Minimally Invasive Resection of Intradural Spinal Tumors

48 Minimally Invasive Resection of Intradural Spinal Tumors

Trent L. Tredway and Mick J. Perez-Cruet

Abstract

Increasingly surgeons are performing more complicated cases using minimally invasive spine (MIS) techniques. This chapter discusses resection of intradural tumors using MIS techniques. Critical to the safe and effective resection of these tumors using MIS techniques is patient selection. Not all intradural tumors can be safely resected using MIS techniques and it is up to the surgeon to carefully select which patient is an appropriate candidate. This chapter reviews commonly seen intradural tumors including meningiomas, ependymomas, and schwannoma.

Keywords: extradural, intradural extramedullary, intramedullary, dural closure, minimally invasive, meningioma, schwannoma

48.1 Introduction

Primary tumors of the spinal cord represent 2 to 4% of all primary tumors of the central nervous system (CNS). It is estimated that there are 850 to 1,700 new adult cases of primary spinal cord tumors diagnosed each year in the United States.¹ The majority of primary spinal cord tumors are classified as low grade (grades I and II) according to the World Health Organization (WHO) pathology classification.

48.2 Types of Spinal Cord Tumors

Primary spinal cord tumors are divided into three categories based on anatomic location²:

• Extradural.

• Intradural extramedullary.

• Intramedullary.

48.2.1 Extradural Spinal Cord Tumors

Most extradural spinal cord tumors in adults are a consequence of systemic metastases originating from lung, breast, or prostate cancer, or lymphoma, and clinically result in epidural spinal cord compression.³^,⁴^,⁵ Extradural primary tumors of the spine can be separated into benign and malignant types. Benign tumors include osteoid osteoma, osteoblastoma, osteochondroma, hemangioma, aneurysmal bone cyst, giant cell tumor, and eosinophilic granuloma. Malignant tumors include chordoma, multiple myeloma, osteosarcoma, chondrosarcoma, Ewing’s sarcoma, lymphoma, soft-tissue sarcomas, and plasmacytoma.

48.2.2 Intradural Extramedullary Spinal Cord Tumors

Schwannomas, neurofibromas, and meningiomas are the most common intradural extramedullary spinal cord tumors. Schwannomas are nerve sheath tumors that arise from the dorsal nerve root. They are benign tumors in the vast majority, although malignant peripheral nerve sheath tumors (MPNST) are quite aggressive and there is an increased incidence in patients with neurofibromatosis type II (NF2). Adolescents and young adults with NF2 often have multiple schwannomas and have a higher risk for malignant transformation. The anatomic location of the tumor is helpful for surgical planning and in determining if a minimally invasive approach is feasible ( Fig. 48.1).

Patients with schwannomas may be asymptomatic. However, most patients present with mild sensory symptoms, radicular pain, or paresthesias. If asymptomatic, schwannomas may be followed with serial imaging given their usual benign behavior. Symptomatic or radiographically enlarging tumors should undergo maximal safe resection. Surgery has minimal morbidity, improves symptoms, and may be curative. Incompletely resected tumors should be followed given the benign growth of the majority of these tumors. Malignant schwannomas (MPNST) should be treated with postoperative radiotherapy, even if total resection was achieved.

Fig. 48.1 Axial image of midthoracic intradural extramedullary meningioma with marked spinal cord compression. This particular tumor was approached via a conventional open multilevel thoracic laminectomy.

Neurofibromas are benign tumors that arise from peripheral sensory nerves. Solitary neurofibromas are discretely localized, globular, or fusiform nodules. Plexiform neurofibromas consist of nerve fiber bundles and tumor tissue intermixed in a disorganized pattern that extends over multiple nerve roots. Neurofibromas encase nerve roots rather than displacing them as is typical of schwannomas. Spontaneous pain and dysesthesias are the most common presenting symptoms. Patients with neurofibromatosis type 1 (NF1) may have multiple spinal cord neurofibromas that often increase in number with age. Patients with NF1 should be followed closely with serial imaging studies as there is a higher incidence of malignant transformation. On imaging, neurofibromas appear as rounded or fusiform tumors that are isointense on T1-weighted (T1W) images, hyperintense on T2-weighted (T2W)/fluid-attenuated inversion recovery (FLAIR) images, and intensely enhanced postcontrast. Since there are no imaging characteristics that differentiate malignant transformation of neurofibromas, a high index of suspicion is required, such as rapidity of tumor growth.

Patients with symptomatic or enlarging solitary neurofibromas should undergo surgical resection. Complete resection with minimal morbidity is usually achieved and minimally invasive techniques may be employed in many cases.⁶^,⁷^,⁸ The clinical results following resection of a plexiform neurofibroma associated with NF1 are poor because complete resection is rarely achieved. Furthermore, plexiform neurofibromas may undergo malignant transformation.

Meningiomas are dural-based tumors that arise from arachnoid cap cells and are found throughout the CNS. Approximately 25% of all primary spinal cord tumors are meningiomas ( Fig. 48.1). Most are slow-growing low-grade tumors (WHO grade 1). Genetic predisposition (NF2) and prior exposure to ionizing radiation are the only definite risk factors. Common presenting symptoms include back pain, motor weakness, sensory disturbance, and incontinence.

Meningiomas appear as solid, well-circumscribed lesions with an attachment to the dura on MRI. The tumor is isoto hypointense on T1 W MRI and slightly hyperintense on T2 W/FLAIR images. Meningiomas display intense, homogenous contrast enhancement, and the majority of all spinal cord meningiomas are intradural extramedullary, while few are extradural.

Asymptomatic patients with spinal cord meningioma can be followed clinically with serial imaging studies. If treatment is indicated, surgery is the primary modality and can be curative with complete resection. Conventional external beam–fractionated radiotherapy or stereotactic radiosurgery is utilized for patients with incomplete resection or recurrence.

48.2.3 Intramedullary Spinal Cord Tumors

Intramedullary spinal cord tumors (IMSCT) constitute 8 to 10% of all primary spinal cord tumors with the majority comprising gliomas (80–90%), of which 60 to 70% are ependymomas and 30 to 40% are astrocytomas.⁹ The third most common IMSCT is hemangioblastoma, representing approximately 3 to 8% of all IMSCT, of which 15 to 25% are associated with von Hippel–Lindau (VHL) syndrome.¹⁰^,¹¹^,¹²

The clinical presentation of primary spinal cord tumors is determined in part by the location of the tumor and in nearly all clinical instances pain is the predominant presenting symptom. In a recent series of IMSCT, pain was the most common presenting symptom (72%) and may manifest as back pain (27%), radicular pain (25%), or central pain (20%). Motor disturbance was the next most common presenting symptom (55%), followed by sensory loss (39%).¹³ Diagnosis of a primary spinal cord tumor requires a high index of suspicion based on clinical signs and symptoms as well as spine-directed MRI.

Astrocytomas and ependymomas represent the most common intramedullary neoplasms. It is estimated that the intracranial-to-spinal ratios for astrocytomas and ependymomas are 10:1 and 3:1 to 20:1 (depending on the histological variant), respectively.¹⁴ The clinical presentation of an intramedullary tumor is variable, but pain and a mixed sensorimotor tract disturbance are usually present.

On MRI, an intramedullary tumor is radiographically recognized by focal, and sometimes holocord, spinal cord expansion with associated T2 W and FLAIR image hyperintensity, T1 W hypoor isointensity, variable contrast enhancement, and occasional tumor-associated syrinx.³

Ependymomas are the most frequent IMSCT in adults.¹^,¹⁵ Histologically, there are two distinct pathological types: cellular (WHO grades 2 and 3) and myxopapillary (WHO grade 1). Cellular (classic) ependymoma arises from the intraspinal canal of the cervical and thoracic cord. Myxopapillary ependymomas arise from the filum terminale and occur almost exclusively at the conus medullaris. The treatment prognosis for spinal cord ependymomas is often excellent as these tumors may be resected completely and in such instances manifest a low recurrence risk.¹³^,¹⁶^,¹⁷

Ependymomas appear as a focal enlargement of the cord and hyperintense on T2 W and FLAIR images and hypoor isointense to normal spinal cord on T1 W images with heterogeneous contrast enhancement.³ These tumors may also be associated with cystic changes, hemosiderin suggestive of previous hemorrhage, and syrinx.

Ependymomas most often are low grade with a benign indolent course, although malignant histological subtypes (anaplastic ependymoma; WHO grade 3) rarely occur. Surgery is the most effective treatment, with complete surgical resection yielding reported local control rates of 90 to 100%, although gross total resection is not achieved in the majority of patients.¹⁷^,¹⁸^,¹⁹ Intraoperative monitoring of motor evoked potentials (MEPs) and somatosensory evoked potentials (SSEPs) is often utilized to assist in achieving a more safe and complete resection.²⁰^,²¹^,²²

Approximately 40% of IMSCT are astrocytomas.¹⁵^,²³ The majority (75%) are low-grade (WHO grade 2) fibrillary astrocytomas with 5-year survivorship exceeding 70%.¹³^,¹⁵^,²⁴ Histology is the most important prognostic variable.²⁵^,²⁶^,²⁷ Juvenile pilocytic astrocytoma, or JPA, is a low-grade (WHO grade I) variant that more commonly presents in younger patients. High-grade spinal cord gliomas (WHO grades 3 and 4, 25%) are less common and associated with a poor survival. Regardless of WHO grade, spinal cord astrocytomas are infiltrative and associated with poorly characterized boundaries and consequently are typically biopsied only.

Astrocytomas appear on MRI as fusiform expansion of the cord with indistinct and occasionally a cystic component.³ Associated edema or syrinx (seen in 40%) may be present. The tumor is hypo- to isointense on T1 W images, and hyperintense on T2 W and FLAIR images, with variable contrast enhancement. In general, the distinction between astrocytomas and ependymomas by MRI is not always possible.

Initial treatment consists of maximal safe surgical resection or biopsy, followed by observation or external beam radiotherapy. Because spinal cord gliomas are infiltrative, gross total resection is rarely accomplished (in ~12% of WHO grade 2 and 0% of grade 3 or 4 astrocytomas).¹³ The optimal extent of surgical resection and need for postoperative radiotherapy is controversial. Tumor histology, extent of resection, and functional status at time of presentation appear to be the primary determinants of outcome.¹⁰^,¹³ Nonetheless, radiotherapy is indicated for patients with high-grade histology, tumors in which a substantial resection cannot be performed, biopsied-only tumors, and those with progressive disease. Though rare in adults, the majority of spinal cord juvenile pilocytic astrocytomas can occasionally be completely resected (up to 80%).¹³

Hemangioblastomas, the third most common IMSCT, are rare vascular tumors that occur as a solitary tumor or as part of VHL syndrome.⁴^,⁵^,⁶^,⁷ Approximately 10 to 30% of patients with hemangioblastoma of the spinal cord have VHL syndrome, an autosomal-dominant disorder caused by a deletion on chromosome 3p. Other tumors associated with VHL include retinal hemangiomas, renal and pancreatic cysts, pheochromocytomas, and renal cell carcinomas. Whether hemangioblastoma occurs as part of the VHL syndrome or solitarily, the clinical and histopathologic characteristics are identical. There is a male predominance, and presentation is usually in the fourth decade.¹¹^,²⁸ Hemangioblastomas have unique imaging characteristics of enhancing nodules in association with nonenhancing cysts ( Fig. 48.2).

The majority of hemangioblastomas arise from the dorsal or dorsolateral portion of the spinal cord.⁹^,¹¹^,²⁸ As such, presenting symptoms are usually sensory, especially slowly progressive proprioception deficits. There may also be other long tract signs and radicular symptoms. Rarely, patients present with subarachnoid or intramedullary hemorrhage.²⁹^,³⁰^,³¹^,³²^,³³

On MRI, the hemangioblastomas appear as a homogenously enhancing hypervascular nodule with associated cyst or syrinx and peritumoral edema.¹¹ Spinal angiography demonstrates enlarged feeding arteries, intense nodular stains, and early-draining veins.³ Hemangioblastoma can be differentiated from ependymoma by the vascular abnormalities on MRI and angiography if performed. Hemangioblastoma is differentiated from a spinal cord vascular malformation by associated syrinx and tumor enhancement on MRI.

Surgical resection is the primary treatment for spinal hemangioblastomas. There are often well-defined margins allowing for a complete resection. Excessive intraoperative bleeding, obscuring the operative field, is the limiting factor for subtotal resection.¹¹ In contrast to posterior fossa hemangioblastomas, preoperative embolization is usually not performed as complications have been reported.³⁴^,³⁵^,³⁶^,³⁷ Serial MRI should be obtained, as de novo lesions can appear in patients with VHL. Stereotactic radiosurgery is an option for patients with recurrent or unresectable tumors.³⁸

Other rare primary IMSCT include primary CNS lymphoma, germinoma, melanoma, primitive neuroectodermal tumor (PNET), paraganglioma, teratoma, dermoid cyst, epidermoid cyst, lipoma, and hamartoma. Treatment following diagnosis is similar to that of intracranial counterparts and in most instances (except for lymphoma, germinoma, and PNET) surgery suffices as primary therapy.

48.3 Surgical Treatment of Intramedullary Tumors

The treatment of IMSCT has been largely based on open surgical biopsy or resection if possible. Successful surgical resections with good clinical outcomes have been reported in the literature with traditional open surgery.³⁹^,⁴⁰^,⁴¹^,⁴²^,⁴³ However, surgical morbidity should also be considered when resecting these lesions.

Open surgical intervention requires adequate exposure with removal of the posterior spinal elements. These structures are also important in maintaining alignment of the spinal column. Extensive removal of the posterior elements can be associated with increased pain and increased blood loss, and can lead to worsening kyphotic deformities of the spine.⁴⁴ This is more pronounced in the cervical spine, but can also occur in the thoracic spine. It has been reported in the literature that there has been an increased risk of kyphosis in the pediatric population after patients underwent surgeries that required greater than three levels of laminectomy for the intradural tumor resection.⁴⁴ Therefore, some neurosurgeons perform laminoplasties in the pediatric population in order to decrease the risk of postsurgical kyphotic deformity as the patient ages.