CHAPTER 89 Intradural Tumors
Intradural Tumors
The era of spinal tumor surgery began in London in 1887 when the influential surgeon Dr. Victor Horsley removed a thoracic intradural extramedullary tumor from a 42-year-old British Army General at the urging of the neurologist Dr. William Gowers.1 The patient presented with a 4-year history of progressive symptoms that had culminated in urinary incontinence, spasticity, and paraplegia. Dr. Gowers diagnosed the lesion as a thoracic compressive lesion on the basis of a history and clinical examination and proposed surgery. After a thoracic laminectomy and removal of a compressive fibromyxoma, the patient made a remarkable recovery, regained the ability to walk, returned to work, and lived 20 additional years without recurrence. As a result, Dr. Horsley became a proponent of surgery as a viable treatment option for spinal tumors.
The next advancement in spinal tumor surgery was the first successful intramedullary spinal cord tumor removal in 1907 by Dr. Anton von Eiselsberg in Austria; however, the first published report of a successful intramedullary tumor removal is credited to Dr. Charles Elsberg in 1911 at the New York Neurological Institute.2,3 Soon thereafter in 1916, Elsberg published his influential book Diagnosis and Treatment of Surgical Diseases of the Spinal Cord and Its Membranes. He later published the first large series of intradural spinal cord tumor resections in 1925.4 In his early writings, Elsberg advocated a two-staged surgery for intramedullary tumors: a laminectomy and myelotomy followed 1 week later by removal of the tumor that had delivered through the myelotomy. However, as his experience grew, he transitioned into performing single-stage operations for intramedullary tumor removal. For these significant early contributions he is remembered as an important figure in spinal cord surgery.
Over the ensuing decades, the widespread practice of intraspinal tumor surgery was hindered by difficulty with tumor localization, infections, neurologic morbidity, and poor outcomes. Throughout the mid 20th century, intradural and especially intramedullary tumors were treated with decompressive laminectomy, biopsy, and palliative radiation.5 In the 1960s, technologic advances such as loupe magnification, widespread use of myelography, and bipolar cautery began making good outcomes after surgery feasible and there was a renewed interest in aggressive surgical management.6 Even in the past 30 years, the evolution of technologies such as the operating microscope, microsurgical instruments, magnetic resonance imaging (MRI), ultrasonic cavitation, and electrophysiological monitoring has made aggressive surgical resection a low morbidity mainstay of spinal tumor treatment.
Epidemiology
The incidence of intradural spinal tumors is reported to be between 1 and 2 per 100,000 population, and they account for 5% to 10% of central nervous system (CNS) tumors.7–9 In adults, extramedullary tumors predominate, accounting for 60% to 75% of intradural masses with intramedullary tumors accounting for the remainder. In children, however, intramedullary tumors are more common, accounting for greater than 35% of intradural masses.9,10 Intradural tumors occur with equal frequency in both sexes, with the exception of a higher incidence of meningiomas in women. The most frequent intradural-extramedullary tumors are nerve sheath tumors and meningiomas with filum terminale ependymomas being a distant third.9–11 Ependymomas and astrocytomas are the most common intramedullary lesions followed by hemangioblastomas and cavernous malformations.9,10,12 One institution’s large experience is summarized in Table 89–1.
Clinical Presentation
Intradural tumors are typically slow growing, benign lesions that may be asymptomatic or minimally symptomatic for years before presentation. The most common presenting symptoms for intramedullary tumors are sensory deficits, motor weakness, gait ataxia, and pain.10 Pain is often worse at night and awakens the patient from sleep, but pain from a tumor may be difficult to differentiate from common degenerative back pain.13 In patients harboring slow-growing tumors, there is an indolent progression of symptoms from nonspecific pain and sensory disturbance to loss of balance, motor weakness, and other signs of myelopathy.14,15 Patients may present with symptoms at any point along this continuum; however, since the advent of the MRI, tumors are being discovered earlier, resulting in less severe deficits at presentation. Intramedullary tumors can be associated with a suspended dissociated sensory loss.7,16 This pattern of sensory disturbance is attributable to dilatation of the central cord by the tumor or an associated spinal cord syrinx causing an interruption of the pain and temperature fibers crossing in the anterior commissure. In fact, spinal axis imaging with contrast is an important step in the workup of an unexplained spinal cord syrinx in order to rule out an underlying spinal cord tumor.
The most common presenting symptoms for extramedullary tumors are as follows: pain, sensory disturbances, motor deficits, and gait ataxia. Schwannomas are derived from the nerve root sleeve and are more likely to present as unilateral radiculopathy, whereas meningiomas, derived from the dura, are more likely to present with diffuse pain or symptoms of cord compression.17,18 Yet these symptoms are not mutually exclusive, and patients often present with a mixture of nonspecific signs and symptoms.
The presenting signs and symptoms of intradural tumors are also influenced by the tumor’s spinal level.19 Both cervical and thoracic tumors can enlarge or compress the cord, resulting in cord impairment with prominent motor weakness and myelopathy. Cervical tumors are more likely to present with occipitocervical pain, arm pain, hand clumsiness, and sensory disturbances. Thoracic tumors often present with myelopathy and sensory disturbances in the lower body but can also produce pain that is mistakenly attributed to the heart and visceral organs. Tumors of the cauda equina typically cause back and leg pain that is classically worse while lying down and relieved by standing. This is thought to be due to positional pressure fluctuations in the epidural venous plexus.20
Radiologic Diagnosis
In the treatment of intradural tumors, the advent of MRI has allowed earlier tumor discovery, superior lesion characterization, precise localization, and improved surgical planning. MRI has supplanted computed tomography (CT) and contrast myelography as the imaging modality of choice in the diagnosis of intradural tumors. In most cases MRI is now a stand-alone imaging modality with CT for certain tumors, and CT myelogram is reserved for use in patients with contraindications to undergoing an MRI (e.g., those with implanted ferromagnetic objects).21 Plain radiographs are rarely important in the setting of intradural tumors but are useful when looking for abnormal alignment or structural instability in patients who have undergone a previous tumor resection.
In the evaluation of spinal tumors, MRI with gadolinium greatly increases the ability to identify a mass, show its relationship to the spinal cord, identify surrounding vascular structures, and give clues as to the pathologic diagnosis. Additionally, it can help differentiate tumors from less ominous lesions such as arachnoid cysts, lipomas, and neurenteric cysts. On T1-weighted images, intradural tumors are typically isointense or slightly hypointense to the spinal cord. In T2-weighted sequences, cerebrospinal fluid (CSF) has high signal and intramedullary tumors are often hyperintense. Spinal cord edema and syrinx associated with tumors are best identified with T2 images.22 After the intravenous administration of gadolinium (paramagnetic ions), T1-weighted MRI detects contrast that leaks through the compromised blood–spinal cord barrier at the tumor interface. The subsequent delay in T1 relaxation time by the gadolinium within the tumor is interpreted as enhancement. Most intradural tumors display some degree of contrast enhancement.
Extramedullary Tumors
Meningiomas
Intraspinal meningiomas are the most common type of intradural extramedullary tumors and occur predominantly in women (75% to 85%). They typically present in the fifth to seventh decades of life with pain, paresthesias, weakness, and long tract signs.23,24 Meningiomas are homogeneously enhancing, smooth-bordered masses on MRI and can contain calcifications that help differentiate them from nerve sheath tumors (Fig. 89–1). With suspected meningiomas, preoperative CT scans are helpful to determine the presence and extent of tumor calcification, which can significantly affect surgical strategy. Seventy-five percent of intraspinal meningiomas arise in the thoracic spine and are typically dorsolateral to the cord; however, cervical tumors (20%) do occur and are frequently located in the upper cervical region and ventral to the cord.20,25 Lumbar meningiomas are the least common. Spinal meningiomas are thought to arise from arachnoid cap cells in the dura near the nerve root sleeve laterally but may also grow from dural fibroblasts or pial cells, explaining their occasional ventral or dorsal location.26 Meningiomas invariably arise and derive their blood supply from a broad dural base that is often visible as a dural tail on MRI. These tumors are typically benign, encapsulated, slow-growing masses, but other growth patterns are seen including en plaque, atypical, and invasive malignant meningiomas.10,27 Treatment for symptomatic tumors is surgical with the goal of gross total resection; however, total removal may not be possible with en plaque or ventral calcified tumors.27,28 Overall, spinal meningiomas are amenable to surgical removal and have low recurrence rates after complete resection.
Nerve Sheath Tumors
Tumors of the nerve sheath account for up to 30% of spinal neoplasms and are categorized as either schwannomas or neurofibromas.28 They typically arise from the posterior sensory nerve root near its entrance into the neural foramen. Although both tumors are thought to be of Schwann cell origin, their significant histologic, epidemiologic, and biologic differences warrant discussing them separately.
Schwannomas are common intradural extramedullary neoplasms that make up 85% of nerve sheath tumors.18 They occur equally in men and women with a peak age of presentation between the fourth and sixth decades. Most appear as solitary tumors and occur equally throughout the spinal canal. Although rare, there are patients with multiple schwannomas but nearly always in the setting of neurofibromatosis 2 (NF2) and schwannomatosis.29 Radiographically, schwannomas are well-demarcated masses associated with a nerve root, intensely enhance, can include cystic areas, and may grow along the root into the extraspinal space (Fig. 89–2A). Up to 30% of schwannomas grow through the neural foramen, giving the classic “dumbbell” shape on imaging (Fig. 89–2B).18 At surgery, schwannomas are smooth, well-demarcated tumors that are discretely attached to an intact nerve root and can usually be separated from the root without nerve injury. The goal of surgery should be complete tumor removal and can usually be achieved. However, if complete resection is not possible, aggressive debulking with preservation of the root can relieve symptoms and the patient can be monitored radiographically for recurrence.
Neurofibromas are less common than schwannomas, accounting for approximately 15% of nerve sheath tumors, and more than half of them are associated with neurofibromatosis 1 (NF1).18,30 Patients with NF1 can harbor single or multiple neurofibromas and are more likely to have malignant tumors.31 Neurofibromas typically, but not exclusively, arise from the sensory root. Consequently, patients most commonly present with pain. Radiographically, neurofibromas are similar to schwannomas but are solid, not cystic. Yet they, too, can extend through the dural root sleeve as a dumbbell tumor (Fig. 89–3). Tumor cells characteristically arise from central nerve root fibers and expand the root, making dissection of the tumor from the root unrealistic. Optimal treatment for symptomatic neurofibromas includes total surgical removal of tumor, often necessitating the sacrifice of the nerve root.28
Ependymomas of the Filum Terminale
Ependymomas of this region are slow-growing tumors that arise from the filum terminale near the junction with the conus. At the terminal filum, ependymomas are thought to arise from ependymal rests ectopically deposited during development. They typically present in the third to fourth decades of life with slowly progressive low back pain, sensory deficits, and even sphincter and sexual disturbances.10 Histologically, filum terminale ependymomas are overwhelmingly of the benign myxopapillary pathological subtype, but cellular and papillary histiotypes do occur.32 On MRI, these tumors exhibit intense contrast uptake and can be anywhere on a spectrum from small, well-circumscribed tumors at the inferior tip of the conus to massive septated tumors filling the spinal canal (Fig. 89–4). An MRI of the neural axis should be obtained because cauda region ependymomas can disseminate along the neural axis via CSF or be the result of intracranial cranial tumor seeding to the spine. Therapy is surgical with a goal of gross total resection; however, this is often not achievable with massive tumors that are adherent to neural structures. Even after gross total resection of larger tumors, the recurrence rate is up to 20% and patients may need multiple surgeries for recurrences.33,34
Less Common ExtramedullaryTumors
Arachnoid Cysts
Although not tumors, these structures can appear as a cystic mass compressing the spinal cord on MRI. They are composed of arachnoid membranes that have become loculated with CSF under pressure. The most common place for symptomatic arachnoid cysts is in the thoracic region. They typically present with pain and signs of myelopathy. Surgical excision relieves spinal cord compression and is the mainstay of treatment for symptomatic lesions.35
Synovial Cysts
These extradural structures are cystic enlargements of the facet capsule that can enlarge to displace the thecal sac, compress the cord, or impinge nerve roots causing radiculopathy. They are most common in the lumbar region, especially at L4-L5, but have been reported in the cervical spine.36 These cysts are thought to result from excess motion at a vertebral level leading to facet capsule hypertrophy and cystic enlargement. These lesions are filled with synovial fluid and can mimic intradural lesions on plain MRI (Fig. 89–5). Large or persistently symptomatic lesions can be managed with image-guided percutaneous drainage or more definitively with surgical removal.37,38
Intramedullary Tumors
Ependymomas
Intramedullary tumors of the spinal cord are rare, accounting for 15% to 30% of intradural spinal lesions. Ependymomas are the most common type of intramedullary spinal cord tumor and usually present in the middle age years with pain, mild sensory changes, motor weakness, and signs of myelopathy.39 There is a curious association of intramedullary ependymomas and NF2. A significant portion of NF2 patients harbor intramedullary ependymomas, and genetic studies on tumor cells have shown NF2 gene mutations in spontaneous intramedullary ependymomas.40–42 Ependymomas of the cord are typically solitary tumors that arise from ependymal lining of the central canal, cause a diffuse enlargement of the cord over several levels, and have an associated syrinx in up to 50% of cases.43,44 Among the four ependymoma histologic subtypes, all are considered benign.12,45,46 On MRI, intramedullary ependymomas are sharply demarcated lesions that are isointense on plain T1 sequences and enhance homogeneously with gadolinium (Fig. 89–6A). Classic radiographic features of spinal cord ependymomas include distinct tumor–spinal cord border, an associated syrinx, cysts within or adjacent to the mass, and hemosiderin deposits or “caps” near the poles of the tumor on T1 and T2 (Fig. 89–6B).47 The treatment of choice is gross total surgical resection. In contrast to intracranial ependymomas, intramedullary spinal cord ependymomas have a good prognosis and a low rate of local recurrence after complete resection, but cases of malignant transformation to malignant pathology after resection have been reported.12,46,48–50
Astrocytoma
Astrocytomas are the second most common intramedullary spinal cord tumor in adults, representing 6% to 8% of intradural tumors. In children, they are the most common histologic type of intramedullary tumors, accounting for 60% to 90% of these lesions.9,51 Patients harboring astrocytomas present most commonly with pain, motor weakness, and gait ataxia that can either be of long duration or of relatively brief onset before presentation.10,52 The average symptom duration before presentation is between 12 and 29 months. Astrocytomas of the spinal cord are infiltrative tumors that are often eccentrically located dorsal or lateral to the central canal. Large tumors can expand and rotate the cord, making identification of landmarks difficult during surgery.12 On MRI, astrocytomas have intermediate T1 signal intensity, can demonstrate heterogenous or homogeneous T1 contrast enhancement, often contain cystic areas, and can rarely grow exophytically into the extramedullary space. The tumor boundaries may be better identified on T2 sequences (Fig. 89–7). In contrast to ependymomas, spinal astrocytomas infiltrate functional cord, have ill-defined borders with normal cord, and are consequently more difficult to remove successfully. Fortunately, intramedullary astrocytomas in adults are largely low grade (Kernohan grade I and II) with approximately 25% considered high grade (Kernohan grade III and IV). In children, where pilocytic astrocytomas predominate, high-grade lesions are even less common, occurring at a rate of 10% to 15%.
