Surgical Management of Intramedullary Spinal Cord Tumors in Adults

Introduction

The neurosurgical literature on intramedullary spinal cord tumors (IMSCTs) contains many case reports and few large series, even for the tumors of glial origin, which are the most numerous. Despite occurring at any age group, these lesions are rare entities only accounting for 2% to 4% of all central nervous system tumors and 15% of all primary intradural tumors in adults. Ependymomas are the most common tumors in adults, and astrocytomas are the most common in children. ^, Hemangioblastomas and cavernomas have peculiar features that require specific strategies. ^, Other tumors of nonglial origin are even more exceptional. It is well known that IMSCTs have no typical clinical presentation.

At present, magnetic resonance imaging (MRI) is the best and, in most cases, the only diagnostic tool required to properly investigate IMSCT patients. However, although being highly accurate, MRI does not always provide a reliable differentiation between ependymomas and astrocytomas. Pathologic diagnosis thus remains the gold standard. Currently, evoked potentials (both sensory and motor) are a standard monitoring tool used during IMSCTs surgery. The ultrasonic aspirator is also used routinely and provides significant assistance.

In cases of benign IMSCTs, the surgical goal is to achieve a complete removal of the lesion whenever possible, irrespective of the histologic type. For malignant variants, infiltration of the spinal cord makes radical surgery not possible in the vast majority of cases.

The treatment of IMSCTs is not routine surgery: the operative duration is often very long and the procedure is always delicate and technically difficult. That is why neurosurgeons specializing in this field of neurosurgery are not numerous.

Anatomy

The spinal cord is located entirely within the vertebral canal. Rostrally, the spinal cord is in continuity with the caudal portion of the medulla at the level of the upper border of C1 nerve roots in front of the middle portion of the anterior arch of the atlas. The spinal cord is roughly cylindrical, ventrally, and dorsally slightly flattened, and its surface is whitish. It is 42 to 45 cm long and 1 cm wide in adults. It has an average weight of 30 g. It presents two enlargements: the cervical enlargement (approximately 10 cm long, extending from C4 to T1) and the lumbar enlargement (approximately 8 cm long, extending from T9 to T12 and in continuity with the conus medullaris). The conus ends at the level of the L1–L2 disc space, and it continues in the filum terminale—an atrophic remnant of the caudal segment of the embryonic spinal cord.

The ventral surface of the spinal cord is marked by the ventral fissure running along the entire length of the spinal cord. This 2 to 3 mm deep fissure splits the ventral aspect of the spinal cord into two symmetrical ventral columns, the lateral border of which gives rise to the ventral roots. The anterior spinal artery runs along the ventral aspect of the cord, sitting in a groove due to the superficial enlargement of the most superficial part of the ventral fissure. The lateral surface of the cord contains the lateral column, located between the entrance of the dorsal roots and the exit of the ventral roots. On the dorsal surface, a dorsal medial sulcus is present. Although it is not a fissure, it is possible to separate its edges to visualize the sulcocommissural arteries, which can be clearly identified under the microscope.

The spinal cord consists of gray matter surrounded by white matter. The gray matter has a typical H shape in cross section, and it is characterized by a vestigial central or ependymal canal, which runs the entire length of the spinal cord. The ependymal canal is the remnant of a larger embryonic central canal, which is nearly always completely obliterated in adults by ependymal cells or neuroglial clusters. Sometimes this vestigial canal persists over a few millimeters in length, but it lies in the central substantia gelatinosa and is lined with ependymal cells. Such an ependymal canal becomes visible on MRI in the shape of a “split central cavity” without pathologic meaning. In presence of IMSCTs, the vestigial canal may be dilated in hydrosyringomyelic cavities or in satellite cysts.

The spinal meninges differ from those of the brain, owing to the presence of a thicker pia mater, which is attached to the inner surface of the dura mater by means of the dentate ligaments. The medial borders of such ligaments are adherent to the lateral columns, all along the spinal cord. The lateral borders are free, with the exception of the areas adjacent to the roots, where the ligaments form relatively thicker membranes whose apices are attached to the dura mater in the space between two root sheaths. The arachnoid consists of a dense impermeable superficial layer in contact with the dura mater and of fenestrated dorsal septa connecting such superficial layer with the pia mater. These structures allow the spinal cord to remain connected to the meninges and the cerebrospinal fluid (CSF) to circulate freely in the subarachnoid space. The spinal dura mater encloses the spinal cord and the cauda equina from the foramen magnum to the sacrum. The diameter of the dural canal is smaller than that of the spinal canal, but it is much larger than the spinal cord. Therefore, the dura mater is separated from the spinal canal by the epidural space, containing fat and epidural venous plexuses. This meningeal structure allows the spinal cord to be slightly mobile within the spinal canal.

The spinal cord is vascularized by the anterior spinal artery, which arises from the vertebral arteries in the upper cervical region, and by the pial anastomotic network, which is supplied by the radiculospinal and radiculopial arteries running with spinal nerves. Because of their size, two ventral radiculospinal arteries have been distinguished: the artery of the lumbar enlargement (the Adamkiewicz artery), which runs with a spinal nerve on the left side in 75% to 85% of cases and between T9 and T12 in 75% of cases, and the artery of the cervical enlargement, which follows the course of a nerve root between C4 and C8. The anterior spinal artery has a mean diameter of 200 to 500 μm. The posterior blood supply is provided by discontinuous arteries of smaller size (100 to 200 μm). The pial network and the radially penetrating arteries supply the white matter, and the central or sulcocommissural arteries arising from the anterior spinal artery supply the gray matter. Finally, the territory of the anterior spinal artery includes the anterior two-thirds of the cord, and the remaining posterior third is supplied by dorsal vessels. The lack of anastomosis between central arteries and the pial network partially justifies the reputation of the midthoracic spinal cord as being “surgically fragile.”

Venous drainage takes place first via the intrinsic vessels, which then drain into the pial veins. The anterior spinal vein lies dorsal to the artery. The posterior dorsal vein, which is often very large (400 to 1000 μm), has a winding pattern, particularly in the thoracic region, and zigzags from one posterior column to the other over the posterior median sulcus.

The surgical approach of the intramedullary tumors depends on the anatomy of the cord and its vessels. The anterior approach to the ventral fissure is blocked by the anterior spinal artery and the branches arising from it. The posterior approach is not blocked by arteries or veins, but there is no open fissure on this side of the spinal cord. However, it is possible to open the dorsal medial sulcus by separating its edges without damaging the dorsal columns and their vessels.

Incidence, Types, and Prognosis

IMSCTs can be classified into three main groups: tumors of glial origin, tumors of nonglial origin, and pseudotumors.

Tumors of Glial Origin

The group of tumors of glial origin is by far mainly represented by ependymomas and astrocytomas. ^, Others, such as oligodendrogliomas, are rare. Of note, ependymomas originate from ependymal and not from glial cells; nevertheless, they are classically considered in this group of tumors.

Ependymomas

Spinal cord ependymomas ( Fig. 173.1 ) constitute the most common IMSCTs in adult patients (approximately 55% of cases in the adult population). ^{, , , , ,} Being usually slow-growing processes, they can reach a considerable size. Most ependymomas are benign masses (WHO grade II), while malignant ependymomas (WHO grade III) are rare. Since most ependymomas have a good plane of dissection, the first therapeutic approach is surgery. A large proportion of these lesions can be removed completely: in the literature, the rate of gross total resections varies from 69% to 97%. ^{, , , , , ,} As an adjuvant treatment, radiotherapy has been proposed by some authors, especially in cases of subtotal resections or recurrences. However, the actual advantage of radiation is still unclear, and other authors even found a negative impact on survival. As a general rule, we advise against performing radiotherapy for benign spinal tumors, and we suggest repeating surgery in case of recurrences. Up until now, no chemotherapeutic strategy has demonstrated significant benefits in the treatment of ependymomas.

The survival of ependymoma patients is good: the 5- and 10-year survival rates range between 83% to 96% and 80% to 91%, respectively. ^{, , , ,} The functional prognosis is also satisfying, as long as patients are treated when their conditions are still good: 10% to 40% of patients improve their neurologic function after surgery, 48% to 75% remain stable, and 9% to 15% deteriorate. ^{, , , ,} The notion that the majority of patients remain stable after surgery highlights the need for prompt treatment, especially in cases of worsening symptoms.

The extent of resection has been demonstrated to be the most important factor influencing the outcome of ependymoma patients. A recent cumulative literature analysis including all the published series with more than 318 patients showed that the probability for patients to be functionally independent after surgery was higher in cases of complete resection compared with subtotal resection. This confirms several previous studies and also our experience. ^{, ,} Furthermore, Tarapore and colleagues found the 10-year survival to be 90% after complete resection and 80% after subtotal resection. Interestingly, a recent analysis has also found that the tumor length is a relevant factor for postoperative outcome: tumors longer than 10 cm showed a reduced chance of gross total resection and an increased risk of postoperative deficits.

Astrocytomas

Accounting for about 45% of cases, spinal cord astrocytomas ( Fig. 173.2 ) are the second most common IMSCTs after ependymomas. ^{, , ,} Nevertheless, in children and adolescents, they are the most frequent subtypes, representing 60 to 80% of all IMSCTs. ^, Pathologically, both low-grade (WHO grade I and II) and high-grade (WHO grade III and IV) astrocytomas can be observed. The prevalence of the different grades varies across studies. In a series of 202 cases, Raco and colleagues found that about half of the tumors were grade II, 30% were grade I, and 20% were higher grades. Aggressive variants (anaplastic astrocytomas or glioblastomas) are more frequent in adults (25% to 30%) than in children (10% to 17%). ^{, , , ,}

Contrarily to ependymomas, radical resection for astrocytomas is most of the time not possible, given their infiltrative behavior. In these cases, surgery usually consists of subtotal/partial resection or, in some circumstances, in biopsy followed by decompression with duraplasty. The reported rates of complete and subtotal removal vary between 11% and 31% and between 21% and 62%, respectively. ^{, ,} The histologic subtype is the most important factor influencing outcome, as the WHO grade is associated with tumor aggressiveness and with the possibility of finding a dissection plane. In low-grade tumors, the 5-year survival exceeds 70%. ^, In high-grade tumors, the 5-year survival rate drops to 18.7%. Prognosis is particularly poor for spinal glioblastomas, where a complete resection can never be achieved and the median survival is only 10 months. A longitudinal database (Surveillance, Epidemiology, and End Results [SEER] database) encompassing 26% of the US population has been published, including 1814 patients suffering from a spinal cord glioma. In this study, age, histology, and grade were defined as significant predictors of outcome. Despite being a relevant factor influencing outcome in low-grade astrocytomas, in many studies, the extent of resection has not been found to be related to progression-free or overall survival for high-grade tumors. ^{, , , , , ,} However, a positive trend was reported for anaplastic astrocytomas.

In case of recurrence, no consensus treatment recommendations are currently available. Repeated surgery and radiotherapy have been proposed. However, the risks associated with radiotherapy of the spinal cord (especially for tumors affecting young patients) and repeated surgery (especially given the incapacity to obtain a gross total resection) make these choices at least controversial. Chemotherapy seems thus to be the most reasonable option. Following improved survival obtained for intracranial astrocytomas, alkylating agents (Temozolomide) have been tested for both high- and low-grade spinal astrocytomas. However, up until now, the results have been modest and associated with adverse events related to the agents used. ^,

Gangliogliomas

Gangliogliomas of the spinal cord are very uncommon lesions mainly reported as case reports and a small series. ^, These tumors develop mainly in children and young adults. In a large series of 56 patients, complete and subtotal resection has been obtained in 82% and 18% of the cases, respectively, with a 5-year survival rate of 88%.

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