Introduction

In the pediatric population, mediastinal masses may be discovered prenatally, incidentally, or because of symptoms. Surgery is a mainstay of treatment in mediastinal masses with the exception to lymphoma, which is treated primarily with chemotherapy.[1] Consideration for a minimally invasive approach depends upon the technical ability of the surgeon, favorable patient anatomy, and tumor location, as well as appropriate oncologic resection and specimen removal.

Masses of the anterior mediastinum can be classified by density into solid, fatty, cystic or mixed lesions. Presentation is primarily dictated by the structure that it impedes with the most common presenting signs and symptoms being dyspnea, stridor, dysphagia, lymphadenopathy, superior vena cava syndrome or malaise.[1] Paraneoplastic syndromes of myasthenia gravis and pure red cell aplasia can be seen with thymomas.[2] Precocious puberty can be induced by hormonally active teratomas.[3]

The first step in diagnosis is a two view chest x-ray followed by cross sectional imaging of CT or MRI for diagnosis and surgical planning. FDG-PET can be specific in determining thymic mass malignancy potential.[4] High AFP levels are associated with malignant potential of teratomas.

Embryology

Mediastinal embryologic development is influenced by the foregut and the thymus. Three weeks after conception, the respiratory groove is recognized in the foregut of the developing mediastinum. At 5 weeks post conception, the esophagus and the trachea have begun to take shape through septation, elongation and epitheliaziation of the lumen. It is believed that a failure of complete tubularization after this time is likely to result in foregut duplication cysts.

Thymic development is derived from the primordia ventral third pharyngeal pouch. In the seventh week from conception, the primordia begin to elongate and descend to its position anterior to the aortic arch. During this time, the lobes of the developing thyroid contain a thymopharyngeal duct that is normally obliterated by the time development is complete. An incompletely obliterated thymopharyngeal duct can result in thymic congenital cysts. Incomplete descent of the thyroid may result in cystic or solid masses in the neck or chest.[5]

Mediastinal Anatomy

The pediatric mediastinum is divided in to four sections similar to adults. There is an anterior, superior, middle, and posterior section. Some references will classify the middle and superior section together.

The anterior mediastinum is bordered by the sternum anteriorly, the sternomanubrial junction superiorly, pericardium posteriorly, diaphragm inferiorly and the parietal pleura laterally. Surgical biopsy for lymphoma is the most common indication for surgery in this space.

The superior mediastinum is bordered by the sternomanubrial joint at T4 inferiorly, the manubrium anteriorly, the thoracic inlet superiorly, vertebrae posteriorly, and parietal pleural laterally. Thymic masses are the most common surgical target in this space.

The middle mediastinum is delineated by the pericardium and the take off of the great vessels. It contains the heart, mesenchymal tissue, and some portions of the great vessels.

Posterior mediastinum is bordered by the pericardium anteriorly, superiorly by the plane of the manubriosternal joint and T4, posteriorly by the vertebral column, inferiorly by the diaphragm, laterally by the mediastinal pleural. Foregut duplication cysts are most commonly found in this space.

Common Surgically Significant Masses

Anterior Mediastinum:

Solid Lesions: Thymoma, Lymphoma, Teratoma

Fatty Lesions: Lipoma

Cystic Lesions: Lymphatic malformation

Middle Mediastinum:

Vascular Lesions : Vascular lesions require a different approach and will not be discussed in this topic.

Congenital foregut duplication cyst: Bronchogenic cyst, Esophageal duplication cyst, Neurenteric cyst

Lymphadenopathy: Primary lymphoma, Metastatic disease (This will not be discussed here.)

Posterior Mediastinum:

Sympathetic Ganglion Tumors: Neuroblastoma, Ganglioneuroblastoma, Ganglioneuroma

Nerve Sheath Tumors: Schwannoma, Neurofibroma, Malignant peripheral nerve sheath tumor

Congenital foregut duplication cyst: Bronchogenic cyst, Esophageal duplication cyst, Neurenteric cyst

Surgical Approach

General Considerations

Before undergoing general anesthesia, the patient should undergo a thorough work up. Severe pulmonary or cardiac symptoms prior to surgery greatly increase the rate of anesthesia related complications. Tumor compression or invasion of critical pulmonary or vascular structures should be accounted for by the anesthesia and surgical teams.[1] Neoadjuvant therapy may be considered for some malignant masses that appear invasive or unresectable on imaging.

Lung ventilation isolation may be used, but is not always necessary. A double lumen tube can be used in children that are of appropriate size, but smaller patients may require either single bronchus intubation or use of a bronchial blocker. It is recommended that either of these techniques be done under direct visualization. If insufflation is used, anesthesia should be vigilant about monitoring for tension pneumothorax physiology and be prepared to alert the surgical team to desufflate immediately.[6]

The patient is positioned to maximize exposure to the operative field and allow gravity assisted retraction. In the lateral decubitus positions a slight break in the bed at the iliac crest will open up the space between the ribs. Arm positioning for patient safety, trocar placement and surgeon/assistant operating location should be considered. The surgeon usually operates from the side opposite of the pathology.

The patient is almost always placed in the lateral decubitus position. Masses on the left side should be approached from the right lateral decubitus position with the left side up. The left side of the chest does have the advantage of avoiding the right phrenic nerve on the lateral aspect of the superior vena cava. Surgical prep should include the entire thorax in the event that  conversion to thoracotomy or median sternotomy is required.

Trocars are placed to allow triangulation of instruments and vision with the thoracoscope to be inline with the surgeon. A vessel sealing energy device of surgeon preference is recommended for securing vessels, but monopolar cautery may be sufficient for dissection.

Thoracoscopic Biopsy/Excision of Mediastinal Mass

Patient Positioning

The patient is prepared for appropriate anesthesia and ventilation by anesthesia colleagues.[6] Correct positioning of the patient is critical to the success of the operation. Gravity should be used to optimize exposure. Left sided, anterior masses should be approached from the right lateral decubitus position with the left side up. Right sided masses should be positioned in left lateral decubitus. Inferior masses benefit from trendelenburg positioning, while superior masses benefit from reverse trendelenburg. A bean bag should be considered for larger patients.

Trocar Placement

Placing the first 5mm trocar in the 4th or 5th intercostal space in the midaxillary line with an open technique assures the safest access to the thorax. Insufflation to a pressure of 5 mmHg is tolerated well and assists with lung deflation. Insertion of a 5 mm 30 degree scope to view the location of the surgical target will help guide subsequent trocar placement. Two additional 5 mm ports should be placed between the anterior and posterior axillary lines to triangulate the lesion. If a stapling device will be used, consider which 5 mm port may be upsized to a 12 mm port to allow this if necessary. Also consider how you will remove the surgical specimen. Removing the specimen through a 5 mm incision is sometimes possible when the trocar has been removed.

Figure 1. Trochar placement for mediastinal mass excision.[7]

Dissection and Biopsy

Blunt dissection with a blunt forceps or suction irrigator is recommended for the majority of the dissection. Pleural adhesion are best released with blunt dissection, but it is important to have electrocautery or an energy device available.[8] When the pathology is encountered a biopsy location should be chosen that is not close proximity to any vascular structures. Aspiration with a small needle can reduce the chance that no vessels will be inadvertently biopsied. A figure of eight silk suture in the biopsy target can facilitate exposure with minimal damage to the specimen.[6] An energy device or scissors connected to monopolar cautery is recommended for dissecting the specimen. Placing the specimen in an endoscopic bag will prevent potential contamination and potential tumor spread. Remembering that the specimen will be significantly smaller than what appears on the magnified laparoscope will ensure that enough of a specimen is biopsied to achieve a diagnosis. If there is any question, frozen specimens for pathology are recommended.

Figure 2. Esophageal duplication cyst.[9]

Figure 3. Opening the pleura and dissecting the duplication cyst free.[10]

Conversion to Open

Consider converting to open if there is uncontrollable hemorrhage, inability to ventilate or poor visualization.

Closure

Placement of a chest drain is determined by surgeon preference and is largely dictated by the type of tissue dissected. Intercostal nerve block with bupivicaine improves pain control. Port sites are closed in layers with absorbable suture.

Postoperative Course

Patients are usually admitted to the surgical floor. Pain control with appropriately dosed acetaminophen and ketorolac in infants and young children is usually sufficient. Older children and adolescents may require narcotic medication. Chest xray is reviewed on postoperative day 1, and if a drain was placed it is usually able to be removed, sparing excessive output. If a small pneumothorax is identified it will usually resolve spontaneously. Most patients are discharged by postoperative day two.[6]

Pearls

1. Correct positioning of the patient and trocars will maximize exposure and retraction.
2. Have clear communication with anesthesia regarding single lung ventilation and recognizing tension pneumothorax physiology.
3. A 12 mm port may be required for the endoscopic bag device and is always required for an endoscopic stapler. There are some 5 mm endoscopic bag devices.
4. Judicious use of monopolar cautery or an energy device will help provide a hemostatic operative field to maximize visualization.
5. Consider having endoscopic ultrasound available for identifying the great vessels.
6. Aspirate any potential biopsy location with a small needle to prevent biopsy of a vascular structure.
7. Confirm with pathology that the pathology specimen is adequate. Utilize frozen sectioning as necessary.
8. Patients requiring support with high positive-pressure ventilation or significant pulmonary parenchymal disease are more likely to benefit from a chest drain.[6]
9. Have surgical clips available to hemostasis. Have instruments to convert to open available in the room.

Pitfalls

1. Bronchial blockers, especially if located in a short right mainstem bronchus may become dislodged and obstruct the entire trachea.
2. Excessive intrathoracic pressures may result in tension pneumothorax physiology.
3. Identify the phrenic nerve and protect it.
4. Biopsy an adequate amount of tissue for diagnosis is the goal, not complete excision.[6]
5. If a duplication cyst shares a common wall with the esophagus, be prepared to repair the esophagus.[11]

REFERENCES

1. Tovar JA: Anterior mediastinal tumors. In Parikh DH, Crabbe DCG, Auldist AW, Rothenberg SS (eds): Pediatric Thoracic Surgery. London, Springer-Verlag, pp 225-231, 2009.
2. Crabbe DCG, Parikh DH: The thymus and myasthenia gravis. In Parikh DH, Crabbe DCG, Auldist AW, Rothenberg SS (eds): Pediatric Thoracic Surgery. London, Springer-Verlag, pp 579-588, 2009.
3. Beresford L, Fernandez CV, Cummings E, Sanderson S, Ming-Yu W, Giacomantonio M. Mediastinal polyembryoma associated with Klinefelter syndrome. J Pediatr Hematol Oncol 2003; 25:321-3.
4. Brink I, Reinhardt MJ, Hoegerle S, et al. Increased metabolic activity in the thymus gland studied with 18 F-FDG PET. Age dependency and frequency after chemotherapy. J Nucl Med 2001; 42: 591-595.
5. Adzick NS, Farmer DL: Cysts of the lungs and mediastinum. In Grosfeld JL, O’Neill Jr JA, Fonkalsrud EW, Coran AG (eds). Pediatric Surgery. Philadelphia, Mosby, pp 955-970, 2006.
6. Little DC, Holcomb III GW: Thoracoscopic biopsy of a mediastinal mass. In Holcomb III GW, Georgeson K, Rothenberg SS (eds). Atlas of Pediatric Laparoscopy and Thoracoscopy with CD. Elsevier Health Sciences, pp 275-283, 2008.
7. Photo courtesy of Jose Prince, MD.
8. Ponsky TA, Rothenberg SS: Foregut duplication cysts. In Parikh DH, Crabbe DCG, Auldist AW, Rothenberg SS (eds): Pediatric Thoracic Surgery. London, Springer-Verlag, pp 383-387, 2009.
9. Photo courtesy of Jose Prince, MD.
10. Photo courtesy of Jose Prince, MD.
11. Dutta S, Albanese CT: Thoracoscopic excision of foregut duplications. In Holcomb III GW, Georgeson K, Rothenberg SS (eds). Atlas of Pediatric Laparoscopy and Thoracoscopy with CD. Elsevier Health Sciences, pp 279-283, 2008.

Authors: Brandon Vanderwel, MD, and Angela M. Hanna, MD