The mediastinum is a complex compartment in the center of the chest hounded by the thoracic inlet superiorly, the diaphragm inferiorly, the sternum anteriorly, the spine posteriorly, and the mediastinal pleura bordering each lung laterally.
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The mediastinum is usually divided into anterior, middle, and posterior compartments to help categorize tumors and diseases according to their site of origin and location (Fig I). Occasionally, a superior subdivision is described and included with the anterior compartment. However, because there are no fascial or anatomic planes separating these compartments, lesions may originate in more than one space, and migration or enlargement of a mass into an adjacent compartment is not uncommon. The anterior mediastinum is defined as the region posterior to the sternum and anterior to the heart and brachiocephalic vessels. It extends from the thoracic inlet to the diaphragm and contains the thymus gland, fat, and lymph nodes. The middle, or visceral, compartment is defined as the space containing the heart and pericardium and including the ascending and transverse aorta, the brachiocephalic vessels, the vena cavae, the main pulmonary arteries and veins, the trachea, bronchi, and lymph nodes. The posterior mediastinal compartment is bordered anteriorly by the heart and trachea and extends posteriorly to the thoracic vertebral margin to include the paravertebral gutters. It also contains the descending aorta, esophagus, azygous veins, autonomic ganglia and nerves, thoracic duct, lymph nodes, and fat.
ISchematic representation of the mediastinal compartments depicting the most common lesions in each subdivision. Neurogenic tumors, thymomas, and benign cysts account for approximately 60% of surgically resected lesions. Neurogenic tumors, enterogenous cysts, and germ cell tumors are the most common tumors in children and adolescents. In adults, approximately 50% of all primary mediastinal tumors are of thymic origin; these are followed by (in order of frequency) neurogenic tumors, cysts, lymphomas, and germ cell tumors.
Clinical Features
The presence of symptoms depends on whether the neoplasm is benign or malignant, on the size and location of the lesion, and on the presence or absence of associated systemic illness. Overall, more than 50% of all patients with mediastinal tumors are asymptomatic. Infants and children are more likely to present with symptoms or findings (78%) because of the relatively smaller dimensions of the mediastinum. Patients with benign lesions are more often asymptomatic (51%) than patients with a malignant neoplasm (15%). The most common symptoms are chest pain, fever, cough, and dyspnea. Symptoms related to compression or invasion of mediastinal structures, such as superior vena cava syndrome, Horner's syndrome, hoarseness, and severe pain, are more indicative of a malignant process. Occasionally, the dominant features at the time of presentation are systemic manifestations, including constitutional symptoms associated with lymphomas, myasthenia gravis with thymomas, and endocrine manifestations from pheochromocytomas, parathyroid adenomas, and germ cell tumors.
Diagnosis and Preoperative Assessment
Posteroanterior and lateral chest x-rays provide important information concerning anatomic location and size of the lesion. Computed tomography (CT) scans with contrast enhancement should be done routinely. In patients with a contraindication to contrast dye, magnetic resonance imaging (MRI) is useful. Tumor disruption of fat planes; irregularity of pleural, vascular, or pericardial margins by tumor; and infiltration into muscle or periosteum are useful in differentiating tumor compression from invasion. MRI may be more useful than CT with certain posterior mediastinal masses, particularly to evaluate their extension into the spinal canal.
Serologic evaluation is indicated in certain patients. Alpha-fetoprotein and beta-human choriogonadotropin serologies should be obtained for men in their second through fifth decades who have an anterosuperior mediastinal mass. A positive serology is indicative of a nonseminomatous germ cell tumor.
Tumors of the posterior mediastinum are often easily accessible to percutaneous biopsy, but we believe that potentially resectable tumors should not be subjected to anything less than a complete excisional biopsy. Dumbbell tumors can be assumed to be of neurogenic origin and do not merit a preoperative tissue diagnosis.
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Patient Selection
Anterior Mediastinal Lesions
Patients with mass lesions less than 3 cm in diameter or with cysts originating in the anterior mediastinum are amenable to video-assisted thoracic surgery (VATS).
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We believe that patients with myasthenia gravis and thymoma are better served by a total thymothymectomy via median sternotomy.1
Middle Mediastinal Lesions
Lesions arising in the visceral compartment, such as pleuropericardial and bronchogenic cysts, are readily accessible through the VATS approach
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(Fig II).
II(A) Chest x-ray of an asymptomatic female patient showing a right mediastinal mass at the level of the aortic arch. (B) CT scan of the same patient showing a smooth right mediastinal mass adjacent to the trachea and esophagus. Note the stippled areas of calcification. This lesion was resected thoracoscopically and proved to be a schwanoma of the vagus nerve. Neurogenic tumors represent 55% to 60% or more of all posterior mediastinal masses and 20% of all primary mediastinal tumors. These tumors are usually located in the posterior mediastinum and originate from the sympathetic ganglia (ganglioma, ganglioneuroblastoma, and neuroblastoma), the intercostal nerves (neurofibroma, neurilemoma, and neurosarcoma), and the paraganglia cells (paraganglioma). Only rarely these tumors locate in the anterosuperior mediastinum or the middle mediastinum where they can originate from the vagus or the phrenic nerves.
Posterior Mediastinal Lesions
Well-circumscribed tumors smaller than 3 cm without evidence of chest wall or bony invasion can be approached thoracoscopically.
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We strongly believe that patients with posterior mediastinal tumors larger than 3 cm or tumors that appear locally invasive should undergo posterolateral thoracotomy and open resection. Moreover, any compromise in surgical technique should not be accepted when performing a thoracoscopic resection of a posterior mediastinal mass. The patient should be prepared physically and mentally to undergo a full thoracotomy before thoracoscopic resection is attempted (III, IV).
III(A) Chest x-ray of a patient with bilateral neurogenic tumors. The right lesion is at the cardiophrenic angle. The left lesion is in the retrocardiac position. (B) CT scan of the chest of the same patient depicting the intraspinal extension of the right lesion.
IVSchematic representation of a dumbbell tumor. Note the intimate location of the tumor to the neurovascular bundle. About 10% of neurogenic tumors have extension into the spinal column (see Fig III) and are termed “dumbbell” tumors because of their characteristic shape resulting from the relatively large paraspinal and intraspinal portions connected by a narrow isthmus of tissue traversing the intervertebral foramen. Although 60% of patients with a dumbbell tumor have neurologic symptoms related to spinal cord compression, a significant proportion of patients without symptoms (40%) underscores the importance of evaluating all patients with a posterior mediastinal mass for possible intraspinal extension using magnetic resonance imaging. VATS removal of the intrathoracic component of the tumor after a posterior laminectomy for microneurosurgical removal of the spinal component is an option for resection of these challenging tumors.
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Anesthesia and Monitoring
General endotracheal anesthesia using a double-lumen tube or a single-lumen tube with a bronchial blocker to effect ipsilateral lung collapse is required. Hemodynamic monitoring with an arterial line is recommended for most procedures because single-lung ventilation is used.
Port Placement
Table 1 shows strategic intercostal access locations for mediastinal VATS.
Table 1Strategic Intercostal Access Locations for Mediastinal VATS
| Area of Interest | Thoracoscope | Retractor/Grasper | Dissector/Stapler | Additional Instrument |
|---|---|---|---|---|
| Apices (dorsal sympathectomy) | 6 mid | 4 ant | 4/5 post | |
| Anterior mediastinum | 5 mid/post | 2/3 mid | 5/6 post | 7 mid |
| Posterior mediastinum | 5 mid | 4/6 ant | 2 ant | 3/4 ant |
| Midesophagus/aortopulmonary window | 5/6 mid | 5 auscultatory triangle | 4 ant | 7 mid |
| Distal esophagus (thoracic splanchnicectomy) | 7 mid | 4 ant | 6/8 post | 7 ant |
| Pericardium (left) | 7 post | 9 mid | 5 post |
NOTE. Numbers in the table refer to intercostal spaces.
Abbreviations: mid, mid axillary lines; ant, anterior axillary lines; post, posterior axillary lines.
Surgical Technique
1The patient is placed in the (A) full lateral decubitus position, and the skin is prepared widely to allow for a posterolateral thoracotomy should the exposure be needed or complications arise requiring conversion to an open procedure. (B) A pillow is placed under the patient's chest at the inferior scapular angle, and the operating table is slanted downward on both sides from the center to ensure maximal stretching of the intercostal spaces. All the pressure points are padded. Optimally, two monitors should be present so that an assistant across the operating table also has a direct view forward. All of the surgical instruments needed for an emergency thoracotomy must be ready in the operating room.
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To provide better exposure of the posterior mediastinum, the patient is rotated anteriorly. This uses gravity to improve exposure by allowing the lung to fall away from the paraspinous region. Similarly, Trendelenburg position may be useful for relatively caudal masses; reverse Trendelenburg, for superior mediastinal masses. Posterior rotation of the table allows the lung to fall posteriorly, enhancing visibility of the anterior mediastinal structures.
2Port placement. The standard positioning of ports is shown here. The configuration for most procedures should be an inverted triangle in which the camera is placed at the apex of the triangle and the working ports are placed at the bases. These ports should be placed at a different level from the telescope so that the instruments do not obscure the operative field, and placed at as wide an angle as possible to prevent “swordfighting.” Moreover, all ports should be placed a sufficient distance from the target area to allow for excursion of instruments and provide a wider field of vision (see Table 1). Sealed ports are not necessary. Placement of the telescope through an open port and direct placement of the instruments through skin incisions without a port are sufficient. Proper port placement also helps prevent levering of the instruments and may reduce postoperative pain because of reduced pressure on the intercostal nerve. The instruments used should be as small as possible, again to minimize intercostal nerve compression.
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Zero-degree telescopes are preferred for most procedures. Angled telescopes occasionally may be required for certain target areas, such as the paravertebral gutters.
3Posterior mediastinal masses. The thoracoscope is placed in the midaxillary line in the fifth, sixth, seventh, or eighth intercostal space, based on the site of the posterior mediastinal mass. Initial entry into the chest is performed bluntly with a finger for safety. Local adhesions are bluntly mobilized. A 30-degree thoracoscope generally provides excellent visibility in all regions of the hemithorax. After the mass has been identified, two or three more 1-cm incisions are made to create the best convergence on the mass. Spinal needles may be passed transthoracically to identify the optimal port position before the incisions are made. Working ports are placed under direct thoracoscopic visualization. Upper posterior mediastinal masses are typically approached via working ports placed anteriorly and posteriorly in the usual triangular contiguration, most often in the fourth, fifth, or sixth intercostal space. These incisions are placed far enough apart to prevent encroachment of instruments and the thoracoscope. Ports should be placed higher on the chest for low posterior mediastinal tumors and lower on the chest for tumors located higher in the chest.
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Thought should be given to the creation of these sites, especially with a view toward utility minithoracotomy to facilitate the removal of noncompressible masses if necessary. The utility thoracotomy also allows the insertion of conventional instruments. The utility thoracotomy, 4 to 6 cm long, is performed between the anterior axillary line and the sternum, in the same fourth or fifth intercostal space that eventually would be used for a thoracotomy should conversion be required. This incision is performed anteriorly because the intercostal spaces are wider and there are no large muscles to incise, except for the intercostals and a few fibers of the serratus anterior. The rib spreader is used only at the end of the procedure to extract a large specimen, to reduce the risk of damaging the intercostal nerve.
The pleura is grasped with endoscopic forceps approximately 1 cm away from the tumor and incised with the hook cautery device or with endoscopic scissors, with care taken to avoid injury to important nearby structures. The phrenic, vagus, and recurrent laryngeal nerves are carefully identified and protected to prevent injury. The pleura is incised circumferentially around the tumor, with care taken to stay 1 to 2 cm away from the tumor to ensure adequate tumor margins. Retraction of the mass is performed with a smooth or a toothed grasper. Retraction allows a combination of sharp dissection with scissors and blunt dissection with mounted sponges to gradually mobilize the mass from surrounding structures. Sharp dissection is often required to develop a well-defined plane between the mass and the endothoracic fascia. The tumor is mobilized until it is tethered only by the neurovascular bundle from which it is seen to arise. Major blood supply to neurogenic tumors is often pedicled and should be clipped and divided. Small vessels (< 2 mm) may be cauterized, whereas larger vessels are clipped with endoscopic clip appliers. The neurovascular bundle is carefully dissected free with the hook cautery device and is secured proximal and distal to the tumor with the endoscopic clip applier. The vascular bundle is double-clipped to ensure secure hemostasis. The pedicle is usually short, and traction is dangerous, so that progress is made in dissecting the pedicle by twisting the mass in either direction to gain exposure. If the lesion has a firm capsule, then a transfixion suture can be used to grip the mass and can then be passed through the chest wall, thereby sparing a trocar. If it were thought that division of the azygous vein would make the surgery safer, an Endo-GIA stapler (Ethicon, Inc, Somerville, NJ) can be used for this; however, care should be taken not to manipulate the stumps, because hemorrhage from dehiscence of the staple lines can occur. Alternatively, double application of the Endo-GIA stapler provides more secure control of the vein and allows the stumps to be carefully manipulated if necessary. Monopolar diathermy should be avoided around the spinal foramen. Bipolar coagulation is mandatory when working close to the foramina to avoid thermal and electrical injuries to the cord. It is helpful to use the cautery extensively to minimize oozing, which can quickly compromise vision in such a restricted field. Strict hemostasis should be achieved to prevent complications of epidural hematoma. Similarly, packing the neural foramen with gelatin foam or oxidized cellulose should be avoided, because the substance may expand within the spinal canal and cause cord compression.
4Anterior mediastinal masses and cysts. A 10-mm zero-degree thoracoscope is placed through the fourth or fifth intercostal space along the posterior axillary line.
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After initial exploration, additional instruments—usually endoscopic grasping forceps and roticulating endoscopic scissors—are introduced through the second or third intercostal space in the posterior axillary line and through the fifth or sixth intercostal space in the anterior axillary line. A triangle is then created with the thoracoport sites as the corners and the cyst or mass projected near the center. A separate lung retractor may be used to enhance visualization if needed.Show full caption
Aspiration with a long spinal needle through one of the working ports under direct thoracoscopic visualization can facilitate grasping and manipulating the cyst and expedite removal through one of the ports. Whenever fluid is aspirated, a specimen should be sent for cytology, culture, and Gram stain analysis if clinically indicated.
A stab incision at the site of percutaneous aspiration may be used to introduce a forceps to grasp and maneuver the cyst during dissection; this frees one of the thoracoport sites for a retractor if necessary. By using blunt and sharp dissection, the lesion is separated from the pleura, adjacent mediastinal structures, or pericardium.
5The phrenic nerve must be clearly identified and spared from injury. This is done by sharp dissection with the endoscopic Metzenbaum scissors, with judicious application of electrocautery throughout the dissection of the lesion. Endoscopic clip ligature of vascular and lymphatic pedicles of the mediastinal lymph nodes is performed as needed. Complete removal of the cyst can be achieved in most patients. Tissue planes are usually well preserved and distinct, making dissection straightforward for benign lesions. Ill-defined tissue planes, suggesting invasion and possible malignancy, warrant consideration of open resection.
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Total excision is the primary aim, and every effort should be made to remove the entire cyst to prevent recurrence. However, complete excision of cysts or tumors that are adherent to adjacent vital structures is technically difficult and generally not advisable. When the cyst cannot be removed completely, partial excision with cautery ablation of the epithelial lining is an acceptable alternative.
6Middle mediastinal lesions and pericardial cysts. Removal of these lesions is usually simple, because there is almost always a good plane of dissection around the lesion. Caution is needed to avoid severing or coagulating the phrenic nerve or breaking the pericardium and injuring the heart or initiating a dysrhythmia. Initially the cyst can be partially detached through a combination of sharp and blunt dissection without the need to grasp it. Attempting to remove the cyst intact is both difficult and unnecessary, so, after as much of the cyst wall as possible is freed, the cyst can be aspirated and then rolled up on an endoscopic clamp. Blunt dissection with a pledget can then be used to detach the remaining wall from the pericardium. If dense adhesions are present close to the phrenic nerve, it is safer to leave a portion of the cyst wall in place to reduce the risk of phrenic nerve injury.
7The tumor is carefully dissected completely free from the posterior chest wall, placed in an endoscopy bag, and removed through one of the working trocar sites or the utility thoracotomy. This incision may need to be enlarged to allow removal of the tumor. The tumor should be delivered through the chest wall in a bag to avoid any potential seeding of the trocar site by a malignant tumor, a rare occurrence that nonetheless is being reported with increasing frequency.
8Once hemostasis is achieved, a single 28–32 F chest tube is placed through the anterior trocar site and directed posteriorly toward the apex under thoracoscopic guidance. Extra holes may be created in the chest tube using a rongeur to facilitate complete drainage of the hemithorax. The trocar sites are inspected for hemostasis by inserting a dry gauze under direct thoracoscopic visualization. The camera trocar is removed, and the remaining incisions are closed in two layers with absorbable suture material. If an epidural catheter was not used, the wounds are infiltrated with 0.25% bupivacaine for postoperative analgesia.
Postoperative Care
Postoperative care is significantly simplified in most patients undergoing VATS procedures. Seldom is a stay in the intensive care unit necessary. Most patients can be returned to a general thoracic surgery ward after a recovery period in the postanesthesia care unit. Optimal postoperative pain control is achieved with epidural analgesia, but most patients can be adequately managed with intercostal nerve blocks placed during the operation supplemented with a patient-controlled analgesic pump for the first 24 hours after the procedure. In the absence of contraindications, an anti-inflammatory medication (e.g., ketorolac, rofecoxib) can improve pain control.
Early ambulation and aggressive pulmonary toilet with incentive spirometry are encouraged to help prevent atelectasis and subsequent pneumonia. Heparin for deep-venous thrombosis prophylaxis is administered until the patient is fully ambulatory. The chest tube is usually removed during the first 24 to 48 hours after the operation once the output drops below 200 mL/day. Patients undergoing thoracoscopic resection are usually ready for discharge on the third or fourth postoperative day.
References
- The mediastinum.in: Shields TW LoCicero J Ponn RB General Thoracic Surgery. ed 5. Lippincott Williams & Wilkins, Philadelphia, PA2000: 1983-1986
- Combined microneurosurgical and thoracoscopic removal of neurogenic dumbbell tumors.Ann Thorac Surg. 1995; 59: 469-472
- Mediastinal cysts.in: Yim APC Hazelrigg SR Izzat MB Minimal Access Cardiothoracic Surgery. WB Saunders, Philadelphia, PA2000: 194-198
- VATS approach to the thymus.in: Yim APC Hazelrigg SR Izzat MB Minimal Access Cardiothoracic Surgery. WB Saunders, Philadelphia, PA2000: 209-220
- Minimally invasive approaches to mediastinal neoplasms.Sem Thorac Cardiovasc Surg. 2000; 12: 301-308
- Video-assisted thoracic surgery for mediastinal disease.Chest Surg Clin N Am. 1993; 3: 283-297
- Video-assisted thoracic surgery for diseases within the mediastinum.Surg Clin N Am. 2000; 80: 1511-1533
- Posterior mediastinal masses.in: Yim APC Hazelrigg SR Izzat MB Minimal Access Cardiothoracic Surgery. WB Saunders, Philadelphia, PA2000: 221-231
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