Intradural Extramedullary Tumors

45 Intradural Extramedullary Tumors

Hani Malone, Richard G. Fessler, and John E. O’Toole

Summary

The resection of intradural extramedullary (IDEM) spinal tumors using minimally invasive techniques has been shown to be safe and effective. Compared to traditional open surgery, minimally invasive surgery (MIS) techniques may reduce complications, morbidity, and cost, in appropriately selected patients. In this chapter, we will present a step-by-step guide on how to use an MIS approach for resection of IDEM tumors. Preoperative planning, technical details, postoperative considerations, and potential complications will be discussed. Surgical workflow will be illustrated by a video case presentation accompanying this chapter (Video 45.1).

Keywords: minimally invasive spine surgery spinal tumor intradural intradural extramedullary MIS IDEM

Video 45.1 Intradural extramedullary tumors.

45.1 Introduction

The increasing availability of advanced diagnostic imaging has brought greater numbers of intradural spinal tumors to surgical attention. Nevertheless, these lesions represent a relatively rare clinical entity with an annual incidence of approximately 1 in 10,000.¹ Intradural tumors are traditionally resected using a standard midline incision, subperiosteal muscle dissection, and bilateral laminectomies. This “open” dissection has been shown to cause denervation and devascularization of the paraspinal musculature leading to significant loss of axial muscle strength.² By comparison, minimally invasive surgery (MIS) techniques utilize a tubular retractor system through a paramedian approach, sparing the midline ligaments, and minimizing damage to the paraspinal musculature.

The microsurgical resection of intradural spinal tumors can be technically challenging. Accordingly, the learning curve associated with performing this already difficult procedure through a tubular retractor system has led some to avoid adopting MIS techniques for intradural pathology. Nevertheless, minimally invasive approaches to spinal tumors have evolved rapidly over the past 10 to 15 years, as more surgeons become comfortable with MIS techniques and seek to avoid the morbidity associated with traditional open surgery.³^,⁴^,⁵^,⁶^,⁷^,⁸^,⁹^,¹⁰^,¹¹^,¹²^,¹³^,¹⁴^,¹⁵ This evolution was driven, in part, by the morbidity and significant complication rates associated with traditional surgical approaches to spinal tumors, particularly for metastatic disease.¹⁶^,¹⁷^,¹⁸^,¹⁹

There is a growing body of evidence that minimally invasive approaches can be used to reduce the morbidity associated with the resection of intradural spinal tumors, without compromising extent of resection or safety.³^,⁴^,⁵^,⁶^,⁷^,⁸^,⁹^,¹⁰^,¹¹^,¹²^,¹³^,¹⁴^,¹⁵ When properly performed, MIS techniques should aim to reduce operative time, blood loss, pain, postoperative immobilization, and length of hospital stay. These benefits should ultimately translate into faster recovery and cost reduction.

45.2 Patient Selection: Indications and Contraindications

Advances in MIS technology and surgical navigation continue to expand the indications and capabilities of minimally invasive spine surgery (MISS) for degenerative disease. This progression has led to corollary advances in MIS for intradural spinal tumors.³^,⁴^,⁵^,⁶^,⁷^,⁸^,⁹^,¹⁰^,¹¹^,¹²^,¹³^,¹⁴^,¹⁵ There have been successful reports of MIS resection of well-circumscribed intramedullary spinal tumors.²⁰^,²¹ Likewise, the treatment of spinal vascular malformations has been shown to be safe and effective through a tubular retractor system.²²^,²³ Nevertheless, minimally invasive approaches to intradural spinal pathology is most commonly used for intradural extramedullary (IDEM) spinal tumors, which will be the focus of this chapter.

A traditional open approach to intradural tumors does provide a number of definitive advantages.²⁴, Midline approaches provide a wide exposure and large surgical corridor. This exposure may be necessary for large lesions that span multiple segments. Dural closure is also more facile when a large surgical corridor is created. However, this exposure comes at the cost of a larger excision with greater soft tissue destruction, blood loss, and recovery time.⁴^,⁵^,⁶ Open surgical approaches also sacrifice the support provided by posterior midline structures, specifically the interspinous ligaments. Compromise of this posterior tension band may predispose patients to segmental instability and/or postoperative kyphosis, necessitating instrumented fusion. The risk of postoperative kyphosis may be particularly significant following surgery for intradural tumors.¹⁶^,²⁵^,²⁶ By comparison, MIS techniques generally utilize a unilateral paramedian approach that preserves the posterior tension band, mitigating the risk of postoperative instability and kyphosis.¹⁶

The fundamental advantages of MISS for degenerative disease (reduced soft tissue destruction, blood loss, mobilization, and hospital stay) have been reproducible in series of patients with intradural tumors.³^,⁴^,⁵^,⁶^,⁷^,⁸^,⁹^,¹⁰^,¹¹^,¹²^,¹³^,¹⁴^,¹⁵ There is also evidence that MIS approaches limit the risk of postoperative cerebrospinal fluid (CSF) leak following intended durotomies, which in turn reduces the risk of wound breakdown and postoperative infection.⁶^,⁷^,²⁷ This is most likely due to the relatively small amount of dead space that remains following removal of an MIS tubular retractor, compared to traditional midline approaches.

The appropriateness of an MIS approach to IDEM tumors is largely dictated by a preoperative assessment of the space required to remove the lesion. For example, tumors that lie ventral to the spinal cord are not amenable to an MIS approach. These lesions often require a larger dural opening to facilitate sectioning of the dentate ligaments and slight mobilization of the spinal cord. By comparison, dorsal and lateral intradural lesions are candidates for MIS. Good visualization of these lesions can be achieved using a tubular retractor from a contralateral paramedian approach (see Video 45.1).³ Size is not necessarily a contraindication, as rather large lesions (up to 4 cm) can be removed with the use of expandable retractors.³ Nevertheless, MIS approaches should be limited to intradural lesions that are one or two levels in length.¹⁵^,¹⁶

There is a learning curve associated with all minimally invasive techniques. This may be particularly true for intradural tumors and some repetition is necessary to become facile with dural closure in a limited corridor.²⁷ However, as MIS for degenerative spine pathology becomes more ubiquitous and is increasingly incorporated into residency training, more neurosurgeons are likely to consider MIS approaches to IDEM pathology.

45.3 Preoperative Planning

For all patients with a known or suspected spinal tumor, clinical evaluation must begin with a detailed history and neurologic examination. Compared to patients with epidural disease or tumors of the vertebral column, those with intradural lesions are relatively less likely to present with severe radicular or axial back pain. However, these patients may develop neurologic deficits from ongoing compression of the spinal cord or nerve roots.²⁸ Magnetic resonance imaging (MRI) is the primary imaging modality used to evaluate intradural spinal tumors.

T1-weighted sequences with gadolinium contrast are useful to define the extent and margins of intradural pathology. Although the degree of enhancement on postcontrast sequences may vary considerably for intramedullary pathology, commonly encountered IDEM lesions, such as schwannomas, nerve sheath tumors, and meningiomas, tend to avidly enhance (Fig. 45.1). T2-weighted sequences are useful for determining the nature and extent of cord deformation and/or nerve impingement, as well as cord edema and syrinx formation. For patients with pacemakers, pain pumps, or other metallic foreign bodies precluding MRI, computed tomography (CT) myelography can be used as an alternative modality.

Fig. 45.1 Magnetic resonance imaging (MRI) of a patient with an intradural extramedullary lesion. The lesion enhances avidly, seen here on sagittal (a) and axial (b) T1 postcontrast images.

When considering a minimally invasive approach to intradural pathology, special attention must be paid to the space required to remove the lesion. It is critical that the MIS retractor used allows for adequate visualization of the full length of the tumor. In ideal conditions, normal (nonpathologic) tissue rostral and caudal to the lesion should also be visualized to allow for accurate identification of tumor margins.²⁴ Inadequate visualization of IDEM tumors may lead to the piecemeal and subtotal resection of lesions that could otherwise be removed en bloc.³^,²⁴ It is important to consider that adjustment of the tubular retractor system is limited after making a durotomy and any attempt to do so may introduce blood into the subarachnoid space and risk injury to exposed neural tissue. Some have advocated that the retractor used should be 5 to 10 mm larger than the planned length of the dural incision, in order to ensure enough length to reliably obtain a watertight dural closure.³^,²⁴ It is also important to consider that the length of the durotomy should generally be 5 to 10 mm longer than the underlying intradural pathology to ensure adequate visualization.

When counseling patients with intradural pathology, expectations and operative goals must be frankly discussed prior to formulating a surgical plan. For intramedullary lesions, the nature of the pathology and the presence or absence of a safe dissection plane often limit extent of resection, making diagnosis the primary goal of surgery. Conversely, for IDEM tumors, surgical resection is often curative. Accordingly, careful surgical planning is critical to ensure maximal resection and definitive treatment when possible. Like all spinal tumor surgery, the primary goals for IDEM tumor surgery are pathologic diagnosis, symptomatic relief, tumor resection/source control, and decompression of the spinal cord and/or roots. The operating surgeon must be confident that these goals can be effectively addressed through an MIS approach before attempting a minimally invasive operation.

45.4 Technique

45.4.1 Patient Positioning

General anesthesia is recommended for all minimally invasive approaches to intradural pathology. We use intraoperative neurophysiologic monitoring with continuous somatosensory evoked potentials (SSEPs), electromyography (EMG), and, when indicated, motor evoked potentials and/or a stimulation probe. Open communication between the anesthesia and neuromonitoring teams is critical to achieve the appropriate balance between muscle relaxation and accurate neurophysiologic recording. Prepositioning potentials may be necessary when intradural pathology has led to significant deformation of the spinal cord, but is generally not necessary.

Following the induction of anesthesia, the patient is placed in the prone position on a standard radiolucent spine table. A Wilson frame (Mizuho OSI | Union City, CA) may be used to open the interlaminar space at lumbar levels, but at thoracic levels it may limit anteroposterior (AP) fluoroscopy and complicate localization. In the thoracic spine, we generally use a combination of standard radiolucent chest, hip, and thigh pads for this reason. With the patient positioned, fluoroscopy is used to identify the appropriate spinal level and mark the intended incision and site of dilation.

45.4.2 Navigation

Challenges to accurate localization with intraoperative fluoroscopy may be obviated with the use of surgical navigation. Recent reports have studied the utility of MRI-based navigation that is co-registered to intraoperative three-dimensional fluoroscopy images.29^,³⁰ In these studies, the authors suggest that in addition to localizing the correct spinal level, navigation minimizes the skin incision and muscle dissection, reduces bone removal, and allows for precise dural openings.²⁹^,³⁰ Nevertheless, in most cases, localization can be achieved with standard fluoroscopy and the details of surgical approach can be aptly extrapolated from careful study of the relevant anatomy on preoperative imaging.

45.4.3 Surgical Technique in Steps

1.Incision and exposure:
We typically approach eccentric IDEM tumors across the spinal canal from a contralateral approach for lumbar tumors and ipsilaterally for cervical and thoracic tumors. An approximately 3 cm paramedian skin incision is made several centimeters lateral to the midline. The position of the lesion in the spinal canal dictates the lateral extent of the incision, which can be measured on MRI preoperatively. An incision made too medial will prevent angulation of the tubular retractor and limit exposure of the midline and contralateral canal. Following skin incision, the opening is carried through the subcutaneous fat with monopolar cautery, achieving hemostasis and leaving the thoracolumbar fascia intact. We generally prefer to cut the fascia sharply prior to dilation, but others traverse the fascia upon dilation with the K-wire and tubular dilators.
Dilation with the tubular retractor system then takes place in a stepwise fashion. We generally initiate dilation and localize with a K-wire, but some favor starting with the smallest dilator to mitigate the risk of passing the K-wire past the interlaminar space. The initial dilator should target the laminofacet junction. Dilators of increasing caliber are then passed over each other using a circular motion to pass deep to the fascia. Lateral fluoroscopic guidance is used to ensure the correct depth at the level of the joint (Fig. 45.2).

2.Localization:
Once the planned dilator width has been achieved and the depth of dilation measured, the corresponding tubular retractor can be placed and secured to the system’s table-mounted articulating arm. Fluoroscopy should be used judiciously during MIS, given the cumulative risk of radiation exposure to the operating surgeon and ancillary staff. However, optimal retractor placement is paramount to success in MIS for IDEM lesions and fluoroscopy should be used as needed until this can be confidently achieved.
The choice of tubular retractor should be based on the location and size of the IDEM tumor, with the retractor diameter ideally at least a few millimeters larger than the length of the lesion. We have experienced using fixed diameter tubes ranging from 20 to 26 mm for intradural pathology, which are significantly larger than the standard 18-mm diameter tubes commonly used for lumbar microdiscectomy. Expandable retractors can be used for larger lesions, which are capable of providing over 4 cm of longitudinal exposure. Once the retractor is secured in place, the microscope is brought into position and focused to the depth of the operative site. We prefer a working distance of 350 mm to allow for facile passing of instruments in and out of the tubular dilator without interference from the operative microscope.

3.Dissection:
At the depth of the tubular retractor, a soft tissue muscle plug is circumferentially sectioned and removed with monopolar cautery, exposing the underlying lamina and medial facet joint. Using a high-speed burr, a standard ipsilateral hemilaminectomy is performed, exposing the underlying ligamentum flavum, which is preserved. With the ligament serving as a protective barrier to the dura, the tubular retractor is then redirected medially. The high-speed burr can then be used to undercut the spinous process. The inner cortex of the contralateral lamina is then removed with a combination of drilling and the use of a Kerrison punch until the contralateral pedicle is visualized. This approach provides access to both the ipsilateral and contralateral sides of the spinal canal, while maintaining the integrity of the spinous process, interspinous ligaments, and posterior tension band.
Next, the ligamentum flavum, which has served as a barrier to the dura during drilling, can be efficiently removed. A straight curette can be used to separate the two bellies of the ligament in the midline, establishing an epidural plane. The ligament can then be freed from its rostral and caudal laminar attachments using a ball tip probe, angled curette, and Kerrison punch, exposing the underlying dura.

4.Tumor resection:
Prior to initiating the dural opening, it is important to ensure that meticulous hemostasis is achieved to prevent blood from running into the operative field and subarachnoid space. We use a long-handled no. 11 blade scalpel to initiate the durotomy in the midline, and then extend the opening rostrally and caudally using a nerve hook. The dural edges are then tacked up using 4–0 Nurolon or silk sutures.

5.Following the dural opening, tumor resection begins with careful dissection of the arachnoid layers overlying the mass and adjacent neural elements. At this stage in the operation, standard microsurgical techniques are used to remove the lesion. We most frequently utilize microscissors and Rhoton dissectors to create a plane around the mass. When addressing tumors in the thoracic and cervical spine, microdissection must free extramedullary tumors from the spinal cord and exiting nerve roots. In the lumbar spine, microsurgical technique must be similarly used to dissect the tumor free from the nerves of the cauda equina.

6.Once an IDEM tumor has been dissected from adjunct neural tissue, the size and location of the mass dictate how it can be then safely and efficiently resected. Large extramedullary lesions that deform the adjacent spinal cord, such as large thoracic meningiomas, generally have to be debulked prior to removal to avoid any additional physical stress to the spinal cord. In some cases, an ultrasonic aspirator with an MIS attachment can be used to perform debulking.

7.Importantly, the ultrasonic aspirator must be used on low power settings to minimize the risk of collateral mechanical injury to the adjacent cord. Although piecemeal tumor removal is less efficient and may increase the likelihood of subtotal resection, it is occasionally necessary to avoid cord injury and neurologic deficit.

8.Conversely, schwannomas and other nerve sheath tumors at the level of the cauda equina can often be safely removed en bloc, without internal debulking.²⁸ For these lesions, microsurgical dissection is used to isolate the tumor and its associated afferent and efferent fascicles from all other roots in the thecal sac (Fig. 45.3). Special care should be taken to ensure that no traversing roots are adherent to the ventral side of the mass, which may be originally difficult to identify. Once isolated, direct stimulation with a unipolar probe is applied to both the afferent and efferent nerve fascicles associated with the mass.

9.In the case of schwannoma, direct stimulation will produce a motor response in only rare cases. If a motor response is recorded, it is often because the nerve stimulated was not the nerve fascicle truly associated with the tumor, but rather a traversing nerve adherent to the mass. If there is no motor response, the afferent nerve is sectioned first and then the efferent nerve. This is to prevent rostral rebounding of the tumor mass above the dural opening from rostral nerve tension. Once the afferent and efferent roots are coagulated and sectioned, the mass can be removed en bloc. While performing microsurgery, surgeons accustomed to using the operating microscope may observe fewer differences than anticipated between MIS and open approaches, as the operation is reduced to a small corridor.

10.Closure:
Dural closure represents one of the more technically challenging components of MIS for intradural tumors. As with open surgery, a watertight dural closure is paramount to avoiding CSF leakage, pseudomeningocele formation, infection, and wound breakdown in the postoperative period. Prior to closing the dura, it is again critical to ensure that hemostasis is achieved, as the drainage of CSF during surgery precipitates bleeding from stretched epidural veins. Although a number of dural closure devices have been developed,³¹ we prefer to repair the dura with a running suture. With elongated instruments adapted for use through an MIS tubular retractor, dural closure can be performed in a manner similar to open techniques. We favor the facility associated with a 6.0 tapered Gore-Tex suture. Some advocate interrupted sutures or running interlocked suture lines, but we prefer a standard running suture (Fig. 45.4). A Valsalva maneuver is performed at the end of the closure to evaluate for any defects in the suture line and to ensure a watertight closure. Hydrogel or fibrin-based dural sealants can be used as an adjunct to reinforce the suture line. At the conclusion of the case, the retractor system is removed slowly, taking care to identify and cauterize any sites of bleeding. The fascia and subcutaneous layers are closed with inverted 0 and 2.0 Vicryl sutures, respectively. We close the skin with topical adhesive glue.

Fig. 45.2 Intraoperative lateral fluoroscopic image demonstrating dilation of the minimally invasive surgical tubular retraction system at the laminar-facet junction at L4/L5.

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