Mathew Thomas and K. Robert Shen
Background and Indications for Surgery
Posterior mediastinal tumors are rare neoplasms. A small percentage of these tumors extend into the neural foramen, causing spinal cord compression and neurologic symptoms. The term “dumbbell tumor” refers to posterior mediastinal tumors that extend into the spinal canal from the thoracic cavity by passing through the narrow spinal foramen (Figs. 30.1A,B). These are mostly benign neurogenic tumors such as schwannomas and neurofibromas. Other tumors that have been reported in individual case reports include meningiomas, lipomas, paragangliomas, hemangiomas, and lymphomas.
Symptoms commonly arise from mechanical compression of the spinal cord by the intraspinal component. The extension into the spinal canal may not be easily detected on routine imaging, and because of the rare nature of these tumors, a high index of suspicion must be maintained to identify neuroforaminal involvement. Intraspinal extension in posterior mediastinal tumors may be suggested by bony erosion of the vertebral body or pedicle.
The diagnosis or suspicion of a posterior mediastinal tumor is enough to warrant further investigation for surgical resection. The long-term course of an untreated dumbbell tumor in asymptomatic patients is unclear. Early resection is advised due to a theoretical increased risk for serious neurologic complications including paraplegia, from continued tumor growth.
CONTRAINDICATIONS
Few contraindications exist for surgical resection of neurogenic dumbbell tumors.
Patients with bleeding dyscrasias are at a higher risk for intraspinal bleeding and such abnormalities must be corrected before surgery.
Those patients who have undergone endovascular or open thoracic aortic aneurysm repairs may have altered spinal arterial anatomy due to occlusion of some vessels. In such cases, there may be an increased risk for postoperative paraplegia if the tumor is close to spinal artery that predominantly supplies the spinal cord. This should be discussed with the patient and extreme care must be taken to avoid injuring the dominant spinal artery during the resection.
Previous parietal pleurectomy may be a relative contraindication to resection. An extra-pleural approach to the thoracic component of the tumor may be required in patients who have undergone mechanical or chemical pleurodesis and have an intact parietal pleura.
Figure 30.1 A,B: MRI images of the chest and thoracic spine in two different patients showing the dumbbell tumor extending into the spinal canal from the posterior mediastinum.
Tumors involving the lower cervical, T1 or T2 vertebrae lie close to the stellate ganglion, increasing the risk for postoperative Horner syndrome. This is not a contraindication to resection but should be discussed preoperatively.
PREOPERATIVE PLANNING
Evaluation of patients with dumbbell tumors begins with a history and physical examination, followed by appropriate tests to evaluate fitness for surgery. Consultation by both thoracic surgeons and neurosurgeons or orthopedic spine surgeons is standard in our practice.
Tests specific for preoperative planning are mainly contrast-enhanced imaging such as computerized tomography (CT) scan of the chest; and magnetic resonance imaging (MRI) of the posterior mediastinum and spine. We stress the importance of obtaining an MRI, as it is the most sensitive method to identify the involvement of the spinal foramen. CT scans have been reported to miss small neuroforaminal involvement.
Once imaging studies identify the vertebra involved, the corresponding rib should be identified to determine the appropriate intercostal space for thoracotomy.
Pulmonary function testing should be considered in all patients. Patients who have undergone previous thoracic procedures on the side of the tumor should be examined carefully to plan proper incision placement.
A preoperative biopsy is not usually required and may potentially be dangerous if bleeding occurs into the tumor.
Resection of highly vascular tumors such as hemangiomas and hemangioendotheliomas can lead to significant blood loss, and adequate amounts of blood products must be available before starting the operation. Preoperative embolization of the feeding arteries to the tumor has been described to help reduce the bleeding risk in the case of vascular tumors. However, this comes with a significant risk of causing spinal cord infarction if a dominant artery is occluded and should only be performed at experienced centers. Temporary test balloon occlusion of the feeding artery with a Fogarty catheter can be helpful in determining whether or not preoperative embolization can be performed without neurologic consequences.
Figure 30.2 Diagram showing the relationship of the artery of Adamkiewicz to the dumbbell tumor and the spinal cord.
Artery of Adamkiewicz
An important part of the preoperative workup of neurogenic dumbbell tumors is identifying the location of the artery of Adamkiewicz (AKA). Also known as the great radicular artery, this is the largest supplier of blood to the anterior spinal artery and can potentially cause paresis or paralysis if it is injured intraoperatively. In the majority of cases, the AKA arises from a left intercostal artery and travels cephalad to join the anterior spinal artery (Fig. 30.2). It is usually located between T8 and L2 vertebral levels and hence is more relevant in tumors located in the lower half of the thorax (T5 to T12) or lumbar region. As mentioned before, a careful search should also be performed to locate critical arteries supplying the spinal cord in patients who have had thoracic or thoracoabdominal aortic aneurysm repair.
Locating the AKA prior to resection of posterior mediastinal tumors helps with the discussion of potential complications with the patient; and more importantly, facilitates surgical strategies to avoid irreversible spinal cord injury. Various methods have been used for localization of the artery, but the gold standard has been spinal angiography. This invasive test is currently being superseded by noninvasive imaging methods such as CT and MR angiography, which also has been reported to have excellent sensitivities.
Intraoperative sensory and motor neuromonitoring should be performed if the preoperative imaging shows that the AKA is at or close to the level of the dumbbell tumor. The intercostal neurovascular bundle must be temporarily clamped with a bulldog vascular clamp for up to 10 minutes and distal somatosensory evoked potentials tested before dividing. If there is loss or significant diminution of neuroresponses, then the neurovascular bundle should not be divided. In such situations, the options include removing the tumor in a piecemeal manner, debulking as much as can be safely done, or abandoning the resection entirely. Postoperative radiation of the tumor should then be considered for all incomplete resections.
SURGERY
Due to the rare nature of dumbbell tumors, there has been no consistent technique for surgical resection. A multidisciplinary team combined of thoracic surgeons and neurosurgeons or orthopedic spine surgeons should collaborate together to determine the best approach for the individual patient.
The various approaches used fall under one of the following two major types:
a. Single stage—where the tumor is approached simultaneously from a posterior direction by an incision overlying the spine and anteriorly through a thoracotomy or thoracoscopy. Some authors have reported using a single posterior incision for spinal resection followed by extrapleural division of the posterior portion of the adjacent rib (costotransversotomy) and resection of tumor through the same incision.
b. Two stage—for large tumors, which have multiple or extensive vertebral involvement. The spinal resection may be done first, followed by thoracic resection in a staged manner, or vice versa.
Anesthetic Considerations
The operation is done under general anesthesia, with a double-lumen endotracheal tube used for lung isolation. If double-lumen tube cannot be easily placed, a bronchial blocker can be used instead.
Preoperative IV antibiotics are given no longer than 1 hour before making the skin incision and redosed during the operation depending on the half-life of the medication. We usually use cefazolin for antibiotic coverage unless the patient has an allergy to cephalosporins or penicillins, in which case vancomycin is used.
Positioning
The positioning of the patient depends on whether the tumor is resected in one or two separate stages.
Single Stage
A lateral decubitus position with a 30-degree ventral rotation is used to allow access to both the chest and spine. A soft roll is placed under the dependent axilla and the arms are extended outward and laid over a pillow or an elevated arm board. The arms must be well padded to protect the radial and ulnar nerves from compression. The bed is then flexed to help open up the rib spaces.
Two Stage
The initial position depends on whether the spinal or thoracic resection is done first. The patient is positioned in a true lateral decubitus for the thoracotomy and completely prone for the spinal resection.
Technique
We describe the more commonly used single-stage technique below.
Incision
The location of the tumor and the extent of spinal involvement should be carefully considered when making the skin incision. An incision that is too high or too low can significantly interfere with exposure and make the operation technically challenging. Intraoperative fluoroscopy is often useful to help precisely identify the correct spinal levels once the patient is positioned on the operating table.
The ipsilateral lung should be isolated before making the skin incision, as this gives it enough time to completely collapse.
Figure 30.3 Incision for the combined approach as initially described by Grillo.
The surgeon stands facing the patient’s back. We use a surgical marker to draw the line of incision, which is usually a hockey stick or J-shaped incision as described by Grillo et al. This is composed of a slanted horizontal incision that extends from the anterior axillary line traveling one or two fingerbreadths below the tip of the scapula, and curves to join with a vertical incision located in the midline posteriorly (Fig. 30.3). The vertical limb overlies the spinous processes and extends 5 cm above and below the level of the foramen involved.
A larger paravertebral vertical incision, as described by Akwari, may be occasionally necessary for tumors with extensive spinal involvement.
Unless it is evident from preoperative imaging studies that a bony resection is absolutely necessary, it is best to start with just the horizontal incision and thoracotomy. The incision can be extended posteriorly and vertically, later, as needed.
The horizontal incision is carried down through the skin and subcutaneous tissue. Musculocutaneous flaps are then raised over the underlying ribs by dividing the latissimus dorsi muscle and sparing the serratus anterior muscle. The serratus muscle is separated from the fatty fascia posteriorly, using cautery. If the tumor is located quite high (above the T4 vertebra), the lower anterior attachments of the serratus muscle may have to be divided to improve exposure for the thoracotomy. Otherwise, a small Balfour retractor or a large Penrose rubber drain usually helps to retract the serratus muscle anteriorly.
After developing the flaps, the intercostal space corresponding to the tumor (as determined preoperatively on imaging) is opened with cautery (Fig. 30.4). The inferior rib may be divided posteriorly under the spinal ligaments with a rib cutter, taking care not to divide the intercostal vessels (Fig. 30.5). A suitable chest retractor is placed and gradually opened. The tumor can then be examined through the thoracotomy, as described below, to decide if it can be removed easily or not. If there is any doubt about this, then the incision is extended posteriorly.
Figure 30.4 The intercostal space corresponding to the vertebral level of the tumor is identified and the intercostal muscles are divided with electrocautery.
The posterior midline incision is carried down to the deep fascia, which is opened sharply. The involved vertebra is exposed by separating the paraspinal muscles from the spinous processes using a Cobbs periosteal elevator.
Figure 30.5 After the intercostal space has been opened, the inferior rib is divided posteriorly to help obtain maximum exposure.
Figure 30.6 The extent of the tumor can most often be assessed initially from the thoracic cavity.
Intraoperative Assessment and Resection
In most cases, the tumor is best assessed initially through the pleural cavity (Fig. 30.6). Most tumors are extradural and the extent of spinal involvement is evaluated intraoperatively with the neurosurgeons. Decision making then revolves around whether any bone has to be resected and if so, how much. In some patients, the spinal foramen is enlarged by the tumor itself, which can be carefully extracted through the chest without having to divide any bone. In others, a spinal foraminotomy may be required if either the foramen is narrow or the intraspinal component of the tumor is larger than the diameter of the foramen. With more extensive spinal involvement or bony erosions, a laminectomy or hemilaminectomy is commonly performed. Ligation with a clip or bipolar cautery coagulation of the feeding arteries to the tumor, from the pleural cavity makes the laminectomy easier.
Once it is determined that a neurosurgical procedure is necessary, it is performed as the next step. If the tumor originates from an intercostal nerve, proximal to the foramen, the spinal nerve root can be sacrificed after ligating it (Fig. 30.7). A bipolar cautery is used to coagulate the divided end of the nerve to prevent cerebrospinal fluid (CSF) leaks. The nerve end can also be clipped. For intradural tumors, the duramater is incised over the tumor, using a no. 11 blade. The tumor is gently separated from the spinal cord using microneurosurgical techniques. After the tumor has been released from the spinal canal, attention is back turned to the intrathoracic portion. At this point it becomes relatively easy to completely resect the tumor enbloc.
If the duramater was opened, it should be closed tightly with a running 4-0 Nurolon suture to prevent postoperative CSF leak and its complications. Fibrin glue is applied over the suture line. Pedicled flaps are created from the pleura, intercostal muscle, or pericardial fat pad and placed over the foraminotomy. The flap is anchored in place to the surrounding vertebral periosteum, using 3-0 silk interrupted sutures.
Spinal stabilization is performed by the neurosurgery team, depending on the extent of bone resection. Failure to do so may cause kyphoscoliosis, especially in children or younger adults. Bone autografts can be obtained from the ribs through the thoracotomy incision.
Hemostasis is critical and every attempt should be made to ensure that the tumor bed and spinal canal are dry. Thrombin solution is applied as a spray or with absorbable gelatin foam (Gelfoam) to help with hemostasis. If used within the foramen, the gelatin sponge should be removed before closing, to prevent spinal cord compression, which may result as a complication of the sponge swelling in a closed space.
Figure 30.7 If the tumor originates from the intercostal nerve root proximal to the neural foramina, the spinal nerve root can be ligated and divided following laminectomy.
After the resection is complete and the surgical field is determined to be hemostatic, a chest tube is placed. It is inserted through a stab incision made just above the diaphragm in the anterior axillary line, directed posteriorly and advanced to the apex of the chest.
It is sutured in place and connected to a water-seal chamber.
The ribs are then reapproximated using interrupted 1-0 absorbable or nonabsorbable sutures. Before tying down the sutures, the lung is inflated and the bed leveled. The serratus and latissimus muscles are closed in separate layers using running 1-0 absorbable sutures. Subcutaneous tissue and skin are also closed in layers.
POSTOPERATIVE MANAGEMENT
We admit these patients to a postsurgical unit where they can be closely monitored for the first 24 hours with hourly neurologic examinations for the first 12 hours and then every 4 hours afterward. The development of neurologic changes is an emergency and a spinal hematoma should be excluded. Emergent neurosurgical consultation for operative decompression or placement of a lumbar drain is required.
Incentive spirometry is prescribed for all patients since atelectasis has been a frequently reported complication.
The chest tube is removed when the 24-hour drainage is less than 300 mL and after ensuring that there is no air leak. There is no well-defined long-term follow-up for benign tumors and yearly evaluation for 2 to 3 years with clinical examination and annual chest CT scan may be all that is necessary.
COMPLICATIONS
Intraoperative Complications
The most critical complication is intraoperative injury to the spinal artery causing bleeding and could result in severe neurologic deficit. The excessive use of force during the removal of the tumor may cause it to break off, leaving the intraspinal component behind. This may lead to intratumoral bleeding and cord compression. It is probably safer to perform a foraminotomy than to try to extract the tumor forcefully.
Spinal canal hematoma was more commonly reported with two-stage operations when the intrathoracic component of the tumor was partially resected in the first stage.
Immediate and Delayed Postoperative Complications
Unrecognized or unrepaired perforation of the duramater will lead to CSF leaks and may require reoperation to repair the leak. Other complications include delayed hematoma and cord compression, atelectasis, pleural effusions, and pulmonary air leaks.
In children and young growing adults, kyphoscoliosis may occur in the long term if spinal stabilization was not performed after laminectomy.
RESULTS
Since neurogenic dumbbell tumors are rare and of varying etiology, the current literature is composed entirely of retrospective review of small case series or individual case reports. Many such reports include these tumors as part of the larger category of posterior mediastinal tumors, making it difficult to accurately assess outcomes. Complete removal of benign tumors has been reported to be curative in almost all cases.
In a review of 16 patients who underwent resection of thoracic dumbbell tumors at the Massachusetts General Hospital, Shadmehr et al. reported complete resection in all, with no spinal cord injury. They were able to identify the intraspinal extension preoperatively in 14 (87.5%) patients and intraoperatively in the rest. In six patients, a laminectomy was not required and the tumor could be removed through a thoracotomy alone.
Single-stage resection is the most common technique utilized currently unless one of the components requires extensive resection. Long-term results after single-stage resection for benign tumors are excellent with few complications reported. Neurologic symptoms improved after the spinal cord compression by the tumor has been relieved.
Resection done in two stages has been reported less frequently but appears to have a higher complication rate, when compared to single stage. This approach has largely fallen out of favor and is mostly of historical relevance.
Recent individual case reports describe the use of thoracoscopic techniques in combination with neurosurgery. Overall reported outcomes using this minimally invasive technique appear to be at least as good as the open technique. However, with thoracoscopy it is difficult to avoid a laminectomy, which is not necessary in about one-third of dumbbell tumors removed through a thoracotomy.
CONCLUSIONS
Dumbbell tumors are often best managed in a single-stage operation involving thoracic surgeons and neurosurgeons/orthopedic spine surgeons. Since they are often benign neurogenic tumors, a curative result is expected. Preoperative planning using CT and MRI scans is essential, and intraoperative attention to avoid neurologic complications can lead to a successful operation with excellent results.
Recommended References and Readings
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