Robotic Surgery of the Mediastinum: A Review

Background: The mediastinum is a complex anatomical region which contains many vital structures. Many aspects of mediastinal surgery, like that for other anatomic regions, have evolved from a maximally invasive approach involving a median sternotomy, anterior mediastinotomy, mediastinoscopy or thoracotomy, to a minimally invasive video-assisted approach. Robotic surgery is pres-ently the most advanced form of minimally invasive surgery. Methods: We reviewed our experience with a robotic approach to mediastinal pathology. In addition, an extensive search was conducted using PubMed, in order to extract references for the application of robotics to surgical conditions of the mediastinum. Results: The first robotic procedure by our group was a mediastinal procedure in 2003. In the past eighteen years, 203 patients have undergone robotic surgery for mediastinal pathology. There were 119 procedures for the Anterior Mediastinum, 33 procedures for the Middle Mediastinum, and 51 procedures for the Posterior Mediastinum. 78 patients underwent robotic thymectomy using a left-sided approach. 43/78 (55%) patients underwent radical thymectomy for Myasthenia Gravis. Thymoma was histologically identified in 32% of patients with Myasthenia Gravis. In patients with thymoma, there was no tumor recurrence. In patients with Myasthenia Gravis, the overall improve-ment rate after robotic radical complete thymectomy was 91% (39/43). Following robotic surgery for the mediastinal disease, the median hospitalization was 3 days, major complications occurred in 0.9% of number of small incisions or ports, while at the same time providing the patient with minimally invasive benefits including shorter hospitalizations, quicker returns to preoperative activity, less pain, less inflammatory response and better cosmesis. The excellent range of motion of the robotic instruments makes them particularly suitable to maneuver around the vital structures and the rigid axial skeleton encountered in various compartments of the mediastinum, and for reaching those “distant” areas of the mediastinum that are difficult to explore and dissect with conventional Video-Assisted Thoracic Surgery (VATS).


Introduction
The mediastinum is the portion of the thorax which lies between the two pleural cavities and is bound laterally by the mediastinal pleura. It extends from the thoracic inlet superiorly to the diaphragm inferiorly and from the vertebral bodies posteriorly to the sternum anteriorly. Historically, the mediastinum is divided in an arbitrary fashion into compartments for the purpose of localizing various lesions. These classifications have been based on the position of the lesion on a lateral chest X-ray, because the mediastinal structures are mainly in the anterior to the posterior axis. However, the demarcation of these compartments has been based on imaginary lines superimposed on chest radiographs.
Historically, two main classifications have been used. The first divides the mediastinum into an anterior, superior, middle, and posterior compartment.
The traditional 4 compartment model ( Figure 1) is based on the lateral radiograph. These compartments are demarcated by dividing the mediastinum into a superior and inferior division, with the latter being divided into anterior, middle, and posterior compartments. The superior mediastinum is the area above an imaginary plane extending from the manubrio-sternal junction posteriorly to the inferior border of the T4 vertebral body. This plane corresponds roughly to the aortic arch and the tracheal bifurcation. It contains all the structures passing through the superior inlet. The rest of the mediastinum is divided into 3 more compartments, anterior, middle, and posterior. The anterior mediastinum is located between the back of the sternal body and the anterior surface of the pericardium. It contains the main body of the thymus and the pre-aortic lymph nodes in stations 5 and 6. The middle mediastinum, located between the anterior and posterior, is occupied by the pericardium, the carina, the proximal main bronchi and the tracheobronchial lymph nodes of stations 2R and 2L, 4R and 4L, and 7.
The posterior mediastinum, located between the back of the pericardium and the anterior spinal ligament contains the esophagus the aorta at the nerves the gan-   The antero-superior compartment is bordered by the sternum anteriorly and extends posteriorly to the pericardial reflection and innominate vein. The middle mediastinum is bordered anteriorly and posteriorly by the pericardium, extending only as high as the pericardial reflection. The posterior mediastinum extends from the ventral aspect of the vertebral bodies to the ribs and includes the costovertebral angles.
A variant of the 3-compartment model was described by Shields in 1991 [1].
This classification which is more relevant to surgical approaches to the mediastinum, consists of an anterior compartment, the visceral compartment, and the paravertebral sulci bilaterally ( Figure 3 The pathology found in each compartment is related to the structures contained within each compartment. The anterosuperior compartment contains the thymus, lymphoid tissue, aortic arch with its branches and the great veins. The

Pathology
The

Clinical Presentation
It is estimated that about two thirds of patients with a mediastinal tumor will be symptomatic at the time of presentation. The most common symptoms include chest pain, cough, dyspnea and fever. Patients presenting with symptoms related to compression of surrounding structures are more likely to have a malignant tumor. Other patients may present with symptoms secondary to hormone production by mediastinal lesions. Patients with a benign tumor are more likely to be asymptomatic.

Traditional Surgical Access to the Mediastinum
Pre vascular zone: Cervical approach with extended mediastinoscopy: A transverse incision in the suprasternal notch traverses the superficial layer of the deep cervical fascia down were dissection allows the transcervical thymectomy and exposure to other tumors in this plane along with lymph nodes in stations 5 and 6 [2].
Anterior mediastinoscopy, Chamberlain procedure: This procedure which was described by Chamberlain in 1966 consist of entering the prevascular substernal space on either side with the parasternal incision carried through the interchondral interspace or through the space of an excised 2 nd costal cartilage [3]. This is particularly useful for biopsies of the preaortic lymph nodes in stations 5 and 6.

Retrovascular, Pretracheal Zone
Classic cervical mediastinoscopy: This procedure was first described by Carlens and 4L, and 7 are located in these areas [5].

Posterior, Prevertebral Zone
Historically conventional approaches to this region of the mediastinum have been limited to drainage of abscesses and mediastinal infections.
Cervical neck approach: A cervical incision is made along the anterior border of the sternocleidomastoid muscle. The middle thyroid vein is divided, and the buccopharyngeal fascia is traversed, the trachea is retracted medially and the carotid sheath is retracted laterally in order to enter the prevertebral, periesophageal space.
This approach to the posterior mediastinum is usually necessitated for the drainage of abscesses located in the superior posterior mediastinum.
In addition, more invasive procedures such as an extended thoracotomy approach to the posterior mediastinum, thoracotomy approach to the mediastinum, and sternotomy for access to the anterior mediastinum have been used.
Clearly the conventional surgical approaches to the mediastinum are hampered , decreased inflammatory response [7], less blood loss [8] and shortened hospital stays with quicker returns to pre-operative function [9].

Robotic Surgical Approach
The surgical approach with the three or four arm port-based DaVinci Si and Xi is illustrated for each of the areas of the mediastinum.

Anterior Compartment or Anterior Mediastinum
Thymic disease: Our preferred approach is a left-sided robotic approach to thymectomy. The patient is placed in a supine position with downward rotation to the right side Forceps. At times we will use a 5 mm port below the camera port in the mid axillary line in the 6 th intercostal space as an Accessory Port. In certain cases, due to technical considerations, the assistant can use the Accessory port for retraction. The right sided port placement and robot positioning is shown in Figure  5.
Non-thymic anterior mediastinal mass: In patients with non-thymic anterior mediastinal disease our robotic approach is slightly different. We use this approach to all other anterior and middle mediastinal disease. Single lung ventilation is utilized with placement of a double lumen endotracheal tube. For a right sided approach, the patient is placed in the  The ports are separated from the adjacent port by one hand's breadth. An accessory port 8mm is placed in the 7 th intercostal space in the anterior axillary line. CO 2 insufflation is used. The positioning of trocars is similar for a left sided approach however the instruments and arms are reversed (Figure 6, Figure 7). The robot is brought in over the head of the patient. A 30˚ camera is used. Instruments similar to the robotic thymectomy procedure are used [12]- [31].

Posterior Compartment: Posterior Mediastinum and Paravertebral Sulcus
In both adults and children, the majority of posterior mediastinal masses are of neurogenic origin. Most of these masses in adults (95%) are benign and are usu- These lesions are approached in our standard fashion as described above. Single lung ventilation utilizing a double lumen endotracheal tube is initiated. The patient is placed in a lateral decubitus position. For posterior mediastinal masses we use a "V" type placement with the camera at the base of the V. Should the tumor be very low in the thorax, an inverted V type configuration would be used with the camera being placed at a higher interspace.
The robot is then brought in and positioned over the head of the patient for mid or superiorly placed lesions. For lower lesions, the robot is brought in alongside the patient's back ( Figure 8).
The pleura adjacent to the mass is incised and the mass is slowly mobilized off the ribs and vertebral bodies. The ability of the robotic endowrist™ to work in 7 degrees of freedom and at 90˚ angles is particularly useful when working between the mass and the ribs. When dealing with an apical mass, the apical portion should be the last area mobilized. Having mobilized all but the apex will allow more traction to be applied while freeing the apical portion and thus avoiding potential damage to the stellate ganglion and subclavian vessels. The neural origin and any vascular supply are clipped with large clips introduced through one of the port sites. The mass is thus freed and placed in a plastic specimen bag. If this is a large mass, this is in turn placed in a second specimen bag and after lubricating the interface between the bags, the mass (within the first bag) is removed through a port site. The site is enlarged as needed in order to extract the lesion. We use one or two subpleurally placed catheters for the constant infusion of bupivicaine 0.5% in the perioperative period. A single chest tube is placed posteriorly in the apex of the hemithorax through the lowest anteriorly port. The lungs are re-inflated under direct vision. Wounds are closed with interrupted polydiaxonone at the muscle layer. The subcutaneous tissues are approximated with absorbable sutures and the skin is closed with a subcuticular absorbable suture.
A Similar approach is used to access the upper portion of the esophagus from the right side.
Another example of esophageal pathology which is amenable to robotic resection is an esophageal duplication cyst.
Fibrous tumors of the paravertebral gutter can be approached in a similar manner.

Conclusion
Robot mediastinal surgery is an evolving field incorporating the well-established principles of conventional open surgery yet utilizing the most advanced minimally invasive techniques and instruments available. While limitations of our current VATS techniques have made certain procedures hazardous and inadvisable and thus limited the overall acceptance of minimally invasive surgical options to the mediastinum, robotic instrumentation and technology have made the minimally invasive approach to mediastinal pathology, safe, and ontologically and immunebiologically efficacious. As experience is gained, it is likely that procedures once considered to be strictly feasible only by open techniques will become routine minimally invasive robotic procedures.

Conflicts of Interest
The author declares no conflicts of interest regarding the publication of this paper.