Minimally Invasive Techniques for Lumbar Disorders

Published on 14/03/2015 by admin

Filed under Neurosurgery

Last modified 22/04/2025

Print this page

rate 1 star rate 2 star rate 3 star rate 4 star rate 5 star
Your rating: none, Average: 0 (0 votes)

This article have been viewed 1150 times

CHAPTER 307 Minimally Invasive Techniques for Lumbar Disorders

Alfred T. Ogden, Richard G. Fessler

Minimally invasive techniques have evolved during the past decade to affect all aspects of lumbar spine surgery, from congenital disorders to adult deformity. Surgical series have demonstrated efficacy for decompressive procedures,1 short-segment lumbar fusions,2,3 and intradural exposures to spinal tumors4–7 and to tethered cords.8 More recent efforts have involved applying this experience to reduce the morbidity of deformity procedures. The advantages of minimally invasive spine surgery include decreased postoperative pain, more rapid postoperative mobilization, shorter length of hospitalization, shorter postoperative recovery times, and less disruption to the paraspinal muscles and ligaments that contribute to the maintenance of proper spine biomechanics.

From a technical perspective, several major themes recur throughout the minimally invasive lumbar spine experience. First, muscle dilators permit the introduction and placement of tubular retractors directly over the site of pathology with minimal soft tissue disruption. Second, a hemilaminar approach using a drill with a dural guard enables contralateral exposure sufficient to perform bilateral decompressive operations and intradural tumor surgery. Third, percutaneous pedicle screw systems now permit placement of posterior stabilization to a theoretically unlimited number of levels without paraspinal muscle dissection. Finally, various other advances in instrumentation placement systems and retractor systems are dramatically decreasing the size of the exposures for many procedures.

Lumbar Diskectomy

Various minimally invasive techniques have been described to treat lumbar disk herniations. The most studied and most accepted of these involves a traditional lumbar microdiskectomy performed through a tubular retractor. An operating microscope or an endoscope can be used for visualization depending on surgeon preference. This procedure is distinguished from the so-called percutaneous diskectomy or endoscopic diskectomy, whereby a trocar is directed into the disk space under fluoroscopic guidance, and disk material is accessed and removed from within the anulus with the aid of specialized instruments. Although good surgical results have been reported using the former technique, it has not gained widespread acceptance and remains conceptually difficult to most neurosurgeons.

Minimally invasive lumbar microdiskectomy, as it is known in the neurosurgical community, involves a similar procedure to traditional microdiskectomy but uses muscle dilators and a tubular retractor to access the interlaminar space with less soft tissue damage. It is performed routinely through tubes ranging from 14 to 22 mm in diameter and has been successfully applied to recurrent disk herniations9,10 and far lateral disk herniations,11,12 in addition to standard disk herniations. Originally developed using an endoscope for visualization, many practitioners use the operating microscope to perform the procedure through the same exposure, and excellent results have been reported.13 Whether a simple lumbar diskectomy using a tubular retractor has significant clinical advantage over an open diskectomy with a conventional subperiosteal muscle dissection and muscle retraction with a Taylor or Thompson-Farley retractor remains a matter of debate.14,15 However, when one considers the increased incisional length and muscle dissection required for proper exposure during a midline approach in heavy patients with typical disk herniations or in all patients with far lateral disk herniations, the advantages of using a tubular retractor centered directly over the site of pathology become obvious.

The procedure is performed as follows. Fluoroscopy is used to center the incision over the correct disk space, about 1 cm off of midline. Initially, a K wire is introduced through a stab incision to center the operation over the junction of the lamina and the inferior articular process of the rostral vertebral level. The incision is extended to accommodate the working channel. Progressively larger muscle dilators are passed, and a working channel of the appropriate length and desired diameter is introduced and fixed to a flexible arm. Cauterization of the remaining soft tissue exposes the inferior lamina, medial facet, and interlaminar space. Now, the procedure is performed in the standard fashion, although bayoneted instruments and an angled drill can be helpful to allow an unobstructed view of the operative field. An endoscope permits a superior view of the operative field and a more comfortable operating position but requires habituation to operating in two dimensions.

Stenosis Decompression

Although the advantages of minimally invasive techniques for microdiskectomy may be debatable, the advantages are clearer for patients with lumbar stenosis requiring a bilateral decompression. Through the same-sized incision as a microdiskectomy, a one-level or two-level stenosis decompression can be performed. Several variations of this procedure have been described, but all share the essential strategy of a bilateral decompression through a hemilaminar approach. This strategy has been proved feasible and effective in large clinical series.1,16

The incision is slightly more lateral than for a microdiskectomy, typically 1.5 cm off midline, to optimize the angle for contralateral decompression, but the process of retractor placement and the final retractor position are the same. An osseous foraminotomy is performed leaving the ligamentum flavum intact for dural protection. After an initial ipsilateral decompression is performed, the retractor is redirected contralaterally. Pulling back the working channel a few millimeters can facilitate this redirection. The base of the spinous process is cleaned of soft tissue and drilled away, using a hemilunar sleeve to protect the dura. The contralateral pedicle and foramen are palpated, and the inner table of the lamina and contralateral facet are drilled away. At this point, the ligamentum flavum is removed, and further osseous decompression of the contralateral foramen can be performed with a drill or Kerrison rongeur as necessary. Fluoroscopy can be useful to confirm the extent of contralateral decompression. After the contralateral decompression, the working channel is redirected again and the ipsilateral foraminotomy is completed.

For two-level decompressions, the initial dilation should be performed at the midpoint of the intermediate lamina. Once the working channel is in place, the largest dilator is used to aim the working channel either rostrally or caudally to access the interlaminar space of the first level to be decompressed. After decompression of the first level, the channel is aimed in the other direction for decompression of the second level. Alternatively, the surgeon can perform separate dilations for each level through the same skin incision.

Lumbar Fusion

There are currently three minimally invasive techniques for lumbar fusion. The most widely practiced is the minimally invasive transforaminal interbody fusion (TLIF). The others are the transpsoas lateral interbody fusion, commercially termed XLIF (Nuvasive, San Diego, CA) and DLIF (Medtronic, Memphis, TN), and the transsacral L5-S1 screw, termed AxiaLIF or Trans1. The Trans1 screw is exclusively for L5-S1 fusion and involves a perirectal, transsacral approach to the L5-S1 disk space. The transpsoas, lateral interbody fusion can be applied to all interspaces except L5-S1. The minimally invasive TLIF is the most versatile and can be applied to all levels.

Minimally Invasive Transforaminal Interbody Fusion

The indications for minimally invasive fusions are the same as for open fusions. The best studied of these techniques is the minimally invasive TLIF developed by Foley and Fessler.2,17,18 Its advantages include less blood loss, decreased perioperative narcotic requirements, and decreased length of stay compared with open lumbar fusions. Beyond a clear perioperative superiority, surgical results from open and minimally invasive fusions are similar in terms of validated self-reported patient outcomes after 1 year.3

The procedure can be considered in two parts (Fig. 307-1): the interbody fusion and the percutaneous placement of pedicle screw fixation. An incision 2.5 to 3 cm long is made 3 to 5 cm off of midline. This distance is determined by the thickness of overlying soft tissues and can be measured on preoperative imaging to provide the desired working angle. A K wire is angled medially 35 to 45 degrees and docked on the facet joint. After muscle dilation, a working channel 22 to 26 mm is introduced directly over and perpendicular to the disk space, spanning the pedicles. After removal of residual soft tissue, the lamina, facet, and interlaminar space are identified. The canal is defined with curets, and a laminotomy is performed, exposing the ligamentum flavum. The inferior articular process is removed using an osteotome, drill, or rongeurs. All bone is saved for the interbody arthrodesis. The superior articular process is removed, as is the ligamentum flavum. The pedicle inferior to the disk space is identified, as are the exiting nerve root and disk anulus. The anulus is cleaned off of fat and epidural veins and incised. A series of pituitary rongeurs, curets, and scrapers are used to remove the intervertebral disk. An interbody graft is placed along with the arthrodesis material of the surgeon’s choice. Hemostasis is achieved, and the working channel is removed.

image

FIGURE 307-1 Minimally invasive transforaminal interbody fusion. A, The K wire is inserted through a stab incision and docked on the facet orthogonal to the disk space. B and C, Sequential dilators expand the opening to accommodate a 28-mm working port. D, After removal of a thin layer of residual soft tissue, the superficial osseous structures overlying the disk space are revealed (f, facet; is, interlaminar space; l, lamina). E, The plane between the lamina and the interlaminar ligaments is developed with an angled curet, and the level is confirmed. F, A laminotomy is performed, exposing the ligamentum flavum (lf). G-I, Using osteotomes under fluoroscopic guidance, the left inferior articular process of L4 is removed, revealing the left superior articular process (sap) of L5. The ligamentum flavum is removed, along with the facet overlying the disk space and nerve root. J, The thecal sac (ts), nerve root (nr), disk (d), and pedicle (p) are identified, and the epidural vessels over the disk space are cauterized and divided. K-M, A wedge chisel is used to enter the disk space (ds), and a diskectomy is performed using a variety of instruments. N and O, After end-plate preparation and packing of the disk space with rhBMP-2 and local bone, the interbody graft (g) is placed under fluoroscopic guidance. The working channel is removed, and pedicle screws are placed percutaneously. P, A hollow biopsy needle is used to guide trajectories using the bull’s-eye technique, and pedicles are cannulated using guidewires. A cannulated tap and screws permit percutaneous pedicle screw placement, and rods are swung into position through separate incisions using the Sextant system. Q and R, Intraoperative x-rays confirm satisfactory construct position.

Percutaneous pedicle screw fixation requires specialized equipment that is now available from a variety of manufacturers. On the side ipsilateral to the interbody fusion, pedicle screws can be placed through the existing incision. On the contralateral side, separate 1-cm incisions are made for each screw. The pedicles are cannulated using the bull’s-eye technique, whereby the center of the pedicle is found with the tip of a large-bore bone biopsy needle guided by anteroposterior fluorography. The needle is then rotated so that the needle shaft is completely in line with the axis of the pedicle. The surgeon ideally will see concentric rings (pedicle, screw cap, and stylet cap) resembling a bull’s eye. The cap and stylet are removed, and a K wire is inserted 2 cm into the pedicle using a power drill. The needle is removed, and another anteroposterior x-ray ensures that the cortex of the pedicle visually circumscribes the K wire. This procedure is repeated for each pedicle.

Next, the C-arm is rotated into the lateral position for screw placement. If the K-wire placement and trajectory is satisfactory on lateral fluorography, the K wires are advanced into the vertebral body. Dilators are placed over the K wire to clear soft tissue. The screw hole is tapped with a cannulated tap, and then a cannulated screw is placed. Screws are attached to specialized extenders that guide rods into the screw heads and ensure proper alignment of locking screws. The mechanism of rod passage varies according to manufacturer, as do mechanisms for compression and reduction.

Transpsoas Lateral Interbody Fusion

The advantages of the transpsoas lateral interbody fusion are the ability to insert a much larger graft, the avoidance of the epidural space, scar tissue, and the absence of any manipulation of the paraspinal musculature. The disadvantages include risk to the abdominal contents; risk to the lumbosacral plexus within the psoas muscle and the genitofemoral nerve, which lies on top of the psoas muscle; and the inability to perform a direct neural decompression. This technique also has the potential to correct deformity in cases of flexible spine, although its role as a corrective technique is still being evaluated.

The transpsoas lateral interbody fusion can be summarized as follows (Fig. 307-2). The patient is placed in the lateral position on a standard operating table with the table break centered between the iliac crest and the bottom of the rib cage. The approach can be performed either from the left or from the right. In cases of scoliosis, graft placement is often facilitated by an approach from the convex side because it provides better access to the disk space. An axillary roll is placed to protect the dependent shoulder. The table is broken to maximize the working space between the rib cage and the iliac crest, and the patient is immobilized using a beanbag and cloth tape. A lateral fluoroscopic x-ray is taken, and the table is adjusted to ensure that the target disk space is perfectly in line with the fluoroscopic beam. The skin is then marked over a point that is in the center of the anterior third of the disk space. A 2- to 3-cm transverse incision is made, and after subcutaneous tissue dissection, the criss-crossing fibers of external and internal obliques are identified, split, and retracted using Army-Navy retractors. After this, the transversalis fascia is seen and sharply divided to expose the fat within the retroperitoneal space. A separate, more dorsal incision can be used to access the retroperitoneum at a point at which the peritoneal contents are more distant. In this instance, the surgeon enters the retroperitoneum through the second incision, develops a space under the true lateral incision, and then dissects down onto the surgeon’s finger.

image

FIGURE 307-2 Lateral transpsoas interbody fusion and posterior instrumentation. A, The patient is positioned laterally over a table break. B, After access into the retroperitoneal space, the abdominal contents are mobilized anteriorly, and safe corridor to the disk space is achieved using tactile feedback, a neurostimulator, and fluoroscopic guidance. A K wire is inserted into the disk space, and the muscle is dilated. C and D, The retractor is placed, the blades are opened, and the retractor is secured over the disk space. E, A diskectomy is performed, and the interbody graft is placed. F, The process is repeated for the next disk space. The interbody grafts are supplemented with pedicle screw fixation placed after a midline incision and suprafascial dissection. H and I, The pedicles are cannulated with K wires, and cannulated taps and screws are used. J, Specialized screw extenders facilitate rod passage. K and L, An L1-5 fusion is performed entirely using minimally invasive techniques.

Once inside the retroperitoneum, blunt finger dissection mobilizes the peritoneal contents anteriorly as the surgeon develops a safe corridor to the psoas muscle. The first landmark palpated is usually a transverse process, then the psoas muscle. A probe stimulator is introduced and advanced into the disk space under fluoroscopic guidance. The lumbosacral plexus tends to lie in the posterior third of the psoas muscle, so the surgical corridor is safest in the anterior third of the muscle, which approximates the anterior third of the disk space. If the probe or stimulator does not indicate a nerve root in the surgical corridor, a K wire is placed into the disk space.

The fluoroscope is shifted to the anteroposterior position, and successive dilators are placed over the K wire. Ultimately, the tubular retractor is placed, expanded, and secured over the disk space. Under direct visualization, the residual soft tissue over the disk space is inspected and tested with a stimulator for nerves; then, if clear, it is dissected away. A diskectomy is performed, and the end plates are prepared. A critical step is the disruption of the anulus and lateral osteophytes on the contralateral side of the disk space. This is effectively and quickly performed using a Cobb retractor guided by fluoroscopy. A graft is then placed with choice of arthrodesis material. The graft should cover the entire width of the end plate and, ideally, sit in the anterior third of the disk space to maximize lordosis. Multiple disk spaces can often be accessed through the same small incision, but each requires a separate muscle dilation. The interbody fusion can be supplemented with vertebral body screws or percutaneous pedicle screws.

Although the earliest systems for minimally invasive lumbar fusion were limited to three vertebral segments, numerous companies now offer technologies that permit longer constructs to a theoretically unlimited length. Such advances have opened the door for minimally invasive techniques to affect deformity. Currently, we place long construct posterior lumbar instrumentation after a midline skin incision and suprafascial dissection. Pedicle screws are placed using the same technique as for percutaneous screws. A combination of minimally invasive posterior and interbody techniques can be used to generate 360-degree fixation over multiple vertebral segments.

Intradural Surgery

The traditional exposure to intradural pathology entails pedicle-to-pedicle removal of the posterior elements, including the spinous processes, intraspinous and supraspinous ligaments, laminae, and part of the facet complex. Although this approach is fairly well tolerated, postlaminectomy deformities can occur at significant rates in high-risk patients, particularly if the exposure involves the thoracolumbar junction. Hemilaminar approaches to intradural tumors were described by Yasargil and Chiou in the early 1970s, and since then, many authors have reported experience with intradural tumor removal using a hemilaminar exposure with a traditional subperiosteal dissection. This approach has been further modified through the use of muscle-splitting tubular retractors, which minimize damage to the paraspinal musculature, and guarded drills, which enable the safe removal of the base of the spinous process and inner table of contralateral lamina, increasing visualization.

The procedure requires a tubular, expandable retractor and has been applied to intradural tumor resections requiring a two-level laminectomy (Video 307-1).

Retractor placement is performed through the same process of muscle dilation previously described. The initial incision is increased, and the blades of the retractor are opened. Hemilaminotomies are performed, and the base of the spinous process and ventral portion of the contralateral lamina are drilled away. The ligamentum flavum is removed to expose the dura. The contralateral pedicles are palpated with a Penfield 4 to ensure that the contralateral exposure is sufficient. The dura is opened in the midline and tacked to the soft tissues, and the tumor is removed using standard techniques. The dura is sutured primarily with a running 4-0 stitch. Specialized, minimally invasive dural closure instruments are commercially available, or a Castro-Viejo needle driver can be used. There are other alternative dural closure devices and materials that may prove to be useful for these cases. The closure can be supplemented with commercially available sealants. In our anecdotal experience, this approach greatly decreases the rate of cerebrospinal fluid leak after intradural surgery because the muscle naturally expands into the void created by the retractor, eliminating the dead space through which the surgery was performed.

Video Minimally Invasive L3-4 Schwannoma Resection

After skin incision and muscle dilation, an expandable tubular retractor is placed. The blades are opened in a rostral-caudal fashion, and additional lateral and medial retractors are added. The soft tissues are dissected off of the ipsilateral laminae and the base of the spinous processes. Hemilaminectomies of L3 and L4 are performed. The base of the spinous process is drilled away along with the ventral cortex of the contralateral lamina. The ligamentum flavum is removed, and a pedicle-to-pedicle exposure is achieved. After hemostasis, the dura is opened in the midline, and the tumor is removed using standard techniques. The dura is closed primarily, in this case with clips, although we usually use a running 4-0 suture. A sealant is added to aid closure.

Conclusion

Minimally invasive techniques have evolved to affect all facets of lumbar spine surgery from pure decompressive procedures for degenerative disease to intradural surgery for tumors. Many of the techniques are nascent and will continue to evolve. The immediate benefits for patients are less postoperative pain and faster recovery times. The attendant preservation of the paraspinal soft tissues may improve long-term outcomes and increase the durability of many procedures.

Suggested Readings

Asgarzadie F, Khoo LT. Minimally invasive operative management for lumbar spinal stenosis: overview of early and long-term outcomes. Orthop Clin North Am. 2007;38:387-399.

Chiou SM, Eggert HR, Laborde G, Seeger W. Microsurgical unilateral approaches for spinal tumour surgery: eight years’ experience in 256 primary operated patients. Acta Neurochir (Wien). 1989;100:127-133.

Fessler RG. Minimally invasive percutaneous posterior lumbar interbody fusion. Neurosurgery. 2003;52:1512.

Foley KT, Holly LT, Schwender JD. Minimally invasive lumbar fusion. Spine. 2003;28:S26-S35.

Foley KT, Smith MM, Rampersaud YR. Microendoscopic approach to far-lateral lumbar disc herniation. Neurosurg Focus. 1999;7:e5.

German JW, Adamo MA, Hoppenot RG, et al. Perioperative results following lumbar discectomy: comparison of minimally invasive discectomy and standard microdiscectomy. Neurosurg Focus. 2008;25:E20.

Harrington JF, French P. Open versus minimally invasive lumbar microdiscectomy: comparison of operative times, length of hospital stay, narcotic use and complications. Minim Invasive Neurosurg. 2008;51:30-35.

Isaacs RE, Podichetty V, Fessler RG. Microendoscopic discectomy for recurrent disc herniations. Neurosurg Focus. 2003;15:E11.

Isaacs RE, Podichetty VK, Santiago P, et al. Minimally invasive microendoscopy-assisted transforaminal lumbar interbody fusion with instrumentation. J Neurosurg Spine. 2005;3:98-105.

Khoo LT, Fessler RG. Microendoscopic decompressive laminotomy for the treatment of lumbar stenosis. Neurosurgery. 2002;51:S146-S154.

Le H, Sandhu FA, Fessler RG. Clinical outcomes after minimal-access surgery for recurrent lumbar disc herniation. Neurosurg Focus. 2003;15:E12.

Oktem IS, Akdemir H, Kurtsoy A, et al. Hemilaminectomy for the removal of the spinal lesions. Spinal Cord. 2000;38:92-96.

O’Toole JE, Eichholz KM, Fessler RG. Minimally invasive far lateral microendoscopic discectomy for extraforaminal disc herniation at the lumbosacral junction: cadaveric dissection and technical case report. Spine J. 2007;7:414-421.

Perez-Cruet MJ, Foley KT, Isaacs RE, et al. Microendoscopic lumbar discectomy: technical note. Neurosurgery. 2002;51:S129-S136.

Schwender JD, Holly LT, Rouben DP, Foley KT. Minimally invasive transforaminal lumbar interbody fusion (TLIF): technical feasibility and initial results. J Spinal Disord Tech. 2005;18(suppl):S1-S6.

Tredway TL, Musleh W, Christie SD, et al. A novel minimally invasive technique for spinal cord untethering. Neurosurgery. 2007;60:ONS70-ONS74.

Tredway TL, Santiago P, Hrubes MR, et al. Minimally invasive resection of intradural-extramedullary spinal neoplasms. Neurosurgery. 2006;58:ONS52-ONS58.

Yasargil MG, Tranmer BI, Adamson TE, Roth P. Unilateral partial hemi-laminectomy for the removal of extra- and intramedullary tumours and AVMs. Adv Tech Stand Neurosurg. 1991;18:113-132.

References

1 Khoo LT, Fessler RG. Microendoscopic decompressive laminotomy for the treatment of lumbar stenosis. Neurosurgery. 2002;51:S146-S154.

2 Isaacs RE, Podichetty VK, Santiago P, et al. Minimally invasive microendoscopy-assisted transforaminal lumbar interbody fusion with instrumentation. J Neurosurg Spine. 2005;3:98-105.

3 Schwender JD, Holly LT, Rouben DP, Foley KT. Minimally invasive transforaminal lumbar interbody fusion (TLIF): technical feasibility and initial results. J Spinal Disord Tech. 2005;18(suppl):S1-S6.

4 Chiou SM, Eggert HR, Laborde G, Seeger W. Microsurgical unilateral approaches for spinal tumour surgery: eight years’ experience in 256 primary operated patients. Acta Neurochir (Wien). 1989;100:127-133.

5 Oktem IS, Akdemir H, Kurtsoy A, et al. Hemilaminectomy for the removal of the spinal lesions. Spinal Cord. 2000;38:92-96.

6 Tredway TL, Santiago P, Hrubes MR, et al. Minimally invasive resection of intradural-extramedullary spinal neoplasms. Neurosurgery. 2006;58:ONS52-ONS58.

7 Yasargil MG, Tranmer BI, Adamson TE, Roth P. Unilateral partial hemi-laminectomy for the removal of extra- and intramedullary tumours and AVMs. Adv Tech Stand Neurosurg. 1991;18:113-132.

8 Tredway TL, Musleh W, Christie SD, et al. A novel minimally invasive technique for spinal cord untethering. Neurosurgery. 2007;60:ONS70-ONS74.

9 Isaacs RE, Podichetty V, Fessler RG. Microendoscopic discectomy for recurrent disc herniations. Neurosurg Focus. 2003;15:E11.

10 Le H, Sandhu FA, Fessler RG. Clinical outcomes after minimal-access surgery for recurrent lumbar disc herniation. Neurosurg Focus. 2003;15:E12.

11 Foley KT, Smith MM, Rampersaud YR. Microendoscopic approach to far-lateral lumbar disc herniation. Neurosurg Focus. 1999;7:e5.

12 O’Toole JE, Eichholz KM, Fessler RG. Minimally invasive far lateral microendoscopic discectomy for extraforaminal disc herniation at the lumbosacral junction: cadaveric dissection and technical case report. Spine J. 2007;7:414-421.

13 Perez-Cruet MJ, Foley KT, Isaacs RE, et al. Microendoscopic lumbar discectomy: technical note. Neurosurgery. 2002;51:S129-S136.

14 German JW, Adamo MA, Hoppenot RG, et al. Perioperative results following lumbar discectomy: comparison of minimally invasive discectomy and standard microdiscectomy. Neurosurg Focus. 2008;25:E20.

15 Harrington JF, French P. Open versus minimally invasive lumbar microdiscectomy: comparison of operative times, length of hospital stay, narcotic use and complications. Minim Invasive Neurosurg. 2008;51:30-35.

16 Asgarzadie F, Khoo LT. Minimally invasive operative management for lumbar spinal stenosis: overview of early and long-term outcomes. Orthop Clin North Am. 2007;38:387-399.

17 Fessler RG. Minimally invasive percutaneous posterior lumbar interbody fusion. Neurosurgery. 2003;52:1512.

18 Foley KT, Holly LT, Schwender JD. Minimally invasive lumbar fusion. Spine. 2003;28:S26-S35.

Related posts:

Brain Abscess Default ThumbnailPeripheral Neuropathies Spinal Cord Tumors in Adults Epilepsy Surgery Medulloblastoma in Children Continuous Electroencephalography in Neurological-Neurosurgical Intensive Care