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Bronchogenic cysts are cystic lesions found it the thoracic cavity arising due to the abnormal separation of localized portions of primitive tracheobronchial tree from the branching embryonic lung(1). The lung bud develops in the 4th week of intrauterine life and then goes on dividing dichotomously to form the tracheobronchial tree. Abnormal bronchi or bronchioles may form large saccular structures, which may later detach to form cystic fluid encased lesions. Since the detachment happens before the formation of the alveoli, the bronchogenic cysts do not have the alveolar structures but have ciliated epithelium with cartilage, smooth muscles and mucus producing bronchial glands similar to the bronchial walls.

Embryology

Bronchogenic cysts are developmental anomalies of the embryonic foregut and along with the enterogenic cysts they are called as foregut duplication cysts (2). The primitive respiratory system starts developing at 28 days of gestation from the foregut. The laryngeal groove forms in the ventral wall of the pharynx. The laryngeal groove deepens to become surrounded with mesenchymal tissue and starts budding dichotomously. The abnormal separation happens between the 4th and 24 weeks of gestation before the formation of alveoli. Some authors argue that the pathogenesis of bronchogenic cysts is similar to that of the pulmonary sequestration (3), cystic bronchiectasis, congenital lobar emphysema, congenital cystic adenomatoid formation and bronchial atresia. Indeed some of these malformations can be found along with bronchogenic cysts in the same patient (4)(5). 

Location

Mediastinal bronchogenic cysts are predominantly found in the middle mediastinum. They can occur in the posterior mediastinum. Case have been reported of bronchogenic cysts in the pericardium (6), thymus, diaphragm(7), retroperitoneum(8), abdomen(9), cervical region(10)(11), esophagus, sternum, subcutaneously(12). The intrapulmonary bronchogenic cysts are predominantly found in the lower lobes in the medial one third of the lung (13). Some studies show they are more common in the right lung (14). They are often solitary and rarely multiple.

Prevalence

Mediastinal cysts account for approximately 20% of all the mediastinal masses (15). Mediastinal cysts are differentiated based on the histological characteristics of the epithelial lining of cyst wall. Bronchogenic cysts are the predominant sub group of the mediastinal cysts accounting for approximately 40% according to one series followed by pericardial cysts (35%), enteric cysts (10%) and non-specific cysts (14%). Other cystic lesions like the thymic cysts, thoracic duct cysts, pancreatic pseudocysts and parathyroid cysts can also occur in the mediastinum. Bronchogenic cysts predominantly occur in the males.

Types

Depending on the location, bronchogenic cysts can be classified into the mediastinal bronchogenic cysts and intrapulmonary bronchogenic cysts.

Symptoms

The location of the cyst and size are important in causing symptoms. Most of the symptoms are caused by the compression of the cyst on the adjacent structures. The symptoms are frequent in the children and rare in the adults. The smaller thoracic volume and the malleable airways of the children predispose them to compressive symptoms earlier when the cyst enlarges. The bronchogenic cysts are asymptomatic in a significant percentage of adult patients.

Some of the common and uncommon symptoms are as follows; Chest pain – substernal, Cough, Dyspnea, Back Pain(16), fever due to recurrent infection in intrapulmonary bronchogenic cysts, Superior vena caval syndrome (17), Pneumothorax, hemothorax and empyema if the cyst ruptures, hemoptysis, Pleurisy, fistulous communication between the cyst and major airway can bring out cyst contents during cough, Cerebral air embolism (18), compression of heart resulting in arrhythmia and can rupture into the pericardial cavity.

In infants it can cause respiratory distress at birth and rapidly progressive major airway obstruction that can be fatal if left untreated (19). In the infants the cyst enlarges due to the ‘check wall’ mechanism and airway compression with differential ventilation affects the oxygenation of the child.

Investigations

Chest X ray

Most of the asymptomatic bronchogenic cysts are discovered during routine screening radiographs taken for other causes. The mediastinal bronchogenic cysts appear as smooth rounded opacity in the mediastinum adjacent to the tracheobronchial tree especially below the carina, near hilum or in the paratracheal region. The intrapulmonary cyst looks like a cyst of variable size within the lung parenchyma. Some of the cyst that lies beneath the carina cannot be visualized by the chest X ray.

CT-Chest

The bronchogenic cysts appear as thin walled, spherical unilocular sharply circumscribed structures and of near homogenous opacity of variable density (20). Infection may result in areas of inhomogenity. . Air is present if the cyst communicates with the adjacent airway of the tracheobronchial tree. This communication invariably leads to infection of the cyst. Consolidation of the surrounding parenchyma and air fluid level can be noted in intrapulmonary cyst. The density of the cystic fluid may vary from watery to high density due to increased mucous, protein and due to infection and this can make it look like soft tissue mass rarely. If doubt exists MRI or ultrasonography can be done to confirm the cystic nature. CT scan also helps in studying the relation of the cyst to adjacent vital structures before decisions regarding the interventions are made. Evidence of calcium deposits can also be made with the CT chest.

Cyst aspiration

To establish the diagnosis of bronchogenic cyst histologically, the cyst fluid could be aspirated under radiological guidance through EBUS or trans thoracic approach and sent for cytology. The presence of mucous and bronchial epithelial cells is confirmatory of bronchogenic cyst.

MRI Chest

 In some cases due to mucus or increased protein content in the cyst fluid or due to infection, the cystic lesion may resemble a solid mass in the mediastinum. The MRI can be reliably used to confirm the cystic nature of the lesion.

Ultrasound

Ultrasound can be used to establish the cystic nature of the mediastinal lesion. The ultrasonography is most useful for the infants and children. In the adults the intervening aerated lung can prevent the optimal visualization of the mediastinal cysts. Sometimes the bronchogenic cyst may be discovered during the antenatal screening ultrasound and post delivery management needs to be planned if any respiratory distress is expected due to the cyst.

Bronchoscopy

Bronchoscopy may sometime reveal features of extrinsic compression of the airways. Rarely the exudates from the cyst, blood clots and communication with the cyst cavity can be found.

Endobronchial Ultrasound (EBUS)

EBUS can be used to ascertain the cystic nature of the lesion. EBUS guided TBNA can be done to aspirate the fluid for cytological analysis to confirm the diagnosis of brochogenic cyst (21). Besides EBUS can be used as a therapeutic tool to aspirate the cystic contents near completely if the patient is not fit for thoracotomy.

Endoscopic Ultrasound (EUS)

If the mediastinal cyst is located close to the gastrointestinal tract, EUS can be used to evaluate the cyst. Mediastinitis is one of the reported complications of this procedure (22).

Barium Studies

Barium studies are useful in ruling out possible fistulous communication with the gastro intestinal tract and to evaluate if the cyst is compressing the GI tract (23). 

Trans esophageal Echo (TEE)

May help in distinguishing the pericardial cyst from the bronchogenic cyst(24).

Pathology

The cyst wall is lined by ciliated pseudostratified columnar epithelium. Cartilage and smooth muscle are seen in the cyst wall. Seromucinous bronchial type glands and fibrotic strands are also seen. The presence of cartilage is essential for the firm diagnosis of bronchogenic cyst as ciliated epithelium can also occur in the enterogenic cysts of gastrointestinal origin and in wall of lung abscess due to the ingrowth of respiratory epithelium from the adjacent communicating airway. In case cartilage and seromucinous glands are not found, the cyst may be classified as non-specific cyst. If the cyst gets infected the normal histological pattern is altered and it is difficult to pathologically assign the embryological origin.

Differential Diagnosis

Despite extensive investigations, the nature of a cystic lesion in thorax cannot be ascertained in a significant percentage of individuals preoperatively.  The following are some of the differentials to bronchogenic cysts;

Mediastinal Bronchogenic cysts - Enteric cysts, Pericardial cysts, Hydatid Cysts, Lymphoma, Cystic degeneration of a mediastinal mass

Intrapulmonary Bronchogenic cysts -Llung abscess, hydatidosis, fungal disease, tuberculosis, infected bullas, vascular malformations, neoplasms

Management

Bronchogenic cyst in fetus, infants and children

In Infants and children it is advisable to remove the bronchogenic cyst regardless of symptoms as the small thoracic volume and malleable airways can result in rapid development of symptoms if the cyst enlarges. If a symptomatic bronchogenic cyst is not intervened in infants it can sometimes be fatal (25).  Asymptomatic cysts in children should be resected to avoid later complications of the cysts, which could make operation more difficult (26).

Routine antenatal screening can sometime detect cystic lesions in the thorax (27). If a large cyst is noted in the prenatal ultrasound, it may be aspirated percutaneously if situation demands (28)(29). They should be referred to be delivered in a center with appropriate neonatal resuscitative expertise and pediatric surgical facilities. Other possible congenital malformations should also be looked in these infants.

Asymptomatic Bronchogenic Cysts in adults

The management of asymptomatic cysts in adults remains controversial (30). Small asymptomatic cysts in the adults may be observed with periodical radiography (31). But prior to opting for observation, a firm diagnosis of bronchogenic cyst should be made ideally by aspirating the cyst and confirming the diagnosis by histopathology. In case of doubt better to go in for surgical resection. 

The inherent disadvantage in this option is that the cysts can enlarge over time and become symptomatic. It is uncertain what proportions of patients with bronchogenic cysts would develop symptoms and it is unpredictable (32). Indeed in a long term study upto 67% of previously asymptomatic individuals with bronchogenic cyst became symptomatic on follow up. The risk of complications with thoracotomy is higher in a symptomatic patient than an asymptomatic patient (30). Besides it is easier to remove the cyst completely when it is smaller as later on enlarged bronchogenic cyst wall may become adherent to adjacent structures like pericardium, great vessels of the thorax, airway wall etc and pose a difficulty in removing. This left over bits of cyst wall can increase the risk of recurrence of cyst. In addition the option of observation carries a minimal risk of malignant transformation later (33).  Adenocarcinoma, squamous cell carcinoma, carcinoid tumours and anaplastic carcinoma are some of the tumors that can likely arise in a bronchogenic cyst as per some case reports. In some studies despite extensive investigations, positive diagnosis of bronchogenic cysts is not made preoperatively and this is an argument in favor of surgery. If confident preoperative diagnosis is not possible, surgical exploration and resection should be done. Hence complete excision is recommended by most surgeons regardless of the symptoms. It is advisable to remove the bronchogenic cyst even if it is asymptomatic during the initial presentation (17). But the patients should be aware of all the options and decisions can be made as per their wish.

Symptomatic Bronchogenic Cysts and patient fit for thoracotomy

Thoracotomy

Complete resection of the cyst without leaving any cystic wall tissue and ligation of communication if any with the tracheobronchial tree is the goal in the management of bronchogenic cysts. Thoracotomy is the ideal procedure to achieve these objectives. During surgery, the surgeons generally aspirate the contents of the cyst before complete removal (34). This makes the cyst technically easier to remove. Thoracotomy helps in removing the peri-cystic adhesions better than other minimally invasive procedures like VATS and mediastinoscopy.  If the cyst wall is adherent to an adjacent vital structure, the cyst wall can be selectively cauterized to prevent later recurrence. Outcomes are better if the thoracotomy is done in the asymptomatic stage.

VATS

Cysts those are accessible through VATS, without significant pericystic adhesions and without communication with the tracheobronchial tree are amenable for resection through VATS. The minimally invasive nature with less post operative morbidity and low conversion rate to open thoracotomy makes it a good option (35).  Thick pericystic adhesion to a major structure and intraprocedure injuries are major indications for converting to open thoracotomy as leaving behind these can result in recurrence(36).

Mediastinoscopy

Mediastinoscopic removal of bronchogenic cyst can be attempted if the cyst is located in certain anatomical locations that are easily accessible through the mediastinoscope especially if the patient is unfit for thoracotomy (37)(38). Similar to VATS it has low post procedure morbidity but may leave behind cyst wall tissue if pericystic adhesions are present.

Symptomatic Bronchogenic Cysts and patient not fit for thoracotomy/VATS/Mediastinoscopy

Trans thoracic aspiration

Trans thoracic aspiration can be done under ultrasound or CT guidance (39). This is more likely a temporary procedure when emergency decompression of the cyst is needed to relieve a significant airway narrowing. In addition the procedure can be done for patients not fit for thoracotomy or other invasive procedure. Sometimes sclerosants like alchol and bleomycin can be injected to prevent cyst recurrence. The disadvantage of the option is the possible increase in the cyst size later and the minimal risk of malignant transformation.

EBUS guided aspiration

Endobronchial ultrasound guided real time aspiration of bronchogenic cyst has been reported (40)(41). The indications are similar to that of trans thoracic aspiration.

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