitously

Complications include superior vena cava syndrome, tracheal compression, pneumothorax, pleurisy and pneumonia
[5]. In the typical case, there is no bronchial communication and the cyst is fluid filled. Repeated infection can cause erosion into a neighboring airway and also an air fluid level. Rhabdomyosarcoma arising in a congenital bronchogenic cyst also has been reported [1].

On plain film examinations, bronchogenic cysts present as spherical or oval masses with smooth outlines and soft tissue density projecting from either side of the mediastinum
[2],[4]. [Figure - 1]

CT is an excellent method for demonstrating the size, shape, position and margin characteristics of cyst. It is also useful for evaluation of mass effect on adjacent structures, cyst attenuation, homogeneity, calcification and patterns of enhancement following intra venous contrast administration of iodinated contrast material

The cysts are classified as water attenuation if their attenuation is less than 20 HU and tissue attenuation if more than 20 HU. Further Mc Adams et al classified bronchogenic cysts as follows: a) cystic - if attenuation is less than that of surrounding soft tissue, lesion is internally homogenous, if there is no internal enhancement and if there is a well defined thin wall. b) Solid: if attenuation is similar to surrounding soft tissues, lesion is internally heterogeneous and if there is no well defined thin wall. c) Indeterminate: if the cysts did not meet above mentioned criteria

Indeterminate borders may be due to obscuration of the margins by associated atelectasis. Mass effect on surrounding structures such as bronchi, esophagus or mediastinal vessels and associated atelectasis or consolidation may also be observed

The bronchogenic cysts appear as round or ovoid masses intimately related to the airway
[4]. They frequently push the carina forward and the esophagus backward - displacements that are almost never seen with other masses [2]. When the cyst has attenuation similar to soft tissues and therefore tumor, the differential diagnosis becomes wider. Rarely the cyst may show uniformly high attenuation, probably due to high protein content or a very high density due to high calcium content (milk of calcium) within [2]. Calcification observed rarely may be in the cyst wall or within the cyst [3].

David S. Mendelson et al studied bronchogenic cysts with CT numbers more than 30 HU and found that they contain turbid mucoid fluid as well as clear and serous fluid. The turbid fluid results in a high CT number . Rarely the cysts become infected and this may also contribute to the relatively high attenuation coefficients
[6]. Hence high Hounsfield units do not exclude the diagnosis of bronchogenic cysts. Infection may also result in thick wall and air fluid level within the cyst. [7].

In the case presented above, a completely extra dural neurogenic tumor too was considered. These are of lower attenuation on CT than muscle, because of their lipid or mucinous matrix or because of cystic degeneration
[8]. They are usually para vertebral in location and may cause smooth expansion of a vertebral foramen [9]. Density varies from hypo to slightly hyperdense. Calcification and hemorrhage are rare[10].

Attenuation differences between mediastinal soft tissues and the cyst contents are accentuated following administration of contrast. The cyst contents do not enhance. Mural enhancement may be seen in some and helps to delineate a thin wall

A thick or irregular wall suggests necrotic tumor or lymph adenopathy .If the lesion is heterogeneous or enhances centrally, neoplasia must be excluded. MRI can be useful to differentiate high attenuation cysts on CT from soft tissue masses. Such cysts are typically iso intense or hyper intense to CSF with all pulse sequences. A lesion that is hypo intense to CSF on T2 W images should be viewed with caution.
[3]. Minimal wall enhancement is expected with gadolinium enhancement [4].

Surgery maybe considered as the treatment of choice even when the cyst is asymptomatic, since complications are not uncommon and definitive diagnosis can be established only on surgical specimen
[5,12]. Observation may be indicated for small, classic, asymptomatic cysts or high-risk patients. Per cutaneous catheter, drainage, sterile alcohol ablation or trans bronchial cyst aspiration have been performed in selected cases [3],[11].

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